California sea lion and harbor seal marks on salmonids : can they be used to estimate relative predation?

• Main Author: Allen, Melissa
• Format: Text
• Language: unknown
• Published: Digital Commons @ CSUMB 2000
• Subjects: Pacific; harbor seal; Phoca vitulina
• Online Access: https://digitalcommons.csumb.edu/caps_thes_restricted/124; https://digitalcommons.csumb.edu/cgi/viewcontent.cgi?article=1123&context=caps_thes_restricted

alifornia sea lion (Zalophus californianus) and Pacific harbor seal (Phoca vitulina) populations are believed to have a negative effect on salmonid populations. Comparing scratches on fish with intercanine distances in pinnipeds has some potential for documenting the level of activity of these predators (Scordino 1993). In order to provide more information on the extent of sea lion and harbor seal predation on a local steelhead (Oncorhynchus mykiss) run, this study examines marks on steelhead trapped in the San Lorenzo River during the 1999 and 2000 seasons. From the marks that could be reasonably measured, in the 1999 season, 36.9% were from harbor seals, 7.2% appeared to be from sea lions, 49% were smaller than the harbor seal intercanine ranges, and 7% were larger than the ranges of the intercanine distances of the sea lions. In the 2000 season, 34% of the marks measured were from harbor seals, 13% from California sea lions, 40% were smaller than the intercanine distances reported for the harbor seal, 11% of the marks were larger than the intercanine distances for the sea lion, and 3% of the marks measured between the largest measurement of the intercanine distance for the harbor seal and smaller than the distance of the intercanines of the sea lion. This study also examined if males or females were being preyed on more. Results from both seasons showed that females had more marine mammal marks than males. If the fish enters and/or exits the mouth at an angle, some marks made by a California sea lion may be interpreted as being made by a harbor seal. Methods for obtaining improved accuracy are suggested. Non-lethal interventions for deterring pinniped predation on salmonids are discussed.