Thesis Project: Variables affecting hard part recovery from scats and spews of California sea lion (Zalophus californianus)

My research focus is on California sea lion (Zalophus californianus) foraging ecology, assessing biases associated with scat analysis in determining sea lion diet. The impetus for this captive feeding study is that recovered hard parts from scats do not accurately represent the diet of pinnipeds. Otoliths and skeletal structures (collectively called hard parts) of prey erode at different rates depending on species (eg. Dellinger and Trillmich 1988, Harvey 1989), because of the length of time hard parts are exposed to digestive acidic solutions (Jobling and Breiby 1986) and the variable robustness of specific hard parts. The digestion of hard parts causes an incomplete recovery rate, where fewer numbers of structures are found in the scat than was consumed, thereby underestimating the number of prey eaten (eg. Treacy 1981, Harvey 1989, Tollit et al. 1997).

I am assessing the affects of single and mixed species meals and meal frequency on passage rates, compare differences in otolith erosion, recovery, and passage among sexes and age classes, and correlate size of prey with size of hard parts recovered. The information collected will be used to provide tools to accurately reconstruct the diet of California sea lions. I will report: (1) passage time of hard parts recovered, (2) numerical correction factors (NCF) for percentage recovery and size correction factors (SCF) for otolith length reduction, and (3) evaluate diet reconstruction indices. Four diet reconstruction indices are commonly used to determine the prey composition of a pinniped population diet: Modified frequency of occurrence (MFO) (Croxall et al. 1993); split sample frequency of occurrence (SSFO) (Olesiuk et al. 1990); variable biomass reconstruction (VBR) (Laake et al. 2002); fixed biomass reconstruction (FBR) (Laake et al. 2002). I will assess these models to determine which best accurately calculates diet.

The motivation for this research stems from a diminishing fish population in our Pacific Ocean. The fishery industry and the natural populations of sea lions and other piscivorians share this common food resource, which only becomes problematic when the resource size decreases. This has challenged scientists to examine in greater detail the diets of pinnipeds, to assess the percentage of the market fish population that sea lions are consuming. Threatened or endangered salmonid species are highly sought after by consumers. Depredation of salmon caught on long lines increased during the 1998 El Niño (Weise 2000), reducing their catch and monetary income. It is uncertain how much salmon and what proportion of the sea lion population is actually suspect of taking salmon. We must turn to scat analysis to properly assess pinniped diet to discover what amount of salmon is being consumed by sea lions and to determine if a few individuals or a wide spread number of individuals are actually consuming the salmon. This study will provide the accurate tools to accomplish this mission.

Acknowledgements: Thank you to The Marine Mammal Center in Sausalito, CA which hosted the collection of this data. A great big thanks to all the volunteers who made this project possible.

Other Research Project Affiliation:
Central Coast Marine Bird Health Study, Marine Wildlife and Veterinary Care and Research Center California, Department of Fish and Game, Santa Cruz

Marine Mammal and Sea Turtle National Stranding Network

Curriculum Vitae

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