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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002 

Is the Reduction in Abundance of
Haliotis fulgens and H. corrugata
(Gastropoda: Haliotidae) at Islas San
Benito (Baja California, Mexico) a
Combined Effect of Overfishing and
Climatic forcing?

J. A. Rodríguez-Valencia1,
F. Caballero-Alegría2,
J. Castro-González2,
N.A. Hernández-Guevara1

1Wadden Sea Station Sylt, Alfred-Wegener-Institute
Foundation, Hafenstraße 43, 25992 List, Germany,
2Instituto Nacional de la Pesca. CRIP-Ensenada.
Carretera Tijuana-Ensenada Km. 107. Parque Industrial
Fondeport, El Sauzal, Baja California, Mexico

Islas San Benito (Baja California, Mexico) are a group of three small islands located in the main abalone fishing area of Mexico. There, the abalone fishery has been controlled since 1990 by assigning yearly fishing quotes, which are calculated considering abundance estimations obtained from fishery-independent surveys. The fishery is maintained by Haliotis fulgens and H. corrugata, and since 1989 the commercial captures of the former species have been eight times higher that those of the latter. H. fulgens is more abundant, has a larger proportion of individuals of harvestable size (60% vs. 30% of H. corrugata), and is more profitable because of larger sizes and a more valuable shell.

The mean density of both species declined significantly between 1989 and 1999. That of H. fulgens decreased from 0.77 to 0.18 abalone•m-2, and that of H. corrugata from 0.49 to 0.27 abalone•m-2. The latest density levels of both species are close to values where recruitment failures may occur, but a slight recovery has recently been observed.

These patterns reflect the decline of the Mexican abalone fishery, observed since 1984. It has been proposed that the mixed effects of increased water temperature during "El Niño" events and recruit overfishing negatively affected the populations (Hypothesis A). After that, other authors proposed that only severe "El Niño" events might have a negative effect, because non-severe "El Niño" events should increase recruitment (Hypothesis B). Both hypotheses are difficult to test, because in most cases it is not possible to discern whether changes occurred by overfishing and poaching or merely climatic forcing. We present data from our surveys, which, in our opinion, illustrate changes attributable to climate forcing. Overfishing or poaching could be neglected at Islas San Benito, because the local fishing cooperative ("S.C.P.P. Pescadores Nacionales de Abulón") has always adhered to the fishery regulations in such a way that it has been considered as a model for more than 30 years. There, poaching of legal sizes and recruits has been rare, because the islands are distant from the coast and strict surveillance programs are applied to locals and strangers.

The mean annual surface temperature increased by 2.1°C between 1989 and 1998 in this area. According to Hypothesis A, the extent of macroalgae beds used as food should have been reduced, leading to weakening and increased mortality due to predation. We have found evidence of this, because the mean condition of both species has significantly decreased between 1989-1998, and this is significantly associated with the increase in temperature. The percentage of mature males and females of both species has also significantly decreased with time, probably due to poor physiological condition. Local fishermen affirm that the extent of macroalgae beds has declined, but we have no data to confirm this.

The time elapsed between settlement of abalone larvae and detection of recruits is normally seven years. According to Hypothesis B, increased recruitment should be detected between 1991 and 1998, because the larvae that originated these recruits settled during low incidence of strong "El Niño" events (1984-1991). Reduced recruitment should be detected after 1998, because the larvae that originated these recruits settled during frequent strong "El Niño" events (1992 up to now). Our data agree only partially with this, since the percentage of recruits of H. fulgens has remained without significant temporal changes. On the other hand, that of H. corrugata has significantly changed, and matches the pattern proposed by Hypothesis B: Increasing percentage of recruits until 1995 and decreasing percentages after 1996. This suggests that the negative effect of severe "El Niño" events on recruitment could be species-specific.

Both hypotheses bring elements to explain the patterns observed in our data. The increase in water temperature caused the reduction of food sources for abalone responsible for significant decline in physiological condition, and leading to the significant decrease of sexually mature organisms in both species. The absence of significant temporal changes in the proportion of recruits of H. fulgens supports our supposition that the effects of recruit overfishing of both species is negligible. The coincidence between the fluctuation patterns of recruitment of H. corrugata with those proposed by Hypothesis B supports the idea that the degree of severity of "El Niño" events determines a positive or negative effect. Nevertheless; this seems to be species-specific, and suggests that H. fulgens may be better able to deal with a damaged condition. Data sets longer than a decade are necessary in order to give more reliable evidence of this.

In our case the abundance of both species has been reduced, not by the combined effects of overfishing and climatic forcing, but by the combined effects of fishing at the authorized levels and climate forcing. It has been a failure not to incorporate the physiological condition and the proportion of sexually mature organisms as variables in the models used to estimate the fishing quotes.

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