. Bulletin. Science. 34 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES. Spring Summer Fall Winter Fig. 6. Mean ± SEM stomach acidity by season for tilapia stomachs at the Salton Sea. Horizontal bars indicate analysis of variance post-hoc groupings (Fisher's LSD, p < ; p = for factor season, 104 df). on plankton and benthic organisms for food, keeping the color of intestinal ma- terial brown, even if food is not digested. Tilapia at the Salton Sea reproduce from the spring to the end of summer (Riedel et al. 2002). According to our stomach acidity data, tilapia feeding in- creases during f
. Bulletin. Science. 34 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES. Spring Summer Fall Winter Fig. 6. Mean ± SEM stomach acidity by season for tilapia stomachs at the Salton Sea. Horizontal bars indicate analysis of variance post-hoc groupings (Fisher's LSD, p < ; p = for factor season, 104 df). on plankton and benthic organisms for food, keeping the color of intestinal ma- terial brown, even if food is not digested. Tilapia at the Salton Sea reproduce from the spring to the end of summer (Riedel et al. 2002). According to our stomach acidity data, tilapia feeding in- creases during fall and winter. In the spring, tilapia are getting ready to reproduce and in the summer tilapia are stressed from anoxia and high temperatures. The critical period for growth may be a short period between the fall and winter before temperatures decrease below suboptimal, and the early spring, before reproduc- tion. During those periods, we observed many fish stomachs full with adult pile worms, further evidence of the importance of this polychaete in determining ti- lapia condition and supporting its population. The Salton Sea recently experienced massive bird mortalities (Tetra Tech 2000). Among the culprits is a bacterium, Clostridium botulinum, which causes botulism in pelicans. Pelican mortality is highest during summer and is possibly mediated by tilapia. Pelicans feed on tilapia infected with the bacterium. Because the bac- terium and its spore are inhibited from growing in acid environments (pH or below), we hypothesize that C. botulinum should be more common in tilapia stomachs during the spring and summer, based on the patterns of tilapia stomach acidity found here (Figure 6). The success of Salton Sea tilapia may be attributed to its high feeding plasticity. Fish derive nutritional value from plants and plankton in the summer, but rely mostly on the pile worms during the fall and winter, making those seasons critical for tilapia growth and production. Because of
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