Crab and lobster fisheries - stock assessments: results 2016 to 2019

4. General Discussion

4.1. Landings

Brown crab remains the most important species, in terms of landed weight, in the crab and lobster fisheries around Scotland. However, brown crab landings have declined in most areas during the last five years with a particularly large decrease observed in 2020. It is unclear if the 2020 decline is related to market prices, which have not increased substantially in the last three years, reduced effort or changes in the abundance of the offshore stocks. The brown crab landings reduction coincides with a decrease in the abundance and recruitment indices calculated for the species (this is discussed in section 4.3.4) and anecdotal evidence from Scottish fishermen who have been reporting decreased catch rates in recent years. The Covid pandemic in 2020 may have affected the fishery but the extent of the reduction in brown crab landings was not observed in either velvet crab or lobster. Total landings of velvet crab have been stable in the last seven years but have declined since the high level recorded in the mid 2000’s. There have been notable declines in reported landings from traditionally important areas including Orkney and South Minch. The spatial distribution of the velvet crab fishery is similar to previous years and the fishery continues to take place in inshore areas. Lobster landings, although still much lower than those of brown and velvet crabs, have more than doubled since 2001, with the East Coast and South East areas making the major contribution to this increase.

Landings of all three species are thought to have been under-reported prior to 2006, before the introduction of the UK ‘buyers and sellers’ legislation. Some major increases in landings evident in the mid-2000s are therefore more likely to be explained by improved reporting than by changes in the abundance of stocks or increased effort. There is little information on changes in fishing effort over the past 30 years, but it is likely that technological advancement and mechanisation of fishing and processing have allowed the crab and lobster fishery to expand and effort to increase. The emergence of European markets, and more recently, eastern markets in China, combined with the ability to transport live animals has increased the demand, particularly for brown crab. However, this was not accompanied by an increase in brown crab prices until 2016, when the price per kilo began to increase, reaching a peak value of £2.6 per kilo in 2018. Market and supply issues contributed to the setting up of a European project ACRUNET (A Transnational Approach to Competitiveness and Innovation in the Brown Crab Industry) in 2012. The project highlighted the disparity of fishery management measures in relation to the latent capacity in the fishery, between the main European crab producer countries, France on one hand and UK and Ireland on the other. Fishermen from both the UK and Ireland fishermen have emphasized the issue of latent effort, pointing out that this was one of the biggest obstacles to crab and lobster management. The latent effort was quantified as part of the ACRUNET project (Mesquita et al., 2015) and this information is likely to be useful in future discussions regarding the management of brown crab fisheries.

4.2. Stock Assessment

4.2.1. Brown Crab

Assessment areas with historically significant brown crab fisheries are relatively well sampled. However, the South Minch area is becoming more important for landings in recent years and is currently not as well sampled as the other key areas. In the northwest of Scotland, female crabs usually make the largest contribution to the landings. This suggests that females are exposed to higher exploitation than males on some of the offshore grounds such as the Hebrides or Papa. Tagging studies suggests that females migrate from deeper offshore grounds, where they live for most of the year, to inshore areas where they moult and mate with males (Edwards, 1979; Jones et al., 2010). Large females also predominate in the landings from offshore areas to the north of Ireland (Tully et al., 2006), and in the fisheries off the east coast of England and in the English Channel, aggregations of ovigerous females have been observed (e.g. Howard, 1982).

The evaluation of size-based indicators relative to reference points, allows for some inferences to be made on stock status in terms of exploitation level. In recent years, the mean size and mean size of the largest 5% of brown crabs were at or above the reference points for both males and females in Papa, Orkney and the Hebrides. This suggests that despite high exploitation in some offshore fisheries, brown crabs appear to have an extended size structure. The mean size of the largest individuals in Shetland increased throughout the time series, in particular for females. The mean size of the largest males has increased in recent years in Orkney and the South East. In contrast, the mean size of the largest individuals decreased in Sule and the North Coast in recent years; particularly for males. This could be indicative of a decrease in the proportion of large individuals in the stock, and possibly an increase in fishing mortality. It could, however, also be a reflection of changes in fishing or discard practices. Without further fishery information, it is difficult to conclude which is more likely.

Geospatial analysis of trawl and dredge survey data has allowed for a better understanding of the distribution of brown crab in Scottish waters. The recruitment (east and west coast) and abundance (all areas) indices show a general increase from 2008 until the 2013-2016 period and a decline in catch rates towards the end of the time series. This trend is similar across the three areas considered and is consistent with the recent reduction in brown crab landings and also with anecdotal reports from the fishing industry describing a general decrease in the fishery catch rates. There is a clear similarity in the crab abundance signal estimated from the dredge and scallop surveys in the east and west coast. The fact that distinct surveys using different gears show the same trend, indicates that active gear surveys can be used to estimate the distribution/abundance of brown crabs, even though dredging/trawling are not the main methods employed by the fishery to capture the species (Mesquita et al., 2021).

The length distributions obtained from the dredge surveys show that the mean size of brown crabs captured in the dredge surveys are smaller than those sampled in the landings. Brown crabs are more rarely caught on trawl surveys (compared with dredge surveys) and the mean size of trawl-caught crabs is generally above the mean size in landings for most years in the different assessment areas. The dredge surveys have a higher sampling effort than the trawl surveys with more stations per area. However, trawl surveys have a very large area coverage and include a variety of different sediment categories where not all ground types will be suitable for crabs. It is likely that the most coastal areas covered by dredge surveys targeting scallops coincide with areas of higher crab density. There are also differences in the catchability of trawl and dredge surveys and this subject is discussed further in section 4.3.

Assessments based on the 2016-19 data (using LCA) were carried out for brown crab in eleven of the twelve assessment areas. Assessment results using 2016-19 data for all assessed areas show a clear decline in fishing mortality estimates compared to those previously reported, however, most stocks are still fished well above F_MSY. The results from the LCA were in general agreement with the indicator evaluation in relation to reference points. In the Papa (males and females), Hebrides (males) and Orkney (females) areas both approaches suggested that these stocks were being fished below the reference points. The mean sizes of female brown crabs in the South East and East Coast assessment areas were smaller than in other areas, as found in previous assessments (Kinnear, 1988; Mesquita et al., 2017; Mill et al., 2009), but showing a slight recovery in the most recent 3 years. It should be noted that the growth parameters used in the assessments are the same for all areas and that this may not be appropriate if individuals in the East and South East are slower growing and reach smaller maximum sizes. Use of inappropriate growth parameters in the LCA could lead to overestimation of F and the conclusion that the stocks are more heavily fished than they actually are. This is discussed further in section 4.3 below.

4.2.2. Velvet Crab

Male velvet crabs were more common in the landings than females and were generally slightly larger. With respect to mean size and mean size of the largest 5% of individuals in 2016-19, all stocks of velvet crabs appeared to have truncated size distributions and are exploited above the F_MSY proxy. The mean size of males in the Hebrides indicated exploitation close to F_MSY (mean size just below L_F=M) in the most recent years. If the landings LFDs are representative of the population size structure and there have been no recent changes in the exploitation pattern, the increase in mean size and mean size of the largest individuals observed in these areas would indicate that the fishing mortality might have declined over the time series. A decrease in the mean size and the mean size of the largest individuals was observed in the South Minch, particularly for males. This may be due to changes in the fishery as fishing mortality estimated from both the LCA and LBI appears to show a downwards trend in recent years. Fluctuating trends in the mean size and the mean size of the largest individuals were observed in the East Coast, which may indicate (assuming no changes in the fishery) an increase in fishing mortality. These areas also show sporadic occurrences of large numbers of small individuals in the length frequency data, which could be due to increased recruitment in these years.

Assessments based on the 2016-19 data (using LCA) were carried out for velvet crab in seven of the twelve assessment areas. Fishing mortality for stocks in most of these areas remains above F_MSY. For all assessed areas, fishing mortality showed a decreasing trend from the 2013-15 assessment estimate (Mesquita et al., 2017) although the Clyde, East Coast and Orkney areas remained above F_MSY for both males and females. In the Hebrides, male velvet crabs were fished below F_MSY and females above F_MSY. Fishing mortality in the South Minch male stock has decreased compared to previous assessments and is now at F_MSY. Results based on the LBI analysis for velvet crabs were less optimistic than the LCA analysis in terms of the relative status of fishing mortality in relation to the reference point. However, the trends in the two methods are generally agreeing on a decreasing trend in fishing mortality in the last four years.

4.2.3. Lobster

Lobster market sampling data from 2016-19 suggest that male and female lobsters were generally landed in equal proportions. For most areas, the mean size has remained stable in recent years. Following the evaluation of size-based indicators relative to the reference points, males appear to be exploited above the F_MSY proxy (mean size less than L_F=M) in all areas. In recent years, the mean size in landings of the largest males has decreased in the Hebrides, Orkney and Shetland, which may indicate an increase in the long term fishing mortality, although this could also be a reflection of changes in the fishing practices. For females, indicators were generally below the reference points in 2016-19, with exception of the mean size of the largest individuals in the Orkney assessment area. In the Hebrides, East Coast, Shetland and Orkney, the mean sizes, mean sizes of the largest females and length frequency data appear relatively stable suggesting a stable stock and fishery.

Size at first capture was above size at first maturity only in the East Coast and South East.

The L_mat is likely to vary between assessment areas but the current estimates were derived from two regions: the East of Scotland (East Coast and South East data) and west of Scotland (Hebrides data). For example, it is expected that lobsters to the west of Scotland as well as at Orkney and Shetland mature at a larger size than to the east of Scotland. Currently, the evaluation of exploitation level on immature individuals should be viewed as preliminary, as size at first maturity is not known for all areas.

Assessments based on the 2016-19 data (using LCA) were carried out for lobster in eight of the twelve assessment areas. Fishing mortality for stocks in most of the areas remains above F_MSY, especially among the males. The results from the LCA were in general agreement with the indicator evaluation. Females in Papa, Hebrides, Orkney and South Minch were fished below F_MSY while in other areas F is above F_MSY. There are differences in mean size in the sampled landings between areas, with lobsters from the North Coast, Orkney and Hebrides being significantly larger than those from the South East and East Coast areas. It should be noted that the growth parameters assumed are the same for all areas, except Shetland. This may not be appropriate if individual growth is generally slower in some areas such the East Coast and South East, and individuals typically attain smaller maximum sizes. The use of inappropriate growth parameters in the LCA could overestimate the degree of growth overfishing. This is discussed further in section 4.3. The LCA results were generally in agreement with the evaluation of mean size and mean size of largest 5% of lobsters carried out for the LBI analysis.

4.3. Management Considerations

4.3.1. Brown crab

The results of LCA assessments for the period 2016-19 showed that brown crab in the majority of the assessment areas in Scotland were fished close to or above F_MSY. In many of the assessment areas, a higher biomass and yield-per-recruit in the long term could potentially be obtained by reducing the level of fishing mortality (effort). LBI results were mostly in agreement with the LCA and indicated similar exploitation levels relative to F_MSY reference points. The evidence from the surveys showed that there was a widespread increase in recruitment until 2014-2015, which likely contributed to the increase in stock size. The increase in recruitment coincides with a period of relatively high fishing mortality and high landings, particularly for the North Sea stocks. This may be an indication that the stocks were not recruitment overfished at that time, although the recruitment indices presented in this report are relatively short and provide no information on historical levels. It is possible that even at a high level of fishing mortality (which has been estimated for a number of years), recruitment and stock biomass continued to increase. A change in stock trend was observed in the mid 2010’s with both abundance and recruitment showing a general decrease in recent years. It is likely that if crab stocks had been subjected to a lower F over the years, this would have allowed the effects of the high recruitment/stock biomass to be sustained over a longer period, possibly generating higher total yields. Most brown crab stocks are being fished close to or above F_MSY, as indicated by the LCA/LBI analysis. It is recommended that effort/fishing mortality should be reduced for those Scottish brown crab stocks for which fishing mortality is estimated to be above F_MSY. Results from a recent study support that the current MLS in Scotland (140-150 mm) is appropriate since brown crab maturity is likely to occur at lower sizes than the MLS (Mesquita et al., 2020). Estimates of effort in creel fisheries are further discussed in section 4.5.1.

Brown crab stock status based on results from the LBI, LCA and survey data.

	Length Based Indicators (LBI)				LCA 2016-2019		Surveys
Area	L̅ / LF=M		Lmax5% /0.9 L∞
	Males	Females	Males	Females	Males	Females
Papa	ok	ok	ok	ok	ok	ok	descending trend	East
Orkney	between	ok	between	between	no	between
East Coast	no	no	no	no	no	no
South East	between	no	no	no	no	no
Sule	unknown	ok	unknown	between	no	no	descending trend	West
North Coast	unknown	unknown	unknown	unknown	no	no
Hebrides	ok	ok	ok	between	no	no
Ullapool	unknown	unknown	unknown	unknown	no	between
Mallaig	unknown	unknown	unknown	unknown	unknown	unknown
South Minch	between	between	no	no	no	no
Clyde	unknown	no	unknown	no	no	between
Shetland	no	between	no	no	unknown	unknown	descending trend

4.3.2. Velvet crab

Assessments have been conducted in seven out of the twelve assessment areas in Scotland, corresponding to those areas where fisheries for velvet crab have been in place. The status of F in relation to F_MSY does not appear to have changed considerably over time in most velvet crab stocks since the 2006-2008 assessments. The results of LCA assessments for the period 2016-19 showed that, in the majority of the areas with sufficient sampling data to conduct assessments, velvet crab remain fished close to or above F_MSY. In some assessment areas (Clyde, East Coast and Orkney), a higher biomass and yield-per-recruit in the long term could potentially be obtained by reducing the level of fishing mortality (effort). LBI results were mostly in agreement with the LCA with the exception of male stocks in the South East, Hebrides and South Minch, where the LCA showed a more optimistic outcome with fishing mortality estimates below or at F_MSY. There is no survey information available for velvet crabs as the species is mostly inshore and not commonly captured by any existing dredge or trawl surveys. Most velvet crab stocks are being fished close to or above F_MSY, as indicated by the LCA/LBI analysis. It is recommended that effort/fishing mortality should be reduced for those Scottish velvet crab stocks for which fishing mortality is estimated to be above F_MSY. Estimates of effort in creel fisheries are further discussed in section 4.5.1.

Velvet crab stock status based on results from the LBI and LCA.

	Length Based Indicators (LBI)				LCA 2016-2019
Area	L̅ / LF=M		Lmax5% /0.9 L∞
	Males	Females	Males	Females	Males	Females
Papa	unknown	unknown	unknown	unknown	unknown	unknown
Orkney	no	no	no	no	no	no
East Coast	no	no	no	no	no	no
South East	no	no	no	no	between	no
Sule	unknown	unknown	unknown	unknown	unknown	unknown
North Coast	unknown	unknown	unknown	unknown	unknown	unknown
Hebrides	no	no	no	no	between	no
Ullapool	unknown	unknown	unknown	unknown	unknown	unknown
Mallaig	unknown	unknown	unknown	unknown	unknown	unknown
South Minch	no	no	no	no	no	no
Clyde	no	no	no	no	no	no
Shetland	no	no	no	no	unknown	unknown

4.3.3. Lobster

Assessments have been conducted in eight out of the twelve assessment areas in Scotland, corresponding to the areas where higher volumes of lobster were reported and market sampling data has been collected. There are also small lobster fisheries in the North Coast, Ullapool and Mallaig and a by-catch of lobster in the Sule offshore brown crab fishery. However, sampling opportunities in these areas are very limited due to the small amounts of reported landings. The results of LCA assessments for the period 2016-19 indicated that lobster in the majority of the assessment areas in Scotland were fished close to or above F_MSY. In all assessment areas, a higher biomass and yield-per-recruit in the long term could potentially be obtained by reducing the level of fishing mortality (effort). There is no survey information available for European lobster as this species is found mostly inshore in rocky areas and not commonly captured by any existing dredge or trawl surveys. Most lobster stocks are being fished close to or above F_MSY, as indicated by the LCA/LBI analysis. It is recommended that effort/fishing mortality should be reduced for those Scottish lobster stocks for which fishing mortality is estimated to be above F_MSY. Estimates of effort in creel fisheries are further discussed in section 4.5.1.

Lobster stock status based on results from the LBI and LCA.

	Length Based Indicators (LBI)				LCA 2016-2019
Area	L̅ / LF=M		Lmax5% /0.9 L∞
	Males	Females	Males	Females	Males	Females
Papa	unknown	unknown	unknown	unknown	no	ok
Orkney	no	no	no	ok	no	ok
East Coast	no	no	no	no	no	between
South East	no	no	no	no	no	no
Sule	unknown	unknown	unknown	unknown	unknown	unknown
North Coast	unknown	unknown	unknown	unknown	unknown	unknown
Hebrides	no	no	no	no	no	ok
Ullapool	unknown	unknown	unknown	unknown	unknown	unknown
Mallaig	unknown	unknown	unknown	unknown	unknown	unknown
South Minch	no	no	no	no	no	no
Clyde	unknown	unknown	unknown	unknown	no	no
Shetland	no	no	no	no	no	no

4.4. Quality of the Assessment and Data

4.4.1. Landings Data

From the range of stock assessment tools available, LCA is one of the least data intensive, and LCA and yield-per-recruit models are commonly used for assessing data-limited shellfish stocks. A major assumption of LCA is that the landings length frequency distribution is representative of the fishery removals from a single cohort of individuals throughout its life. However, since the length frequencies are derived from a single year of sampling, rather than from the lifetime of a single cohort, this assumption is only true if the population is in a steady state or at equilibrium, i.e. that recruitment and exploitation rate are constant. An average of the length frequency data (2016-19) was used in order to limit the effects of these variations. Landings from most of the Scottish assessment areas tend to fluctuate, which may reflect year to year variation in recruitment and/or fishing effort. Results of the dredge surveys seem to indicate reductions in the recruitment and stock biomass of brown crab over the time period covered by the latest LCA. Systematic changes in exploitation rate or recruitment over the four-year period could potentially result in biased estimates of fishing mortality.

Landings are generally well sampled, for length and sex composition in the most important fishing areas. However, in some areas such as South Minch (brown crab), the Clyde (brown crab), Ullapool (brown crab), Papa (lobster) and East Coast (velvet crab), size data are sparse, and sampling levels remain relatively low with few sampling trips taking place. Length frequencies derived for these areas are often similar to those in other adjacent areas and are therefore assumed to be representative, despite the low sampling levels. However, the effect of this assumption on LCAs has not been investigated and the results of these assessments should be interpreted with caution.

It was concluded based on data recorded in FISH1 forms that vessels fishing in the South Minch land in ports on the west of the Kintyre peninsula while fishing trips landing into ports on the east side are associated with fishing activity in the Clyde. Historically the split of landings in 40E4 between South Minch (west) and Clyde (east) was based on a zone code entered by fishery officers and recorded in the FIN database. This procedure was discontinued in the new iFISH database and it is expected that the port allocation method (described in section 2.2) will continue to be used in the future. Analysis of historical data (2000-2016) has shown that the previous allocation method based on the zone code was likely to overestimate 40E4 landings in the Clyde until 2016. Since 2017, the split of landings between South Minch and Clyde in rectangle 40E4 is based on the location of the port, which is considered a more reliable method compared to the previous use of zone codes.

4.4.2. Biological Parameters

LCA is frequently used for assessing crustacean species for which ageing techniques are not yet fully developed. Discontinuous growth within a cohort of animals (growth increments and frequency of moults) results in growth rates varying between individuals within a stock. This can make it difficult to track cohorts through length frequency data and hence assessment methods which translate between size-structured and age-structured data have not been widely applied to crustaceans (e.g. Sheehy et al., 1996; Sheehy and Prior, 2008; Kilada, 2012; Kilada and Acuña, 2015).

In addition to landings length frequency distribution data, LCA also requires estimates of other biological parameters, including von Bertalanffy growth parameters and natural mortality. LCA is very sensitive to these parameters and the choice of input parameters may critically influence the results obtained (Lai and Gallucci, 1988; Jones, 1990), such as the perception of the state of the stock (in terms of the position of the current exploitation rate in relation to F_MSY). Natural mortality (M), for example, has a marked effect on the shape of the relative yield-per-recruit curve. Using lower values for M results in a more pessimistic stock assessment, with current fishing mortality estimated to be higher in relation to F_MSY (or vice versa). The values of the von Bertalanffy growth parameters (K and L_∞) also affect the shape of the yield-per-recruit curve and estimation of the value of F in relation to F_MSY. Using growth model parameters that result in growth rates that exceed the true growth rate (i.e. using values of K and L_∞ which are too large), results in the current exploitation rate (F) being over-estimated in LCA and could lead to the erroneous conclusion that a stock is growth-overfished (or vice versa).

For the LCA assessments, the same biological parameters have been applied across all areas except Shetland (Chapman, 1994; Mesquita et al., 2011; Mesquita et al., 2016; Mesquita et al., 2017; Mill et al., 2009). Studies of velvet crabs in Shetland (Tallack, 2002; Mouat et al., 2006) provided much higher estimates of M (0.58 compared to 0.1 elsewhere) and a higher value of K (0.46 compared to 0.1 elsewhere). For brown crab in Shetland, M is estimated to be higher than elsewhere (0.25 compared to 0.1) and the sex-specific von Bertalanffy K parameters are also different (lower at Shetland for males, but higher for females). The high value of K estimated for Shetland velvet crabs implies a very fast growth rate for young crabs (for example a 30 mm male crab would be expected to grow to nearly 60 mm carapace length in a single year). This seems unrealistic and merits closer scrutiny of the original data and methods used to estimate the growth parameters for this stock. For example, Electronic Length Frequency Analysis (ELEFAN) is a system of stock assessment methods which may be used to estimate L_∞ and K from length frequency data in the future (Mildenberger et al., 2017). For brown crab, the parameters derived from studies in Shetland are closer to those used by MSS for other assessment areas, but some notable differences persist, in particular for females. The difference in parameters is sufficient to explain the disparity in the results obtained for the velvet and brown crab assessments in Shetland when compared to other stocks with relatively similar length frequency distributions. Velvet crabs in the Clyde, East Coast and Orkney are estimated to be growth overfished, whereas in Shetland they are estimated to be fished below F_MAX (as estimated from a LCA using Shetland parameters), despite the estimated F in Shetland being much higher than in other areas. The F calculated from a LCA applied to Shetland data using the rest of Scotland parameters is much lower but estimated to be above F_MAX. For brown crab, as M and K parameters from Shetland and elsewhere are closer, F estimates are similar but the estimated F_MAX, taken as a proxy for F_MSY, from Shetland parameters is always higher. This is reflected in the shape of the YPR curves generated from the LCAs using Shetland parameters, which are more flat topped resulting in a higher F_MAX and hence a higher reference point. Because of the differences in the estimates of F obtained using the different sets of growth parameters and the associated issues of interpretation for the purposes of this report, we have described the stock status for Shetland velvet crab and brown crab as unknown. Owing to the uncertainty around appropriate input parameter values, care is required in drawing firm conclusions regarding the status of crab and lobster stocks, particularly for velvet crab. To progress this discussion it would be worthwhile holding a joint Shetland UHI/MSS crab working group in the near future.

Differences in size composition across areas, particularly the relatively small size of brown crab and lobster landed in the South East and East Coast compared to the north and west, suggest that area specific parameter values may be more appropriate and it is possible that the extent of growth overfishing of brown crab and lobster in the East Coast and South East is overestimated. Estimation of growth parameters for these areas would, however, require a large scale tagging project using tags that could be reliably retained on moult, with seasonal measurements of length and weight.

4.4.3. Size-based Indicators

ICES has previously suggested a multiple-indicator-based approach, including LPUE (landings per unit effort), size-based indicators and recruitment indices, as a potential way forward in the provision of advice on stock status for crab stocks (ICES, 2009). The exploratory analysis presented here makes use of commercial length frequency data and biological parameters in an approach for evaluating the status of data limited stocks proposed by the ICES WKLIFE V workshop

(ICES, 2015a; Miethe et al., 2016). More recently, ICES have introduced some new approaches for the provision of advice for data limited stocks (ICES, 2020) which may be relevant for crabs and lobsters, although the latter are currently non-quota species. The new methods proposed by WKLIFE X are further discussed in section 4.5.5. In the assessments presented here, size-based indicators were calculated and compared to the respective reference points. In some cases it was possible to relate variations of the mean size and mean size of largest animals to trends in fishing mortality. The results are highly dependent on the quality of estimates of life history parameters by assessment area. In most cases, results were in agreement with the LCA results and indicated similar exploitation levels relative to F_MSY reference points.

Mean sizes of the largest individuals are quite variable from year to year, particularly in areas where data collection is more sporadic. This is likely to reflect sampling variability rather than changes in the population. The sampling levels achieved in some areas remain low. Improved sampling and better information on fishing activity and fishers’ behaviour could help to develop robust size-based indicators for assessment purposes.

Assumptions for biological parameters (M/K, L_∞ and L_mat) are also important for the interpretation of size-based indicators. The estimated mean size indicator reference point L_F=M depends on the ratio between natural mortality and the growth parameters. The collection of data on growth rate in different areas, in order to derive area-specific parameters, would help to improve the assessment using size-based indicators. With the exception of Shetland, the M/K ratios used for the LCA were below 1. A lower M/K ratio would result in a higher, more restrictive, size-based reference point, with an expectation of more large individuals in the size distribution of an unexploited stock (Hordyk et al., 2015; Jardim et al., 2015).

4.4.4. Survey Analysis

While the survey analysis provides an indicator of trends in abundance and of the distribution of brown crab in Scotland, currently these are only available for three large areas (east coast, west coast and Shetland) rather than for each of the brown crab assessment areas. The data collected from surveys around the Shetland Islands has been more limited than that from the east and west coast areas. This is due to the low number of trawl stations around Shetland combined with very low catch rates of crabs in the area. The Shetland dredge survey has also a number of missing points related to difficulties in conducting the surveys under adverse weather conditions. Additionally, very few crabs below 100 mm CL were caught in the Shetland dredge survey and hence the trawl abundance and dredge recruitment indices were not derived for Shetland brown crab. It is unlikely that trap surveys will be introduced in Scotland in the near future. Therefore, dredge and trawl surveys targeting scallops and whitefish, which already take place annually in the waters around Scotland, are likely to continue to be the main source of fishery independent data on brown crabs, providing useful information on distribution and trends in abundance and recruitment. Brown crabs are amongst the species more often caught as bycatch in these surveys, unlike velvet crabs and lobsters, which are rarely captured. The models applied in this study could be potentially improved by including other environmental and oceanographic variables normally associated with crustacean distribution, such as seabed temperature or currents strength.

An advantage of using dredge and trawl gears over passive gears such as traps for obtaining standardized catch rates is that the catchability of active gears is not affected by factors such as season, bait, current strength and direction (Mesquita et al., 2021). In addition, catch rates calculated from active gears eliminate issues of trap avoidance by berried females (Howard, 1982), and are not affected by saturation effects resulting from the first animals which enter the traps. The main limitation associated with the use of active gear surveys applied to benthic crustaceans is that towed nets or dredges are limited to muddy/sandy seabed types, which may introduce biases given that crustaceans may also live in rocky habitats (Smith and Tremblay, 2003).

4.4.5. Reference Points

LCA provides long term equilibrium predictions and assumes constant recruitment and exploitation rates. It is therefore advisable to complement the outputs with additional data, which can provide information on trends in abundance, typically catch per unit effort data or exploitation rate. Effort data in terms of numbers of creels fished are not currently available for Scottish creel fisheries, precluding calculation of catch per unit effort. In an attempt to gain additional information on variation of fishing mortality from the available data , the mean overall size and the mean size of largest 5% of individuals were explored (ICES, 2014b; ICES, 2015a).

The landings at length data used as the input for the LCA were averaged over 2016-2019 which covers periods of time both pre- and post- increase in MLS. Data exploration focusing on comparing the left tails of the length frequency distributions (Annex B) with those from previous years (Mesquita et al., 2017) do not show major differences in the size of first capture for the three species, despite the MLS increase. Additionally, in some areas, particularly for the velvet crabs, it is clear that the first size of capture in the most recent data is below the old MLS. The changes to MLS therefore appear not to have resulted in a major selectivity change, at least in the first two years after being introduced. Despite these observations, some selectivity changes, even if minor, would be expected in the fisheries, and to account for that, the F_MSY reference points were re-estimated with a yield-per-recruit analysis for all stocks using the most recent LCA input data (2016-2019).

The conclusions in this report are based on estimates of fishing mortality in relation to a reference point for each stock, to infer whether or not a stock is fished above the level that would in theory result in MSY (in the long term). For the purposes of consistency in this report, all discussion relates to the F_MSY proxy (F_MAX). Although LCA and yield-per-recruit analysis give an indication of current F relative to the fishing mortality required to optimise yield (from a particular cohort), they provide no indication of whether or not a stock is recruitment overfished (i.e. whether fishing is compromising recruitment). F_MSY, the fishing mortality which gives the maximum sustainable yield (high long term yield with low risk of stock depletion), is difficult to estimate, requiring good estimates of spawning stock biomass and recruitment. In the absence of such estimates, the survey based recruitment index, when considered in combination with estimates of current F, may in future provide an indication of periods of potential recruitment overfishing for brown crab.

In cases where F_MSY cannot be estimated directly, proxy values based on yield-per-recruit analysis are often used. ICES advises that in cases where the peak in the yield-per-recruit curve is well defined and there is no evidence of poor recruitment at this level of fishing mortality, then F_MAX may be an appropriate proxy for F_MSY (ICES, 2010). In cases where the peak is less well defined and the curve is more flat topped, then F_0.1 is likely to be a more appropriate proxy (Jennings et al., 2001). Another potential reference point is F_30%SpR which is defined as the fishing rate, which results in combined spawning biomass per recruit equal to 30% of the un-fished level. F_0.1 is usually the most conservative reference point while F_MAX is generally above F_30%SpR, depending on the relative shape of the YPR and BPR curves. F_MSY proxies for Nephrops stocks assessed by ICES have been selected from these three candidate reference points (F_0.1, F_35%SpR or F_MAX) for each stock independently according to the perception of stock resilience, typical population density, biological knowledge and the nature of the fishery (e.g. ICES, 2015b). Most crab and lobster fisheries have been in existence for several decades with little evidence of between-stock differences in resilience. Therefore, despite some stocks showing very flat topped YPR curves (which might suggest F_0.1 as the most appropriate proxy for F_MSY), F_MAX was selected as a proxy for F_MSY for all stocks (Mesquita et al., 2017). However, these reference points remain preliminary and may be revised in the future as further data become available.

In most areas around Scotland, the crab and lobster stocks are being fished at levels which result in yield-per-recruit values not far below the maximum. However, in some cases, the estimated fishing mortality is substantially above F_MSY, making it more likely that these stocks are recruitment overfished as well as growth overfished. It should be noted, however, that so far lobster stocks have not showed signals of systematic changes in sex ratio, which has been associated with recruitment overfishing in other lobster species.

4.5. Data Gaps and Future Research Priorities

From the discussion above it is clear that there are a number of areas where research or additional data collection would improve Scotland’s crab and lobster stock assessments.

4.5.1. Fishing Effort

Prior to 2016, no useful measures of creel fishing effort were available from official log sheets with the exception of the Shetland area where the Shetland Regulating Order requires licensed fishers to return logbook information to the SSMO, detailing the catch location (at the 5 nmi scale) and the number of creels or pots fished. This has precluded the use of LPUE data as an indicator of abundance for the crab and lobster stocks around Scotland. Fishing effort for most finfish species can be estimated as fishing time (days fishing or KW days) using days absent from port and vessel power, but these are not particularly useful measures of effort in creel fisheries. The number of creels used or hauled when fishing for crabs and lobsters is considered to be a much more useful measure of effort in the fishery. The recent changes to reporting on the FISH1 form include the introduction of a mandatory field for the number of creels hauled. This provides new effort data for vessels under 10 m fishing around Scotland and allows for calculation of LPUE indices. These newly available effort data have recently been used by MSS to provide scientific support for the Outer Hebrides Inshore Fisheries Pilot (OHIFP). As part of this pilot project, work was carried out using FISH1 data to obtain effort information (total number of vessels and trip numbers) and preliminary estimates of LPUE for brown crab, velvet crab and lobster in the Outer Hebrides area which show a general decrease between 2017 and 2020. It has been highlighted that the reliability of this type of analysis depends heavily on the quality and accuracy of data reporting by fishers (Bell et al., 2022).

VMS data have become available for larger vessels (over 15 m from 2008 and over 12 m from 2012), and could potentially be integrated with logbook landings information to obtain indicators of LPUE for the offshore fleets. However, these monitoring tools do not cover the majority of the inshore fleet, which is mostly composed of smaller vessels (under 10 m).

The results of a series of inshore fisheries pilot projects to support sustainable Scottish fisheries, funded by the European Fisheries Fund suggest that many of these data deficiencies could be addressed through self-sampling and electronic monitoring technology (Course et al., 2015). A number of work packages within the Scottish Inshore Fisheries Integrated Data System project (SIFIDS) (funded by the European Maritime Fisheries Fund, EMFF), were concerned with improving data collection from inshore creel vessels through the use of new technology. One of the project outcomes was a system designed by the University of St Andrews (Ayers R. et al., 2019; James et al., 2018) which is currently being utilized as part of the OHIFP pilot project. These tracking devices collect and transmit spatial fishery data including effort. Remote Electronic Monitoring (REM) is a component of the Future Fisheries Management Strategy (FFM) and the Bute House Agreement includes a commitment to cover all commercial fishing vessels by the end of the current parliamentary session. Proposals for REM for Inshore vessels will be subject to a Scottish Government consultation in 2022. Data collection would, however, need to be coordinated and maintained, to build up useful time series.

4.5.2. Discard data

Discards in crab and lobster fisheries are not sampled on a regular basis and fishing mortality associated with non-retained catch is not taken into account in assessments.

There are only a few studies estimating discards of brown crab in Scotland. A recent study showed that the percentage of discards in total catch may be close to 80% in the inshore crab fishery and just under 45% in the offshore fishery (Mesquita, 2020). Data for velvet crab and lobster are not currently available. Discarding practices are influenced by a combination of different factors, particularly fishing location, market requirements and the condition of the catch. More regular sampling to obtain information on catches of undersized animals could provide an indication of inter-annual variation in recruitment, which could support those data obtained from the dredge survey analysis. By-catch data collected on MSS scallop surveys may potentially provide information on variation in recruitment in areas where the scallop and the crab and lobster distributions overlap. Work on by-catch data from surveys targeting other species has been successfully carried out for brown crab but again, not for velvet crab or lobster. Discard studies of brown crab in Orkney estimate discard survival in creel trials at 92.7% (Rodrigues et al., 2021). However, further discard studies are also required to obtain estimates of discard survival and to help understand more fully the reasons for discarding in crab and lobster fisheries.

4.5.3. Population Structure

The population structure of crab and lobster stocks around Scotland, and the rest of the UK, is not well understood. The current assessment areas are empirical, based largely on past fishing patterns. Brown crabs are known to undertake extensive seasonal migrations in some areas while in contrast, velvet crabs and lobsters appear to make relatively limited movements. MSS previously conducted a tagging study of brown crab to the north of Scotland (Jones et al., 2010). The results suggest linkage between inshore and offshore crab stocks to the north and west of Scotland. Fishermen support the idea that crabs migrate between (and across) the ‘windsock’ and inshore grounds around Orkney, although there is only a limited fishery in the area in between. Large catches of female crab have also been reported on the shelf edge at depths greater than 200 m. Tagging studies of brown crab in Orkney indicated that female crabs make both localised movements and long distance migration across stock assessment boundaries (Coleman and Rodrigues, 2017). Further work being undertaken through the Scottish Regional Inshore Fisheries Group network (RIFGs) in collaboration with OSF should provide further evidence regarding the structure of brown crab stocks to the north of Scotland. Ideally, such studies should be followed up by population genetics/morphology studies and consideration of larval dispersal. It has been suggested, for example, that brown crab populations in the Irish Sea may be closely linked to the larger populations on the Malin shelf, which are contiguous with the Hebrides and South Minch in western Scotland (Tully et al., 2006; ICES, 2007). The current brown crab assessment areas may be reviewed by ICES WGCRAB in the future.

4.5.4. Growth studies and maturity

The currently used growth parameter estimates for crustaceans in Scotland were derived from tagging studies which took place in the 70’s in a few selected areas. More recent studies were carried out in Shetland and these show some relevant differences to those values previously estimated, in particular for the parameter K in crabs. Given the sensitivity of LCAs to the input growth parameters, further work in this area is required, especially for velvet crabs for which available data suggest very different growth rates between Shetland and elsewhere. Field studies based on tagging methods (using tags retained on moult) and subsequent evaluation of parameters would be desirable.

More recently, a number of studies concerning brown crab maturity have been carried in Scottish waters providing updated values for L_mat in the east coast, west coast and Orkney (Haig et al., 2016; Mesquita et al., 2020). The results have shown that brown crab maturity is likely to occur at lower sizes than the current MLS of 150 mm CW (140 mm in Shetland), implying that crabs may be able to reproduce at least once before being harvested (Mesquita et al., 2020). However, it is important to note that not all areas in Scotland have specific size-at-maturity estimates and that regional variations should be taken into account to inform the setting of appropriate MLS for all stocks.

4.5.5. Approaches to advice provision

The crab and lobster stock assessments presented in this report use length-based assessment methodologies with LCA and size indicators providing stock status for stocks with available data. For brown crab, additional stock size and recruitment indicators are available based on trawl and dredge surveys. The current survey indices for brown crab are based on GAMs and these models may be further developed in future to estimate abundance indices at the stock level. Developing such indices is complicated by the fact that dredge/trawls surveys are directed at other species (scallops and gadoids) and therefore, the survey design and spatial distribution of the survey stations do not cover all crab assessment areas. This mismatch is more evident in the west coast while the two main areas in the east of Scotland (East Coast and South East) appear to have an acceptable spatial coverage from the dredge surveys. Trawl surveys were also used to calculate abundance indices but the spatial discrepancy between crab stocks and sampling stations in these surveys is even more evident with only a small percentage of stations carried out in inshore waters. The development of abundance indices from the dredge surveys may provide an opportunity to develop stock specific harvest rates as those presented for Nephrops species, defined by the ratio between the catch and the biomass index (e.g. ICES, 2021).

Within the ICES assessment framework, a recent development implied that from 2022, advice provided for certain categories of data limited stocks should be based on the methods documented in the WKLIFE X report (ICES, 2020). These methods include production models such as the SPiCT model (Surplus Production model in Continuous Time (Pedersen and Berg, 2017) and a number of empirical harvest control rules (HCR) which have been tested through management strategy evaluation (MSE) simulations and were designed to follow the ICES precautionary approach, where the long-term risk of stocks falling below a limit reference point does not exceed 5% (Fischer et al., 2020). SPiCT models use catch and fisheries independent data to model stock dynamics, providing estimates of the exploitable biomass and fishing mortality as well as reference points. The new empirical catch rules make use of length data and the individual growth rate of the target species and therefore require an estimate of K (the von Bertalanffy growth parameter). The advice is then based on the product of a pre-existing catch advice (or recent catches) and a number of multipliers which may include biomass ratios (e.g. surveys, reflecting stock biomass trends), fishing mortality proxies relative to F_MSY (derived from length data) and biomass safeguards (ICES, 2020). These new advice approaches implemented by ICES may potentially be applied to the crab and lobster stocks considered in this report as both SPiCT and the HCR rules were designed to be used in data-limited species without full analytical assessments, reference points, or short-term forecasts, and requiring only some knowledge of life history and length data (which are available for Scottish stocks). At the latest ICES WGCRAB meeting (2022), preliminary SPiCT assessments were trialed for brown crab stocks off Ireland and Scotland using survey indices, commercial CPUE (catch per unit effort) and landings as input data. This WG is planning to consider how to adapt the currently available and commonly used ICES assessment frameworks to crustacean stocks given their biological and gear specificities.

4.5.6. Other Factors

The interpretation of trends in indicators derived from fishery dependent data would be helped by improved understanding of the economic and environmental factors that influence fishers’ decision-making with respect to fishing location and target species. A component of the Lot 1 EU project (Armstrong, 2010) involved conducting questionnaires and interviews to establish the main factors in fishers’ decision making. As well as providing information on historical changes in fishing practices, the interview responses suggested that the Scottish brown crab fishery has been influenced more by the market than by management measures. Additional information on factors affecting catchability such as bait type, creel density and soak time could also be collected by engaging with fishers and industry. Work being conducted within the RIFGs liaising with fishers through the inshore fisheries pilots, for example, collecting spatial data on fishing location, ought to provide an improved understanding of the drivers of fishing behaviour.

Acknowledgements

The authors would like to express their thanks to Marine Scotland Science staff who collected and collated the data used in this report, Shetland UHI and Orkney Sustainable Fisheries Ltd for providing size frequency data for the Shetland and Orkney/Papa areas, and the Marine Institute for providing brown crab landings from Ireland in ICES Subarea 6.