Crab and Lobster Fisheries in Scotland: Results of Stock Assessments 2009-2012: Scottish Marine and Freshwater Science Vol 7 No 9

2. Data Collection and Methods

2.1. Assessment Areas

For assessment purposes, the Scottish creel fishing grounds are divided into 12 assessment areas as shown in Figure 1. These areas are based on the district and creek system for reporting Scottish landings data (Thomas, 1958), subsequently revised to include two offshore areas - Papa, which lies to the west of Scotland, and Sule, which is to the north and west of Orkney and includes Rona, Sulisker and Sule-Skerry banks. Some Scottish assessment areas extend outside Scottish Territorial Waters. On the east of Scotland the South East assessment area extends beyond the Scottish border, while on the west coast the Clyde assessment area stops short of the Irish border. There is some fishing on grounds outside the assessment areas. Currently, data from these grounds are not included in the assessments but may need to be in the future, particularly when a high volume of landings is recorded from a single statistical rectangle.

2.2. Landings Data

The assessments use official landings data, which detail the location, the species and the weight landed into ports in Scotland. These data are collated by Marine Scotland Compliance from sales notes and EU logbook and FISH1 forms, and held in the Marine Scotland Fisheries Information Network ( FIN) database and in the Marine Scotland Science ( MSS) Fisheries Management Database ( FMD). Data for brown crab landings from the Republic of Ireland (collected by the Irish Sea-Fisheries Protection Authority - SFPA) were compiled and provided by the Irish Marine Institute. These data were not used in the assessments but have been included to illustrate brown crab landings by nation on a statistical rectangle basis to the west coast of Scotland.

2.3. Catch-at-length Data

2.3.1. Landings-at-length

Landings length-frequency data were collected by MSS as part of its market sampling programme. Shetland sampling data were provided by the North Atlantic Fisheries College ( NAFC) Marine Centre with the permission of the Shetland Shellfish Management Organisation. Since 2010, data from fisheries in Shetland have been collected and provided by staff at the NAFC Marine Centre under the Memorandum of Understanding (MoU) between NAFC Marine Centre and MSS. All the data are held in the MSS Fisheries Management Database ( FMD).

Sampling measurements are taken as carapace length ( CL) for lobsters, measured from the eye-socket to the centre of the base of the thorax carapace, and carapace width ( CW) for brown and velvet crabs, measured across the widest part of the body, not including any spines.

In general, sampling effort is focused in those areas where fisheries are most important. However, the timing of landings is rather unpredictable and sampling is to some extent opportunistic, factors which explain the variability in numbers sampled and the occurrence of zeros for certain species in some areas. Brown crabs and lobsters may be retained for a period of time in holding tanks after being 'landed' which makes them easier to access for sampling. In contrast, velvet crabs are often landed in remote harbours and promptly dispatched to fishing processors or shipped abroad. This makes it more difficult to get samples, particularly in the Inner Hebrides (South Minch) in the West of Scotland.

MSS aims to run crab and lobster stock assessments every three years using length-frequency data from the most recent three years. However, delays associated with processing the sampling data for years 2010 and 2011 meant that when these data became available, data for year 2012 were also accessible. The stock assessments reported here use sampling data collected between 2009 and 2012. The numbers of animals measured, number of trips and percentage of sampled fishing trips (quotient between the number of trips sampled and the total number of trips extracted from FIN) by assessment area are shown, respectively, for each species in Table 3, Table 4 and Table 5.

2.3.2. Discards

There are currently no discard data regularly collected for the crab and lobster fisheries in Scotland and any mortality due to discarding practices is not taken into account in these assessments. Anecdotal information and recent ad hoc studies suggest that crab and lobster discard rates in the target creel fisheries are variable and occasionally high (>50% by number) but survival rates are also high. Discards typically comprise those animals that do not meet landing regulations such as undersized individuals, post-moult animals with soft shells, v-notched female lobsters or berried female crabs. Therefore, for assessment purposes landings are assumed equal to catch.

2.3.3. Raising and Data Quality

Length frequency data obtained from market sampling and official landings data were combined to provide a raised annual landings-at-length distribution. Data were averaged over a number of years (2009-2012) and aggregated into 5 mm length classes for brown crab and lobster and 3 mm length classes for velvet crab for use in the LCA.

Landings-at-length data were not available for the three species in all assessment areas. The decision-making process to select which areas had sufficient data to run stock assessments is presented in Annex A. Four parameters were defined to categorise the quality of the sampling data: number of trips/animals sampled; number of years for which data is available (maximum 4 years - 2009-2012); sampling seasonality; Length frequency distribution ( LFD) shape. For each species/area, these parameters were classified in three categories ("poor"/"ok"/"good"). Stock assessments were not run for areas where one or more of the parameters was classified as "poor" (see Annex A).

2.4. Biological Data

Information about the growth of British crabs and lobsters used in the LCA comes mainly from tagging studies carried out in the 1960s and 70s (Hancock and Edwards, 1966; Hancock and Edwards, 1967) ( Table 6). Estimates of the von Bertalanffy growth parameters: asymptotic length (L _∞) and instantaneous growth rate ( K) have been estimated using Ford-Walford plots (Chapman, 1994; Tallack, 2002; Mouat et al., 2006). Length-weight relationships (parameters a and b shown in Table 6) are from MSS (unpublished) market sampling measurements of length and weight. Different, area specific, biological parameters were applied for Shetland assessments as these are available from research on these species conducted in Shetland (see Leslie et al., 2010 for data sources).

2.5. Size Indicators and Sex Ratio

In theory, size based indicators can provide information about the effects of fishing pressure on a population. Temporal trends in the mean size of landed individuals were explored for the period over which sampling data are available (brown crab and lobster - from 1981; velvet crab - from 1987). Any years in which the number of sampled animals was less than 50 were excluded from these analyses. A decrease in mean size may be due to a reduction in the number of large individuals in the population due to fishing pressure but may also be as a result of high recruitment, which causes an increase in the number of small individuals. To mitigate the effects of variable recruitment, trends in the largest 20% (above the 80 ^th percentile of size) of individuals in the sampled landings (Mesquita et al., 2011) were examined. A declining trend in the mean size of largest animals can be associated with a reduction in the abundance of large individuals due to the effects of size selective fishing mortality. In addition to trends in size-based indicators, trends in the average annual sex ratio in the sampled landings were explored for each of the three species, again excluding years where less than 50 animals were sampled.

2.6. Length Cohort Analysis ( LCA)

Age determination is generally not possible for animals that moult, and therefore the application of age-structured assessment methods to crustacean stocks is problematic (Smith and Addison, 2003). Length Cohort Analysis ( LCA) (Jones, 1984) is a commonly used method of assessing stocks for which commercial catch length frequency distribution data are available. LCA was used by MSS in previous assessments of the Scottish crab and lobster stocks (Mill et al., 2009; Mesquita et al., 2011) and also by the NAFC Marine Centre to assess crustacean stocks around Shetland (Leslie et al., 2007; Leslie et al., 2010).

The LCA method uses the commercial catch size composition data (length frequency data) and estimates of growth parameters and natural mortality to estimate fishing mortality at length. The key assumption of the approach is that the length distribution is representative of a typical cohort over its lifespan. However, this is only true of length frequency data from a single year if the population is in equilibrium, therefore, LCA is usually applied to data averaged over a number of years during which recruitment and exploitation rates have been stable. LCA also assumes uniform growth among animals. The results of LCA can be used to predict changes in the long-term (equilibrium) stock biomass and yield-per-recruit based on changes in mortality, potentially resulting from fishing effort reductions or changes to minimum landing size regulations. The approach gives an indication of the exploitation of the stock in terms of growth overfishing, but not recruitment overfishing.

An LCA (Jones, 1984) was applied to all stocks where sampling data were collected and considered to be sufficient ( Annex A). To account for the differences in growth and length-weight relationships ( Table 6), sexes were assessed separately. The LCA provides estimates of fishing mortality (F) for each length class which are averaged over a fixed length range for each species and sex to give an estimate of average fishing mortality for each stock (Fbar). Using a fixed length range in the calculation of Fbar enables comparisons of the current F value (2009-2012) with those calculated in previous assessments (2002-2005 and 2006-2008) and the potential to detect trends in F.

2.7. Reference Points

The results of LCA were used to calculate yield-per-recruit ( YPR) and biomass-per-recruit ( BPR) relative to changes in fishing mortality which give an indication of stock status in terms of growth overfishing. The relationship between YPR and F is typically dome shaped - low levels of F result in low landings as few individuals are caught, whilst high levels of F may also result in a reduction in yield (in addition to biomass) from a particular cohort as animals are caught before they have had time to grow to a size that would contribute much weight to the yield (growth overfishing). In between these lies F _MAX, the fishing mortality rate that maximizes YPR for a particular selection pattern. For data-limited stock such as crabs and lobsters it is not possible to directly estimate the maximum sustainable yield ( MSY) and hence, in line with previous assessments, F _MAX was used as a proxy for F _MSY. This approach has been widely used by ICES ( ICES, 2010) for other crustaceans such as Nephrops (e.g. ICES, 2014a). Other potential reference points such as F _0.1 (F at which the slope of the YPR curve is 10% of the slope of the YPR of the unexploited stock) and F _30%SpR (F which corresponds to 30% of the virgin SPR) were also calculated and plotted along with F _MSY for comparison. F _MSY proxy values were calculated from a per-recruit analysis from a LCA of 2006-2008 landings-at-length data. Where 2006-2008 sampling data were not available, data from the current assessments (2009-2012) were used instead. All F _MSY proxy values remain preliminary and may be modified following further data exploration and analysis. A summary of stock status in terms of fishing mortality in relation to F _MSY was provided for each stock. A stock was classified as being "at F _MSY" when the estimated F was within 10% of F _MSY.