The influence of density on adults and juveniles of the estuarine fish, the sheepshead minnow (Cyprinodon variegatus)

The relationship between population density and demographic rates (e.g. survival, reproduction, and growth) is critical to understand population dynamics and has been widely studied in fishes. Estuarine species are regularly exposed to dramatic changes in density with daily, monthly, and seasonal tide cycles, yet knowledge on how density influences demographic rates of fish within these systems is largely lacking. We evaluated the influence of density on the sheepshead minnow (Cyprinodon variegatus) survival, growth, maturation, and reproduction in a series of laboratory studies. Four experiments evaluated the impact of density on demographic rates with and without food limitations and with single (adult) and mixed cohorts (juveniles and adults). The proportion of juveniles to reach sexual maturity was significantly affected by density but was not related to size, indicating that maturation is a function of both the presence and the density of adults. Fecundity was significantly less in the mid-range and high densities compared to the lowest densities tested. There was minimal mortality of both adults and juveniles across all densities, and juvenile growth was only affected at extremely high densities (> 500 fish m− 2). Fish collected from marsh sites using 1 m2 box samplers provided estimates of the frequency of high fish density in estuarine systems. Highest field densities coincided with densities where sexual maturation and fecundity were altered by density treatments in laboratory fish. These studies provide a baseline assessment of the influence of density on previously unmeasured demographic endpoints for an estuarine species and demonstrate both density dependence and demographic plasticity.

► Impacts of high density on an estuarine fish was assessed in laboratory studies.
► Endpoints previously unmeasured in density studies were impacted by density.
► Sexual maturation was affected by adult density independent of sex and size.
► Reduced fecundity in high density was driven by interference competition.
► Survival and growth were tolerant to extremely high densities.