. Bulletin. Science. 30 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES 11%. ? Pelagic ? Nearshore ? River Mouths Fig. 3. Average percentages of ingested macrophytes by sampling area present in tilapia stomachs sampled at the Salton Sea. Pelagic n = 11, nearshore n = 32, river mouth n = 109. Mean values were calculated from individual stomachs. tows) and the observed plankton percentages (tilapia stomachs) for barnacles, copepods, and rotifers during the summer (p > ), fall (p > ), and winter (p > ; Figure 5). Pile worms and nematodes were highest in the spring and fall (Table 1; Kru
. Bulletin. Science. 30 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES 11%. ? Pelagic ? Nearshore ? River Mouths Fig. 3. Average percentages of ingested macrophytes by sampling area present in tilapia stomachs sampled at the Salton Sea. Pelagic n = 11, nearshore n = 32, river mouth n = 109. Mean values were calculated from individual stomachs. tows) and the observed plankton percentages (tilapia stomachs) for barnacles, copepods, and rotifers during the summer (p > ), fall (p > ), and winter (p > ; Figure 5). Pile worms and nematodes were highest in the spring and fall (Table 1; Kruskal- Wallis test statistic = , p = (pile worms), , p < (nematodes), 3 df). Ciliates were highest during the summer (Kruskal-Wallis test statistic = , p < , 3 df). Pile worms in stomachs were more abundant at the river mouths (Table 1; Kruskal-Wallis test statistic = , p = , 2 df). There was no pattern between intestinal color and stomach acidity nor was there evidence of stomach acidity varying according to time of day. Fisher's LSD post-hoc groupings were spring-summer and fall-winter (Figure 6). Stomach acid- ities were lower during the fall-winter period compared with the spring-summer period (Figure 6; split-plot ANOVA, p = for factor season, 104 error df)- Discussion Our observations show that the summer at the Salton Sea is a stressful period for tilapia. We commonly observed tilapia in the summer gulping air for oxygen and spending most of their time at the surface (Costa-Pierce and Riedel 2000; Riedel et al. 2002). Water temperature often surpasses 32°C and oxygen inter- mittently drops to zero over large areas during that season. We do not, however, exclude the winter from also being a stressful period for tilapia because water temperatures during winter may drop to their minimum lethal limit (Costa-Pierce and Riedel 2000). The seasonal variation of the sediment quantity in tilapia stomachs indicates that fish spend the leas
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