Publication Abstract

Dr Walne Memorial Lecture

A review of shellfish resources and their management
Dr Colin Bannister
CEFAS, Lowestoft Laboratory

Mr President, My Lords, Dr Edwards, SAGB members, friends and colleagues. Thank you for your kind introduction. I am greatly honoured by being invited to deliver this very last Walne Lecture, which I consider to be an honour for the whole shellfish team. I recall Peter Walne as being a member of the old school, who did not suffer fools gladly, but he was a first rate scientist. He strongly believed in the future of the shellfish industry, and that it could be greatly assisted by a combination of biological knowledge, scientific method, and practical common sense. This belief is as valid now as it was in Peter's day, and it applies equally well to the management of the wild resources, which are the subject of my lecture.

The lecture covers the following aspects:

• The growth of the shellfish industry
• Crustacean fisheries and stocks
• Mollusc fisheries and stocks
• Supporting science
• Current management
• Future issues.  

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 world-wide. 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

Table 3. Factors Influencing the Effects of Exploitation in Crustacea

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.

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 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)

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.

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.

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.

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

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, 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.

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 furth