Analysing environmental and fishing effects on a short-lived species stock: the dynamics of the octopus Octopus vulgaris population in Senegalese waters

  • M Thiaw
  • D Gascuel
  • D Thiao
  • OT Thiaw
  • D Jouffre

Abstract

Short-lived species are extremely dependent on the seasonal and interannual variability of environmental conditions, and determining their stock status is  often difficult. This study investigates the effects of environmental  variability and fishing pressure on the stock of octopus Octopus  vulgaris in Senegalese waters over a 10-year period from 1996 to 2005. Monthly catches-at-age were estimated based on catch-at-weight data and a  polymodal decomposition constrained by a given growth curve. Octopus recruitments and fishing mortalities were then estimated using a catch-at-age analysis performed on a monthly basis. Yield and biomass per recruit were  simulated using a Thompson and Bell model and used to generate a diagnostic of the fishery’s impacts. Results indicate that the high interannual and seasonal variability of the octopus stock biomass is linked to the spring recruitment event, the annual intensity of which was significantly correlated with the coastal upwelling index and sea surface temperature. Yield per recruit varied  seasonally but remained almost unchanged from one year to the next. Even when catches vary strongly according to recruitment, the octopus stock appears to be consistently fully exploited, or slightly overexploited in some years. In this context of environmental variability, usual indicators such as the maximum yield per recruit, and the related fishing mortality and spawning potential ratio, remain useful for fisheries management purposes.

Keywords: environment, fishery, indicators, population dynamics, Senegal, stock assessment, West Africa

African Journal of Marine Science 2011, 33(2): 209–222

Author Biographies

M Thiaw
Institut de Recherche pour le Développement, US 007 Osiris, route des hydrocarbures, BP 1386, 18524 Dakar, Sénégal; Université Européenne de Bretagne, UMR 985 Agrocampus Ouest / Inra (Ecologie et Santé des Ecosystèmes), 65 route de Saint Brieuc, CS 84 215, 35 042 Rennes cedex, France
D Gascuel
Université Européenne de Bretagne, UMR 985 Agrocampus Ouest / Inra (Ecologie et Santé des Ecosystèmes), 65 route de Saint Brieuc, CS 84 215, 35 042 Rennes cedex, France
D Thiao
Centre de Recherches Océanographiques de Dakar-Thiaroye (CRODT, ISRA-Sénégal), Route du Front de terre, Dakar, BP 224, Sénégal
OT Thiaw
Institut Universitaire de Pêche et d’Aquaculture (IUPA), Université Cheikh Anta Diop, BP 5005, Dakar, Sénégal
D Jouffre
Institut de Recherche pour le Développement, US 007 Osiris, route des hydrocarbures, BP 1386, 18524 Dakar, Sénégal; Institut de Recherche pour le Développement (IRD), Laboratoire ECOSYM (UMR 5119), Université Montpellier II, Place E Bataillon, 34095 Montpellier Cedex 5, France
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Articles

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eISSN: 1814-2338
print ISSN: 1814-232X