Dynamics of ENSO events in the Indian ocean: To what extent would recruitment and catchability of tropical tunas be affected ?
Le Blanc, J.-L.
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In this paper, the authors explore functional relationships between environmental fluctuations and some key parameters used in stock assessment such as recruitment and catchability. Yellowfin tuna (Thunnus albacares) is taken as an example but the concepts developed in this study could be widened to other tropical tunas. The French purse seine data-set is used to estimate a recruitment index from adjusted CPUE of age 0 fishes, and the effect of the environment on catchability is assessed using CPUE of age 3 fishes which provide the bulk of adult catches. The ACE algorithm is used to describe a non-linear relationship between the recruitment and the turbulence (wind speed cubed). There is a detrimental effect of an increasing turbulence on recruitment. The optimal transformation of this simple regression explains more than 25 % of the variance of recruitment. The analogy, with the fundamental triad and the optimal environmental window that explain the variability of small pelagic fish populations, is discussed. Regarding catchability, there is clearly a non-linear relationship between the CPUE age 3 and the depth of the isotherm 20°C (denoting the core of the thermocline), with a negative effect of deeper thermocline. These results are used to assess the potential effects of the El Niño-Southern Oscillation (ENSO) events on tuna stocks and their exploitation. The dynamics of ENSO in the Indian Ocean are described, and it appears that the latest event (1997-98) will remain classified in this ocean as extremely intense. The fishing pattern of purse seiners changed drastically: an abnormal easterly wind stress along the equator induced a reversal of the slope of the ocean (and consequently that of the thermocline. Rising of thermocline in the eastern basin generated increased catchability for purse seine gears. It is also suggested that ENSO would promote success of recruitment for tropical tunas. However, the consequence for the stock is not straightforward, as the response might be related to its level of exploitation.