As part of a comprehensive striped bass population dynamics study conducted in the Potomac River in 1976, we examined the stomach contents of 703 juvenile striped bass (Motone saxatilis) to ascertain the importance of feeding patterns of this life stage to regulation of population size. Analysis of stomach data indicated that juveniles (25-100 mm) were flexible, nonselective feeders consuming mostly insect larvae, polychaetes, larval fish, mysids, and amphipods, and that food habits did not change appreciably with size up to 100 mm. Greater abundance and higher feeding success (weight of food items per individual) at nearshore collection sites indicated these to be the preferred habitat of juvenile striped bass. The composition of diet reflected changes in nature! estuarine community composition associated with prevailing salinity patterns. These results suggest that juveniles can adapt to alternative food items readily, which may be an important mechanism in lowering juvenile mortality rates compared to those of earlier postlarva! stages, and thereby in stabilizing year-class strength at this developmental stage.
An analysis of the spatial and temporal abundance patterns of developing striped bass (Morone saxatilis) ichthyoplankton stages in the Potomac Estuary, including a Markovian description of transport, indicates that annual differences in the distribution of spawning fish are not likely to account wholly for the wide fluctuations in year-class success. Year-class success can be decomposed into the product of two factors — one extrinsic, acting upon the eggs and larvae, and the other, a behavioral property of the adult stock. The effect of extrinsic environmental conditions upon year-class success can be measured relative to the maximum computed survival of eggs to post-finfold larvae among all locations and times during a given year. Apparently, the behavior of the spawning adult fish is not well matched with the environmental conditions favorable to ichthyoplankton survival, and therefore, the actual spawning distribution yields only a fraction (the spawning fitness) of the maximum production possible during that year. Spawning fitnesses were estimated to be small (< 0.02 out of 1.0) and varied by less than a factor of two over the three seasons observed. In contrast, the year-class success as measured by post-finfold production differed 35-fold over the same 3 yr. It appears most likely that the large range in success is due primarily to the extrinsic, density-independent environmental factors which determine the optimum survivals in combination with spawning behavior. However, no strong case can be made for behavioral compensation by spawning fish to offset changes in the annual optimum survival conditions for ichthyoplankton.Key words: anadromous, ichthyoplankton development, Markovian transition probabilities, optimal survival, spawning behavior, spawning fitness, striped bass, transport model, year-class success
Peak abundances of larval striped bass, Motone saxatilis, in the Potomac estuary during 1974 through 1977, and of juvenile striped bass in 1975 and 1976, were found either in the area of peak spawning activity (Maryland Point, river kilometer 107 to Indian Head, km 139) or upstream, in spite of continued downstream river flow. Greatest abundances of larval striped bass occurred in 1977, thottgh greatest egg abundances were found in 1974. Zooplankton densities were generally greater in 1977 than in 1976, the only 2 years sampled. Fin-fold and postfin-fold striped bass were concentrated from Douglas point, km 116, to Sheridan Point, km 158, in 1977, which were the areas of greatest zooplankton abundance.Spatial and temporal distributions of larval striped bass expected from egg densities did not always materialize. We suggest that three phenomena in combination account for the observed spatial and temporal distribution of striped bass early life history stages in the Potomac estuary:(1) continued upstream migration of the spawning stock; (2) differential mortality of egg and larval stages throughout the spawning season; (3) ability of older larvae and juvenile striped bass to maintain longitudinal position within the estuary.
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