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Revised editon: Fish and Shellfish Stocks: 2014 Edition

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Nephrops Stocks - Biology and Assessment

Latin name : Nephrops norvegicus

Common names : Norway lobster, scampi, Dublin Bay prawn, langoustine

Introduction

The fishery for Nephropsin Scottish waters has developed from landings of a few tonnes in the early 1960s to over 30,000 tonnes in the mid 2000s. Landings in 2012 of just under 22,000 tonnes had a first sale value of £82 million making Nephropsthe most valuable species landed into Scotland. 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 Catches (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 meters 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 because 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.

Assessment

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 conversion factor to the relative abundance index, an estimate of the absolute abundance of Nephrops is obtained. A proxy for fishing mortality known as the 'harvest rate' is derived as the ratio of total catch to absolute abundance.

RELATIVE DENSITY OF NEPHROPS BURROWS ON A RECENT UWTV SURVEY OF THE FLADEN GROUND

North Sea : ICES Advice on Management

Information Source: ICES advice 2013 http://www.ices.dk/sites/pub/Publication%20Reports/Advice/2013/2013/Neph-IV.pdf

2014 Position : TAC 15,499 tonnes UK share 13,424 tonnes

2012 Landings : 13,600 tonnes

Stock Health : In most stocks abundance has declined to around MSY Btrigger and fishing mortality is above the FMSY proxy

Importance : Key shellfish species

Location : Widespread on muddy grounds

Latest Assessment : April 2013

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 accounted for over 30 % of the total North Sea landings in 2012.

Total International Landings (Tonnes) Of North Sea Nephrops

Total International Landings (Tonnes) of North Sea Nephrops

State of the stocks

In 2013, ICES conclusions about Nephropsstock status were based on trends in the UWTV survey, fishery and catch size composition data.

Farn Deeps: the UWTV survey indicates that the stock declined after 2005 and has been fluctuating near MSY Btrigger since 2007. Changes in survey methodology in 2007 make exact comparison with the preceding series difficult, but the general trend is considered reliable.

Fladen Ground: the stock has declined from the highest observed value in 2008 and is now just below the MSY Btrigger. The harvest rate has fluctuated in recent years, and fell to approximately 4% in 2012 which is below FMSY.

Firth of Forth: the stock remains above MSY Btrigger but has declined since 2008. The harvest rate remains above FMSY.

Moray Firth: the stock is declining but remains just above MSY Btrigger. The harvest rate was above FMSY in 2011 and decreased in 2012, although it is still above FMSY.

Distribution of Scottish Nephrops Landings (Tonnes) in The North Sea in 2012 (UK Vessels into Scotland

Distribution of Scottish Nephrops Landings (Tonnes) in The North Sea in 2012 (UK Vessels into Scotland)

Nephrops North sea stocks UWTV abundance and harvest rate

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 (FMSY). Target harvest rates corresponding to fishing between F0.1 and FMAX (proxies for FMSY) 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 FMSY).

On the basis of this approach, ICES has advised the following limits on landings (for the main FUs of Scottish interest) for 2014[1]:

Farn Deeps Landings: 1,026 tonnes
(Catch = 1,169 tonnes)

Fladen Landings: 8,959 tonnes
(Catch = 9,059 tonnes)

Firth of Forth Landings: 1,417 tonnes
(Catch = 1,646 tonnes)

Moray Firth Landings: 739 tonnes
(Catch = 796 tonnes)

In order to provide information for implementation of the discard ban, ICES has also given estimates of future catch (shown in brackets above) in addition to landings for 2014. These are based on the assumption that discard rates do not change from the average of the last three years (2010-2012). Note that the catch estimate includes discards expected to survive the discarding process - assumed to be 25 % of the total number of discards.

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 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

Information Source: ICES advice 2013 http://www.ices.dk/sites/pub/Publication%20Reports/Advice/2013/2013/Neph-VIa.pdf

2014 Position : TAC 15,287 tonnes UK share 14,925 tonnes

2012 Landings : 14,300 tonnes

Stock Health : Stocks above Btrigger and fishing mortality fluctuating around the FMSY proxy

Importance : Key shellfish species

Location : Widespread on muddy grounds

Latest Assessment : May 2013 (advice released in autumn to account for 2013 surveys)

West of Scotland stocks

On the west coast of Scotland, there are Nephrops fisheries in the North Minch, South Minch, the Clyde and the the Sound of Jura and to a lesser extent at Stanton Bank. 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 about 15 % of landings in the North Minch and 20 % in the South Minch in 2012. 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 can 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

Total International Landings (Tonnes) of West of Scotland Nephrops.

State of stocks

In 2013, ICES conclusions about Nephrops stock status were based on trends in the UWTV survey, fishery and catch size composition data.

North Minch: The stock has been above MSY Btrigger for more than 15 years. The results from the UWTV survey indicate that the abundance has decreased in 2012 and recovered in 2013 to an abundance similar to those observe in 2010-11. The historical harvest ratios (removals/UWTV abundance) have fluctuated around the FMSY proxy. The harvest ratio in 2012 increased to 17.9 % and is above the FMSY proxy.

South Minch: The stock fell below MSY Btrigger in 2012 but increased in 2013 and is now above MSY Btrigger and at a level similar to that observed in 2011. The harvest ratio (removals/UWTV abundance) has increased to 15.8 % in 2012 and is now above FMSY proxy.

Clyde: UWTV abundance remains above the MSY Btrigger. Harvest rates for Nephrops in the Firth of Clyde have increased in 2012 to 26.0 % and remain above the proposed FMSY proxy. Harvest rates for Nephrops in the Sound of Jura have been well below the proposed FMSY 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 Btrigger.

Distribution of Scottish West Coast Nephrops Landings Landings (Tonnes) in 2012 (UK Vessels into Scotland

Distribution of Scottish West Coast Nephrops Landings Landings (Tonnes) in 2012 (UK Vessels into Scotland

Nephrops west coast stocks UWTV abundance and harvest rate

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 FMSY). Target harvest rates corresponding to fishing at F0.1 to FMAX were recommended (proxies for FMSY). 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 FMSY).

On the basis of this approach, ICES has advised the following limits on landings (by FU) for 2014:

North Minch Landings: 3,485 tonnes
(Catch = 3,702 tonnes)

South Minch Landings: 5,211 tonnes
(Catch = 5,394 tonnes)

Clyde Landings: 6,265 tonnes
(Catch = 6,959 tonnes)

In order to provide information for implementation of the discard ban, ICES has also given estimates of future catch (shown in brackets above) in addition to landings for 2014. These are based on the assumption that discard rates do not change from the average of the last three years (2010-2012). Note that the catch estimate includes discards expected to survive the discarding process - assumed to be 25 % of the total number of discards.

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