Netherlands Journal of Sea Research最新文献

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.

We evaluate the joint effects of temperature, hours of daylight, prey production, spawning regimes, and mortality rates on recruitment of winter flounder, Pleuronectes americanus, by means of an individual-based simulation model. These factors were varied to approximate conditions at three latitudinally distinct sites in winter flounder's geographic range from New Jersey, USA (southernmost site) to New Brunswick, Canada (northernmost site). Most life historical and demographic response variables from simulation output differed substantially among sites. At year's end, the recruits were larger but fewer at lower latitudes than at higher ones. We further assessed the effects of season duration, prey production, and mortality rate on recruitment by means of a 3³-factorial numerical experiment. This analysis provided estimates of the proportion of variance in the response variables (the number and sizes of recruits, larval and juvenile growth rates, larval period duration) due to each of the three manipulated factors and their interactions. Season duration explained most of the variation in the responses except for the sizes of recruits at year's end which was influenced most by the timing of prey production. Interaction effects of season duration and prey production were evident in all responses, reflecting the importance to flounder recruitment of the phenology of prey production relative to flounder early life history. Even though mortality rates were assigned as part of the factorial design structure, they accounted for only 10% of the variance in the number of recruits. These simulation results are in need of empirical verification and we suggest ways to proceed. We also propose means of enhancing data on winter flounder life history and ecology, and methods for expanding our model and analysis. We conclude that our approach proved useful for 1. appraising multiple, interacting recruitment processes, 2. locating areas where incomplete knowledge of winter flounder life history and the habitat it occupies limits a more satisfactory assessment of recruitment processes, and 3. identifying recruitment hypotheses that warrant further empirical evaluation.

Vertical movements are a fundamental part of tidal transport, a migration strategy found in many coastal fishes. Because light is considered important to vertical migration, the ontogeny of eye development, phototaxis and vertically oriented behaviour were examined for different metamorphic stages of Japanese flounder larvae (Paralichthys olivaceus). The timing of rod recruitment to the retina and the development of the ability to dark adapt were investigated histologically. Phototaxis experiments were conducted to determine when during development phototactic behaviour changed. Observations of vertical movements were made using infrared sensitive video equipment. Larvae raised at S=32 were observed at S=30, 32 and 34 under changing light conditions. Light was increased from nocturnal to daytime levels in log steps while swimming and settling behaviour was observed. Settling behaviour was apparent during the early stages of metamorphosis and increased with development. Larvae in the early stages of metamorphosis responded to night time light levels by swimming up. Larvae were more active in water of higher salinity relative to their acclimation salinity and were more likely to settle in water of lower salinity. During the late stages of metamorphosis, the importance of light to activity decreased, increasing the relative importance of salinity. Results are consistent with the hypothesis that tidal migration develops as diurnal vertical migration controlled by light is replaced by tidal movement cued by changes in salinity. Our findings suggest that efficient transport of larvae inshore depends on a clear salinity signal and that variability in such a signal may influence migration success.

Experimental studies with farmed juvenile halibut, Hippoglossus hippoglossus, were undertaken to optimize rearing procedures. Extended feeding period and/or day length did not significantly increase growth rates of juvenile halibut of 5 to 20 g wet weight. Growth was strongly affected by temperature, and highest growth rates were obtained at 13°C followed by 10, 16 and 7°C for juveniles of 20 to 90 g. Growth rate declined with size in one of two experiments in this size interval. Q₁₀ of daily growth rate between 7 and 10°C varied from 2.5 to 3.0. Individual growth always varied highly within the temperature treatments (overall range −0.3 – 3.5%·d⁻¹), but significant size rank correlations were maintained during the 12-week experiment. Juvenile halibut grew approximately isometrically from 20 to 90 g. Weight-specific oxygen consumptions of 80 g juveniles averaged 140 and 200 mg O₂ (kg·h)⁻¹ at 10 and 16°C, respectively, and this is comparable to rates measured for other flatfish species.