ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on January 8, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(2):332-345; doi:10.1093/icesjms/fsl035
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Migrations, fishery interactions, and management units of sea bass (Dicentrarchus labrax) in Northwest Europe
1 The Centre for Environment, Fisheries, and Aquaculture Science (Cefas), Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK
2 69 Langstone Drive, Exmouth, Devon, EX8 4HZ, UK
3 IFREMER, Centre de Brest. Département Sciences et Technologies Halieutiques, Laboratoire Unité de Populations et des Peuplements, BP 70, 29280 Plouzané, France
Correspondence to M. G. Pawson: tel: +44 1502 524436; fax: +44 1502 526351; e-mail: mike.pawson{at}cefas.co.uk
Pawson, M. G., Pickett, G. D., Leballeur, J. Brown, M., and Fritsch, M. 2007. Migrations, fishery interactions, and management units of sea bass (Dicentrarchus labrax) in Northwest Europe. – ICES Journal of Marine Science, 64: 332–345.A total of 4959 sea bass was tagged between 2000 and 2005 around the coasts of England, Wales, southern Ireland, and the Channel Isles to investigate whether movement patterns documented in 1987 have changed and to estimate the level of competition for sea bass between inshore and offshore fisheries. Most (54%) of the recaptures of adult bass (fish >40 cm) tagged inshore and made between May and October were within 16 km of the release positions. From November on, an increasing proportion was recaptured at least 80 km south or southwest of tagging sites. Bass tagged on offshore spawning grounds in March and April showed reciprocal movement, 75% of recaptures between May and October being at least 80 km from the release site. These observations reaffirm the hypothesis that adult sea bass may move considerable distances to offshore winter spawning areas, but there was little evidence of the spawning migrations between the North Sea and the western Channel that were observed in the early 1980s. Just 3% of the recaptures of bass >36 cm tagged in UK inshore fisheries were reported from the offshore pairtrawl fishery, whereas half the recaptures of bass tagged in that fishery were made inshore along the UK coast. When weighted by the respective catches, this suggests that the effects of management measures implemented in the UK inshore fishery are largely restricted to that fishery. A proposal for stock units for assessment and management of NW European sea bass fisheries is presented.
Keywords: fishery interactions, migrations, sea bass, stock units, tagging
Received 8 August 2006; accepted 22 November 2006; advance access publication 8 January 2007.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Pawson et al. (1987) described the distribution and movements of sea bass (Dicentrarchus labrax) around England and Wales on the basis of mark-recapture studies conducted during the late 1970s and early 1980s. They proposed a working hypothesis that, once sea bass mature, they either occupy well-defined (usually inshore) feeding areas or move south and west in autumn to prespawning and spawning areas that tend to be offshore. Movement between areas appeared to be relatively rapid and took place at the end of spawning in April/May and as the water cooled between October and December, when adult female bass seek water warmer than
9°C. Observations in the field have been used to support and extend the hypothesis. Ripe adult sea bass have been caught by pelagic trawling in the western English Channel during the months of January–March (Cefas, the Centre for Environment, Fisheries and Aquaculture Science, and IFREMER, l'Institut Français pour la Recherche et l'Exploitation de la Mer, unpublished data). Planktonic egg surveys (Thompson and Harrop, 1987; Jennings and Pawson, 1992) and studies on geographical and seasonal patterns of feeding, condition, and gonad development of bass sampled around England and Wales (Pawson and Pickett, 1996; Masski, 1998) show that bass spawn offshore in the English Channel and eastern Celtic Sea from February to May.
Larvae of sea bass resulting from offshore spawning recruit from June on into coastal and estuarine nursery habitats on the south and west coasts of the UK (Jennings and Pawson, 1992; Reynolds et al., 2003). Juveniles emigrate from these nursery areas at 36 cm total length (used throughout) and a mean age of 4–5 y depending on growth, often dispersing well outside the "home" range of adult bass caught in these areas during summer, and do not necessarily recruit to their parent spawning stock (Pawson et al., 1987; Pickett et al., 2004). It appears that there is substantial mixing at this stage throughout large parts of the distribution range of sea bass. The species matures at about 35 cm for males and 42 cm for females (Kennedy and Fitzmaurice, 1972; Pawson and Pickett, 1996), although ambient water temperatures in winter may influence the onset of maturity in female bass (Pawson et al., 2000).
During the past 20 y, there has been an expansion of effort by French and UK pairtrawlers directed at prespawning and spawning bass that gather in winter offshore in the western English Channel, the Bay of Biscay, and the eastern Celtic Sea. Although concern has been raised over the impact of this exploitation on inshore fisheries for sea bass, and on the sustainability of the bass fishery as a whole, Pawson et al. (2007) show that bass stocks around the coasts of England and Wales increased during the 1990s and early 2000s following several years of good recruitment and moderate levels of exploitation.
Catch data and anecdotal information collected since the mid-1980s as part of a Cefas monitoring programme (Pawson et al., 2007) also suggest changes in the seasonal distribution of bass around England and Wales. According to the hypothesis of Pawson et al. (1987), the movement to spawning areas from summer feeding areas during warmer winters occurs later and over shorter distances, and the survival of first-year bass might also be expected to be enhanced (Pawson and Eaton, 1999; Kelley, 2002). The combined effect would allow self-sustaining bass populations to become established in areas not previously occupied by the whole of the species' life cycle.
To describe further the movements and distribution of sea bass, and to determine the relationship between the winter offshore fishery for prespawning and spawning bass and the summer inshore fisheries, Cefas instigated a mark-recapture exercise on bass >36 cm in 2000. During this exercise, sea bass were tagged in summer and autumn in inshore commercial and recreational fisheries at a wide range of locations around England, Wales, the Channel Isles, and southern Ireland, and during the winter in the UK and French offshore pairtrawl fishery in the western English Channel.
The objectives of this paper are to show, by the pattern and number of recaptures of tagged sea bass in relation to the fisheries in which they are likely to be caught, whether the geographical range and movements of bass off Northwest Europe have changed from those described by Pawson et al. (1987), to evaluate the level of interaction in terms of sea bass between offshore and inshore fisheries, and to propose assessment and management units in relation to the areas and fisheries in which the species is now found.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The fisheries
In any mark-recapture study, knowledge of the distribution of the "sampling effort" of the fishery in which tagged fish are likely to be recaptured provides a semi-quantitative basis for interpreting tag-recapture data in relation to the actual distribution of the fish population being studied. To this end, data were compiled on the number of vessels involved in fisheries that target sea bass, or take the species as a significant bycatch, the gear used (distinguishing commercial from recreational fishing) by area (ICES Division) and season, and the bass catch over the main duration of recaptures during the present study (in this case, 2000–2004).
In England and Wales, only a small proportion of the bass catch landed passes through major ports, and the best estimates of annual catch are obtained by integrating official statistics with estimates of total landings by inshore fishing vessels, including charter and casual angling boats, obtained from a voluntary logbook scheme (Pawson et al., 2007). Estimated annual landings of sea bass in the UK ranged from 1224 to 2211 t between 2000 and 2004, whereas French landings of bass from the offshore fishery in the English Channel ranged from 529 to 1024 t over the same period.
Tagging
Bass >36 cm (the current European minimum landing size), but with emphasis on fish >40 cm, were tagged at 17 inshore sites around England, Wales, the Channel Isles, and southern Ireland during summer and autumn of the years 2000–2005 (Figure 1), chiefly by members of the Bass Anglers Sportfishing Society (BASS), who were trained in tagging techniques by Cefas staff. Sea bass were also tagged aboard commercial fishing vessels and charter boats, and tagging in the offshore fishery was conducted by Cefas staff who accompanied UK pairtrawlers fishing in the western Channel (ICES Division VIIe) in early 2000. Scientists from IFREMER in Brest, France, tagged bass on board French pairtrawlers in the same fishery between March 2000 and March 2004 (Fritsch et al., in press).
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Several types of tag have been used and tested with bass (Pawson et al., 1987), but for this study, we used large internal/external (IEX/WAD or "New York" pattern) abdominal anchor tags (Hallprint Pty Ltd, Holden Hill, South Australia), which allow the fish freedom of movement, have good retention properties, and are relatively quick and easy to apply (Pickett et al., 2004). The tags consist of a yellow or orange streamer 105 mm long, bearing a unique serial number and a message printed in English and French requesting recapture details to be returned to Cefas or IFREMER, bonded via a short nylon sheath (WAD) or directly to a T-bar insert anchor (New York).
Sea bass were caught by trawl and held for at least 1 h in a tank of refreshed seawater to ensure that only fish most likely to survive were tagged. While being measured and tagged, fish were calmed and kept moist by having a wet cloth or towel placed over the head and opercula, and gently restrained. Tags were attached as described in Pickett et al. (2004), on the left flank of the fish midway between the distal tip of the pelvic fin and the vent, 2–3 scale rows up from the mid-ventral line. A few scales were taken for age determination, and all fish were examined for evidence of running eggs or milt.
Data on size and age of fish at release and release position and date were stored on the Cefas Fish Tagging Database, together with details of size, date, and position of recaptures. A summary of numbers and positions of bass recaptures was updated each month on a Website (www.cefas.co.uk/Basstagging) that contained a French version and was shared by Cefas, BASS, and IFREMER via linked web pages. Several bass recaptured by BASS tagging team members were returned alive to the water with the tag intact (having been identified and recorded). Therefore, some repeat recaptures of individual fish were recorded, but only the first recapture event was recorded in the database. Details on recapture position were not available for all fish reported.
In addition to the Website, posters and other publicity announcing the scheme throughout Northwest Europe encouraged anyone catching a sea bass to examine it and, if tagged, to return the tag to Cefas or IFREMER, the BASS coordinator, or a local Fishery Office, giving details of the tag number, place and date of capture, and size of fish. A reward of £6/
10 was given for each tag returned.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Sea bass fisheries in Northwest Europe
For the purpose of this study, the bass fisheries in Northwest Europe are presented by groups of ICES Divisions that encompass the major differences in patterns of bass migration and seasonality of exploitation. Commercial sea bass fisheries can be split into inshore and offshore components. In the inshore fishery, small boats make daily trips and use a wide variety of fishing methods with relatively little activity in winter, whereas the offshore fishery is based mainly on pairtrawling, often directed solely at sea bass, and takes place chiefly between January and April. Catches of bass taken by rod and line (angling) constitute a substantial part of the overall landings into southern Britain, France, and Ireland (Pickett et al., 1995; ICES, 2004), where the species is widely regarded as the most important for sea-angling. The average numbers of UK and French boats employed in commercial fishing for sea bass, in season, during the main period of this study are given by main gear type and sea area in Table 1.
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North Sea: ICES Divisions IVb and c
Although sea bass are caught by angling around northern Scotland, those fish are towards the periphery of the species' normal range. South along the English coast into Yorkshire, relatively few sea bass are taken as bycatch in trawls and in nets set for migratory salmonids and, increasingly, by directed angling. From Norfolk, East Anglia, south, sea bass are caught in fixed nets and driftnets and by trawls and lines, and are targeted in estuaries and around wrecks and offshore banks from May to November. There has been a resurgence in charter-boat angling for sea bass in recent years, and many boats now use rod and line commercially for sea bass. Bass are caught in the southern North Sea by French and Dutch vessels using bottom trawls and by both shore and boat-anglers along the Belgian, Dutch, and German coasts.
Eastern English Channel: ICES Division VIId
Bass are caught inshore along the English coast east of the Solent (tagging site E) by beach-launched boats that fish trammel-nets and gillnets for a mixture of species; several trawlers (often in pairs) occasionally take large catches of bass. In the Solent area, the bass fishery uses fixed gillnets, driftnets, and longlines between April and November, and small trawlers also use locally designed, high-headline bottom trawls to catch the species. There are commercial and recreational angling fisheries for bass all along this coast, and some of the larger charter boats take out groups of anglers specifically to catch the species, going offshore as far as the Channel Isles. Bass are taken as a bycatch by French and Dutch bottom trawlers in the eastern Channel and inshore by netters and liners.
Western English Channel: ICES Division VIIe
Along the English coast, bass are taken mainly by rod and line (both recreational and commercial, with some charter angling trips) off estuary mouths and headlands and by fixed gillnetting and driftnetting inshore all year round. Recently, there has been an increase in activity in the traditional fisheries trolling and handlining with artificial sandeel bait for bass around offshore rocks and reefs.
The largest fishery for sea bass in the western English Channel uses pairtrawls from November to April to catch bass shoaling offshore prior to spawning and has involved a few boats from ports on the Cherbourg Peninsula and up to 20 pairs of teams from ports in the Bay of Biscay, up to 5 UK pairs, and the occasional Danish or Dutch midwater trawler.
There is a commercial bass fishery in many tide races and overfalls around the Channel Isles, deploying drifting longlines and using rod and line, as well as trolling and gillnetting around the rocky reefs. There is also a small recreational angling fishery for sea bass.
Along the French coast west of the Cherbourg Peninsula, small boats take sea bass as bycatch all year round and also target the species using gillnets and longlines in spring and summer, and by trolling, as on the southwest coast of England. Otter trawlers frequently land sea bass, which are also taken by recreational boat anglers around northern Brittany.
Celtic Sea and Bristol Channel: ICES Divisions VIIf and VIIg
The main fishery for sea bass along the north coasts of Cornwall and Devon uses nets and rod and line in summer in estuaries and tidal rips. Also, a few small boats troll close inshore in winter, and local trawlers take bass as a bycatch throughout the year. In late winter, spawning sea bass are sometimes targeted by French midwater pair teams, as in Division VIIe.
Along the coast of South Wales, sea bass are taken by various netting methods, including driftnetting, and by rod and line or handline from May to November, and some of the larger vessels may take out angling charters or fish for large bass around offshore reefs and areas of tidal overfalls.
Irish Sea: ICES Division VIIa
Off west Wales, sea bass are taken by small boats setting gillnets, trammel-nets, and stake-nets close inshore, and some bass have recently been landed by trawlers. Farther north, commercial and recreational angling tends to dominate, together with fixed gillnet or trammel-net and driftnets, along the north Wales and northwest English coasts, where bass are also caught in nets and by lines set intertidally and as a bycatch in nets or traps set for flatfish or salmon throughout the Solway Firth.
Few sea bass are caught along the Northern Irish coast of the Irish Sea. Commercial exploitation of bass has been prohibited in the Republic of Ireland since 1990, but they are caught by recreational anglers in estuary mouths, off rocky headlands and reefs, and along open storm beaches.
Tagging
Between 2000 and 2005, 1881 sea bass >36 cm were tagged inshore in England and Wales, 179 in Ireland, and 360 in the Channel Isles. Fish were mainly tagged in summer and autumn, but a few also in winter. Another 582 bass were tagged in March and April 2000 in the UK offshore fishery operating in the western Channel (Division VIIe), and a further 1957 on French pairtrawlers operating in the same fishery between March 2000 and March 2004 (Fritsch et al., in press). The numbers of bass tagged at each release site are listed in Table 2.
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The bass tagged inshore around England and Wales ranged from 36 to 81 cm, with a mode at 42 cm, and Figure 2 shows how the size distributions varied by sea area. Substantial numbers of bass <40 cm were tagged only in Divisions IVb and VIId, where a subsidiary aim was to investigate exploitation by specific local fisheries.
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The sea bass tagged in the UK offshore fishery in Division VIIe had the same length range as those tagged inshore (36–81 cm), but the modal size group was larger (47 cm; Figure 3a). Those fish were predominantly ripe males up to 65 cm, although some ripe females of 42–79 cm were also tagged. Bass tagged on French vessels in Division VIIe ranged from 36 to 60 cm and had a modal size of 43 cm, although larger fish in the catch were not selected for tagging (Figure 3b).
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Whereas all areas produced bass up to
14 y old, >98% of the bass tagged were <12 y old. Fish tagged offshore included many of the 1989 year class as 11-year-olds (26%) and the 1994 year class as 6-year-olds (23%) (Table 3). No age data were available for French-tagged fish. The bass tagged inshore off the UK and Ireland also contained many fish of the 1989 and 1994 year classes (8 and 18%, respectively), but were dominated by the 1995 year class (37%), which appears to have only partially recruited to the offshore fishery in 2000.
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Recaptures
Up to the end of February 2006, 225 recaptures of sea bass tagged at >36 cm had been reported (Table 2). Figure 4 shows the distribution of recaptures of fish tagged at >40 cm by release region, distinguishing between recaptures reported in May through October ("summer") and November through April ("winter").
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Inshore-tagged sea bass
Most of the 142 recaptures of fish >40 cm tagged inshore were made between April and October, generally within 16 km of the fish's release position, indicating limited summer movement (Table 4). Those fish that were reported outside the respective release area in summer tended to be recaptured to the east along the English coast of the Channel, to the north in the North Sea, or to the west and north of release areas along the west coasts of England and Wales. Two fish tagged on the English coast of the Channel moved to the French coast.
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Some fish were recaptured in their release areas as late as February, but most of the 53 recaptures from November on were made south or southwest of tagging sites, nearly half >80 km from their release position. The only fish recaptured in winter east of the release site had been released in the central English Channel (including the Channel Isles) or the North Sea.
There was no evidence that sea bass released at >40 cm in summer along the east English coast left the southern North Sea in winter (Figure 4a). Winter recaptures of bass tagged in summer along the English coast of the Channel were either inshore close to the release area (10 fish) or in the western Channel (three inshore and four offshore, plus one near Guernsey). The exceptions were a bass that had moved to the north Cornish coast and the one that had moved from Portland Bill in September to south Brittany in late December. Bass tagged in summer near the Channel Isles tended to be recaptured nearby in winter (16 fish), although one fish was reported offshore in the western Channel, and another, released in September, was recaptured <2 months later in the southern Bay of Biscay, a minimum distance of 1200 km. In contrast, 8 of the 11 sea bass tagged in summer on the Welsh and northwest English coasts that were recaptured in winter showed long-distance movements to waters off the north and south Cornish coasts in the Bristol Channel and western English Channel (Figure 5). The exception was a sea bass released in summer at Anglesey that was recaptured in winter off southwest Ireland.
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Sea bass tagged off southern Ireland were generally recaptured near inshore release sites, although one fish released off southwest Ireland in October was recaptured 3 weeks later 60 km off the southeast coast of Ireland.
Offshore-tagged sea bass
There were 33 recaptures of bass >40 cm tagged offshore in winter in Division VIIe. Eight fish were recaptured during the first month after tagging and three fish reported recaptured between November and February in the following 2 y were taken by pairtrawlers in the same fishery, and two were reported closer inshore in winter, one off south Cornwall and one off Guernsey (Figure 4c). There were 14 recaptures between May and October inshore along the UK coast between the eastern Channel (Division VIId) and northwest Wales (Division VIIa), one offshore in the eastern Channel and another in the southern North Sea (Division IVc), two near Guernsey, and two along the French coast (Divisions VIId and VIIe). This pattern of recaptures closely reflects that of bass tagged inshore in summer, indicating substantial movement between offshore spawning areas in Division VIIe (in winter) and feeding areas inshore along the English Channel coast and the west coasts of England and Wales. The sea bass recaptured near the Channel Isles and along the French coast tended to have been released closer to the French than the English coasts of the Channel.
Distribution of recaptures between fisheries
Up to the end of 2005, 131 (73%) recaptured bass that were tagged and released at >36 cm inshore along the English and Welsh coasts were reported within 3 miles of the UK coast (Table 5), two-thirds by commercial métiers, and one-third by recreational anglers (Table 6). Of these recaptures, 40 were reported from inshore fisheries between November and April. Another 26 fish (14%) were recaptured between 3 and 12 miles offshore, split roughly 60/40 in winter and summer, and 23 (13%) were retaken outside the UK 12-mile zone, mainly in winter, 6 by pelagic trawlers, 11 by bottom trawlers, and 6 by offshore netters operating in Division VIIe.
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A similar analysis for fish tagged offshore in winter (Table 6) shows 14 bass (41% of recaptures) recaptured inshore along the English and Welsh coasts between the southern North Sea (Division IVc) and west Wales (Division VIIa). Another 6 (18%) were recaptured in summer between 3 and 12 miles offshore, or along the French coast, and 14 (41%) were retaken in winter outside the UK 6-mile zone, 11 in the offshore pairtrawl fishery in Division VIIe, and 3 by bottom trawlers and offshore netters.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Migration and distribution
The hypothesis originated by Pawson et al. (1987) explains the autumn and spring migrations of adult sea bass around England and Wales as a function of the influence of sea temperature on maturation and spawning. This was based on studies in the late 1970s and early 1980s, when many adult bass tagged on the south coast of England and in the southern North Sea during summer and autumn migrated southwest to prespawning areas in the western Channel. Then, spawning started in the mid-western Channel during March, when the temperature range associated with bass egg distributions was 8.5–11°C, and appeared to spread east through the Channel as the surface water temperature attained 9°C (Thompson and Harrop, 1987). Although sea bass were not tagged on their spawning grounds, it was inferred that the spent fish moved north and east onto feeding grounds in the eastern Channel and southern North Sea (Pawson and Pickett, 1996).
Similarly, many adult sea bass appeared to move south in autumn from the Irish Sea and Bristol Channel towards their prespawning areas off Cornwall, where they spent winter. Tagging off south Cornwall in autumn showed that, from April on, those fish migrated to summer feeding areas along the Welsh and northwest English coasts.
The present study confirms that migration patterns of adult sea bass that spend summer along the coasts of the English Channel and west and northwest England and Wales remain largely unchanged since 2000. The main change since the earlier work is the lack of recaptures offshore in the western Channel of bass > 40 cm tagged in the North Sea, whereas 13 of 17 winter (December–April) recaptures of adult bass tagged in the North Sea in the early 1980s were from the offshore fishery in the western Channel (Pawson et al., 1987). Only one fish tagged in the latter fishery between 2000 and 2004 was recaptured (just) in the southern North Sea. However, preliminary results of a more recent study (Sendall, D. pers. comm.), in which bass >40 cm were tagged in February and March 2006 near Guernsey, include several recaptures in summer along the coast of southeastern England, which suggest that a migratory link still persists between the North Sea and the western Channel. This is also the case for smaller bass: of 39 recaptures of bass tagged at <36 cm in the early 1990s in the Thames Estuary, 10 (26%) were recaptured well to the west of the Dover Straits, though just two (<2%) pre-adult bass moved from the English Channel into the North Sea (Pickett et al., 2004). Therefore, although the seasonal migration of adult bass between the North Sea and offshore in the western Channel is now much less evident, there may still be considerable mixing between sea bass in the North Sea and populations farther west.
A comparison of the recaptures of bass >40 cm tagged inshore and reported by Pawson et al. (1987) and in the present study in the respective release areas (Divisions IVb and c; VIId, VIIe, VIIf and g, VIIa) and in other sea areas, raised by the recaptures per release (Table 7), shows remarkably similar proportions in summer between the two periods, whereas the distribution of winter recaptures was obviously different only for fish released in the North Sea (Divions IVb and c; no recent fish in VIIe) and possibly in the Irish Sea (Division VIIa; more recent fish in VIIe and VIIf and g). However, these comparisons do not bear statistical analysis, because the relative numbers released at different sites within each sea area differed between the two studies and, more importantly, there have been considerable changes in the geographic pattern of fisheries taking bass between the two periods.
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In both Pawson et al. (1987) and the present study, most sea bass >40 cm tagged inshore and recaptured between May and October were recaptured within 16 km of their tagging site (76% in the 1980s; 54% in 2000–2005). Further evidence of this fidelity to summer feeding sites is presented by the repeat recaptures in successive years of at least 17 fish by BASS tagging team members close to the original tagging locations in Ireland, Wales, Devon, Cornwall, Dorset, and the Isle of Wight, usually within two calendar months of the release date. In particular, 5 of 40 fish tagged and released on 27 and 28 July 2001 at a fixed station on Christchurch Ledges were recaptured at the same position in May or June 2002.
The proportion of bass >40 cm tagged inshore reported from within the UK's 3-mile zone between November and April of 2000–2005 (30%) was much higher than that in the 1980s (10%), some fish being recaptured in those areas as late as February. This suggests that the duration of residence of adult sea bass in summer feeding areas may have lengthened and, at least in the North Sea, may have allowed bass to spawn farther east and north than their main spawning areas in the western Channel. This fits well with the hypothesis that the movement from summer feeding areas to prespawning areas to the south and west will be delayed (and probably take place over a shorter distance) during warmer winters (Pawson and Pickett, 1996). This possibility is supported by reports of adult bass being caught in February and March inshore off Cumbria (Division VIIa) and in the central and southern North Sea (Divisons IVb and c) (J. Linstead, pers. comm.) and by an observation that the mean sea temperature for January–March at Southwold (52°19' N 01°41'E) on the English coast of the southern North Sea has risen from 4.7°C in 1980–1984 to 6.3°C in 2000–2004 (Joyce, 2006). Finally, the results of the offshore tagging in winter have confirmed a reciprocal movement of sea bass between prespawning and spawning areas in the western English Channel to a wide range of summer feeding areas along the western and southern coasts of the UK, from Wales through the English Channel and (just) into the southern North Sea.
Fisheries interactions
Results of this study provide some insight to the partition of bass exploitation patterns between UK inshore and international offshore fisheries. One problem with using mark-recapture data for quantitative analysis is the patchy distribution of fisheries likely to catch sea bass, as well as uncertainty about the levels of reporting recaptures. The overall reported recapture rate of sea bass tagged inshore in this study was 7.9%, compared with 9.2% reported by Pawson et al. (1987) for bass tagged at >36 cm in the late 1970s and early 1980s. It is highly unlikely that exploitation rates have declined in the interim (Pawson et al., 2007), and anecdotal information indicates that, in some fisheries, fishers catching tagged bass have withheld the information to try to minimize awareness of their particular fishery from competitors.
Returns of recaptured fish tagged in the offshore fishery are much lower,1.4%, which suggests that the survival rate of even selected sea bass from pairtrawl catches is relatively poor (by a factor of around 0.2, compared with bass tagged inshore, given that around 2400–2500 fish were tagged in each fishery). Nevertheless, it can be assumed that the recaptures of fish tagged inshore and offshore were derived from the same fishing effort within the various fisheries and that the distribution of returns can be used to indicate relative exploitation levels (Dunn and Pawson, 2002).
A comparison of the distribution of recaptures of bass tagged inshore between the four main commercial métiers (enmeshing nets, otter and beam trawls, lines, offshore pairtrawls), and estimates of the catches of sea bass by these gear groups in Subareas IV and VII over the period 2000–2004 (Pawson et al., 2007), shows similar relative rates of recapture in otter/beam trawls (2.19 tagged fish returned per 100 t of bass landed), lines (1.74 per 100 t), and nets (1.92 per 100 t). This suggests that sea bass that spend the warmer months inshore around the UK are equally vulnerable to these fisheries. Bass tagged in the winter offshore pairtrawl fishery had similar catch rates in both inshore and offshore fisheries, at 0.35 and 0.33 per 100 t, respectively, and therefore appear equally available to these fisheries. The comparable catch rate of bass tagged inshore and recaptured in the winter offshore pairtrawl fishery was 0.5 per 100 t. Given their much higher rate of survival following tagging compared with the sea bass caught by pairtrawls, and because levels of tag reporting in any fishery should be the same irrespective of the origin of the tagged fish, this relatively low catch rate of inshore-tagged sea bass in the offshore fishery (some 25% of their catch rate inshore) probably reflects a true difference in the availability of these fish to the respective fisheries.
Taken with the respective catch levels (the annual bass landings from the UK inshore bass fishery were more than twice those of the offshore bass pairtrawl fishery between 2000 and 2004), we believe that a relatively small part (10%) of the bass population that is available to and exploited by the inshore fishery around the English and Welsh coasts is also exploited by the offshore fishery. Note that, in the early 1980s, some 6% of recaptures of sea bass >36 cm released inshore around England and Wales were reported from the offshore pairtrawl fishery in the western English Channel, whereas the corresponding figure from the present study is 3%.
These results, taken with those of an earlier study on the recruitment patterns of bass from UK nursery areas (Pickett et al., 2004), suggest that the main benefits (in terms of yield) of management aimed at protecting juvenile sea bass in coastal waters of England and Wales accrue chiefly to fisheries operating within the UK 6-mile zone. Nevertheless, about half the recaptures of sea bass tagged in the winter offshore fishery in the western Channel were reported in English and Welsh inshore fisheries, suggesting that any controls that result in reduced fishing mortality on the prespawning and spawning population of bass in the western Channel will provide commensurate benefits for the inshore fisheries.
Stock identity
Although population genetic studies have shown evidence of some structuring of sea bass populations in several sub-basins of the Mediterranean Sea (Patarnello et al., 1993; Garcia De León et al., 1997; Bahri-Sfar et al., 2000), Atlantic populations appear genetically homogeneous over wide areas (Castilho and McAndrew, 1998; Naciri et al., 1999; Bonhomme et al., 2002). Fritsch et al. (in press), using microsatellite analysis of up to eight polymorphic loci typed on adult bass captured at the main spawning grounds and juveniles inshore, found a lack of genetic differentiation among populations inhabiting the Bay of Biscay and the English Channel, but suggested a separate population of sea bass around Ireland. Given the extent of mixing between bass subpopulations at earlier life stages attributable to larval drift (Jennings and Pawson, 1992) and the patterns of emigration of pre-adults (Pickett et al., 2004), genetic structuring/differentiation between spawning grounds and between adults and juveniles in the Northwest European bass population is likely to be minimal.
These observations led Fritsch et al. (in press) to question the validity of the "stock" assessment units used by the ICES Study Group (ICES, 2004). Although supporting the Bay of Biscay as being demographically distinct from the English Channel (but noting the lack of genetic differentiation between populations in the Bay of Biscay and the English Channel), and the independence of the stock in Irish waters, Fritsch et al. (in press) suggest that both the genetic results and tagging data made it difficult to validate distinct stocks within the English Channel, the Celtic Sea, and the Irish Sea. However, assessment and management units do not have to be biological entities. In Figure 5, we propose areas within which fishery and biological data could be used in assessments and for which management advice may be given, based on a consideration of both the current patterns of seasonal movements of sea bass in the exploited populations (i.e. >36 cm), as indicated by tag recaptures, and the characteristics of the seasonal fisheries taking them. Given the evidence for diminished interchange of adult sea bass between the Channel and the North Sea (other than around Guernsey in winter; Sendall D., pers. comm.), we propose that bass in the North Sea be considered as a separate management unit.
There appears considerable mixing to the east and west within populations of sea bass along the French and English coasts of the English Channel. However, just two bass in the present study moved from the English coast to the French coast of the Channel and northern Bay of Biscay, and no bass released near the Channel Isles in summer was recovered in UK waters, although four fish moved between the Channel Isles and the offshore winter fishery area in the western Channel. Although this would suggest that bass around North Brittany and the Channel Isles should be treated separately from fish in the northern part of Division VIIe, nearly half the recaptures of sea bass tagged in winter 2006 near Guernsey and recaptured in the following summer were in UK waters in the eastern Channel and southern North Sea (Sendall D., pers. comm.). Despite this connection between populations around the Channel Islands and along the French and English coasts of the eastern Channel, the respective fisheries are different in character and are prosecuted by separate fleets, so we suggest that the boundary between ICES Divisions VIId and VIIe be retained for sea bass assessments.
The continued extensive movement of juvenile and adult sea bass along the west coasts of England and Wales indicates that bass populations in English and Welsh waters of ICES Divisions VIIa, f, and g and the northern part of Division VIIe could be regarded as a single management unit. Although one bass tagged there was recaptured off southern Ireland, we consider that the bass population around Ireland could still be regarded as a discrete stock for management purposes.
This separation of stock units is supported by an assessment of the dynamics of bass populations around the coasts of England and Wales (Pawson et al., 2007). That analysis shows similar biomass trajectories in the western Channel and along the west coast of England and Wales, and rather different patterns in the eastern English Channel and North Sea. Although recruitment time-series from these assessments show common features regarding year-class strength in Divisions IVb and c, VIId, VIIe and h, and VIIa, f and g, this does not necessarily indicate that stocks of sea bass around England and Wales are linked biologically: it is more likely that the abundance and survival of juvenile sea bass is controlled by large-scale environmental patterns. Bearing in mind the movement of sea bass between Divisions VIIa, f, g, and e (northern part, both inshore and offshore) and between Divisions IVb and c, VIId, and VIIe (southern part), we suggest that models be developed to allow these movements of sea bass to be taken into account in future assessments.
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This study was funded by the UK's Department for the Environment, Food, and Rural Affairs (Defra). We thank the 68 members of BASS, working in tagging teams led by Alan Aubert, Kevin Bain, Roger Bankhead, George Bernowicz, Roger Bayzand, Kenny Bowles, Robin Bradley, Malcolm Brindle, Steve Butler, John Darling, John Hall, Arthur Heckler, Don Kelley, Mike Ladle, Phil Marshall, Joe Monaghan, John Morgan, Bob Moss, Andrew Pascoe, John Rawle, Stewart Smalley, Andrew Syvret, Mike Turner, and Alan Vaughan, who tagged and measured bass, took scale samples, and recorded these details meticulously, and to the skippers who kindly let us tag on board their vessels.
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  M. G. Pawson, S. Kupschus, and G. D. Pickett The status of sea bass (Dicentrarchus labrax) stocks around England and Wales, derived using a separable catch-at-age model, and implications for fisheries management ICES J. Mar. Sci., March 1, 2007; 64(2): 346 - 356. [Abstract] [Full Text] [PDF]
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