Ain, Najma Binti Hashim. 2020. Stomach Analysis of Top Aquatic Predators in a Headwater Stream Network. Corvallis: Oregon State University. 28 p. B.S. thesis.
Top predators play important roles in regulating communities and ecosystem processes in freshwater ecosystems. In the headwater stream network, it is crucial to understand the interspecific interaction between multiple predators. In Oregon forested ecosystems, Coastal Giant Salamander (Dicamptodon tenebrosus) and cutthroat trout (Oncorhynchus clarkii) are the top predators in many perennial headwater streams. Both feed on aquatic invertebrates and thus, they are potential competitors. In this study, we assessed the diet variation in headwater stream predators. The research objectives are to understand the dietary composition of both predators in different locations within the headwater stream and to compare the similitudes and differences based on their diet and body size. The stream evaluated was Lookout Creek, located within the H.J. Andrews Experimental Forest, Oregon. We collected 91 Coastal Giant Salamanders, 100 Cutthroat Trout, and 10 Rainbow Trout, and applied the gastric lavage method to extract the stomach contents. A total of 4,897 identifiable prey items were found belonging to 104 prey types. Trout diets contained the highest overall proportions of terrestrial prey. Although H’ indicates a similar trend of prey diversity between size classes, we detected a difference in the stomachs contents in terms of composition (or proportion) of semiaquatic and terrestrial insects. Our findings show that there is no major differences in diets between upstream and downstream for each predeator species, and sampling over relevant spatial and temporal scales is needed to understand the feeding behavior of trout and salamanders.