Fish and Shellfish Stocks 2012

SCOTTISH SHELLFISH STOCKS

NEPHROPS STOCKS — BIOLOGY AND ASSESSMENT

Latin name : Nephrops norvegicus
Common names : Norway lobster, scampi, Dublin Bay prawn, langoustine

Introduction

The fishery for Nephrops in Scottish waters has developed from landings of a few tonnes in the early 1960s to almost 30,000 tonnes in 2010. Landings valued at £79.6 million made Nephrops the second most valuable species landed into Scotland in 2010. 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.

Biology and life cycle

Nephrops distribution is limited by the extent of suitable muddy sediment in which animals construct burrows. There are populations in the North Sea and waters to the west of Scotland, in open waters and sea lochs, at depths ranging from a few metres 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. They are opportunistic predators, feeding primarily on crustaceans, molluscs and polychaete worms. Female Nephrops usually mature at three years of age and reproduce each year thereafter. Mating takes place in early summer. Females spawn in September, and carry eggs under their tails (described as being 'berried') until they hatch in April or May. The larvae develop in the plankton before settling to the seabed six to eight weeks later. Reproductive timing may be slightly delayed in the deeper areas of the Fladen Ground.

Nephrops in different areas grow at different rates and mature at different sizes. This variation is related to the density of animals and sediment type. On the softest mud, Nephrops densities are low, but the animals grow relatively fast and reach a larger maximum size. The largest animals are colloquially referred to as clonkers. On sandier mud, Nephrops density is much higher, but the animals grow relatively slowly and are smaller ('beetles'). In the North Sea there are differences in growth between stocks, while on the west coast there are also differences between areas within the same stock.

Since most Nephrops fishing is by trawling, and animals are protected from trawls when in burrows, the emergence patterns affect catch rates. The timing of emergence to feed appears related to light level, and greatest catches are often taken at dawn and dusk, although this may vary with water depth and clarity. As 'berried' females rarely come out of the burrow, they are naturally protected from trawlers. Males dominate trawl catches for most of the year, and are more heavily exploited than females.

Assessments

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 which they inhabit. Unlike fish, Nephrops cannot be aged directly and therefore 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.

NORTH SEA : ICES ADVICE ON MANAGEMENT

2012 position : TAC 21,929 tonnes; UK share 18,994 tonnes
2010 Landings : 20,800 tonnes
Stock Health : Most stocks appear to be stable and fished at around F _MSY
Importance : Key shellfish species
Location : Widespread on muddy grounds

Latest Assessment : May 2011

North Sea stocks

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 are caught by trawlers targeting Nephrops. In some areas, particularly the Fladen Ground, they are also caught by whitefish trawlers. The Fladen Ground is by far the largest FU and accounts for more than 50 % of the total North Sea landings.

Total international landings (tonnes) of North Sea Nephrops.

State of the stocks

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 UWTV survey indicates that the stock status has been fluctuating around MSY B _trigger since 2007. Changes in survey methodology in 2007 make comparison with the preceding series difficult.

Fladen Ground: the stock remains at a high level, well above MSY B _trigger. The harvest rate (removals/ TV abundance) has been increasing but is still below F _MSY .

Firth of Forth: the stock remains at a high level, well above MSY B _trigger. The harvest rate (removals/ TV abundance) remains slightly above F _MSY .

Moray Firth: the stock remains above MSY B _trigger. The harvest rate has declined since 2006 and is now at F _MSY .

Distribution of Scottish Nephrops landings (tonnes) in the North Sea in 2010 ( UK vessels into Scotland

Management advice

The 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 ( F _MSY ). Target harvest rates corresponding to fishing at F _0.1 to F _MAX (proxies for F _MSY ) were recommended. Where the advice implies a reduction in harvest rate, ICES has advised on the basis of incremental reductions in fishing mortality towards the target (transition to F _MSY ).

Management action

ICES management advice is formulated for Nephrops FUs, 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.

WEST COAST : ICES ADVICE ON MANAGEMENT

2012 position : TAC 14,091 tonnes; UK share 13,758 tonnes
2010 Landings : 12, 200 tonnes
Stock Health : Stocks appear to be stable and fished at around F _MSY
Importance : Key shellfish species
Location : Widespread on muddy grounds
Latest Assessment : May 2011

West of Scotland Stocks

On the west coast of Scotland, there are Nephrops fisheries in the North Minch, South Minch, the Clyde and to a lesser extent at Stanton Bank and in more offshore areas on the shelf edge. Most of the catch is taken by small inshore trawlers targeting Nephrops, but some are caught by larger twin-rig vessels. Creel fishing accounted for 24% of landings in both the North and South Minch in 2010. Creel-caught Nephrops are generally larger and in better condition than those caught by trawling. They attract high prices in the live export market and provide an important source of income for small local boats. Creels are used mainly in inshore areas and sea lochs, where access by trawlers may be limited by the seabed or legislation. In some areas both fishing methods are used and gear conflicts sometimes occur.

Total international landings (tonnes) of West of Scotland Nephrops.

State of stocks

In 2011, ICES conclusions about Nephrops stock status were based on trends in the UWTV survey, fishery and catch size composition data.

North Minch: The harvest ratio (dead removals/ UWTV abundance) has fluctuated around the F _MSY proxy. The stock has been above MSY B _trigger for more than 10 years. (Both MSY B _trigger and historical abundance estimates were rescaled in 2011 to account for revisions to the estimated area of population distribution based on VMS data).

South Minch: The harvest ratio (dead removals/ UWTV abundance) has fluctuated around the F _MSY proxy. The stock is above MSY B _trigger.

Clyde: Harvest rates in the Firth of Clyde have been above the proposed F _MSY proxy since 2007. UWTV abundance remains well above the B _trigger. Harvest rates in the Sound of Jura have been well below the proposed F _MSY proxy in recent years. UWTV abundance remains higher than observed at the start of the series, but the series is too short and patchy to propose an MSY B _trigger.

Distribution of Scottish west coast Nephrops landings landings (tonnes) in 2008 ( UK vessels into Scotland

Management advice

The 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 (equivalent to fishing at F _MSY . Target harvest rates corresponding to fishing at F _0.1 to F _MAX were recommended (proxies for F _MSY ). Where the advised harvest rate implies a significant reduction in catch, ICES has advised on the basis of incremental reductions in fishing mortality towards the target (transition to F _MSY ).

Management action

ICES management advice is formulated for Nephrops FUs, whereas management continues to be applied to the larger ICES finfish areas. STECF supports the ICES ACOM advice for management at FU level and has emphasised the need for whitefish by-catch mitigation measures in Nephrops fisheries.