Feeding Habits and Ontogenetic Diet Shifts of Mackerel Tuna (Euthynnus affinis) in the Beibu Gulf, South China Sea

Abstract:

To study the mackerel tuna (Euthynnus affinis) feeding habits and its ontogenetic diet shifts, 435 mackerel tuna were collected for an annual research from September 2008 to September 2009 in the main fishing ports such as Beihai, Basuo, Sanya and Jianghong of the Beibu Gulf, South China Sea. Results showed that the mackerel tuna of the Beibu Gulf was omnivorous and it predated not only the small pelagic fish but also the benthic fish, crustaceans and cephalopods. Pisces were dominant in the diets with the percentage of Index of Relative Importance (IRI%) of 98.57%. Among these, 21 species were identified. As indicated with IRI%, Bregmaceros rarisquamosus (45.50%), Stolephorus zollingeri (11.81%), Theyssa setirostris (5.89%), and Sardinella aurita (5.87%) were principal in the food cons­ituent. The main species changed monthly and it was Sardinella aurita in January; Bregmaceros rarisquamosus in February, March, November, December; Unidentified Pisces from April to July; Sardinella aurita in August; Leignathus riviulatus in September; Stolephorus zollingeri in October. The prey weight and numbers increased when body size of mackerel tuna was less than 340 mm (SL<340 mm), but there were decreased when body size of mackerel tuna was from 341 to 520 (341-520 mm). The spawning periods of the mackerel tuna were spring and summer, mainly from April to August. There were two spawn peaks, June and August, which were recognized by the female gonad somatic index (GSI) that maintained strong feeding intensity during the reproduction periods. Moreover, the peak periods were consistent with the water temperature of pelagic layers provided by France CATSAT fisheries remote sensing system.