A conceptual model for examining density dependence in the growth of stream trout

Abstract

AbstractDetermination of the occurrence and importance of densitydependent responses is central to understanding stream trout population dynamics. I propose a conceptual model of growth, based on a distribution of feeding site quality, that considers the effects of density on growth. The site quality model assumes that trout select the best feeding sites available and, as the number of trout increases, they will be forced to use less energetically profitable sites, resulting in decreased growth, but also an increase in variance of size or growth. Results from a 3‐year study of a brown trout (Salmo trutta) population show that growth to age 1 was significantly reduced for the 1981 year class, which was about 4 times more dense than other year classes. The reduction in growth was not due to a decrease in the growth of all trout but rather an increase in the number of slower growing fish. These results are consistent with my site quality distribution model and suggest that individual fish growth, the distribution of growth, and the variance of growth rates should be considered in addition to mean size and growth.