- Email: [email protected]
The 1996 Flatfish Symposium; a summary report
ELSEVIER
Journal of Sea Research 40 (1998) 173–177
The 1996 Flatfish Symposium; a summary report Henk W. van der Veer a,Ł , Ru¨diger Berghahn b , John M. Miller c , Adriaan D. Rijnsdorp d a Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg Texel, Netherlands Federal Environmental Agency, Institute for Water, Soil and Air Hygiene, Field Station Marienfelde, Schichauweg 58, 12307 Berlin, Germany c Zoology Department, North Carolina State University, Campus Box 7617, Raleigh, NC 27695, USA d Netherlands Institute for Fisheries Research (RIVO-DLO), P.O. Box 68, 1970 AB IJmuiden, Netherlands b
Received 21 March 1998; accepted 19 May 1998
Abstract A summary report of the 1996 Flatfish Symposium is presented. Three topics of the Symposium are discussed: taxonomy and systematics, recruitment processes, and management issues. It is strongly recommended that a taxonomic advisory committee should be formed that will facilitate the transfer of information between different scientific disciplines focused on increasing our knowledge about flatfishes. Recruitment variation in flatfish appears to be dominated by density-independent factors operating at a local scale on the eggs and larvae. Variability in hydrodynamic circulation is suggested to be an important factor. Density-dependent processes are likely to be restricted to the smaller size classes which are not affected by fishing. The debate on the species range hypothesis is continuing. The patterns in latitudinal variation observed in fish did not correspond with the expectations. However, the species range hypothesis could neither be accepted nor rejected, because other sources of variation such as water temperature, sampling strategy, age composition of catches, inshore–offshore gradients and shifts in species ranges are biasing and maybe overruling the patterns in recruitment. Evaluations of the management of different flatfish stocks stressed the importance of the interactions between management measures and flatfish population structures. This field should be strengthened in the future. 1998 Elsevier Science B.V. All rights reserved. Keywords: flatfish; symposium; recruitment; taxonomy; management
1. Introduction The Third International Symposium on Flatfish Ecology was held at the Netherlands Institute for Sea Research (NIOZ) in November 1996. The Symposium was built on the themes of the first two Flatfish Symposia: Life Cycle (1990) and Recruitment (1993). The central theme of the 1996 Flatfish Symposium was ‘Systems dynamics of flatfish’. This Ł Corresponding
author. E-mail: [email protected]
theme reflected the desire to emphasise both the environment and the dynamic processes affecting the population dynamics (especially recruitment) of flatfishes. The principal goals of the 1996 Flatfish Symposium were to logically extend and focus the results of the first two symposia and to try to close the loop on population dynamics by developing insight into the inter-relationship of life history characteristics among the different stages. The scientific programme of the 1996 Flatfish Symposium was based on the results of the 1993
1385-1101/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII S 1 3 8 5 - 1 1 0 1 ( 9 8 ) 0 0 0 2 7 - 6
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H.W. van der Veer et al. / Journal of Sea Research 40 (1998) 173–177
Flatfish Symposium, where the following areas of future research were suggested (Rijnsdorp et al., 1995): (1) Classification of flatfish populations in ‘ecological equivalents’, including the habitat requirements of the successive life history stages. (2) Comparative studies among species and populations of recruitment processes. (3) Analysis of environmental factors determining the survival of pelagic eggs and larvae. (4) Density-dependent habitat selection of demersal juveniles in relation to growth and mortality. (5) Analysis of the habitat characteristics of species for pelagic eggs and larvae, demersal juveniles and adults, including drift of eggs and larvae, transport from spawning towards nursery areas and adult migration. (6) Simulation studies of relevant population dynamical processes to explore quantitatively the importance of the various processes and to establish the necessary precision with which these should be known. This report summarises the progress made by the 1996 Flatfish Symposium especially with respect to the study of recruitment. The results of the 1993 Flatfish Symposium were evaluated by Rijnsdorp et al. (1995). An overview of the progress made by the three Flatfish Symposia will be presented separately (Van der Veer et al., 1998a)
2. Structure of the 1996 Flatfish Symposium The Symposium was structured around a number of topics, each consisting of a combination of invited lectures, scientific papers and poster sessions. In many cases, the abstracts submitted did not correspond with the areas of future research suggested by Rijnsdorp et al. (1995). As a consequence, the preliminary programme had to be re-arranged and the following topics were selected on the basis of the response: (1) flatfish taxonomy; (2) key issues in flatfish dynamics; (3) hydrodynamics; (4) quality of eggs and larvae; (5) behaviour and physiology; (6) stock structure; (7) community analysis; (8) management issues.
3. The progress made In total over 100 oral and poster contributions were presented. It is out of the scope of this summary to present an extensive overview of all these contributions. Only the main outlines — taxonomy and systematics, recruitment processes, and management issues — will be discussed. 3.1. Taxonomy and systematics With renewed interest in recent years, much progress and change has occurred in our understanding of the evolutionary relationships, systematics and nomenclature of flatfishes. Questions on systematics and evolutionary ecology continue increasingly to be posed in the light of new developments in systematic approaches (Hensley, 1997). Advances in systematics lead to recognition of problem areas and help to provide a framework useful in posing meaningful questions. Phylogeny and taxonomy provide a necessary framework for understanding adaptation and constraint and their roles in structuring ecological patterns, life histories and behaviour. Integration of information developed in these fields can clearly benefit through insights provided within a systematic framework. Variation, whether it is observed in ecology, behaviour or life history of flatfishes, obviously has a genetic basis. The determination of the diversity and of the relevant time scales represented by this variation can be advanced through better understanding of phylogenetic relationships among flatfishes. As ecologists and fisheries biologists branch out into areas of high taxonomic diversity, such as the tropics, the need for increased knowledge on the taxonomy and systematics of flatfishes occurring in these areas has become apparent. The exchange of information between ecologists and systematists has never been more necessary. Ecologists recognise that taxonomic expertise is necessary for accurate identifications, and likewise, systematists can benefit from collaborative studies with ecologists through the exchange of ecological information and specimens necessary in conducting systematic studies. It is recommended that a taxonomic advisory committee should be formed that will facilitate the transfer of information between different scientific disciplines focused on increasing our knowledge about flatfishes.
H.W. van der Veer et al. / Journal of Sea Research 40 (1998) 173–177
3.2. Recruitment processes At the previous Flatfish Symposium Leggett and DeBlois (1994) concluded that year-class strength in flatfish and in marine fish in general is determined by mortality operating during the pre-juvenile stage of the life history, the pelagic stages of eggs and larvae. A recent review supported and expanded this view: recruitment variation appears to be dominated by density-independent factors operating at a local scale on these pelagic stages (Leggett and Frank, 1997). The role of hydrodynamics during transport of eggs and larvae in explaining variability in yearclass strength of fish was stressed by Werner et al. (1997). Quantitative understanding of the physical processes that affect the cross-shelf transport and exchange is still a fundamental problem. Coupled physical–biological models offer the possibility to examine population-level implications of environmental conditions. Although it appears to be difficult to identify explicit links between physical variability and recruitment, this field of research seems promising. Nielsen et al. (1998) provided evidence for such a link: analysis of a long-term data set covering the period 1957–1994 showed that the abundance of settled 0-group plaice Pleuronectes platessa along the Danish coast of the Kattegat did depend on transport from the Skagerrak. Moreover, studies on spawning of North Sea plaice in the Southern Bight suggested that variability in circulation patterns during the early pelagic stages in the open sea might be a key factor in determining year-class strength (Van der Veer et al., 1998b). Attention was also paid to a number of other processes that could affect recruitment. Variations in the reproductive potential already start with the influence of maternal effects, i.e. variation in growth and condition of mature female fish. Although clear changes in the viability of the offspring have been found, there is no convincing evidence so far that maternal effects have any impact on recruitment variability (Solemdal, 1997). In addition to the processes operating during the pelagic egg and larval stages, metamorphosis and settlement might be a critical period. Eye migration, asymmetrical pigmentation and a 90º rotation in posture are developmental changes that unify all flatfishes and facilitate a transition from a pelagic to a benthic existence. Settlement places
175
the fish in new environmental conditions, and these imply different performances for survival compared to those needed in the plankton. Preliminary comparisons suggest that flatfishes may have accelerated development of their mechanosensory system relative to pelagic species (Fuiman, 1997). Furthermore, the unfavourable period of eye migration and transition to the benthic habitat requires food reserves and a rewiring and=or recalibration of vision and gravityassociated structures utilised previously by the still symmetric larvae for e.g. food detection (Osse and Van den Boogaart, 1997). However, whether or not the process of metamorphosis and settlement is a key process in determining recruitment still remains unsolved. The physical characteristics of the environment at least appear to influence the early life-history patterns by modifying the behaviour of immigrating larvae (Burke et al., 1998). Habitat selection may play a crucial role in determining the distribution patterns on a small spatial scale. Food supply and predator abundance (Wennhage and Gibson, 1998) as well as sediment characteristics (Neuman and Able, 1998) may influence the distribution of early life-history stages in some flatfish species. Also in subsequent life stages, behaviour patterns remain involved in changing between habitats (Gibson, 1997). Studies of recruitment variability over the range of a species formed another important topic. The species range hypothesis (Miller et al., 1991) — predicting that recruitment variability in flatfish should be most variable at the northern edge of the species range, least near the centre of the range, and intermediate near the southern limit — was tested for a number of species in different areas. Leggett and Frank (1997) could not find support for the species range hypothesis. Their results were generally inconsistent with the hypothesis. The patterns in latitudinal variation observed in fish recruits in northwest Europe also did not correspond with the expectations. However, Philippart et al. (1998) could neither accept nor reject the species range hypothesis, because in their opinion the expected relationships between the recruitment variation and latitude might have been overruled by other sources of variation such as water temperature, sampling strategy, age composition of catches, inshore–offshore gradients and shifts in species ranges. In addition, Bailey (1997) and Rice and Kronlund (1997) stressed the importance of
176
H.W. van der Veer et al. / Journal of Sea Research 40 (1998) 173–177
community analysis and of the structural dynamics and ecology of flatfish populations. So far research data such as recruitment time-series are still interpreted as if populations were closed systems. Rice and Kronlund (1997) showed that evidence of competitive interactions may be removed by the effect of fishing. These simulation results were supported by the analysis of Piet et al. (1998), who inferred interspecific competition among juveniles or small-sized flatfish species from differences in spatial distribution and diet. In most species, basic information on migration mechanisms, colonisation and extinction events, gene flow, such as the work done on sole Solea solea (Exadactylos et al., 1998) and Pacific black halibut Reinhardtius matsuurae (Diakov, 1998), and density-dependent movements between subpopulations is still rudimentary despite its importance. This lack of information on recruitment of well-defined (sub)populations means that the debate will continue for some time. 3.3. Management issues Management issues formed another topic of the symposium. Daan (1997) evaluated the Common Fisheries Policy of the European Union, adopted in 1983 and primarily focused on a science-based total-allowable-catch (TAC) management system with fixed national quota shares. This analysis showed that the Policy has not been able to restrict fishing mortality and as a consequence the level of over-exploitation has only increased. The autonomic economic developments within the fisheries suggest that the fishery can be more effectively managed for the cost side rather than the profit side. An analysis of the yellowtail flounder (Pleuronectes ferruginea) fisheries on the Grand Bank of Newfoundland from 1975 to 1995 suggested a strong impact of fisheries on the distribution of the yellowtail flounder. Brodie et al. (1998) observed a contraction of the stock as a function of low stock size, which resulted from increased fishery activity in the mid to late 1980s. The potential impact of fisheries on recruitment was stressed by Berghahn and Purps (1998). They consider discard mortality of fisheries in flatfish nursery areas to have an effect on ultimate recruitment. These examples illustrate how information on flatfish populations can be used to improve management approaches.
Acknowledgements All participants of the 1996 Flatfish Symposium are acknowledged for their contribution to the Symposium. This is NIOZ publication No. 3281.
References Bailey, K.M., 1997. Structural dynamics and ecology of flatfish populations. J. Sea Res. 37, 269–280. Berghahn, R., Purps, M., 1998. Impact of discard mortality in Crangon fisheries on year-class strength of North Sea flatfish species. J. Sea Res. 40 (this issue). Brodie, W.B., Walsh, S.J., Atkinson, D.B., 1998. The effect of stock abundance on range contraction of yellowtail flounder (Pleuronectes ferruginea) on the Grand Bank of Newfoundland in the Northwest Atlantic from 1975 to 1995. J. Sea Res. 39, 139–152. Burke, J.S., Ueno, M., Tanaka, Y., Walsh, H., Maeda, T., Kinoshita, I., Seikai, T., Hoss, D.E., Tanaka, M., 1998. The influence of environmental factors on early life history patterns of flounders. J. Sea Res. 40 (this issue). Daan, N., 1997. TAC management in North Sea flatfish fisheries. J. Sea Res. 37, 321–341. Diakov, Y.P., 1998. Population structure of the Pacific black halibut Reinhardtius matsuurae Jordan et Snyder. J. Sea Res. 40 (this issue). Exadactylos, A., Geffen, A.J., Thorpe, J.P., 1998. Population structure of the Dover sole, Solea solea L. in a background of high gene flow. J. Sea Res. 40 (this issue). Fuiman, L.A., 1997. What can flatfish ontogenies tell us about pelagic and benthic lifestyles? J. Sea Res. 37, 257–267. Gibson, R.N., 1997. Behaviour and the distribution of flatfishes. J. Sea Res. 37, 241–256. Hensley, D.A., 1997. An overview of the systematics and biogeography of the flatfish. J. Sea Res. 37, 187–194. Leggett, W.C., DeBlois, E.M., 1994. Recruitment in marine fishes: is it regulated by starvation and regulation in the egg and larval stages? Neth. J. Sea Res. 32, 119–134. Leggett, W.C., Frank, K.T., 1997. A comparative analysis of recruitment variability in North Atlantic flatfishes — testing the species range hypothesis. J. Sea Res. 37, 281–299. Miller, J.M., Burke, J.S., Fitzhugh, G.R., 1991. Early life history patterns of Atlantic North American flatfish: likely and (unlikely) factors controlling recruitment. Neth. J. Sea Res. 27, 261–275. Neuman, M.J., Able, K.W., 1998. Experimental evidence of sediment preference by early life history stages of windowpane (Scophthalmus aquosus). J. Sea Res. 40 (this issue). Nielsen, E., Bagge, O., MacKenzie, B.R., 1998. Wind-induced transport of plaice (Pleuronectes platessa): early life-history stages in the Skagerrak–Kattegat. J. Sea Res. 39, 11–28. Osse, J.W.M., Van den Boogaart, J.G.M., 1997. Size of flatfish larvae at transformation, functional demands and historical constraints. J. Sea Res. 37, 229–239.
H.W. van der Veer et al. / Journal of Sea Research 40 (1998) 173–177 Philippart, C.J.M., Henderson, P.A., Johannessen, T., Rijnsdorp, A.D., Rogers, S.I., 1998. Latitudinal variation in fish recruits in Northwest Europe. J. Sea Res. 39, 69–77. Piet, G.J., Pfisterer, A.B., Rijnsdorp, A.D., 1998. On factors structuring the flatfish assemblage in the southern North Sea. J. Sea Res. 40 (this issue). Rice, J.C., Kronlund, A.R., 1997. Community analysis and flatfish: diagnostic patterns, processes, and inference. J. Sea Res. 37, 301–320. Rijnsdorp, A.D., Berghahn, R., Miller, J.M., Van der Veer, H.W., 1995. Recruitment mechanisms in flatfish: what did we learn and where do we go? Neth. J. Sea Res. 34, 237–242. Solemdal, P., 1997. Maternal effects — a link between the past and the future. J. Sea Res. 37, 213–227. Van der Veer, H.W., Berghahn, R., Miller, J.M., Rijnsdorp, A.D.,
177
1998a. Recruitment in flatfish, with special emphasis on North Atlantic species: progress made by the Flatfish Symposia. ICES J. Mar. Sci. (submitted). Van der Veer, H.W., Ruardy, P., Van den Berg, A.J., Ridderinkhof, H., 1998b. Impact of interannual variability in hydrodynamic circulation on egg and larval transport of plaice Pleuronectes platessa L. in the southern North Sea. J. Sea Res. 39, 29–40. Wennhage, H., Gibson, R.N., 1998. Influence of food supply and a potential predator (Crangon crangon) on settling behaviour of plaice (Pleuronectes platessa). J. Sea Res. 39, 103–112. Werner, F.E., Ouinlan, J.A., Blanton, B.O., Luettich Jr., R.A., 1997. The role of hydrodynamics in explaining variability in fish populations. J. Sea Res. 37, 195–212.
Journal of Sea Research 40 (1998) 173–177
The 1996 Flatfish Symposium; a summary report Henk W. van der Veer a,Ł , Ru¨diger Berghahn b , John M. Miller c , Adriaan D. Rijnsdorp d a Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg Texel, Netherlands Federal Environmental Agency, Institute for Water, Soil and Air Hygiene, Field Station Marienfelde, Schichauweg 58, 12307 Berlin, Germany c Zoology Department, North Carolina State University, Campus Box 7617, Raleigh, NC 27695, USA d Netherlands Institute for Fisheries Research (RIVO-DLO), P.O. Box 68, 1970 AB IJmuiden, Netherlands b
Received 21 March 1998; accepted 19 May 1998
Abstract A summary report of the 1996 Flatfish Symposium is presented. Three topics of the Symposium are discussed: taxonomy and systematics, recruitment processes, and management issues. It is strongly recommended that a taxonomic advisory committee should be formed that will facilitate the transfer of information between different scientific disciplines focused on increasing our knowledge about flatfishes. Recruitment variation in flatfish appears to be dominated by density-independent factors operating at a local scale on the eggs and larvae. Variability in hydrodynamic circulation is suggested to be an important factor. Density-dependent processes are likely to be restricted to the smaller size classes which are not affected by fishing. The debate on the species range hypothesis is continuing. The patterns in latitudinal variation observed in fish did not correspond with the expectations. However, the species range hypothesis could neither be accepted nor rejected, because other sources of variation such as water temperature, sampling strategy, age composition of catches, inshore–offshore gradients and shifts in species ranges are biasing and maybe overruling the patterns in recruitment. Evaluations of the management of different flatfish stocks stressed the importance of the interactions between management measures and flatfish population structures. This field should be strengthened in the future. 1998 Elsevier Science B.V. All rights reserved. Keywords: flatfish; symposium; recruitment; taxonomy; management
1. Introduction The Third International Symposium on Flatfish Ecology was held at the Netherlands Institute for Sea Research (NIOZ) in November 1996. The Symposium was built on the themes of the first two Flatfish Symposia: Life Cycle (1990) and Recruitment (1993). The central theme of the 1996 Flatfish Symposium was ‘Systems dynamics of flatfish’. This Ł Corresponding
author. E-mail: [email protected]
theme reflected the desire to emphasise both the environment and the dynamic processes affecting the population dynamics (especially recruitment) of flatfishes. The principal goals of the 1996 Flatfish Symposium were to logically extend and focus the results of the first two symposia and to try to close the loop on population dynamics by developing insight into the inter-relationship of life history characteristics among the different stages. The scientific programme of the 1996 Flatfish Symposium was based on the results of the 1993
1385-1101/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII S 1 3 8 5 - 1 1 0 1 ( 9 8 ) 0 0 0 2 7 - 6
174
H.W. van der Veer et al. / Journal of Sea Research 40 (1998) 173–177
Flatfish Symposium, where the following areas of future research were suggested (Rijnsdorp et al., 1995): (1) Classification of flatfish populations in ‘ecological equivalents’, including the habitat requirements of the successive life history stages. (2) Comparative studies among species and populations of recruitment processes. (3) Analysis of environmental factors determining the survival of pelagic eggs and larvae. (4) Density-dependent habitat selection of demersal juveniles in relation to growth and mortality. (5) Analysis of the habitat characteristics of species for pelagic eggs and larvae, demersal juveniles and adults, including drift of eggs and larvae, transport from spawning towards nursery areas and adult migration. (6) Simulation studies of relevant population dynamical processes to explore quantitatively the importance of the various processes and to establish the necessary precision with which these should be known. This report summarises the progress made by the 1996 Flatfish Symposium especially with respect to the study of recruitment. The results of the 1993 Flatfish Symposium were evaluated by Rijnsdorp et al. (1995). An overview of the progress made by the three Flatfish Symposia will be presented separately (Van der Veer et al., 1998a)
2. Structure of the 1996 Flatfish Symposium The Symposium was structured around a number of topics, each consisting of a combination of invited lectures, scientific papers and poster sessions. In many cases, the abstracts submitted did not correspond with the areas of future research suggested by Rijnsdorp et al. (1995). As a consequence, the preliminary programme had to be re-arranged and the following topics were selected on the basis of the response: (1) flatfish taxonomy; (2) key issues in flatfish dynamics; (3) hydrodynamics; (4) quality of eggs and larvae; (5) behaviour and physiology; (6) stock structure; (7) community analysis; (8) management issues.
3. The progress made In total over 100 oral and poster contributions were presented. It is out of the scope of this summary to present an extensive overview of all these contributions. Only the main outlines — taxonomy and systematics, recruitment processes, and management issues — will be discussed. 3.1. Taxonomy and systematics With renewed interest in recent years, much progress and change has occurred in our understanding of the evolutionary relationships, systematics and nomenclature of flatfishes. Questions on systematics and evolutionary ecology continue increasingly to be posed in the light of new developments in systematic approaches (Hensley, 1997). Advances in systematics lead to recognition of problem areas and help to provide a framework useful in posing meaningful questions. Phylogeny and taxonomy provide a necessary framework for understanding adaptation and constraint and their roles in structuring ecological patterns, life histories and behaviour. Integration of information developed in these fields can clearly benefit through insights provided within a systematic framework. Variation, whether it is observed in ecology, behaviour or life history of flatfishes, obviously has a genetic basis. The determination of the diversity and of the relevant time scales represented by this variation can be advanced through better understanding of phylogenetic relationships among flatfishes. As ecologists and fisheries biologists branch out into areas of high taxonomic diversity, such as the tropics, the need for increased knowledge on the taxonomy and systematics of flatfishes occurring in these areas has become apparent. The exchange of information between ecologists and systematists has never been more necessary. Ecologists recognise that taxonomic expertise is necessary for accurate identifications, and likewise, systematists can benefit from collaborative studies with ecologists through the exchange of ecological information and specimens necessary in conducting systematic studies. It is recommended that a taxonomic advisory committee should be formed that will facilitate the transfer of information between different scientific disciplines focused on increasing our knowledge about flatfishes.
H.W. van der Veer et al. / Journal of Sea Research 40 (1998) 173–177
3.2. Recruitment processes At the previous Flatfish Symposium Leggett and DeBlois (1994) concluded that year-class strength in flatfish and in marine fish in general is determined by mortality operating during the pre-juvenile stage of the life history, the pelagic stages of eggs and larvae. A recent review supported and expanded this view: recruitment variation appears to be dominated by density-independent factors operating at a local scale on these pelagic stages (Leggett and Frank, 1997). The role of hydrodynamics during transport of eggs and larvae in explaining variability in yearclass strength of fish was stressed by Werner et al. (1997). Quantitative understanding of the physical processes that affect the cross-shelf transport and exchange is still a fundamental problem. Coupled physical–biological models offer the possibility to examine population-level implications of environmental conditions. Although it appears to be difficult to identify explicit links between physical variability and recruitment, this field of research seems promising. Nielsen et al. (1998) provided evidence for such a link: analysis of a long-term data set covering the period 1957–1994 showed that the abundance of settled 0-group plaice Pleuronectes platessa along the Danish coast of the Kattegat did depend on transport from the Skagerrak. Moreover, studies on spawning of North Sea plaice in the Southern Bight suggested that variability in circulation patterns during the early pelagic stages in the open sea might be a key factor in determining year-class strength (Van der Veer et al., 1998b). Attention was also paid to a number of other processes that could affect recruitment. Variations in the reproductive potential already start with the influence of maternal effects, i.e. variation in growth and condition of mature female fish. Although clear changes in the viability of the offspring have been found, there is no convincing evidence so far that maternal effects have any impact on recruitment variability (Solemdal, 1997). In addition to the processes operating during the pelagic egg and larval stages, metamorphosis and settlement might be a critical period. Eye migration, asymmetrical pigmentation and a 90º rotation in posture are developmental changes that unify all flatfishes and facilitate a transition from a pelagic to a benthic existence. Settlement places
175
the fish in new environmental conditions, and these imply different performances for survival compared to those needed in the plankton. Preliminary comparisons suggest that flatfishes may have accelerated development of their mechanosensory system relative to pelagic species (Fuiman, 1997). Furthermore, the unfavourable period of eye migration and transition to the benthic habitat requires food reserves and a rewiring and=or recalibration of vision and gravityassociated structures utilised previously by the still symmetric larvae for e.g. food detection (Osse and Van den Boogaart, 1997). However, whether or not the process of metamorphosis and settlement is a key process in determining recruitment still remains unsolved. The physical characteristics of the environment at least appear to influence the early life-history patterns by modifying the behaviour of immigrating larvae (Burke et al., 1998). Habitat selection may play a crucial role in determining the distribution patterns on a small spatial scale. Food supply and predator abundance (Wennhage and Gibson, 1998) as well as sediment characteristics (Neuman and Able, 1998) may influence the distribution of early life-history stages in some flatfish species. Also in subsequent life stages, behaviour patterns remain involved in changing between habitats (Gibson, 1997). Studies of recruitment variability over the range of a species formed another important topic. The species range hypothesis (Miller et al., 1991) — predicting that recruitment variability in flatfish should be most variable at the northern edge of the species range, least near the centre of the range, and intermediate near the southern limit — was tested for a number of species in different areas. Leggett and Frank (1997) could not find support for the species range hypothesis. Their results were generally inconsistent with the hypothesis. The patterns in latitudinal variation observed in fish recruits in northwest Europe also did not correspond with the expectations. However, Philippart et al. (1998) could neither accept nor reject the species range hypothesis, because in their opinion the expected relationships between the recruitment variation and latitude might have been overruled by other sources of variation such as water temperature, sampling strategy, age composition of catches, inshore–offshore gradients and shifts in species ranges. In addition, Bailey (1997) and Rice and Kronlund (1997) stressed the importance of
176
H.W. van der Veer et al. / Journal of Sea Research 40 (1998) 173–177
community analysis and of the structural dynamics and ecology of flatfish populations. So far research data such as recruitment time-series are still interpreted as if populations were closed systems. Rice and Kronlund (1997) showed that evidence of competitive interactions may be removed by the effect of fishing. These simulation results were supported by the analysis of Piet et al. (1998), who inferred interspecific competition among juveniles or small-sized flatfish species from differences in spatial distribution and diet. In most species, basic information on migration mechanisms, colonisation and extinction events, gene flow, such as the work done on sole Solea solea (Exadactylos et al., 1998) and Pacific black halibut Reinhardtius matsuurae (Diakov, 1998), and density-dependent movements between subpopulations is still rudimentary despite its importance. This lack of information on recruitment of well-defined (sub)populations means that the debate will continue for some time. 3.3. Management issues Management issues formed another topic of the symposium. Daan (1997) evaluated the Common Fisheries Policy of the European Union, adopted in 1983 and primarily focused on a science-based total-allowable-catch (TAC) management system with fixed national quota shares. This analysis showed that the Policy has not been able to restrict fishing mortality and as a consequence the level of over-exploitation has only increased. The autonomic economic developments within the fisheries suggest that the fishery can be more effectively managed for the cost side rather than the profit side. An analysis of the yellowtail flounder (Pleuronectes ferruginea) fisheries on the Grand Bank of Newfoundland from 1975 to 1995 suggested a strong impact of fisheries on the distribution of the yellowtail flounder. Brodie et al. (1998) observed a contraction of the stock as a function of low stock size, which resulted from increased fishery activity in the mid to late 1980s. The potential impact of fisheries on recruitment was stressed by Berghahn and Purps (1998). They consider discard mortality of fisheries in flatfish nursery areas to have an effect on ultimate recruitment. These examples illustrate how information on flatfish populations can be used to improve management approaches.
Acknowledgements All participants of the 1996 Flatfish Symposium are acknowledged for their contribution to the Symposium. This is NIOZ publication No. 3281.
References Bailey, K.M., 1997. Structural dynamics and ecology of flatfish populations. J. Sea Res. 37, 269–280. Berghahn, R., Purps, M., 1998. Impact of discard mortality in Crangon fisheries on year-class strength of North Sea flatfish species. J. Sea Res. 40 (this issue). Brodie, W.B., Walsh, S.J., Atkinson, D.B., 1998. The effect of stock abundance on range contraction of yellowtail flounder (Pleuronectes ferruginea) on the Grand Bank of Newfoundland in the Northwest Atlantic from 1975 to 1995. J. Sea Res. 39, 139–152. Burke, J.S., Ueno, M., Tanaka, Y., Walsh, H., Maeda, T., Kinoshita, I., Seikai, T., Hoss, D.E., Tanaka, M., 1998. The influence of environmental factors on early life history patterns of flounders. J. Sea Res. 40 (this issue). Daan, N., 1997. TAC management in North Sea flatfish fisheries. J. Sea Res. 37, 321–341. Diakov, Y.P., 1998. Population structure of the Pacific black halibut Reinhardtius matsuurae Jordan et Snyder. J. Sea Res. 40 (this issue). Exadactylos, A., Geffen, A.J., Thorpe, J.P., 1998. Population structure of the Dover sole, Solea solea L. in a background of high gene flow. J. Sea Res. 40 (this issue). Fuiman, L.A., 1997. What can flatfish ontogenies tell us about pelagic and benthic lifestyles? J. Sea Res. 37, 257–267. Gibson, R.N., 1997. Behaviour and the distribution of flatfishes. J. Sea Res. 37, 241–256. Hensley, D.A., 1997. An overview of the systematics and biogeography of the flatfish. J. Sea Res. 37, 187–194. Leggett, W.C., DeBlois, E.M., 1994. Recruitment in marine fishes: is it regulated by starvation and regulation in the egg and larval stages? Neth. J. Sea Res. 32, 119–134. Leggett, W.C., Frank, K.T., 1997. A comparative analysis of recruitment variability in North Atlantic flatfishes — testing the species range hypothesis. J. Sea Res. 37, 281–299. Miller, J.M., Burke, J.S., Fitzhugh, G.R., 1991. Early life history patterns of Atlantic North American flatfish: likely and (unlikely) factors controlling recruitment. Neth. J. Sea Res. 27, 261–275. Neuman, M.J., Able, K.W., 1998. Experimental evidence of sediment preference by early life history stages of windowpane (Scophthalmus aquosus). J. Sea Res. 40 (this issue). Nielsen, E., Bagge, O., MacKenzie, B.R., 1998. Wind-induced transport of plaice (Pleuronectes platessa): early life-history stages in the Skagerrak–Kattegat. J. Sea Res. 39, 11–28. Osse, J.W.M., Van den Boogaart, J.G.M., 1997. Size of flatfish larvae at transformation, functional demands and historical constraints. J. Sea Res. 37, 229–239.
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