Octopus-red grouper interaction in the exploited ecosystem of the northerncontinental shelf of Yucatan, Mexico

Octopus (Octopus maya) and red grouper (Epinepheha morio) are the most impo rtant fisheries resources on the northern continental shelf of Yucatan, Mex ico, with annual yields fluctuating between 9000 and 16 000 t. Octopus is a n important component of the diet of red grouper, particularly when the abu ndance of octopus increases during summer and autumn in shallow waters. A p revious mass-balanced model using the Ecopath program described the main fl ows of biomass in this ecosystem, with emphasis on the commercially importa nt stocks. In this paper, a dynamic structured model of exploited ecosystem s, Ecosim, is used to simulate changes in vulnerability of octopus to preda tion by the red grouper under three scenarios of fishing mortality (F): (1) F low and constant; (2) gradual increments in F reflecting historical deve lopment of the octopus fishery; and (3) high values of F representing the c urrent state of the fishery. A fourth scenario is also presented, based on scenario 2, to observe biomass patterns of other groups. For all scenarios, when vulnerability of the octopus to predation was reduced, their biomass pattern was inverse to that when their vulnerability to predation increased . A similar behavior was found at high levels of vulnerability when octopus were submitted to high fishing effort. The impact of the combined effect o f changes in vulnerability with fishing intensity suggests the presence of important compensatory mechanisms tending to maintain a thermodynamic stabi lity, where growth efficiency and consumption play important roles. Even wh en fishing intensity appears to produce larger impact on the octopus biomas s, vulnerability has an important role when it is reduced and when the stoc k is submitted to very high levels of exploitation. After simulations, the biomasses of octopus and red grouper exhibited a similar picture to histori cal catch trends, suggesting that Ecosim could provide useful guidance for fisheries managers. (C) 2000 Elsevier Science B.V. All rights reserved.