The California Current is a productive eastern boundary current that is an important habitat for a number of highly migratory shark species that depend on the current as a nursery and foraging ground. Three of the most abundant juvenile sharks in the California Current are the shortfin mako, blue and the common thresher sharks. These sharks differ in swimming speed, tooth and jaw structure, and fishing strategies. Antonella Preti, a biologist at the Southwest Fisheries Science Center (SWFSC), has been conducting stomach content analysis to characterize niche separation and the ecological role of these three overlapping species. From 2002-2010, Preti analyzed the stomach contents of over 700 sharks at the Highly Migratory Species lab in La Jolla, working with a number of colleagues and volunteers. The results from these analyses were recently published in a special issue on the Feeding Ecology of Elasmobranchs in the journal Environmental Biology of Fishes.
In the study, stomach contents of the three shark species were identified to the lowest (most specific) possible taxonomic level revealing distinct differences in foraging preferences across species. In the stomachs of shortfin mako sharks, Preti found 43 prey taxa with jumbo squid and Pacific saury as the dominant prey taxa. In blue sharks, Preti found 38 prey taxa, with jumbo and Gonatus squids as the most important prey. Finally, in thresher sharks, a narrower range of prey were recorded (18 prey taxa) with the small schooling fish, northern anchovy and Pacific sardine, dominating the diet.
The analyses of stomach contents across the three species reveal that while the there is some overlap in the diets, the differences provide insights into niche separation. Thresher sharks had the most specialized diet and forage on small pelagic fish. Their diet is likely linked to their foraging strategy of stunning fish with their tail before eating them. Blue and mako shark diets were most similar although blue sharks consumed a greater proportion of squid than the mako sharks. Blue sharks are thought to forage deeper in the water column on species associated with the deep scattering layer, a layer of abundant marine organisms that migrates daily from depths below the photic zone to near the surface at night. The sharks’ diets are linked to how they are using the California Current and will help us understand their ecological role and predict how the blue, shortfin mako and thresher sharks may be impacted differently by changes in the forage base or environmental conditions.
Link to "Preti, A., Soykan, C.U., Dewar, H., Wells, R.J.D., Spear, N., and Kohin, S. 2012. Comparative feeding ecology of shortfin mako, blue and thresher sharks in the California Current. Enviromental Biology of Fishes, 95(1): 127-146. DOI: 10.1007/s10641-012-9980-x"
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