Why Do Fish Populations Vary?

Abstract

Fish populations vary because of density-dependent and -independent processes that determine recruitment, growth, and natural mortality, and in response to fishing. Most of the natural (non-fishing) variability is associated with recruitment, presumably the density-independent effect of fluctuating environmental factors.

Numerous empirical models have been used to explain recruitment variability. While “statistically significant” correlations are plentiful, most empirical studies are flawed because they a) use an inappropriate proxy for recruitment, b) are data exploration exercises that are not based on a plausible a priori hypothesis, c) do not consider, simultaneously, physical variables and spawning biomass, and/or d) fail to test predictions on independent data (i.e., not used to establish correlation). As a result, many empirical models fail to predict post-publication events. Furthermore, fundamentally different empirical models may be indistinguishable because they account for virtually the same proportion of variability in recruitment.

Process-oriented studies of recruitment variability have focused on starvation, particularly of first-feeding larvae. Starvation may be related to the amount of suitable food, the contagious nature of its distribution, currents which transport eggs and larvae, the match or mismatch of the annual reproductive cycles with the annual production cycle of prey, and the stability of the environment.

Only recently has a new hypothesis emerged, namely, that predation is the major cause of prerecruit mortality. It is supported by a) evidence of prey concentrations which are adequate for a high survival of larvae as indicated by laboratory and modeling studies, b) lack of evidence of starvation for field-collected larvae, c) a high survival rate of larvae in large, predation-free enclosures, d) the high mortality rate of eggs and yolk-sac larvae which are not subject to starvation, and e) the identification of fish and invertebrates as predators of egg, larval, and post-larval stages.

At least for some systems, fish consume most of their own production. Two important implications are that fish populations are probably able to compensate for fishing, and post-larval mortality must be high, thus affecting recruitment. The lack of correlation between larval abundance and recruitment also implies that year-class strength is not established until the post-larval stage.

Physical factors are presumed to be responsible for recruitment variability. There are numerous possible mechanisms if recruitment is determined by starvation of larvae. It is less apparent how physical factors are related to variability in predation mortality of post-larvae.