The 1999 Flatfish Symposium: where to go from now?

A summary report of the 1999 Flatfish Symposium is presented. The Symposium was built on the themes of the previous Symposia, whereby the link is through processes related to recruitment. As in the previous Symposia, the papers presented have a strong focus on temperate species and basic information on (sub)tropical species is still lacking. The lack of insight into (sub)stock structure is still a problem and more insight is necessary into the methodologies for determining and describing stock structure. Another important unresolved issue concerns the quantitative understanding of the physical processes that affect the cross-shelf transport and exchange of eggs and larvae. Finally, there is a need for bioenergetic approaches to habitat quality and quantity both at the individual and population level to assess the qualitative and quantitative use of habitats in space and time. Applying global information systems (GIS) might be promising in this respect. With respect to management issues two items are of utmost importance: the collapse and recovery of stocks and the problems relating to re-stocking.     

The effect of salinity on growth and weight loss of juvenile plaice (Pleuronectes platessa, L): An experimental test

Previous population estimates of the 0+ plaice (Pleuronectes platessa L.) in the Firth of Forth, east central Scotland, did not take account of the Forth estuary west of the Forth bridges. Previous work found plaice in the estuary grew as fast as, or faster than, the outer firth plaice. It was hypothesised that salinity may affect growth rates of early 0+ plaice. This hypothesis was tested in a laboratory experiment, by exposing juvenile plaice to three different, but naturally — experienced by the juveniles, salinities; 25, 30 and 35. Plaice fed a minimum ration did not grow in length. Mean weight decreased at all three salinities, however, the lowest weight loss was found at the lowest salinity (25) and the highest weight loss was found at the highest salinity (35). The minimum feeding ration was halted and plaice were then fed ad libitum. Consumption rates were not significantly different during the ad libitum feeding, while significant differences in mean weight change were found between the highest and lowest salinities.     

Differences in habitat selection of male and female megrim (Lepidorhombus whiffiagonis, Walbaum) to the west of Ireland. A result of differences in life-history strategies between the sexes?

The sex ratio in the catches of megrim (Lepidorhombus whiffiagonis, Walbaum) varied systematically with depth on three independent trawl survey series off the west coast of Ireland. Female megrim dominated the shallow catches, while males were more common in catches from deeper waters. The size difference between the sexes alone cannot explain this pattern because it remained evident when fish length was taken into account. Therefore size-specific habitat preferences or size-selective fishing mortality cannot fully explain the observed trend in the sex ratio of megrim. Female megrim grow to a larger size, at a faster rate than males and it is likely that their differences in habitat preferences are related to this. Shallower waters are warmer during the growing season and are likely to provide better conditions for fast growth. An understanding of the mechanisms behind these patterns is an important consideration in the management and conservation of this fish stock, which might be particularly vulnerable because the commercial landings are to a large extent dominated by female megrim.     

Ecophysiology of juvenile flatfish in nursery grounds

Relationships between biotic and abiotic factors and the ecological performance of late larval and juvenile flatfish in nursery grounds are examined from ecophysiological viewpoints. The first events in the nursery are metamorphosis and settlement. Development of organs, osmoregulation and behavioural changes during metamorphosis, and size at metamorphosis are regulated by environmental factors. Various hormones play critical roles in this regulation. Effects of environmental conditions on individual growth in the nursery grounds are described on the basis of Fry's five environmental factors: limiting, controlling, masking, directive and lethal factors. The main limiting factors are food and dissolved oxygen; controlling factors are temperature and body size; masking factors are salinity and pollutants; lethal factors are extreme environments; and directive factors are food, predators and dissolved oxygen. In addition to temperature, it has been indicated that dissolved oxygen seems to be relatively important for flatfish of the eastern US and northern European countries, while food abundance appears to be more critical for Japanese flounder. The feasibility is discussed of ecophysiological modelling to predict individual growth and subpopulation production based on the assessment of the role of environmental variability using the above classification, which organises and integrates environmental effects.     

Ecological and biological strategies of Etropus crossotus and Citharichthys spilopterus (Pleuronectiformes:Paralichthyidae) related to the estuarine plume, Southern Gulf of Mexico

Differences in the biological and ecological strategies of two tropical flatfishes, Etropus crossotus (fringed flounder) and Citharichthys spilopterus (bay whiff) are discussed. The comparative analysis was based on the seasonal distribution of relative abundance of the two species and flatfish movements utilizing estuarine plume influenced areas as part of their life cycles. Growth parameters of the von Bertalanffy equation and recruitment were estimated (FISAT software) to compare life cycle patterns. The recruitment patterns illustrate the key difference between “estuarine-dependent” and “estuarine-related” nekton strategies. Both species have a short life cycle with a continuous recruitment activity. E. crossotus is an estuarine-related species, with two recruitment pulses (winter frontal season), in the estuarine plume on the shelf. C. spilopterus is an estuarine-dependent species, with a main recruitment pulse during the rainy season in the estuarine plume, and a residual secondary pulse inside the adjacent estuarine system (Terminos Lagoon). The results show that the ecological success of these tropical coastal marine flatfish, with similar biological patterns, is based upon the sequential use (in time and space) of estuarine plume influenced habitats, suggesting that fish migration to shallow waters is related to food availability changes as a strategy towards optimum recruitment.     

The impact of intrapopulation variability in reproductive traits on population reproductive potential of Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea)

In geographically extensive fish populations the potential exists for reproductive traits to vary over the population’s range but, the impact that such intrapopulation variability has on overall population reproductive potential has not been formally assessed. Here intrapopulation spatial variability in size at maturity and fecundity are demonstrated for Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea). Recognition of intrapopulation variability in these reproductive traits coupled with spatial variability in abundance resulted in an increase in estimated population total egg production (TEP) by as much as 10¹⁴ eggs for American plaice and 10¹⁵ eggs for yellowtail flounder as compared to assessment of TEP for the population as a whole. Results highlight the need to explore variability in life history traits not only between but, also within populations and emphasizes the need for sufficient spatial coverage during sampling in order to assess the reproductive potential of fish populations.     

Intra- and interspecies comparison of energy flow in North Atlantic flatfish species by means of dynamic energy budgets

The quantitative aspects of growth and reproduction in four flatfish species (plaice, flounder, dab, sole) in terms of energy flow are described on the basis of a dynamic energy budget (DEB theory). This theory consists of general assumptions about energy uptake, storage and utilisation and describes an individual by two state variables: structure and reserve, whereby body size exerts its influence through the ratio between surface area and volume. Comparison between model estimates and field data shows that the DEB model successfully describes the energetics of growth and reproduction in a number of flatfish species. Differences between species could be captured in the same model using different parameter values. Intraspecific differences in growth between males and females are mainly caused by differences in maximum surface area-specific ingestion rates. Differences between species are reflected in the surface area-specific maximum ingestion rate, the energy partitioning over growth and reproduction, and in egg volume. According to these parameters at 283 K (10°C), the species could be ranked as follows: surface area-specific maximum ingestion rate (W m⁻²) plaice: 56.6; flounder: 54.5; sole: 45.1 and dab: 36.1 W m⁻². Fraction of energy allocated to reproduction (–): flounder: 0.35; plaice: 0.15; dab: 0.15 and sole 0.10. As a consequence of these differences in surface area-specific maximum ingestion rate and in the fraction of utilised energy allocated to reproduction, the gonad masses (g) of females of 0.5 kg wet mass differ considerably: flounder: 149 g; plaice: 86 g; sole: 70 g; and dab: 69 g. However, due to differences in egg size between species, the potential annual egg production shows a completely different pattern: dab: 2200 10³; flounder: 1560 10³; sole: 343 10³ and plaice: 130 10³ eggs.