Dietary composition and fatty acid content of giant salmonflies (Pteronarcys californica) in two Rocky Mountain rivers

Many aquatic invertebrates are declining or facing extinction from
stressors that compromise physiology, resource consumption, reproduction,
and phenology. However, the influence of these common stressors
specifically on consumer-resource interactions for aquatic invertebrate
consumers is only beginning to be understood. We conducted a field study
to investigate Pteronarcys californica (i.e., the ‘giant salmonfly’), a
large-bodied insect that is ecologically and culturally significant to
rivers throughout the western U.S. We sampled gut contents and
polyunsaturated fatty acid (PUFA) composition of salmonflies to compare
resource consumption across river (Madison or Gallatin, Montana), sex
(male or female), and habitat (rock or woody debris). We found that
allochthonous detritus comprised the majority of salmonfly diets in the
Gallatin and Madison Rivers, making up 68% of the gut contents on average,
followed by amorphous detritus, diatoms, and filamentous algae. Diets
showed little variation across river, sex, or length. Minor differences in
diets were detected by habitat type, with a higher proportion of diatoms
in the diets of salmonflies collected from rocky habitat compared to woody
debris. Fatty acid composition generally supported the results of gut
content analysis but highlighted the importance of primary producers. The
presence of eicosapentaenoic acid (EPA; 20:5n-3) and alpha linolenic acid
(ALA; 18:3n-3) suggest consumption of diatoms and filamentous green algae,
respectively. Our research underscores the importance of a healthy
riparian zone that provides allochthonous detritus for invertebrate
nutrition as well as the role of algae as an important source of fatty
acids.