Loss of stability owing to a stable age structure skewed toward juveniles

This study analyses the effect on sustainability of a strategy of exploitation under age-specific survival probabilities fixed through time. Exploitation can be considered an artificial selective mortality that affects the stability of ecological communities. For the exploitation of any single species, it is necessary to obtain some indices that permit to check when the exploitation strategy may become an overexploitation strategy. For fisheries, supposing an asymptotically stable equilibrium exists, the number of recruits per spawning biomass units at the steady state need be bounded, such a boundary being dependent on reproductive parameters. In a previous study, this is shown to be a necessary condition for sustainable exploitation. Also, it was shown that when the stable age structure is correctly balanced then the necessary condition for sustainability is verified. Experience say that in order to maintain the sustainability, it is necessary to have diversity of individuals in the age structure. In this paper, the analysis of how the stable age structure should be, relative to reproduction parameters in order to induce the population collapse is performed. The results show that if the age-specific survival probabilities for ages that contribute to recruitment are too low, then the population tends to extinction. Assuming a stable age structure, there exists an extreme value omega(1)(*), for the first component of the stable age structure (omega(1)), related to the age-specific survival probabilities that do not contribute significantly to recruitment. When these survival probabilities are low, omega(1)(*) is large and conversely. If omega(1) > omega(1)(*) then maintaining through time the exploitation strategy, or in other words the age-specific survival probabilities, forces the population to the extinction. This is a sufficient condition for a nonsustainable exploitation strategy. In addition, this result shows that an unbalanced stable age structure as characteristic of nonsustainability. Besides, it is possible to see that the necessary condition for sustainable exploitation is not a sufficient condition. We take the Argentine hake (Merluccius hubbsi) fishery as an example where the necessary condition is true and, nevertheless omega(1) > omega(1)(*). The analysis allows to reach the conclusion that during the period 1986-1999 this fishery stock was overexploited. The loss of adult specimes and depletion of spawning stock biomass, biomass, and numbers of individuals are characteristic of that time period. Also in this paper, a proposal for an exploitation strategy is analysed, through which recovery and sustainable exploitation for this hake stock are expected. This exploitation strategy contemplates a specific fishing gear and suitable closed areas. (C) 2003 Elsevier B.V. All rights reserved.