Digestion of pelagic Ctenophora and Cnidaria by fish

The diet of chum salmon (Oncorhynchus keta) can range from a primarily arthropod to a primarily gelatinous composition. Because no data exist for the digestion rates of gelatinous prey in any fish, it has not been possible to convert data on stomach contents to feeding rates. We measured rates of digestion to address this issue. In freely feeding age-0 chum salmon, the ctenophore Pleurobrachia bachei was digested almost completely within 1 h, whereas 70% of shrimp sections of similar mass remained after 5 h at 13°C. To examine the fast progress of digestion at smaller intervals, age-0 and age-1 fish were also force-fed with whole P. bachei or sections of the hydromedusa Aequorea victoria. These data emphasize the necessity of examining fish stomachs very quickly after capture. Although gelatinous organisms have high water and salt content relative to their organic content, given the rapid rates of digestion observed, coelenterates might rival arthropods as energy sources.