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Salmonid population dynamics: stability under weak density dependence?



Lobón-Cerviá and Rincón analysed the dynamics of a stream-resident brown trout (Salmo trutta) population using stock-recruitment relationships, and reported no indication of density dependence but strong effects of water discharge on juvenile production. The authors concluded to have demonstrated an example of a population where “… endogenous density-dependent mechanisms played, at most, a limited role” for population dynamics (p. 643), and that the population persists with “… little or no operation of endogenous regulatory mechanisms” (p. 641). Further, they “… question the extent to which generalizations claiming that most natural animal populations should be subject to the operation of regulatory feedback mechanisms hold for fish populations” (p. 644). If these conclusions were accurate, it should radically alter our view of fish population dynamics and how such populations should be managed. However, here I argue that these conclusions should be treated cautiously, and suggest alternative interpretations which are more consistent with theory and previous empirical research. Furthermore, using the study of Lobón-Cerviá and Rincón as an illustrative example, I suggest that additional insight into population dynamics may be attained by shifting the focus away from the conventional absolute distinction between density-dependent and -independent factors towards studies that address the potential for interactions between external forces and density.