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

Executive summary

This report presents the results of Scottish regional brown crab (Cancer pagurus), velvet crab (Necora puber) and lobster (Homarus gammarus) stock assessments carried out by Marine Scotland Science (MSS) based on length cohort analyses (LCAs) applied to commercial length frequency data, for the period 2016-19.

The Fisheries

The Scottish creel fisheries are long established and tend to be mixed species fisheries, with brown crab, velvet crab and lobster as the main target species. The importance of each species varies regionally and in relation to season and market demand. The landings of the combined fishery into Scotland in 2020 were 9,000 tonnes with a first-sale value of £31M.

Brown crab:

The most important crab species, in terms of weight and value, landed in Scotland is the brown crab which is found all around the Scottish coast. In recent years, reported landings of brown crab have decreased and were under 7,000 tonnes in 2020. The principal fishing areas for brown crab in Scotland are the Hebrides, Orkney, East Coast and South Minch; landings from these areas accounted for over 60% of the total brown crab in recent years.

Velvet crab:

Velvet crabs are caught in the inshore creel fishery with lobster and brown crab, although few fishermen fish solely for velvet crab. In recent years, landings for velvet crab have fluctuated around 1,600 tonnes. Most landings were taken from inshore areas.

Lobster:

Assessments and stock status

Temporal trends in landings were explored using several size-based indicators for the period over which sampling data are available for each species, by sex and assessment area. The size at first capture, the mean size in the landings of individuals above the size at first capture and the mean size of the largest 5% of individuals were compared to the respective reference points. In several of the well sampled areas it was possible to relate variations of the mean size and mean size of largest animals to trends in fishing mortality. In most cases, results were in agreement with the LCA results and showed similar exploitation level relative to F_MSY reference points.

Length based assessments (LCAs) and length based indicators (LBIs) were used for assessing Scottish crab and lobster stocks. LCAs use commercial catch size composition data and estimates of growth parameters and natural mortality to estimate fishing mortality at length. LBIs were used to explore trends in the sampled landings for each of the three species. Assessments are conducted on a regional basis, with males and females assessed separately.

LCAs were conducted for each species with sufficient data (2016-1019), by assessment area. There was a change in the minimum landing size (MLS) for the three species in 2018 but it is not expected that this has resulted in a major selectivity change. Nevertheless, F_MSY reference points were re-estimated for all stocks with available data, using the most recent LCA input data (2016-2019). A summary of stock status in terms of fishing mortality in relation to reference points is provided for each stock.

Brown crab:

Brown crab fishing mortality for both males and females was estimated to be above F_MSY in the East Coast, Hebrides, North Coast, South East, South Minch and Sule areas. In the Clyde, Orkney and Ullapool, fishing mortality for females was at F_MSY while males were fished above F_MSY. In the Papa area, fishing mortality was below F_MSY for both males and females. In Shetland, the fishing mortality was deemed to be inconclusive due to a lack of consistency between alternative assessment parameters. No assessments were performed for the Mallaig area. In the latest stock assessments, fewer stocks are being fished above F_MSY than in the 2013-2015 assessments.

Analysis of trawl and dredge survey data for brown crab indicates that recruitment and abundance are declining in recent years. Similar trends are apparent in the brown crab survey abundance estimates for the east and west coast.

Velvet crab:

Velvet crab in the Clyde, East Coast, Orkney and South Minch were estimated to be fished at levels above F_MSY (both males and females). In the Hebrides and South East, recent fishing mortality for males was at F_MSY while females were fished above F_MSY. In Shetland, fishing mortality estimates were inconclusive. No assessments were performed for the Mallaig, North Coast, Papa, Sule and Ullapool areas.

Lobster:

Fishing mortality was estimated to be above F_MSY for both males and females in the Clyde, South East, Shetland and South Minch areas. In the East Coast, Hebrides, Orkney and Papa, the fishing mortality estimated for females was at or below F_MSY while males were fished above F_MSY. No assessments were performed for the Mallaig, North Coast, Sule and Ullapool areas as the sampling data collected were considered insufficient to run LCAs.

Management considerations

The results of assessments for the period 2016-19 indicated that in the majority of the assessment areas, brown crab, velvet crab and lobster were fished close to or above F_MSY. It is recommended that effort/fishing mortality should be reduced in those crab and lobster stocks where fishing mortality is estimated to be above F_MSY.

Data and recommendations

With the exception of Shetland, only limited effort data (number of pots fished) have been collected from creel fisheries. Recent changes in the FISH1 form for vessels under 10 m provide additional data which may be useful for deriving estimates of fishing effort. For larger offshore vessels (over 12 m), Vessel Monitoring System (VMS) data integrated with logbook landings could be used to derive a time series of landings-per-unit-effort which could potentially be used as an indicator of trends in stock size.

Discards in crab and lobster fisheries are sampled only on an irregular basis. More regular sampling and information on catches of undersized animals could provide an indication of inter-annual variation in recruitment.

The population structure and some aspects of the biology of crab and lobster are not well understood and some of the assessment areas as currently defined may be inappropriate. Brown crab tagging studies are currently underway in Orkney and these may shed light on potential links between the offshore stock at Sule and inshore Orkney stock.

Given the sensitivity of LCAs/LBIs to the input growth parameters, further work in this area is required. Field studies based on tagging methods and subsequent evaluation of parameters would be desirable.

Information on factors affecting catchability could also be collected by engaging with fishers and the fishing industry. Work being conducted within the RIFGs, for example through the inshore fisheries pilots, ought to provide an improved understanding of the drivers of fishing behaviour.