The importance of comparative studies in stock analysis

Abstract

The identification and discrimination of stocks of fishes lies at the interesting juncture of fishery biology and fish microevolution. Fish have been “taken apart” in the search for features that discriminate between stocks. Early stock studies focused on morphological characters (meristic counts and morphometric measurements) that are expressed phenotypically, and hence, benefit from incorporation of both genetic and environmental information, but suffer from the absence of information on their respective contributions. However, the ability to discriminate stocks has risen with the emergence of more sophisticated laboratory and statistical tools. Some of these techniques are mostly sensitive to environmental information, i.e., microchemical analysis of otoliths. Others are purely genetic (i.e., various forms of DNA sequence analysis), and reflect time, selection pressure, and the degree of isolation among populations. But despite this diversity of approaches, all are based on a common principle: the signal from the among-stock variation must exceed the noise of within-stock variation. Much of the uncertainty concerning the application of stock concept approaches stems from the lack of synchronicity in the development of the many potentially differentiating features between fish stocks. If so, then this field would benefit from a greater understanding of the trajectories of differentiation of analyzable features-gained from comparison between approaches from a microevolutionary point of view. Because only heterogeneity, and not homogeneity of fish stocks can be demonstrated, and because sophisticated stock identification is costly, it is imperative to select an appropriate approach from the outset. Unfortunately, comparative studies among stock identification approaches have only rarely been conducted. Although this “black art” could continue to muddle along, there is no doubt that useful information on the relative sensitivities of stock analysis techniques would be gained via well-planned intercalibration studies.