Evaluation of the Shepherd and Cushing (1980) model of density-dependent survival: a case study using striped bass (Morone saxatilis) larvae in the Potomac River, Maryland, USA

Abstract

Abstract Quantifying the degree of density-dependence in stock–recruit relationships is critical to understanding fish population dynamics. The Shepherd and Cushing (1980) model couples a simple model of density-dependent larval growth with a constant rate of mortality to predict numbers surviving to recruitment. The model has not been evaluated using field data, nor have its predictions been compared with those from other models. Here, the S&C model, an individual-based model (IBM), and a regression model are applied to 8 years of field data for striped bass larvae in the Potomac River, Maryland, USA, to predict larval carrying capacity (K) and percentage of recruitment lost as a consequence of density-dependent growth. The IBM and the regression model were corroborated by comparing their predictions of average growth rates of larvae and relative recruitment strengths to observed values for the 8 years of field data. Although the IBM and the regression model differed in their predictions of several important intermediate variables, both models predicted higher values of K and lower values of density-dependent growth than did the S&C model. Over the 8 years, the IBM and the regression model predicted an average of 0.3 and 1.8% recruitment lost as a result of density-dependent growth, respectively. In contrast, the S&C model predicted much higher recruitment lost (average of 27%). Slight differences in the assumed rate of mortality used in the S&C model resulted in values of carrying capacity similar to those predicted by the IBM and the regression model. Difficulties in estimating parameters of the S&C model from field data are discussed.