Marine Subsidies Fortify a Stronghold for a Freshwater Megafish (Hucho taimen)

Abstract

Taimen—the largest salmonids in the world—are among the most at-risk groups of freshwater megafish. The Tugur River in the Russian Far East is perhaps the greatest stronghold of Siberian taimen (Hucho taimen) and large runs of anadromous chum salmon (Oncorhynchus keta) to this ecosystem may contribute to this conservation success. We combined analyses of stable isotopes, allometry, and body condition to assess the role of nutrient subsidies from salmon in sustaining this megafish stronghold. Siberian taimen relied on marine-derived nutrients throughout ontogeny, but the assimilation rate increased by a factor of > 5.5 once they grew large enough to consume adult salmon whole. This marine resource constituted up to nearly 60% of the lifetime diet and 97% of the seasonal diet for large taimen. Salmon provided episodic pulses of direct nutrition and year-round ecosystem-wide benefits (including taimen prey that also assimilated marine-derived nutrients), enabling taimen to persistently maintain high body condition. Our findings underline key conservation lessons regarding the role of prey to guide conservation of freshwater megafish and other imperiled megafauna elsewhere.