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Investigaciones marinas

versión On-line ISSN 0717-7178

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

Optimal Environmental Window and
Positive Effects of `El Niño' Events on
Recruitment and Sustainability of
Common Hake (Merluccius gayi gayi)
off Central Chile

Ignacio Payá Contreras

Instituto Fomento Pesquero, Huito 347, Valparaíso,
Chile, E-mail:

The common hake fishery is located in the neritic zone between 27 and 41°S off Central Chile. This fishery began in the 1940s, and its catches quickly increased to 80 thousand tons in the mid-1950s, then fluctuated, achieving their maximum historical level of 128 thousand tons in 1968. After that, a strong drop in catches occurred, with a decrease to 30 thousand tons in 1976. They remained at that level until 1988, but have since been increasing to a current level of 120 thousand tons.

Several hypotheses have been postulated to explain the collapse of the fishery in the 1969-1970 period, including a decrease in market demands due to the prohibition of fish meal production from hake, environmental changes and recruitment failure due to overexploitation. These hypotheses have remained untested and scientists, fishermen and managers have considered the collapse of 1969-1970 an important historical upper limit to the catch. However, the strong increase in biomass during the last 10 years has brought this empirical criterion into question.

The aim of this presentation is to show how the abundance of hake has been increasing because of positive environmental conditions during the last 10 years, in which El Niño events have been important in the recruitment success and have contributed to the improvement of the present status of hake stock, which is quite different to that found before the 1969-1970 collapse.

Hake inhabits the 100-400 m depth layer, mainly in the Günter current, which flows from north to south and has low oxygen and high nutrient contents. This water is upwelled, explaining the high productivity of the ecosystem. Hake spawns throughout its distribution and spawning is concentrated in 5 areas associated with upwelling zones. Hake spawns in August,concurrently with minimum levels of upwelling and turbulence indices, decreasing the probability of egg and larval death due to offshore Ekman transport and starvation by the de struction of food patches by turbulence.

Historical abundance was estimated by means of virtual population analysis, tuned with numbers at a given age calculated from hydroaccoustic surveys and a relative abundance index from commercial catches. An ADAPT framework based on least squares was used in the tuning. This stock assessment includes data collected since 1968, the year of the maximum historical catch.

Biomass varied around 300 thousand tons from 1968 to 1983. Since 1984 the biomass has been increasing and in the year 2001 it reached 1.4 million tons. The fishing mortality peak value was in 1968, after which it decreased rapidly until 1976, and has remained at a low level since then.

Total recruitment of 3-year-old individuals decreased between 1968 and 1976 and has since been increasing, showing three major peaks: 1987; 1994 and 2000, which correspond with individuals born in 1984, 1991 and 1997, respectively.

Denso-dependent recruitments were estimated using the Welch filter (1986), which is based on Fourier analysis. This procedure filters denso-dependent recruitment as a function of reproductive effort by age. Ricker's stock-recruitment model was fitted with total recruitments and denso-dependent recruitments. Denso-dependent recruitments were closer to the model than total recruitments. The largest positive deviations of total recruitment from the stock-recruitment model correspond to the "El Niño" events of 1982, 1991 and 1997.

An Optimal Environmental Window (OEW, Cury and Roy, 1989) for total recruitment was analyzed by means of the Alternating Conditional Expectation Algorithm (ACE) and additivity and variance stabilizing transformation (AVAS). These procedures transform both response and predictor variables in order to equal the sum of transformed predictor variables with the transformed response variable. The OEW was found to be associated with the turbulence during the spawning season, with similar values of turbulence (200-300 m3/s3) or wind speed (5-6 m/s2) found for small pelagic resources in upwelling systems.

Therefore, successful recruitments are associated both with the El Niño events and with upwelling and turbulence during the spawning season. The pelagic behaviour of larvae and small hake explains why hake is quite similar to small pelagics, like sardine and anchovy, in terms of having an OEW of turbulence.

The contribution of the environment to improving hake production by recruitment and the effects of environmental changes in the biological reference points are discussed. Finally, the empirical historical limit of 128 thousand tons catch is reassessed considering the present status of the stock, biological reference points and the current levels of total allowable catch.

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