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

3. Results and Discussion by Species

3.1 Brown Crab

3.1.1. The Fishery

The brown crab fishery is long established and landings, although variable, have increased significantly over the last 40 years ( Figure 2a). Reported landings averaged around 2,000 tonnes in the late 1970s, increased to a maximum of about 12,000 tonnes in 2007, decreasing in 2008 to around 9000 tonnes and fluctuated in recent years around 11,000 tonnes. It is difficult to establish how accurate the pre 2006 data are as landings are thought to have been under recorded before the introduction of the 'buyers and sellers' legislation. The value of landings has increased in line with the tonnage landed. However, the price per kilogram has changed little over this time ( Figure 2b and Figure 2c). One kilogram of brown crab was sold for an average of £1.14 at first sale during the period 2009 to 2012.

The annual brown crab landings by assessment area are shown in Figure 3. The principal fishing areas for brown crab in Scotland are the Hebrides, Sule, Orkney, South Minch and East Coast; landings from these areas accounted for over 70% of the total in recent years ( Table 7). Landings from the offshore areas of Sule and Papa have increased since the 1990s when the fishery expanded, but seem to have stabilized in the last four years. Landings from Orkney, East Coast and Ullapool have increased in the last four years. The spatial distribution of brown crab landings by ICES statistical rectangle (including Irish landings) is shown in Figure 6. There were no major changes in relation to the most important rectangles for brown crab landings. Landings by non- UK vessels taken from the Scottish assessment areas were relatively low (mostly from Ireland) and usually confined to some grounds in Sule, Hebrides and South Minch. There is an important fishery for brown crab on the Malin shelf (Irish assessment area) that is mostly exploited by Irish vessels, with most landings being taken from the Donegal region (rectangles 40E2 and 39E2). There were no other regions of importance for brown crab landings outside assessment areas.

3.1.2. Sampling Levels

The number of sampled brown crabs, number of trips and percentage of sampled fishing trips are shown in Table 3. Good sampling coverage was achieved for most assessment areas, with samples being obtained throughout the year in the period 2009-2012. In the Hebrides, a decline in the number of sampling trips was noted in 2011-2012. The percentage of trips sampled was generally less than 2% in assessment areas where daily inshore trips are common. In assessment areas with offshore grounds such as Orkney, Sule and Papa, sampling percentages were higher ( Table 3) as the fishery is dominated by larger vessels which tend to make fewer but longer trips. Sampling data from Papa (which was not included in the previous round of assessments) became available in recent years allowing to run an LCA, however, the number of sampling trips in this area is still lower than elsewhere. Sampling data were considered to be insufficient (low numbers and infrequent sampling) for running assessments in Clyde, Mallaig and Ullapool ( Annex A). Of these, only Clyde was included in 2006-2008 assessments. Of the assessed areas, the North Coast had the lowest sampling levels as no data were recorded in 2010 and 2011. However, 2009 and 2012 data included a relatively high number of sampled animals and the averaged length frequency ( Annex B) was assumed to be representative of the size distribution over the four year period.

3.1.3. Mean Size and Sex Ratio

The mean size of brown crab in landings from each assessment area is shown in Figure 9. The data are typically noisy. Crabs from the South East and East Coast are generally smaller than in other areas. There is some evidence of an increasing trend in the mean size of largest male individuals throughout the time series in the East Coast, Hebrides, Orkney and Shetland and more recently in the North Coast (both sexes). In Clyde, mean size in landings also appears to have increased recently (both sexes) but sampling has been relatively poor compared with other areas. A decreasing trend was observed among larger individuals in the last ten years for both sexes in the South East (males: 185 to 170 mm CW; females: 187 to 172 mm CW). The sex ratio in landings of brown crab varied greatly between assessment areas but in most cases it did not change through time ( Figure 12). Consistently higher percentages of females in landings over the time series is evident in areas which include offshore fishing grounds such as Papa (87%), Sule (85%), North Coast (84%) and Hebrides (88%). The South East is mostly a male dominated fishery (69% males). Orkney has shown a decrease in the male percentage in landings from over 70% in the 80s to around 50% in recent years, whereas the sex ratio in Shetland displayed the opposite trend.

3.1.4. Stock Assessment

Results of assessments based on LCAs and per recruit analysis, summarising estimates of fishing mortality in relation to the F _MSY proxy, are shown below. Trends in fishing mortality in relation to previous assessments are presented in Figure 15 (males) and Figure 16 (females). Brown crab biomass and yield-per-recruit plots given changes in fishing mortality for each assessment area are shown in Figure 21 (males) and Figure 22 (females).

Brown crab stock status in terms of the relationship between F and F _MSY proxy for 2002-2005, 2006-2008 and 2009-2012.

Of the nine assessed areas, six were fished above the F _MSY proxy to some extent ( Table 10). Fishing mortality was estimated to be above F _MSY for both males and females in South Minch, Orkney, East Coast and South East. In Sule and Hebrides, fishing mortality for males was at F _MSY or below while females were fished above F _MSY. In the North Coast, Papa and Shetland, recent fishing mortality was around F _MSY or lower.

3.1.5. Comparison with Previous Assessments

Previous assessments for brown crab in Scotland were run in 2002-2005 (Mill et al., 2009) and 2006-2008 (Mesquita et al., 2011). The current assessment revises the two previous assessments by including a fixed length range that is used to calculate an average fishing mortality. This allows fishing mortality to be compared with previous estimates. F shows a downward trend in the Hebrides, North Coast and Sule (males) and has increased substantially for females in the East Coast, South East and South Minch ( Figure 15 and Figure 16).

3.1.6. Management Considerations

Overall, assessments for the period 2009-2012 showed that most brown crab assessment areas in Scotland were fished close to or above the F _MSY proxy. In many of the assessment areas, a higher yield and biomass per recruit in the long term could potentially be obtained by reducing the level of fishing mortality (effort).

3.2 Velvet Crab

3.2.1. The Fishery

Velvet crabs are often caught in the inshore creel fishery with lobster and brown crab. Very few fishermen fish solely for velvet crab, although some target the species at certain times of the year. Commercial exploitation was first documented in 1984 with reported landings of just over 300 tonnes and a price per kilogram of around £1 at first sale. Between 2009 and 2012, annual landings into Scotland were about 2,200 tonnes ( Figure 2a) and the value had increased to £2.30 per kilogram ( Figure 2b). It is not clear, however, if the recent increase in landings reflects the introduction of the "buyers and sellers" legislation. The three areas that have historically had significant velvet crab fisheries are the Hebrides, Orkney and South Minch, although the two latter have shown a slight decrease over the last four years ( Figure 4). More recently, landings have increased in the East Coast, South East and Shetland. These six areas accounted for over 90% of velvet crab landings in Scotland in the period 2009-2012 ( Table 8). Figure 7 shows the spatial distribution of velvet crab landings 2009-2012. Most landings were taken from inshore areas; only very small amounts were reported from offshore grounds in Papa and Sule. There were no significant landings of velvet crab reported from outside the assessment areas.

3.2.2. Sampling Levels

The numbers of sampled velvet crabs, number of trips and percentage of sampled fishing trips are shown in Table 4. The percentage of trips sampled was generally lower than that achieved for other species. In the Clyde, East Coast, Hebrides and Orkney, sampling coverage was good with samples being obtained throughout the year in the period 2009-2012. No data were available for Shetland in 2009 (this being prior to the agreement for data collection between MSS and the NAFC). The South Minch is one of the important areas for the velvet crab fishery where the data collection has been lower than anticipated (no data in 2010 and few samples in 2011). It has proved difficult to access and measure crabs caught and landed in the Inner Hebrides. However, an assessment was conducted for the South Minch because 2009 and 2012 data included a relatively high number of sampled animals, and the averaged length frequency ( Annex B) was assumed to be representative of the size distribution over the four year period. Sampling data were considered to be insufficient (low numbers and infrequent or no sampling) for running assessments in Mallaig North Coast, Papa, South East, Sule and Ullapool ( Annex A). Of these, only South East was included in the previous round of assessments (2006-2008).

3.2.3. Mean Size and Sex Ratio

Velvet crab mean sizes in landings by assessment area are shown in Figure 10. The time series of sampled landings is shorter than for brown crab and lobster. Mean size data were generally noisy with considerable inter-annual variation. Among the areas where sampling has been consistent over the years, an upwards trend was observed in the Hebrides. A marked decrease in the size of the largest individuals was noted in the East Coast from 2008 for both males (83 to 76 mm, CW) and females (82 to 74 mm, CW). The sex ratio for velvet crab showed little evidence of trend with males dominating the landings, representing 65% to 80% (by number) in the well sampled areas ( Figure 13).

3.2.4. Stock Assessment

Results of assessments based on LCAs summarising estimates of fishing mortality in relation to the F _MSY proxy are shown below. Trends in fishing mortality in relation to previous assessments are presented in Figure 17 (males) and Figure 18 (females). Velvet crab biomass and yield-per-recruit plots given changes in fishing mortality for each assessment area are shown in Figure 23 (males) and Figure 24 (females).

Of the six assessed areas, velvet crab in Shetland was fished below the F _MSY proxy. In the Clyde, Orkney and East Coast, both males and females were fished at levels above F _MSY. In the Hebrides, the fishing mortality for males was below F _MSY while females were fished above F _MSY. In the South Minch, females were fished below F _MSY while males were fished above F _MSY ( Table 11).

Velvet crab stock status in terms of the relationship between F and F _MSY proxy for 2002-2005, 2006-2008 and 2009-2012.

3.2.5. Comparison with Previous Assessments

The last two stock assessments for velvet crab in Scotland were run in 2002-2005 (Mill et al., 2009) and 2006-2008 (Mesquita et al., 2011). The assessment presented in this report revises the two previous assessments by including a fixed length range that is used to calculate an average fishing mortality. This allows recent fishing mortality to be compared with previous estimates.

In the East Coast, F increased to values above the F _MSY proxy while in the South Minch a downwards trend was observed for females. In the other areas where assessments were run, the position of F relative to the F _MSY proxy remains unchanged ( Figure 17 and Figure 18). Shetland's velvet crab results are only presented for the 2009-2012 assessment. The MoU between MSS and NAFC Marine Centre for data provision means that recent data (sampled landings) are not directly comparable with data provided prior to 2009 (previously NAFC provided raised catch data which included discards). Therefore, the new F time series starts at the latest assessment. Velvet crab F estimates for Shetland are much higher than those estimated elsewhere ( Table 11, Figure 17 and Figure 18). This is due to the use of different growth rate ( K) and natural mortality ( M) parameters specific to Shetland ( Table 6). To further examine these differences, LCAs were run for all areas using both the Shetland and the rest of Scotland biological parameters and results compared ( Annex C). Shetland's higher natural mortality rate results in flat-topped YPR curves with higher estimates of current F and F _MAX (the F _MSY proxy). This is further discussed under section 4.3.

3.2.6. Management Considerations

Overall, assessments for the period 2009-2012 in areas with sufficient sampling data showed that most velvet crab assessment areas in Scotland were being fished close to or above the F _MSY proxy. In some assessment areas, a higher yield and biomass per recruit in the long term could potentially be obtained by reducing the level of fishing mortality (effort).

3.3 Lobster

3.3.1. The Fishery

The total tonnage of lobster landed in Scotland has consistently been much lower than that of crabs. However, reported lobster landings have increased substantially in recent years, from 415 tonnes in 2001 to about 1,100 tonnes in 2012 ( Figure 2a). The average price per kilogram of lobster is much higher than that of crab, and the value of the lobster fishery has almost matched that of the brown crab fishery in recent years ( Figure 2b). Average prices of lobster have been stable at over £10 per kilogram in the last four years ( Figure 2c). The annual lobster landings by assessment area are shown in Figure 5. Over the past 40 years the majority of landings of lobster in Scotland have been from the Hebrides, Orkney, South East, South Minch and (more recently) the East Coast areas. The period between 1999 and 2004 was characterised by lower landings from all areas. This can be related to an increase in minimum landing size to 87 mm in 1999, with the effect on landings being evident for the following years. Landings in 2006 do not seem to be comparable with those in the preceding years particularly in the East Coast and South East. This could be due to the introduction of "buyers and sellers" regulations, before which landings may have been under-reported. Landings from the South East and the East Coast, however, increased sharply from 2006 to 2011 and accounted for over 50% of landings into Scotland in the last four years ( Table 9). Figure 8 shows the spatial distribution of lobster landings around Scotland in the period 2009-2012. ICES rectangle 41E7 in the South East had consistently the largest amount of landings. Small quantities of lobster were landed from grounds outside the assessment areas, including ICES rectangles to the west of the Hebrides and South Minch, to the south of Clyde and just outside the South East area.

3.3.2. Sampling Levels

The numbers of sampled lobsters, number of trips and percentage of sampled fishing trips are shown in Table 5. The percentage of trips sampled was generally less than 2% in most assessment areas, where daily inshore trips are common. An improvement in sampling levels in the South Minch and Papa allowed assessments in these areas not attempted in the previous round of assessments (2006-2008). However, the number of sampling trips carried in these areas remains lower than elsewhere. Sampling data were considered to be insufficient (low numbers and infrequent sampling) for running assessments in Mallaig, North Coast, Sule and Ullapool ( Annex A).

3.3.3. Mean Size and Sex Ratio

Lobster mean sizes in landings by assessment area are shown in Figure 11. The data are very noisy, and although males and females appeared to generally follow the same pattern of variation, there is little evidence of any trends in the majority of areas. Lobsters in the South East and East Coast were noticeably smaller than in other areas over the full time series. A marked decline in the mean size of larger animals over the past four years is evident in the Hebrides (males: 132 to 101 mm CL; females: 126 to 102 mm CL) and South Minch males (113 to 105 mm CL). The sex ratio in landings for lobsters was close to 50% in all areas and showed no trends ( Figure 14).

3.3.4. Stock Assessment

Results of assessments based on LCAs summarising estimates of fishing mortality in relation to the F _MSY proxy are shown below. Trends in fishing mortality in relation to previous assessments are presented in Figure 19 (males) and Figure 20 (females). Lobster biomass and yield-per-recruit plots given changes in fishing mortality for each assessment area are shown in Figure 25 (males) and Figure 26 (females).

Lobsters in all the areas assessed were fished above the F _MSY proxy to some extent, particularly males. Fishing mortality was estimated to be above F _MSY for both males and females in Clyde, South Minch, East Coast and South East. In the Hebrides, Orkney and Papa, fishing mortality for females was at F _MSY or below while males were fished above F _MSY. In Shetland, males were fished below F _MSY and females above F _MSY ( Table 12).

Lobster stock status in terms of the relationship between F and F _MSY proxy for 2002-2005, 2006-2008 and 2009-2012.

3.3.5. Comparison with Previous Assessments

The last two stock assessments for lobster in Scotland were run in 2002-2005 (Mill et al., 2009) and 2006-2008 (Mesquita et al., 2011). The assessment presented in this report revises the two previous assessments by including a fixed length range that is used to calculate an average fishing mortality. This allows fishing mortality to be compared with previous estimates. Shetland's lobster results are only presented for the 2009-2012 assessment. The MoU between MSS and NAFC for data provision means that recent data (sampled landings) are not directly comparable with data provided prior to 2009 (raised catch data including discards). In the South East, estimated F for males increased substantially to values above the F _MSY proxy. An upwards trend in F for males was also evident in the Clyde, Hebrides and Orkney. The East Coast F for males decreased in 2009-2012 but is still above the F _MSY proxy. Estimates of F for female stocks were generally lower than males, but show an increase in relation to the previous assessments in the Hebrides, Orkney and South East ( Figure 19 and Figure 20).

3.3.6. Management Considerations

Overall, assessments for the period 2009-2012 show that most lobster assessment areas in Scotland were fished close to or above the F _MSY proxy. A higher yield and biomass per recruit in the long term could potentially be obtained in all assessment areas by reducing the level of fishing mortality (effort).