Allee effects and compensatory population dynamics within a stock complex

Sparked by the observation that (i) many collapsed stocks have failed to recover despite the apparent prevalence of compensatory population dynamics and (ii) many stocks have a complex structure, we devised a simulation model illustrating that the underlying behavior of substocks is masked when the data are aggregated and evaluated at the scale of the management unit. The model consisted of several substocks within a stock complex, each having its own stock and recruitment (S-R) relationship with Allee effects. Incorporation of Allee effects into the S-R relationship was achieved by specifying a critical spawning stock biomass threshold below which no recruitment occurred. The simulation model revealed that it was possible for the aggregate S-R relationship to appear compensatory, even though no substock exhibited this behavior. If the conclusions we draw from our modeling are correct, biological reference points developed from aggregated data from multiple substocks and used in conventional fisheries management and stock assessment models are likely to be inaccurate and possibly nonconservative. More research in support of the delineation of substock structure, biophysical modeling, and metapopulation theory is advocated.