Density and Migration of Age 0 Striped Bass throughout a Natal Estuary: an Ideal Free Distribution?

The heterogeneous distributions of fishes in their nursery habitats results from a complex combination of differential transport, growth, and survival. Relatively small differences in distribution can lead to large variations in recruitment and resulting year-class strength. In this study, we used a number of techniques to both characterize and explain the distribution of age-0 striped bass Morone saxatilis in the Patuxent River, a tributary to the Chesapeake Bay and important natal estuary for this diadromous species. We fit the observed distributions (i.e. distance of occurrence from hatching location) of age 0-striped bass over three years (2007, 2009, and 2010) to mathematical functions of exponential decline (diffusion model) to determine the amount of variance explained by density dependent dispersal. To explore the dynamics of an ideal free distribution, we used multivariate statistical analyses to determine effects of dispersal on juvenile condition and growth. We also determined the effects on juvenile distribution, condition, and growth of other site-specific variables, including habitat type, habitat quality, temperature, salinity, dissolved oxygen, and density and diversity of co-occurring species. Habitat quality was estimated from prey assemblages, age-0 striped bass trophic position, and effective densities. Available prey assemblages were identified from stomach contents; trophic position was determined using stomach contents and stable isotope analysis. We used otolith microchemical analysis of Sr:Ca to determine life-time habitat use patterns of age 0 striped bass. Profiles of dispersal histories revealed three statistically distinct patterns: (1) fish that dispersed as early juveniles into brackish habitats and remained there; (2) fish that remained resident in the freshwater habitat; and (3) fish that dispersed down-estuary and then “retreated” back to freshwater, resulting in a dome-shaped dispersal pattern. These patterns represented distinct modalities, confirmed with repeated measures multivariate analyses of variance. Both down- and up-estuary dispersal mostly occurred over narrow size ranges indicating that migration may be conditioned on ontogenetic thresholds. Individuals collected in brackish water were of significantly higher condition than those collected in freshwater, which may indicate growth advantages for migratory juveniles. In summary, juvenile distributions, site-specific condition and growth rates, and the distinct migration patterns of individual fish represent dynamics at different scales that contribute to both year-class strength and stability, conferring resilience to the population.