Netherlands Journal of Sea Research最新文献

The population dynamics of three 0-group species, plaice (Pleuronectes platessa L.), flounder (Platichthys flesus L.) and sole (Solea solea L.) in the Dollard (Ems Estuary, Wadden Sea) were investigated in 1992. The instantaneous rate of decrease in catch density of plaice was 0.011·d⁻¹, which corresponded with other calculated mortality rates of plaice in the western Wadden Sea. Catch densities of 0-group flounder decreased at a rate of 0.018·d⁻¹. The rate of decrease in catch density of 0-group sole was estimated at 0.011·d⁻¹, but was less accurate and probably reflected migration. The rate of increase in mean length of 0-group sole was in agreement with experimental growth studies under excess of food. The observed rate of increase in mean length of plaice and flounder appeared to decline from the beginning of June onwards in comparison with simulated growth in length. A number of factors that may be responsible for the observed differences are discussed.

Field growth experiments were conducted in cages during June–August 1993, to compare growth rates of juvenile southern flounder, Paralichthys lethostigma (standard length (SL) 37 to 70 mm), in historically utilized (two creeks) and underutilized (two creeks) low salinity nursery areas within the Pamlico River Estuary of Pamlico Sound, North Carolina. Growth rates from five sequential trials were used to estimate the nursery areas' productive capacities for juveniles of this species. Instantaneous daily growth rates, measured as a function of changes in weight, showed that southern flounder placed in underutilized creeks exhibited significantly higher mean instantaneous daily growth rates (3.94·10⁻²±0.47) than southern flounder placed in utilized creeks (1.66·10⁻²±0.49). Thus, other factors such as colonization rate must be limiting production in nursery areas. Temperature was positively correlated with growth rate and accounted for about 20% of the variability in growth rates. The approach used in this study provides an alternative method of assessing critical nursery habitats for juvenile flatfish in estuarine environments. Assessment of nursery value in terms of potential productive capacity rather than realized production should prevent underassessment of value due to low levels of colonization in some years.

Results presented in an earlier paper showed that differences in temperature and salinity could explain approximately 30% of the variability in growth rates of juvenile southern flounder (Paralichthys lethostigma) in different creeks of the Pamlico River estuary. In addition, the maximal growth rate was lower than expected from published laboratory results, suggesting food limitation. In the present paper, the obtained mean and maximal growth rates are related to simultaneously collected data on the amount of food available in the creeks. Thirteen different groups of food items were observed in the stomachs of the flounder. Most of the stomachs of flounder collected outside the cages contained only mysids (65 to 75%). Inside the cages, 25 to 45% of the flounder stomachs contained only mysids, while 85 to 100% of the stomachs contained both mysids and other food categories. Individual growth rates of the founder were only significantly related to the number of mysids in the stomachs, and not to any of the other food categories. Stomachs of other fish species mostly showed food categories other than mysids. This indicates that interspecific competition for food did not occur. The four creeks of the cage experiment showed significant differences in abundance of mysids. However, in only one of the two trials the abundance of mysids was significantly related to the observed growth rates. It can be concluded that mysid abundance does not unequivocally explain the variability in growth rates of juvenile southern flounder in low salinity nursery areas of Pamlico Sound, and that the hypothesis of food limiting maximal growth is not supported by the results of this study.

This paper summarizes the developments in flatfish recruitment studies over the last decade with emphasis on the general patterns that have emerged from the contributions to the two Flatfish Symposia of 1990 and 1993. Recruitment variability is largely generated by density-independent factors acting during the pelagic egg and larval phases. Effects of variability generating processes tend to be amplified towards the edges of the distribution range and appear to be related to abiotic conditions. Density-dependent feedback processes occur in the demersal juvenile phase, when flatfish become highly specialized benthic feeding fish. There is evidence that density-dependent feedback processes may also occur during the adult phase in not or lightly exploited populations. Areas of future research that emerge from this symposium are: 1. the 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 with 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; and 6. simulation studies of relevant population dynamical processes to explore quantitatively the importance of the various processes and the necessary precision with which these should be known.