Responses to Density Reduction in a Fishery Stock: Consequences for Management Models

To sustain harvesting over time, stocks must have compensatory density dependent (CDD) mechanisms. We studied CDD responses in greenlip abalone, at all life stages from settlement to reproduction, using controlled, replicated experiments on farms and in the wild. Settlement of larvae onto suitable algae showed no evidence of CDD, but post-larval mortality and growth are both strongly CDD. Juveniles seeded into replicated rock reefs showed weak CDD mortality (after initial mortality, from 3-9 months after seeding), but strongly CDD growth rates, and faster growth led to earlier maturation. Natural mortality of adults is known to be low.  At reduced densities, small adults increased growth markedly, but growth still declined to zero as they approached the asymptotic length for the site. All males increased testis size, but in females extra gonad growth increased progressively as they approached the asymptotic size. Overall, the strongest effect of reduced densities is CDD growth.

CDD growth is increasingly recognized as important, and is evident in lobsters and many fish stocks. Our simulations of population dynamics under fishing that incorporate strong CDD growth and weak CDD mortality and reproductive effects show that standard stock assessments may well be biased so as to increase the risk of fishery collapses, depending on the way fishing of adults would affect growth of smaller fish. Even if maturity is age determined, CDD will shorten the effective generation time as density is reduced under fishing, and standard models cannot fit this process.