Cultivation, Allee effect and resilience of large demersal fish populations

In the marine environment, small forage species can predate on, or compete with, the pre-recruits of their larger predators. The "cultivation effect" hypothesis proposes that this mechanism can slow down the recovery of depleted populations of large demersal fishes by creating Allee effects that lower their recruitment success. Using an individual based fish community model applied to the southern Benguela ecosystem, we simulate situations of potential cultivation effect on a population of shallow water hake Merluccius capensis. We search for evidence of Allee effects due to cultivation and investigate how complex trophic interactions could influence their underlying mechanisms and impact. The resilience of the shallow water hake population was measured by following the variations of the ratio R/S (the number of recruits over the number of spawners) when population size decreases and the predators and competitors of shallow water hake pre-recruits were identified. Simulations suggest that cultivation effects are likely to emerge within the fish community of the southern Benguela ecosystem. Our twelve species model emphasizes that cultivation effects result from complex influences of predation and competition on the different pre-recruit stages, whose trophic role depends on both body size and geographical distribution. For realistic linkages between forage species and shallow water hake pre-recruits, cultivation effects occurring in the limits of the southern Benguela fish community are predicted to delay population recovery by several decades. These significant delays are not characterized by a decrease of R/S at low stock size, which is the sign usually tracked when looking for Allee effects. Our study suggests that cultivation effects could play a major role in the dynamics of overexploited large demersal fish populations and pleads for the development of improved detection techniques for these processes.