The Influence of Environmental Factors on Longline Catch Rates of Yellowfin Tuna Thunnus albacares and Bigeye Tuna Thunnus obesus in the South China Sea

Abstract

The fluctuation of ocean environment affect distributions and productions of fishes have been known. This paper describes an investigation of possible associations between longline catch rates of commercially important tunas, yellowfin tuna Thunnus albacares and bigeye tuna Thunnus obesus, and environmental conditions in the South China Sea from 1990 through 2004. During this period catch per unit effort (CPUE) of yellowfin tuna decreased with increasing sea surface temperature (SST) (n=15, r=-0.53, p＜0.05), whereas CPUE of bigeye tuna increased with increasing SST (n=15, r=0.59, p＜0.05). Between 1997 and 2004 chlorophyll-a (Chl-a) concentration and sea surface temperature (SST) in the South China Sea showed a significant negative correlation (n=83, r=-0.42, p＜0.05). There was a significant positive correlation (n=33, r=0.46, p＜0.01) between the changes of Chl-a concentration and CPUE of yellowfin tuna during the periods of abnormally high temperature (anomaly SST ＞0.4℃) in the South China Sea (Sep., 1997-Apr., 1999; Dec., 2000-Jun., 2001; Nov., 2002-Apr., 2003). But variations of Chl-a concentration and CPUE of yellowfin tuna during the whole period (Sep., 1997-Dec., 2004) did not show a significant relationship (n=83, r=0.12, p＞0.05). The results indicate that increased SST is correlated to a reduction of Chl-a concentration and indirectly is correlated to the decrease of CPUE of yellowfin tuna. However, bigeye tuna seemed to be unaffected by changes in Chl-a concentration during either the whole period or periods with abnormally high temperatures.