Lipid content in the prey of bull trout: an avenue to restoration?

Jon Bryant Anderson

Reproductive success is a major contributor to population dynamics through recruitment and stability. Environmental conditions, predator/prey relationships, and numerous parental effects all affect the surviving number and condition of progeny. For oviparous populations, parental diet contributes to reproductive success and fitness in females by way of egg provisioning and in males by the rate of fertilization through the structure and function of sperm. We studied the possible prey items of bull trout, Salvelinus confluentus, in Swift Reservoir of Southwest Washington, USA, for a potential relationship between prey quality and reproductive success. Bull trout abundance on the North Fork Lewis has been monitored since 1995 with a documented population decline beginning in 2005. We propose a key factor relating to the decline was a change in reproductive success and over-winter survival resulting from a dietary shift away from prey rich in energetic value and unsaturated fatty acids stemming from changes in hatchery plantings. To test this, we quantified the lipids of potential bull trout prey items in 13 groups from 7 species to compare their metabolic value. Total lipid was used as a proxy for catabolic potential, while a suite of unsaturated fatty acids represented anabolic potential. Total lipid was determined gravimetrically and unsaturated fatty acids of the lipid extracts were identified and quantified using Gas Chromatography Mass Spectroscopy. Hatchery fingerling rainbow trout provided the greatest catabolic potential of the known prey groups (Kruskal-Wallis P< 0.01)). MANOVA and linear discriminant analysis showed hatchery fingerling rainbow trout had the greatest anabolic potential, mostly from elevated levels of long chain omega-3 and omega-6 fatty acids. The results support our hypothesis, that hatchery fingerling rainbow trout were a unique high quality forage item present in the reservoir prior to, but absent at the time of and since the bull trout population’s decline. Specifically, we show that hatchery fingerling rainbow trout are high in dietary components known to improve reproductive success in teleost fishes. Our findings suggest that lipid analysis and monitoring of forage species may be a significant aid in the management of many oviparous populations.

Lipid content in the prey of bull trout: an avenue to restoration?

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