Complementary Approaches to Analyzing the Trophic Ecology of Juvenile Pacific Salmon in the California Current

Much information has accumulated on the foraging ecology of juvenile salmon when they first enter marine waters along the west coast of the US and Canada.  We review and discuss assumptions, strengths, and weaknesses of three approaches commonly used to infer feeding habits of juvenile salmon: 1) stomach contents, 2) stable isotopes, and 3) fatty acids.  The most direct and common way has been through visually characterizing prey items in the stomach.  This method allows for detailed taxonomic analysis and is quantifiable by number and weights or volumes of individual prey taxa. A disadvantage is that this only provides information on recent feeding history and is biased by the rapid deterioration or digestion of soft prey such as gelatinous zooplankton and the retention of hard parts such as bones and otoliths for extended periods of time. Stable isotopes and fatty-acids are both biochemical approaches that can provide a longer-term history of feeding and assimilation than available from direct stomach analysis, but they require more development to make them more reliable indicators of diet composition. In particular, these methods assume that the predator is in equilibrium with prey in its environment which is not likely with constantly migrating salmon.