Aquatic Living Resources

Table of Contents - July 2008 - Volume 21 - Issue03 (Special Issue (Advances in Fisheries Analysis and Modelling – Reversible and Irreversible Changes in Fish Resources and their usage))

Research Article

Reversible and Irreversible Changes in Fish Resources

Cultivation, Allee effect and resilience of large demersal fish populations

« Cultivation », effet Allee et résilience des populations de grands poissons prédateurs démersaux

Vergnon, Rémia1a2, Shin, Yunne-Jaia1 and Cury, Philippea1

a1 IRD Institut de Recherche pour le Développement, CRH-Centre de Recherche Halieutique Méditerranéenne et Tropicale, Research Unit 097 “Upwelling Ecosystems”, avenue Jean Monnet, BP 171, 34203 Sète Cedex, France

a2 University of Sheffield, Department of Animal and Plant Sciences, Western Bank. Sheffield. S10 2TN, UK

Abstract

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.

Résumé

Dans l'environnement marin, les petites espèces-fourrages peuvent se nourrir des juvéniles (pré-recrues) de leurs prédateurs plus grands, ou entrer en compétition avec eux. L'hypothèse de l'effet de « cultivation » suppose que ce mécanisme peut ralentir le recouvrement de populations effondrées de grands poissons démersaux en créant des effets Allee qui diminuent le succès de leur recrutement. Grâce à un modèle individu-centré de communauté de poissons appliqué à l'écosystème du sud-Benguela, nous simulons des situations d'effet de « cultivation » potentiel au sein d'une population de merlu Merluccius capensis. Nous cherchons des preuves de la présence d'effets Allee et étudions comment des interactions trophiques complexes peuvent influencer leurs mécanismes sous-jacents et leur impact en terme de dynamique des populations. La résilience de la population de merlu est mesurée en suivant les variations du rapport R/S (le nombre de recrues sur le nombre de femelles prêtes à pondre) lorsque la taille de la population décroît et les prédateurs et compétiteurs des pré-recrues de merlu sont identifiés. Les simulations suggèrent que des effets Allee, tels que ceux décrits par l'hypothèse de l'effet de « cultivation », sont susceptibles d'émerger dans la communauté de poissons de l'écosystème du sud-Benguela. Notre modèle montre que ces effets résultent d'influences complexes de la prédation et de la compétition sur les différents stades de pré-recrues, dont le rôle trophique dépend à la fois de la taille corporelle et de la répartition géographique. Pour des intensités d'interaction trophique réalistes, les effets Allee apparaissant dans les limites de la communauté de poissons du sud-Benguela retardent le recouvrement de la population de merlu de plusieurs décennies. Ces délais significatifs ne sont pas caractérisés par une diminution du rapport R/S, ce qui est le signe habituellement considéré comme symptomatique d'un effet Allee. Notre étude suggère que les effets Allee peuvent jouer un rôle majeur dans la dynamique des populations surexploitées de grands poissons démersaux et plaide pour le développement de méthodes de détection améliorées de ce processus.

(Received September 18 2007)

(Accepted June 10 2008)

(Online publication July 26 2008)

Key Words:

  • Marine ecosystem dynamics ;
  • Fishery exploitation ;
  • Recruitment ;
  • Spawning abundance ;
  • Fish competitors ;
  • Predators ;
  • Model ;
  • Benguela system

Correspondence:

c1 R.Vergnon@sheffield.ac.uk

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