Netherlands Journal of Sea Research最新文献

The long-term dynamics to be expected of fish populations controlled by density-dependent processes in the early life-history compared with in the adult phase are explored. It is suggested that those species (as exemplified by European plaice, Pleuronectes platessa) which tend to concentrate spatially into nursery grounds during their early life-history may be ‘saturating’ the carrying capacity of those juvenile habitats though the adult part of the population is still relatively small and not limited by the adult habitat. This is called the concentration hypothesis and, if valid, it would explain some features of the long-term dynamics of different species. Tests based on the characteristics of the stock and recruitment data series to be expected from ‘concentrating’ vs ‘non-concentrating’ species are proposed, with some provisional results.

The ecology of two flounders belonging to the genus Paralichthys was studied in the Bay of Coquimbo (29°57′S), Chile. P. adspersus grows larger than P. microps. In both species, the females were significantly larger than the males. The species differed in their reproductive cycles as revealed by their gonadosomatic indices. This difference was also reflected in larval abundance and spatial distribution of the species. No significant changes were found in the abundances of juveniles and adults in the Bay of Coquimbo either throughout the year or interannually. However, when the analysis included the proportion of specimens between the two species and sex by season, the differences were apparent in most cases. The results show that the environmental characteristics of the marine system which the Bay of Coquimbo is a part of provide good feeding, spawning and nursery grounds for these two species of flounder.

The nursery areas of two species of flatfish, the solenette Buglossidium luteum and the scaldfish Arnoglossus laterna, in the southern North Sea, were investigated by analysing the patterns in abundance and distribution in surveys previously carried out in the open North Sea, in the Dutch coastal zone and in the estuarine Dutch Wadden Sea. Juveniles of both species were only found in the open North Sea in waters deeper than about 15 m. 0-group solenette was present over a depth range of 10 to 50 m, with highest numbers at about 25 m. 0-group scaldfish occurred from 15 m to 70 m with peak numbers at about 35 m. Both species lacked special nursery areas in the southern North Sea and the distribution patterns of juveniles and adults overlapped. Adults were rarely found in the coastal zone and in the deeper channels of the estuarine Wadden Sea. Densities of 0-group fish were rather low (less than 10 ind·1000 m⁻²). Mean length increased to an average of 4 cm for 0-group solenette and of 5 cm for 0-group scaldfish in autumn.

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.