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International Council for the The Life History, Dynamics and Exploitation Exploration of the Sea of Living Marine Resources: Advances in Knowledge and Methodology CM 2001/J:14

Larvae surveys of edible crab (Cancer pagurus) off the east coast of England: implications for stock structure and management

D. R. Eaton, J. T. Addison, S. P. Milligan, J. Brown, L.J. Fernand

The Centre for Environment, Fisheries and Aquaculture Science (CEFAS), Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT. United Kingdom. [tel: +44 (0)1502 562244, fax: +44 (0)1502 513865, e-mail: [email protected]

Summary

Edible crab (Cancer pagurus) fishing areas are distributed along the English east coast with significant fisheries offshore of North Norfolk and the River Humber. Previous tagging studies showed directed northerly movements of mature females suggesting that recruitment was strongly influenced by larval drift southwards. Comparison of results from a larvae survey undertaken in July 1999 with two previous surveys in 1976 and 1993 showed that the epicentre of crab spawning, as perceived by the density of stage I larvae, has remained consistent over the 23 year period, being centred to the south-west of The Dogger Bank approximately 70km offshore. Recent hydrographical evidence suggests that there is a seasonal jet-like circulation which extends southwards along the north-east coast of England as far as Flamborough Head before turning offshore along the northern edge of The Dogger Bank. The front forms in April/May and breaks down again in October/November, and once established there is very little interchange between water masses north and south of the Dogger Bank. South of the Dogger Bank the circulation is weak with only a slow easterly residual flow. The areas to the north and south of the front are effectively isolated during the period of crab spawning activity, challenging the accepted wisdom that the main crab fishery is reliant upon recruitment from spawning areas to the north. Indeed, the crab population south of the Dogger Bank may be a separate, self-sustaining stock providing recruitment of adult crabs to northern areas.

Introduction

Pot fisheries for edible crab (Cancer pagurus) occur along almost the entire length of the British North Sea coastline and traditionally these fisheries have been essentially inshore in nature. Many crab tagging studies have been carried out in the North Sea, dating back as far as 1900, the results of which have been summarised by Edwards (1979). Tagging seemed to show that whilst males were relatively sedentary, there was a northerly migration of adult female crabs along the east coast of England and Scotland. This northerly movement of mature females was interpreted as part of an inherent behaviour pattern which resulted in the southerly transport of crab larvae to areas suitable for their survival and subsequent settlement. The observation that the mean carapace width of female crabs increased from south to north in the North Sea

© British Crown copyright, 2001 1 supported a northward movement of mature females (Edwards, 1967), and this observation has since been repeated using English data from crab landings since 1990. It was also stated that the northerly movement was accompanied by an offshore movement into deeper water in the autumn prior to spawning, followed by an inshore movement in the spring when the eggs were hatched. All releases and recaptures of tagged crabs were made in the inshore zone because at the time the crab fisheries were restricted primarily to the inshore area. More tagging experiments were carried out in the Norfolk inshore fishery in the 1990's and the few long distance recaptures of tagged crabs (5 of 68 recaptures from 4009 releases; CEFAS unpublished data) were all females taken in inshore fisheries along the Yorkshire coast, supporting the previous observations of northward migration.

However, since the late 1980's offshore crab fisheries have developed, rapidly becoming the largest in the North Sea. They revealed the presence of large quantities of mature female crabs in the offshore areas. This was unexpected given the prevailing theory concerning the perceived mechanism of spawning and recruitment of edible crab along the English North Sea coast. In consequence, a crab larvae survey was undertaken in the North Sea in 1999 and we have re-examined the results of previous crab larvae surveys carried out in the area by the CEFAS Lowestoft Laboratory.

Crab larvae surveys

Methods

In 1999 a plankton survey undertaken by the CEFAS research vessel Cirolana investigated the distribution of edible crab larvae in the western central North Sea, between latitudes 52o37'N and 55o15'N and west of longitude 3oE. Over a two week period in July, 199 stations were sampled using a 53cm high speed tow net fitted with a Guildline CTD monitoring system.

Results and discussion

No edible crab larvae later than stage IV were found and production levels of larvae other than stage I were low; (<15/m2/day-1.) Stage I larvae were almost entirely confined to an area south-west of the Dogger Bank and approximately 70 - 100 km east of Flamborough Head, with a discrete area of production at levels between 40 and 60/m2/day-1 forming the epicentre.

The observations of the distribution of crab larvae in 1999 were compared with those from previous CEFAS surveys. A monthly series of plankton surveys in 1976 showed that the crab larvae first appeared in late May or June, had effectively disappeared from the plankton by late October, and that the main area of spawning was centred around 54oN, 1oE (Nichols et al, 1982). A further CEFAS survey in July 1993, and one by the University of Hull Institute of Estuarine and Coastal Science (IECS) in July 1994, were timed to coincide with the period of larval hatching. They showed that the epicentre of spawning as inferred from the distribution of stage 1 larvae had remained remarkably consistent around latitude 54oN, longitude 1oE centred approximately 70km to the south-west of the Dogger Bank (Fig. 1). To the north of this epicentre there was very little production other than low levels along the north-east English coast and the northern edge of the Dogger Bank. The only other area of significant production was to

2 the south-east of the main area. There appeared to be a close correlation between the areas of highest density of stage 1 larvae and the main fishery (Fig. 2), and between the distribution of larvae and the temperature of bottom water greater than 8oC, particularly in June when hatching was assumed to commence (Fig. 3). The distributions of larvae later than stage 1 indicated that there was only a slow drift to the east away from the main spawning area and a slow south-easterly drift in spawning areas nearer the north Norfolk coast (Nichols et al, 1982; CEFAS unpublished data).

Hydrography of the English North Sea coast

During the months when crab larvae are present within the water column the region is divided into two distinct hydrographic regimes. The deep (> 50 m) northern part of the central North Sea is thermally stratified in summer (from late April until November) because there is insufficient tidally-generated turbulent energy to maintain mixing against the input of surface buoyancy through increased solar heating (Simpson and Hunter, 1974). Here, a pool of cold bottom water is isolated beneath the thermocline in which temperatures seldom rise above 7oC (Brown et al., 1999). South of latitude approximately 54°N and bordering the north-east coast of England, the combination of comparatively shallow water and strong tidal currents (> 0.5 m s-1 amplitude) maintains a vertically mixed water column, where water temperatures typically exceed 10oC by early June. Separating the regions is a zone of strong horizontal density gradients, which in the south is known as the Flamborough front. Associated with these fronts is a strong and persistent density driven, seasonal jet-like circulation (Brown et al, 1999; Brown et al, 2001) which extends south along the English east coast as far as Flamborough Head before moving offshore and along the northern edge of the Dogger Bank (Fig. 1).

Once the fronts are established there is very little interchange across them. Evidence for this comes from the trajectories of drogued satellite-tracked drifters deployed in the vicinity of the north-east coast and to the north of the Flamborough front (Brown et al., 1999; Brown et al., 2001). The drifters followed the boundary of the front passing southward parallel to the north-east coast and moving offshore to skirt the western and northern flanks of the Dogger Bank (Fig. 4). An element of this work examined the incidence and possible origin of paralytic shellfish poisoning (PSP) on the east coast of the UK. (Brown et al, 2001.) There are sporadic outbreaks of PSP in shellfisheries along the north-east coast of the UK, caused by toxins produced by the dinoflagellate Alexandrium tamarense. However, despite the presence of extensive shellfisheries south of Flamborough, there are no recorded instances of PSP in this area. Combined with supporting data on water masses, it appears that when established, the front acts as an effective barrier to the transport of water-borne materials northwards or southwards across it.

Drifter deployments to the south of the front indicate that in the main area of crab spawning there is a weak eastward advection of water. An instrument deployed during the larvae surveys in 1999 moved at a mean speed of 2.8 cm/sec (Fig. 4), in close agreement with velocities determined from drifter deployments during August 1989 (Hill et al., 1993). Further south the residual drift is still very weak, running parallel to the north Norfolk coast before either turning north-east towards the Dutch coast, or stagnating. There is no evidence of any significant penetration into the Southern Bight. There is some interchange between the eastern English Channel and the Southern Bight (Thompson et al, 1995), but except under conditions of prolonged easterly or southerly

3 winds, the residual drift is north-easterly along the French and Belgian coasts and away from the English east coast (Fig. 5).

Discussion

Spawning and larval development in the edible crab are both dependent upon temperature and neither are triggered below 7 or 8oC. The observed spatial pattern of the crab fishery closely matches the observed distribution of stage 1 larvae, which in turn mirrors the observed distribution in time and space of sea-bed temperatures above 7oC at the onset of hatching. If the distribution of stage 1 larvae can be assumed to be indicative of the distribution of spawning females then it can be hypothesised that, in the area studied at least, spawning female crabs are generally restricted to the shallow waters south of the Dogger Bank and to the warmer coastal waters to the north. The effective barrier provided by the Flamborough front ensures, however, that spawning products south of the front are retained in the natal area and there is essentially no exchange with areas to the north. Similarly, crabs spawning to the north of the front would appear to occur in marginal areas of the cold water pool and the transport of larvae across the front is likely to be minimal. In addition there does not appear to be a transport mechanism whereby larvae from spawning in the eastern English Channel can contribute to significant recruitment in the North Sea. There are only very low catches of edible crabs (< 60 tonnes annually) taken in the English fishery in the North Sea south of Norfolk.

In essence, the area of maximum edible crab spawning activity in the North Sea, as identified by a series of crab larvae surveys, appears to be effectively isolated from the rest of the North Sea by the hydrographic features described, throughout the period from hatching of the eggs to settlement of the megalopae. If this isolation does occur, then the accepted wisdom of a contra-natant movement by females providing recruitment to the fishery from spawning areas in the central and northern North Sea to the north, is evidently untenable. It would also suggest that the crab population in the area of the main fishery is a separate, self-sustaining stock and may actually provide recruitment of adult crabs to the northern areas through the ontogenic movement of mature females northwards.

Genetic analysis of edible crabs around the British Isles using microsatellite markers is currently being undertaken and may provide more information regarding stock identity and origins. Also, very little is known about post-larvae recruitment processes. In particular, few megalopae have been sampled during the surveys (with the exception of the series of cruises in 1976, the surveys have been targeted at early stage larvae) and those that were found were not in close proximity to the coast. Further investigations into the settlement, distribution and the movements of early juvenile stages would provide therefore a clearer understanding of the population dynamics of crab stocks in this region. The residual drift of surface water south of the Flamborough front suggests that it would also be advantageous to have some knowledge of what, if any, crab fisheries and populations exist in the eastern North Sea along the Dutch and Danish coasts.

4 References

Brown, J., Hill, A. E., Fernand, L. and Horsburgh, K. J. (1999) Observations of a seasonal jet-like circulation at the Central North Sea cold pool margin. Estuarine Coastal Shelf Sci., 48: 343 – 355.

Brown, J., Hill, A. E., Fernand, L., Horsburgh, K. J., Hill, A. E. and Read, J. W. (2001) Paralytic shellfish poisoning on the east coast of the UK in relation to seasonal density- driven circulation. J. Plankton Res. 23: (1) 105 – 116.

Edwards, E. (1967) The Yorkshire crab stocks. Lab. Leafl. Fish. Lab. Burnham-on- Crouch (New Series). No. 17, 34pp.

Edwards, E. (1979). The edible crab and its fishery in British Waters. Fishing News Books Ltd., Farnham, Surrey, England.

Hill, A.E., James, I.D., Linden, P.F., Matthews, J.P., Prandle, D., Simpson, J.H., Gmitrowicz, E.M., Smeed, D.A., Lwiza, K.M.M., Durazo, R., Fox, A.D. and Bowers, D., 1993. Dynamics of tidal mixing fronts in the North Sea. Phil. Trans. R. Soc. Lond., A343, 431-446.

Nichols, J. H., Thompson, B. M. and Cryer, M. (1982). Production, drift and mortality of the planktonic larvae of the edible crab (Cancer pagurus) off the north-east coast of England. Netherlands Journal of Sea Research 16: 173-184.

Simpson, J.H. and Hunter, J.R., 1974. Fronts in the Irish Sea. Nature, Lond., 250, 404- 406.

Thompson, B.M., Lawler, A.R. and Bennett, D.B. (1995). Estimation of the spatial distribution of spawning crabs (Cancer pagurus L.) using larval surveys in the English Channel. ICES mar. Sci. Symp., 199: 139 – 150.

5 R. Humber

Norfolk

Figure 1. The combined distributions of stage 1 edible crab larvae from the 1993 and 1999 surveys. The epicentre of larvae production is denoted by the darkest shading, with the position of the Flamborough front indicated by arrows.

6 Figure 2. Distribution of English edible crab fisheries in the North Sea showing the level of landings in 2000.

7 Figure 3. The mean bottom temperature in the North Sea in June from 1990-1999.

8 Figure 4. Trajectories of sub-surface drifters deployed in the survey area in July, 1999.

9 Figure 5. Trajectories of sub-surface drifters in the southern North Sea, May – June 2000.

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