Conserving populations at low abundance: delayed functional maturity and Allee effects in reproductive behaviour of the queen conch Strombus gigas

Effectiveness of conservation measures for diminished populations depends on the mechanism producing low abundance. In queen conch, which is heavily exploited, reproductive activity is depressed where conch density is low, which may be due to the Allee effect, poor habitat quality, or delayed functional maturity. To determine the mechanism underlying impaired reproduction in shallow seagrass beds, mature conch were translocated from ‘source’ sites with high and low ambient conch density (~1000 and ~20 conch ha–1, respectively), and kept at high density in enclosures within highand low-density ‘host’ sites. We monitored reproductive activity, and modelled conch population dynamics under exploitation, a reproductive Allee effect, and delayed functional maturity. In the field, conch from high-density source sites had significantly higher reproductive activity than those from low-density sites, although conch density was the same in all treatments. Thus, reduced reproductive activity at low conch density in shallow seagrass habitats is not due strictly to an Allee effect. Physiological condition of conch did not differ between treatments, regardless of habitat quality. Conch from high-density source sites had thicker shell lips, indicating that they were older, and they displayed significantly higher reproductive activity than younger conch with thinner lips. The field, morphological, and physiological evidence is therefore consistent only with a mechanism of delayed functional maturity. In model simulations, either an Allee effect or delayed functional maturity, combined with exploitation, produced non-linear population collapses. Young adult queen conch that are not functionally mature occur at low density in shallow seagrass habitats, and are heavily exploited before reproducing. Conservation efforts must be directed at these habitats, and not only to habitats where older functionally mature adults are at high density and not susceptible to Allee effects.