Dr Walne Memorial Lecture

A review of shellfish resources and their management

The shellfish industry has come a long way from the sepia-tinted quayside photographs of its Victorian past to the colourful and mouth-watering displays of the modern seafood restaurant or supermarket outlet. Figure la shows that in the last 30 years the percentage contribution of shellfish has risen progressively from 5% to 30% of the total first sale value in England and Wales, almost on a par with the 38% which shellfish contribute to the total value of fish worldwide. Table 1 shows that shellfish are well represented in the top 20 species, whether in England and Wales or the UK as a whole. In the UK, Nephrops is second only to cod, and in England and Wales edible crab is third only to cod and sole. The total contribution is £52 million in England and Wales, and £153 million in the UK, and this takes no account of the value added by processing, packaging, or selling at the higher end of the market. In the UK potting sector alone there may be 2500 vessels under 10 metres, and more than 500 vessels over 10 metres. Shellfishing therefore provides substantial employment, as well as contributing scenic value to our harbours. In England and Wales the main shellfishing areas are in the Channel, with more modest contributions from the North Sea coast and the west coast (Figure 2). 1 am in no doubt that this is an industry worth conserving. It has significant economic value, substantial vessel numbers, and although there are significant shellfisheries outside 12 miles, many of the stocks are within sight of the coast and can be managed under national legislation. Figures 14, 15 and 16, at the end of the figure section at the rear of the text, show the location of the main fisheries in England and Wales.

Table 2. £million of Shellfish in England and Wales, 1998

Crustaceans	28.56	Molluscs	29.47
Edible crabs	13.95	Scallops	13.86
Lobsters	7.17	Cockles	4.05
Nephrops	4.40	Mussels	2.77
Spider crabs	1.70	Native oysters	1.42
Brown shrimps	1.11	Whelks	0.88
Crawfish	0.07	Queens	0.42
Pink shrimps	0.05	Periwinkles	0.18
Velvet crabs	0.02	Cephalopods	5.72
Others	0.09	Others	0.34

Table 3. Factors Influencing the Effects of Exploitation in Crustacea

	Nephrops	Lobster	Edible Crab
Availability	Emergence from burrows	Foraging out of shelter	Foraging, and migration of females
Sex ratio of landings	Mainly male	Equal	Mainly female
Fishing rate	Moderate	High-very high	High-very high
Size at first maturity	23-30 min CL	80-90 min CL	100-130 min CW
Maximum size	60 min CL	150+ min CL	200, or 240 + min CW
Egg number	500-5,000	400-40,000	200,000-3 million

The Shellfish Resources

Table 2 ranks the landed value of the main shellfish species in England and Wales in 1998. The most valuable crustacean landings are the edible crab, lobster and Nephrops. Scallops, cockle and mussel, and the various cephalopods, are the most valuable mollusc landings. These values strongly influence the priorities for scientific investigation and management. The following review deals principally with the fisheries of England and Wales, but also occasionally refers to the Scottish fisheries.

Crustacea

Cefas assessments of crustacean fisheries and stocks are based on official MAFF statistics of landings, and the collection of monthly size measurements at the main ports. Official recording of shellfishing effort is patchy, so Cefas scientists collect log book data from approximately 40 potters round the coast, in order to monitor catch per effort trends. They also undertake sampling voyages at sea, and carry out a range of scientific studies, as described briefly later. In the case of Nephrops, assessments are carried out every two years by the International Council for the Exploration of the Sea (ICES). Other assessments are carried out nationally on a periodic basis as priority dictates. These assessments (Figure 2) enumerate how many animals are landed in each size group, and convert this to an age structure by applying a growth curve. The age structure is then used to calculate the fishing rate, and to predict the effects of changes in either the fishing rate (F) or the minimum landing size (MLS), or both. These procedures require the use of numerical and statistical models, and assumptions about stock structure, natural death rate, and juvenile abundance. Their accuracy depends on the quality of the landings data, and how well the size distribution and growth data represent the biological reality of complex life cycles.

Nephrops

Nephrops live mainly in muddy substrates, and are caught when they emerge periodically from their complex burrow systems. The ICES Nephrops Working Group assesses over 20 stocks of different sizes distributed at varying depths from Norway to Portugal. Cefas is responsible for collecting the assessment data for the eastern Irish Sea and the Farn Deeps stocks, and contributes to the co-operative assessment of the various other stocks. The ICES working group assessments are carried out every second year.

In most of the fisheries, the rate of fishing is moderate to high (40-80%) on male Nephrops, but fairly low (20-30%) on female Nephrops, suggesting that females may emerge less frequently, especially during the egg-carrying phase. In the western Irish Sea, however, fishing rate is high (60%) on females also. In the large Fladen area of the northern North Sea, where the stock is large and the fishery still developing, the fishing rate is still low overall, although it may be quite high in the most heavily fished areas. The ICES assessments show that Nephrops stock biomass (the total weight of the stock) and recruitment (the number of new juveniles entering the exploitable size range) are generally fluctuating without trend, so that most stocks and fisheries appear to be stable. The exceptions are in Portugal, where several stocks are declining. The ICES recommendations are to contain the fisheries at the present level by setting status quo TACs, and these have not changed appreciably over the last six years or so. Although scientists calculate TACs for individual stock units, these are aggregated by the EU to give blanket TACs for major sea areas (Table 4). This potentially weakens the degree of control that can be exerted on each stock unit, although this does not appear to have caused major problems yet. For the last decade, Figure 3 shows the trends in officially recorded landings in the main TAC areas, ICES area IV (North Sea) and ICES area VIa (West of Scotland), where the major landings are by the UK, and ICES area VII (Irish Sea and south west), where the major landings are outside the UK. Landings are stable due to the stable stocks and the status quo TAC recommendations.

Table 4. Summary of Nephrops TACs for 1998/9
(All quantities are tonnes)

Stock Unit	1996	1997	1998/99	1998
	Landings	TAC	ICES Advice	EU TAC
Moray Firth/Noup Head (IV a)	1861		2400
Fladen (IV a)	6176		> 5000
Farn Deeps/Firth of Forth (IV b,c)	4490		4170
Botney Gut QV b,c)	944		> 875
Sub-total	13471	15200	> 12445	15200
Minch/Clyde (V1 a)	10754		11300
Rockall. (V b)	0		zero
Sub-total	10754	12600	11300	12600
Irish Sea (VII a)	8433		9400
Channel (VII d,e)	0		zero
Porcupine/W Ireland (VII b,j,k)	3747		4000
Celtic Sea (VII fg,h + some VII a)	4926		3800
Sub-total	17106	23000	17200	23000
Total	41331	50800	> 40945	50800

Lobster

Potting is widely distributed round the coast, but there are particular concentrations in the western Channel and along the north east coast (Figure 4). The lobster seeks shelter at all stages of the life history, and is primarily associated with cobble, boulder or reef habitat, which is distributed extensively but sparsely along all coasts. It is difficult to identify separate stocks, and lobster assessment data are therefore usually grouped geographically, or by Sea Fisheries Committee districts. Most traditional lobster fisheries are located within sight of land, but in the last decade fast work-boats fitted with GPS systems have enabled potters to spread their effort offshore, particularly on parts of the east coast and in the Channel. Potting efficiency has also increased as metal parlour pots have become more popular. Pot numbers, pot days, and potting efficiency have probably increased in most districts, on both the inshore and offshore grounds.

Figure 5a shows that most lobster landings come from the North Sea coast and the Channel. The North Sea coast was dominant in the 1960s, then declined, and in the 1980s landings from the Channel increased. Channel landings have since been stable, but in the 1990s landings from the North Sea coast have risen dramatically. The regional contributions are shown in more detail in Figure 5b. (Note the different scales). On the east coast, the decline in the 1960s affected both Northumberland and Yorkshire. In the 1990s, the main increase occurred in Yorkshire, and to a lesser extent East Anglia, but landings in Northumberland have remained low. In the Channel, the main contribution comes from the areas east of Devon (principally Sussex and Dorset), and to a lesser extent North Cornwall, and the increase in the 1980s is very clear. In the west the major contribution has switched from Cardigan Bay to South Wales. Landings from North Wales, which were significant in the 1960s and 1970s, have dwindled.

Inshore, lobster fishing is intensive and catch rates are generally low. Offshore, where catch rates are higher, fishing is intensifying, and catch rates must be expected to decline fairly rapidly as the newly exploited stocks are fished down. Large lobsters are rare except on the new offshore grounds, and the catch depend mainly on lobsters just above legal size. Size distributions thus imply that the fishing rate is high (at least 60-70%) in almost all districts, and is even higher in some traditional areas. The fishing rate everywhere is beyond the optimum on our yield curves, so that increasing effort will only increase landings in the short term, before they fall back to the previous level. Recruitment (the number of young lobsters entering the stocks) still appears to be stable, but the models predict that at the present rate of fishing, stock biomass in most areas will be less than a quarter of the virgin level, and in some areas may be as low as 10% of the virgin level. This is why there is concern about the effect of further increases in potting effort on spawning stocks, and hence on sustainability. At present we cannot predict exactly how much more effort can be supported before recruitment starts to decline, so a precautionary approach is warranted. This point is emphasised by Figure 6, which shows the trend in lobster landings since 1895. Total landings were very stable from 1895 to the 1980s, but the recent increase has taken landings well above any previous maximum.

Edible Crab

Potting for edible crab has intensified in some inshore areas, but the major change has been a widespread dispersion of effort onto offshore grounds, principally in the western Channel, west of Scotland and Ireland, but also off the Humber. In the last 20 years large crabbers have increased in size and efficiency, and many now carry up to 1500-2000 pots. Figure 7a illustrates the resulting rise in English landings from the Channel. The Channel is also fished by vessels from France and the Channel Islands, but I have not been able to assemble their trend in landings.

Regional landings of edible crab are illustrated in Figure 7b. On the east coast, crab landings in the 1960s were similar in Northumberland, Yorkshire and East Anglia, but there has since been a prolonged decline in Northumberland. A recent increase in Yorkshire and East Anglia corresponds to the development of offshore fisheries. In the Channel, landings expanded in the 1970s in South Devon, but although this is still the major component of the fishery, recorded landings have declined somewhat, and have been replaced by landings further east. There may be some distortion here because English landings direct into France have increased, but are not well recorded. There has been a modest increase in landings in south Cornwall. In the west, the principal landings occur in north Cornwall, but landings in South Wales have also increased.

After some years, Cefas is switching attention back to crabs, which were previously studied in the 1960s. Work by Dr Eric Edwards on the east coast, and Dr David Bennett in the Channel, showed that fishing rates in the 1970s were in the order of 30-40%. Preliminary new results suggest that these rates may have more than doubled in the principal fisheries. In the expanding Channel and east coast fisheries, recruitment is still seemingly stable, but as with lobster there is concern about the effect of increasing effort on spawning stocks, especially as in the Channel the main fishery is for spawning females prior to the overwintering egg-carrying phase. There is concern about the long term decline in Northumberland, as yet unexplained.

The assessment of crab stocks is more problematical than for lobster. Lobster biology is very similar around the coast, and lobsters generally migrate very little, so that even though lobster stock structure is not well understood the geographic grouping of data is robust and does not influence the assessment results unduly. In contrast, edible crabs show significant regional differences in growth rate, which is much higher in the western Channel than elsewhere, but probably also varies between other parts of the coast. This causes regional differences in the maximum size range available for capture, and also in the minimum landing size. In addition, tagging shows that female crabs migrate during summer and autumn, when they are heavily exploited by the fisheries, causing strong seasonal and geographical differences in size composition. Crab assessments are being conducted regionally, but the groupings of data within regions are still too coarse to represent individual local fisheries, and it is also uncertain how well they represent the effect of exploitation on the migratory part of the stock. The location of individual spawning grounds, and their links to nursery areas, are poorly known, and this makes it difficult to study the additional effects of gravel extraction in areas alleged to be the main sites where female crabs overwinter.

Spider Crab, Crawfish, and Brown Shrimp

Cefas has no analytical assessments of fishing rate for these three species, but the main trends in landings are shown in Figure 8. Spider crabs migrate offshore into deeper water during the winter, and return inshore in the spring and summer to spawn. The main fishery intercepts them on the inshore grounds, using either traps or nets. Over the years, the fishery has been subject to recruitment variations, possibly caused by enhanced recruitment following warm years. Recently the main landings (Figure 8a) have come from the mid-coast area of the Channel, east of Devon, where recruitment has been notably higher.

Crawfish are caught mainly in the most western part of the Channel, and in South Wales. It is not known if the fishery is supported by recruitment originating from native stocks, or by larvae transported from elsewhere, such as south of Ireland. Figure 8b shows a pulse of landings in north Cornwall in the 1970s, followed by a significant decline, and a similar but much smaller pulse in the 1980s. Figure 9 shows that this is actually part of a prolonged sequence of fluctuating but progressively declining landings originating in the 1920s, compared to which the present landings are very low indeed. The fluctuations may represent a sequence of recruitment pulses, but the reason for the prolonged decline, which clearly pre-dates the introduction of netting in the 1960s, has not yet been investigated.

For brown shrimp (Figure 8c), the landings from west coast areas have gradually declined, and east coast landings have increased substantially. The latter increase stems mainly from the development of processing capacity at King's Lynn serving the Dutch market. The effects of fishing on the stocks are still not well understood, but it is considered that the various cycles in landings are most likely to be natural fluctuations caused by the dynamics of this short-lived species.

Scientific Studies on Crustacea

Crustacean assessments are supported by a range of research studies of which those outlined below are just a selection.

• Studies on Nephrops include the counting of burrows directly using underwater TV, plus a study of the physical processes that may allow gyres to transport Nephrops larvae back to their settlement sites.
• The pioneering work by MAFF on lobster stock enhancement is now very well known, but because the success of enhancement will depend on the outcome of competition between juveniles, new studies have been started to estimate the carrying capacity of cobble habitat for newly settled juveniles of 'matchstick' size.
• Studies on maturity and egg production of lobster and crab have been used to support minimum size calculations.  
• Cefas has pioneered studies on whether the size range and catch rate of lobster and crab caught in pots tell a true story about the state of exploitation. Dr Julian Addison has shown that pot design can influence size distributions, and that the number of crustacea caught by traps depends not only on density, but also on competition between individuals within and between species. MAFF-funded work at Southampton University has also used tracking techniques to study the activity and mobility of lobsters near traps set on an artificial reef.  
• Cefas is pioneering methods to estimate the absolute number of crabs present on the seabed. This is based on measurements of the capture efficiency of a trap, and its radius of attraction, which can be used to convert catch per effort to density and total abundance. Preliminary results indicate that at Race Bank off Norfolk, for example, the number of crabs present in the summer could comprise 14 to 24 million animals (equivalent to one crab every 5 square metres). This will be used to estimate fishing rate directly from landings, a calculation which is much easier for fishers to understand than stock assessments which analyse size-distributions using models. Cefas hopes to further develop this approach in the future, and to link it to the results of new studies on crab ageing to be carried out by our colleague Dr Sheehy at Leicester University.

Molluscs

Mollusc stocks are monitored using trends in landings, estimates of relative abundance obtained from dredge surveys of subtidal stocks, or absolute counts made by quadrat surveys of intertidal stocks.

Scallop

Scallop, the primary commercial mollusc species, is dredged on a large number of beds scattered round the coast in the Channel, the Irish Sea, and in west and east Scotland. Fishing persists, but fluctuates according to variations in scallop abundance on the individual beds as settlements come and go. The main English fishery is in the Channel, but significant catches also come from the Irish Sea and Manx waters (Figure 10). Large scallopers are working more and more dredges, and landings from the western Channel have risen substantially over the last few years (Figure 1 la). Landings are lower and more variable in the eastern Channel. In Manx waters, and in the Irish Sea, landings have been declining (Figure 1 lb).

The sustainability of scallop fisheries depends on the spatial extent of the stocks, and how successfully they are renewed from residual stocks left behind when fishers move on. In the Channel, effort and catch fluctuate in proportion, as shown by Figure 1 lc for vessels over 10 metres, landing at Plymouth. It appears that the number of beds, and hence fishing effort, rise and fall as recruitment varies. Vessels have so far maintained a steady catch rate by moving from one bed to the next as each bed becomes depleted, so that there are as yet no conventional signs that the stocks are overfished. The Isle of Man stock, which is very heavily exploited, has been the cause of concern for many years, however, and in 1998 Scottish scientists also expressed concern that recent research vessel surveys had shown the beginning of stock decline in several Scottish fisheries. This has recently led to new scallop conservation initiatives, as described later.

Cockle

Modern cockle fishing is a mixture of traditional hand working, some tractor dredging, and powered fishing using continuous delivery hydraulic suction dredges. Suction dredgers have a large capacity, and can produce economic returns from a much lower density than hand workers. For a period in the 1970s vessels in The Wash also lifted cockles 'blown out' by the rotating screw of vessels at anchor on the beds.

Figure 12a shows that in the 1970s cockle landings came equally from The Wash and the Thames, but in the 1990s The Wash fishery collapsed, and the main landings have since come from the expanding Thames fishery. In South Wales, landings from the locally important Burry Inlet hand-worked fishery have remained stable.

Scientific data for cockles come from Cefas stock surveys in the Burry Inlet, and Sea Fisheries Committees surveys in other areas. The data are reviewed annually by a joint SK-Cefas cockle working party. Following The Wash collapse, and concerns about the very rapid growth of the Thames fishery, Cefas has recently been analysing historical data in detail. Preliminary conclusions were published in Shellfish News Vol. 6 (November 1998) and Vol. 7 (May 1999). In the Burry Inlet, Cefas survey data show that the fishing rate is moderate, usually less than a quarter of the stock. The stock is stable, despite also serving as an important food source for oystercatchers, and settlement has so far never failed. In contrast, in The Wash, where hand raking gave way firstly to blowing and then suction dredging, stock collapse occurred in the 1990s, when a long run of poor recruitment followed the high fishing rates of the 1980s. After several years of very poor stocks and tight controls, a modest recovery may now be in progress. In the Thames, exploitation increased in the 1990s, and concerns about sustainability led to the present licensing regime established under the Regulating Order. Spatfall in the Thames has usually been very regular, but has fluctuated substantially in the last three years making management more difficult.

Historical data show that although a single large cockle spatfall can promote a rapid stock recovery, even from a very low stock level, such spatfalls are very infrequent. They generally occur when larvae are retained in a local system by favourable winds, or when predation on the young stages is reduced, as when winter or spring temperatures are cold. Although some spatfalls survive better when stocks are low, good and poor spatfalls are generally produced by low and high stocks alike, and it is difficult to prescribe what minimum biomass level is required to protect stocks from collapse. It is conceivable that this needs to be higher in a suction dredge fishery than in a hand fishery because juvenile cockles drawn into the delivery pipe are subjected to mechanical and physiological stress before being discarded at deck level. The effect of this on juvenile survival needs further study.

Native Oyster

Cefas still surveys the main oyster beds in the Solent annually. In the eastern Solent, adult stock size is fairly stable or increasing slightly, but the modest stocks in the western Solent continue to decline (Figure 12 b).

Mussels

Some years ago mussel landings were dominated by those from The Wash, used for human consumption, bait, and seed exports to France. In the 1990s The Wash stocks collapsed completely, however (Figure 12c). As with cockles, this was probably caused by very heavy fishing coupled with a long run of years without significant spatfall. Some spat were present in the water, but survival was poor. Attempts to revive The Wash are now being made using seed. Since 1992, overall landings have been maintained by mussel cultivation in Poole Harbour, and by a concerted re-laying and harvesting programme in the Menai Straits, where the SAGB and the North Western and North Wales S17C have played a significant part. The Menai Strait fishery, which supplies both home and Dutch markets, shows what can be achieved by a major re-laying programme.

Whelk

Whelk landings (Figure 12d) have been driven by sudden market changes. After years of low level activity in Norfolk, the Thames and Sussex, whelk potting expanded suddenly in the mid1990s, as a result of demand from the Far East market. Fishing spread to Yorkshire, the western Channel and South Wales. This caused concern for stocks because whelks have limited mobility, and they produce only a few juveniles, which are benthic, and do not disperse. Consequently there is considerable potential for local depletion. Two years ago Cefas showed that the mean size of maturity varies markedly round the coast (Figures 13) so that no single MLS is suitable. Of late, the conservation problem has receded because the whelk market has collapsed and landings have declined steeply.

Scientific Studies on Molluscs

As with crustacea, mollusc assessments have been supported by scientific studies including the following:

• Scallop dredge selectivity and efficiency, used to convert survey estimates to biomass  
• Scallop growth curves based on analysing the biochemical composition of shells  
• Scallop seasonal cycles of condition and maturity  
• Effects of temperature, wind, and freshwater inflow on cockle and mussel spatfall in The Wash  
• Detailed analysis of cockle and mussel biology, spatfall, and exploitation in The Wash, and the effects of bird predation on juveniles  
• Cockle population regulation in the Thames estuary and the Burry Inlet.

Management of Stocks

The most pressing question for the market is always whether landings can be sustained in the long term, and the question for catchers is whether sustainability can be achieved without a loss of fishing opportunity. The answer to the first question depends partly on the nature of the life cycle of the species, and the answer to the second depends on the level of exploitation, how far stocks have declined, and what management methods are appropriate.

The Influence of Life History

Given effective management, most major crustacean stocks should provide stable and sustainable landings. Apart from shrimps, the major species are relatively long lived (from 20 up to 74 years in the extreme case of lobster), they appear to produce juveniles every year, and they are naturally well protected from predators (e.g. Nephrops in burrows, lobster in cobble and reefs). The susceptibility of the individual species to recruitment failure probably depends on the factors compared in Table 3. In Nephrops, for example, the average number of eggs carried by a female is very low. This might imply that Nephrops survive well in their burrows, and also that there is a low rate of larval loss due to the gyres found in some Nephrops areas. A low reproductive rate obviously reduces the resilience of a population to exploitation, however, so it is fortunate that in most fisheries the fishing rate on females is rather low. The low reproductive rate of Nephrops is therefore balanced by the behavioural factors which determine female catchability. In the lobster, fishing poses a larger risk, because females are caught almost as readily as males, but this is compensated by a considerably higher egg production per mature individual. The growth rate of lobsters varies considerably between individuals, so that the effect of heavy exploitation on the size class just above minimum size is partly buffered because it contains several age groups. For the edible crab, the potential threat from fishing is very high in those areas such as the Channel where fishing is predominantly on spawning females. Fortunately, the egg production of individual crabs is much higher than in lobster or Nephrops, and the distribution of crab larvae is wide, which may explain the resilience of crab stocks so far.

Molluscs are less stable than crustacean stocks. Their life cycle is generally shorter, and there are usually substantial natural fluctuations in recruitment year to year as well as spatially from bed to bed. These variations may be caused by variations in temperature, dispersal, food supply and survival. In some stocks, recruitment may also be affected by additional damage mortality on juveniles caused by dredging. Mollusc fisheries fluctuate in sequence with the natural yearclass fluctuations, and there is always a risk that if fishing effort increases in response to a large spatfall, it will then coincide with a subsequent run of poor years. For most mollusc stocks scientists and managers are very much at the mercy of random events, and more stable landing patterns can only be obtained by curtailing the harvest in good years, in order to carry stock over into poor years. Where intertidal mollusc stocks support bird predators, as well as fisheries, this needs to be taken into account in defining a minimum spawning stock biomass.

Management Measures

The principles of good management are to set a minimum size of first capture above the size of first maturity, and to control harvesting rate in order to prevent growth overfishing, or to stop it leading to recruitment failure. Size of first capture can be controlled by setting a minimum landing size, or a mesh size, or by defining the selection characteristics of dredges and riddles. In molluscs, this may also be achieved by closing beds periodically when they contain undersized juveniles. The principal control of harvest rate is by TAC, or by the licensing of fishing effort. In some mollusc fisheries, catch and effort may also be controlled using closed areas, closed seasons, and various limitations on the number, size and efficiency of gear units, but the efficacy of these technical measures depends very much on the circumstances. Based on these considerations, a hierarchy of management measures is currently or soon to be in force, as summarised below.

EU TAC Regulations

• Nephrops

EU Technical Regulation 850/98 (in force 1/1/2000)

• MLS lobster (in force 1/1/2002)  
• MLS Nephrops, edible crab, spider crab, crawfish, scallop, whelk  
• Restrictions on landing crab claws  
• Mesh size bands and target species percentages  
• Two net rule, twin rigs, square mesh panels.

National Technical Measures (in force 1/1/2000)

• MLS lobster, edible crab, spider crab, crawfish, velvet crab, whelk  
• Ban on landing v-notched lobsters and crawfish

Other National Measures

• Over 10 metre scallop licensing (review in 2000)  
• Proposed technical measures for scallop: permitted number of dredges, dredge width, tooth number, ring size and mesh size, ban on French dredges inside 6 miles, restricted scallop bycatch in queen fishery, MLS adjustments.

Regulating and Several Orders

• Oyster and cockle fisheries: licensing, seasonal closures, bed closures, daily catch limits, riddle sizes.

Local Sea Fishery Committee Bylaws (out to 6 miles)

• Vessel size and gear restrictions  
• Closed seasons, areas, beds  
• Higher MLS (e.g. 90 min CL for lobster in some districts)  
• Permit schemes.

There is clearly a considerable amount of international, national, and local regulation of our fisheries.

Nephrops

Nephrops stocks are managed by a combination of TACs, mesh size and minimum landing size regulations, and restrictions on the use of twin rigs and square mesh panels. The mesh size and MLS determine the size of first capture, although because of the anatomy of a Nephrops the selectivity of nets is notoriously poor, necessitating frequent discarding in some areas. Constraints on the use of twin rigs help to control fishing power, whilst square mesh panels are aimed at the release of undersized whitefish. The TACs are status quo TACs (Table 4) which aim to contain the fishing rate at its present level. Scientists would like to see these large TACs dis-aggregated and reallocated down to the level of individual stock units, especially as under-reporting of some TACs is alleged to be a problem in some areas, despite the fact that the agreed EU TACs are still somewhat above the scientific recommendations. The new EU Technical Measures Regulation 850198 will change the current practice on mesh size ranges, percentages of target species, twin rigs, and square mesh panels. This requires corresponding amendments to the more restrictive national legislation on these aspects, and this is presently being discussed with the industry. It is too early to say how these technical changes will affect the level of fishing on Nephrops stocks, or the pattern of selection and discarding. Note that new Nephrops TACs will be negotiated this autumn.

Other Crustacea

For crustacean stocks other than Nephrops, the only major conservation measures in force are minimum landing sizes, and these are set to be amended by the EU Regulation, and by the complementary changes to national legislation (Table 5). The agreed national sizes specify a size of first capture the same as, or larger than those in operation now, and in slightly different sea areas. Cefas will monitor the effect of these size changes, and in particular will assess the benefits of a further increase in the lobster MLS to 90 mm CL (already in place in some SFC districts). It will also re-examine the case for a maximum size in lobsters in those areas where increased fishing on the offshore refuges may be a threat. Except for lobster, there is probably little scope for any further MLS changes in crustacea, especially as new provisions will shortly make it illegal to retain or land a v-notched lobster or crawfish. This will enable fishers and/or SM to pursue v-notching schemes in order to boost spawning stocks. Cefas scientists are currently carrying out tank experiments to determine the longevity of a lobster v-notch. If required we can also advise on the technical aspects of v-notching, and on the interpretation of v-notching data. Dr Addison has contributed in this way to the North Eastern SFC v-notching scheme.

Table 5. EU and National Minimum Landing Sizes

Scallop

Following the concerns expressed in Scotland about the onset of stock decline in some Scottish scallop fisheries it was decided in 1998 to license the over 10 metre fleet in order to restrict scallop fishing capacity. This will be reviewed in a couple of years. In addition, consultation is in progress to define a suite of new technical measures on the permitted type, number, size, and selective properties of the dredge. The details are still being finalised but the aim is to limit total swept area, and to improve selectivity for both scallops and whitefish. It is also proposed to ban French dredging within six miles of the coast. Cefas and SOAEFD will monitor if these changes benefit stocks.

Cockle

Cockle dredge fisheries have a very high fishing capacity. This needs to be controlled by limiting the number of licensees and their catch, and by protecting juvenile and or spawning stocks, using closed seasons and closed areas to regulate the pattern of fishing as circumstances require. Science cannot predict very precisely the minimum average cockle biomass required to maintain recruitment, so that a precautionary approach is needed, involving a 'hands-on' year-by-year approach using survey data. There is considerable pressure for managers to permit high levels of harvesting, but in my view managers should strongly resist this. It is true that low cockle stocks sometimes recover rapidly following a very large spatfall, but, as was seen in The Wash, consistently depleting stocks to low levels greatly increases the risk posed when a run of poor spatfalls occurs. The contrast between the consistency of settlement in the Burry Inlet, where exploitation is restricted to a moderate level, and the unstable sequence of recruitment in The Wash in the 1990s, is very striking. The study of these issues in cockle management will continue.

Mussel

The earlier reference to the Menai Straits mussel fishery showed the benefits of establishing an active rehabilitation of the stock based on a long-term re-laying programme, and this is likely to be the best option for other areas such as The Wash.

Whelk

For whelk, both the EU Regulation and the national proposals prescribe a single MLS of 45 min shell height. This is too low for areas such as Yorkshire or South Wales where the size of first maturity is about 70 min, but it was, unavoidably, determined by the small size of whelks found in the eastern Channel and the Thames Estuary (Figure 13).

Future Issues

Throughout this lecture, I have mentioned topics which will continue to be active for Cefas, and these are included in the following list.

• Stock-specific TACs for Nephrops
• Monitoring the effects of changes in MLS
• Assessing the benefits of a 90 min CL size for lobster
• Assessing the benefits of a maximum size for lobster
• Assessing if technical measures are enough: the case for a potting licence Scallop, edible crab and lobster assessments
• Further development of studies on cockle management
• Assessing the impact and sustainability of an Ensis fishery
• Age determination of lobster and edible crab.

Licensing Pot Fisheries

The most important item in the list is the question of licensing pot fisheries. Minimum landing sizes can and do play an important role in managing crustacean stocks, hence the efforts which officials put into negotiating the new EU sizes in Brussels, but there is nevertheless genuine scientific concern about whether this is enough to safeguard the future. So far there is nothing to control the number of potting vessels, or the number of pots that they use, and there is little doubt that the benefits of minimum size changes can easily be neutralised by such increases. In almost all lobster and edible crab stocks the present fishing rates are well above the optimum point on their yield curves. The stocks are therefore substantially depleted, but scientists cannot say just how much extra effort can be accommodated safely, since we cannot yet identify the most likely collapse point. Given the economic value of these fisheries, the scientific uncertainties about recruitment failure, and the diminishing financial benefits of adding more effort, I believe that it makes great sense to cap effort at its present level. This would reduce the risks of a stock collapse, and in particular could avoid the later painful elimination of effort required by a future recruitment crisis.

It is well known that previous discussions on licensing in 1995 and 1996 identified such problems as:

• verification of entitlement claims from a large number of small vessels  
• the distinction between the 'true' effort which generates most of the mortality, and the large amount of 'latent effort' which has previously only fished occasionally  
• the desire by the larger mobile vessels to have track records in several different fishing areas
• the effectiveness of trying to enforce a pot limitation scheme.

These problems will not go away, and there is no magic solution, but because of scientific concern about recruit failure ('we can hear the sound of breakers, but we cannot locate the cliff edge in the dark') they deserve further detailed consideration. The move to crustacean effort limitation in Western Australia and Canada has already encountered similar problems, but has been successful.

Partnerships

I end by stressing the word 'partnership'. Cefas has already fostered joint projects with university scientists, and with Sea Fisheries Committees, whose permit schemes provide good data on catch and, more importantly, effort. Cefas is also assisting the development of SFC programmes, as already noted. This lecture clearly illustrates the breadth of the shellfisheries portfolio, and the number of issues that need to be dealt with. In their role as advisors, Cefas scientists will continue to work across this broad field, but it is clear that there is also increasing scope for joint ventures, particularly at the local level.

Concluding Remarks

In conclusion I wish to say 'thank you', to the SAGB for inviting me to give this lecture; to the shellfish team, ever ready to get dirty in the field; to our many fishery contacts on the coast, who give their opinions honestly and forthrightly, but who have also respected our position; and finally, to Dr Edwards, for his constant interest in the conservation of shellfish, and for his regular encouragement to Cefas scientists.

Figures

Fig 1a. The percentage value of shellfish in England and Wales

Fig 1b. Value of shellfish landings in England and Wales, 1998

Fig 2. The key elements of an assessment

Fig 3. Nephrops landings by TAC area

Fig 4. Number of potters under and over 10m in 1995
(Source: SFIA and SFCs, EU study contract 94/076)

Fig. 5a. Lobster landings in England and Wales

Fig. 5b. Lobster landings by region

Fig. 6. Lobster landings and value, England and Wales, since 1895

Fig 7a. Edible crab landings in England and Wales

Fig 7b. Edible crab landings by region

Fig 8a. Spider crab landings

Fig 8b. Crawfish landings

Fig 8c. Brown shrimp landings

Fig 9. Crawfish: estimated landings since 1920

Fig 10. Scallop catches by UK vessels landing in England and Wales in 1998

Fig. 11a. UK scallop landings from the English Channel

Fig. 11b. Anual scallop landings from the Irish Sea

Fig. 11c. Catch v effort, Plymouth, vessels over 10 m, 1976-98

Fig. 12a. Cockle landings in England and Wales

Fig 12b. Oyster surveys 1981-1998

Fig. 12c. Mussel landings in England and Wales

Fig. 12d. Whelk landings in England and Wales

Fig. 13. Average size at first maturity in whelks

Fig. 14. Location of the main mollusc fisheries

Fig. 15. Location of the Nephrops, crab abd crawfish fisheries

Fig. 16. Location of the lobster and shrimp fisheries