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The fishery for Langoustine or Nephrops (Nephrops norvegicus) in Scottish waters has developed from landings of a few tonnes in the early 1960s to over 21,000 tonnes in 2011. Landings valued at £80.3 million made Nephrops the second most valuable species landed into Scotland in 2011. There are Nephrops fisheries on various grounds around Scotland, the largest being the Fladen Ground in the North Sea. Most Nephrops are caught by trawlers, but creel fisheries are also important, particularly on the west coast of Scotland. Scotland is allocated the majority of the Total Allowable Catch (TAC) in both the North Sea and on the Scottish west coast and takes over one third of the landings worldwide.
Nephrops occur in areas of suitable muddy sediment in which they construct burrows. Populations are found in depths as shallow as a few meters in sheltered sea lochs, down to over 500 m on the shelf edge, west of the Hebrides. Nephrops spend most of their time in burrows, only coming out to feed and look for a mate. When they are inside their burrows, most animals are protected from trawls so that the emergence patterns are very important from a fisheries perspective. The timing of emergence appears to be related to light level, and greatest catches are often taken at dawn and dusk, although this varies with water depth and a number of other factors. Males tend to dominate trawl catches for most of the year and are more heavily exploited than females which rarely come out of the burrow when carrying eggs ('berried').
Female Nephrops mature at about three years of age and usually reproduce annually. Mating occurs in early summer followed by spawning in September, females remain berried until they hatch their eggs in April or May. Reproductive timings may be slightly different in deeper waters like the North Sea Fladen grounds. The larvae develop in the plankton before settling to the seabed six to eight weeks later.
Growth and maturity in Nephrops in different areas appears to be related to the density of animals which appears to vary with sediment type. In soft mud, density is low, but the animals grow relatively fast, and reach a larger maximum size. On sandier mud, Nephrops density is far higher, but the animals grow more slowly, and are smaller ('beetles'). The diet of Nephrops is extremely varied and includes many bottom living animals and species swimming close to the seabed. Nephrops also scavenge.
For the purposes of stock assessment, Nephrops around Scotland are split into a number of stocks or 'functional units' (FUs) based on the discrete patches of mud in which they live.
In common with other crustaceans, Nephrops cannot be aged and so cannot be assessed using methods commonly employed for fish stocks exploited in Scotland. Instead, the assessments make use of size composition data from catches, combined with information on stock abundance obtained from underwater television (UWTV) surveys. UWTV cameras are used on research vessel surveys to estimate Nephrops burrow density on the seabed. The information gathered provides an index of stock abundance for each FU which is independent of the fishery and burrow emergence patterns. By applying a number of 'correction' factors to the index, an estimate of the absolute abundance of Nephrops is obtained.
In the North Sea, Scottish fishermen exploit Nephrops in the Farn Deeps, Firth of Forth, Moray Firth, Fladen Ground and to a lesser extent at the Noup and Devil's Hole. Most Nephrops are caught by trawlers, the Fladen Ground is by far the largest FU and accounts for more than 50 % of the total North Sea landings.
In 2011, ICES conclusions about Nephrops stock status were based on trends in the UWTV survey, fishery and catch size composition data.
Farn Deeps: the survey indicates that the stock status has been fluctuating around MSY Btrigger since 2007. Changes in the survey method in 2007 make comparison with the preceding data difficult.
Fladen Ground: the stock remains at a high level, well above MSY Btrigger. The harvest rate (removals/TV abundance) has been increasing but is still below FMSY.
Firth of Forth: the stock remains at a high level, well above MSY Btrigger. The harvest rate (removals/TV abundance) remains slightly above FMSY.
Moray Firth: the stock remains above MSY Btrigger. The harvest rate has declined since 2006 and is now at FMSY.
West of Scotland
On the west coast of Scotland, there are Nephrops fisheries in the North Minch, South Minch and the Clyde. Most of the catch is taken by small inshore trawlers targeting Nephrops, but some are taken by larger vessels. Creel fishing accounted for 24% of landings in both the North and South Minch in 2010.
North Minch: The harvest ratio (dead removals/ UWTV abundance) has fluctuated around the FMSY proxy. The stock has been above MSY Btrigger for more than 10 years. (Both MSY Btrigger and historical abundance estimates were rescaled in 2011 to account for improved revisions to the estimated population area).
South Minch: The harvest ratio (dead removals/ UWTV abundance) has fluctuated around the FMSY proxy. The stock is above MSY Btrigger.
Clyde: Harvest rates in the Firth of Clyde have been above the proposed FMSY proxy since 2007, abundance remains well above the Btrigger. Harvest rates in the Sound of Jura have been well below the proposed FMSY proxy in recent years. Abundance remains higher than observed at the start of the series, but the series is too short and patchy to propose an MSY Btrigger at this time.
ICES advice on future landings is provided on the basis of a fixed proportion ('harvest rate') of the UWTV abundance estimate. ICES advised that this 'harvest rate' should be at a level which is consistent with high long term yield and low risk of depletion of production potential ( FMSY). Target harvest rates corresponding to fishing at F0.1 to FMAX (proxies for FMSY) were recommended.
ICES management advice is formulated for Nephrops Functional Units (FU's), whereas management has continued to be applied to the larger ICES finfish areas. STECF supports the ICES ACOM advice for management at a smaller scale and has emphasised the need for whitefish by-catch mitigation measures in Nephrops fisheries.
The agreed Total Allowable Catches for Nephrops are as follows:
North Sea 21,929 18,994
West of Scotland 14,091 13,758
Marine Scotland Science (MSS) has been heavily involved in the development of underwater television (UWTV) surveys for estimating Nephrops abundance for a number of years and this continues with the refinement of both the technological aspects and analysis of video recorded material.
During 2009 MSS made major contributions to an ICES workshop which developed a method for providing TAC advice from the UWTV estimates of abundance.
Work continues in 2012, working closely with industry to assess the development of whitefish by-catch mitigation measures in Nephrops fisheries. To date, this has resulted in the development of highly improved new trawl designs which significantly reduce whitefish catches, whilst still remaining viable in the exploitation of Nephrops.