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Habitat Assessment, Marine Chemistry, and Genetics Program

Winter Energy Dynamics in Two Populations of Juvenile Eulachon in Southeastern Alaska

Eulachon (Thaleichthys pacificus), an anadromous smelt occurring from northern California to southwestern Alaska, are probably best known for their high lipid content. With lipid comprising up to one-half of their dry mass, eulachon are one of the most energy-dense fishes in the North Pacific Ocean. Dried and equipped with a wick, eulachon have even been used as a light source, earning them the common name candlefish.

Spring spawning aggregations of eulachon attract large concentrations of marine and terrestrial predators at a critical time of the year when winter-depleted energy supplies need replenishment. In southeastern Alaska a single spawning run can attract hundreds of harbor seals, more than 1,000 Steller sea lions and bald eagles, and tens of thousands of gulls. Eulachon are also highly valued by native peoples along the Pacific Northwest coast, many of whom continue the tradition of harvesting eulachon for their rich oil.

Despite their ecological and cultural importance, the factors that influence eulachon populations are poorly understood. Size-selective winter mortality from starvation may play an important role in the recruitment success and, ultimately, abundance of eulachon. In north temperate fishes, lipid stores accumulated during summer and fall provide energy during winter when food abundance is low. Relative to older individuals, age-0 fish store less lipid prior to winter and metabolize energy faster, thereby increasing their susceptibility to winter starvation.

In 2007 AFSC scientists studied the overwinter (January-April) energy dynamics of age-0 and age-1 eulachon in two southeastern Alaska bays, Berners Bay and Fritz Cove. Age-0 eulachon in both bays began mid-winter with relatively less lipid than older eulachon despite allocating more of their surplus energy to lipid reserves.

Midwinter lipid reserves were larger in Berners Bay fish than in Fritz Cove fish. However, fish from Fritz Cove maintained their energy levels from January to April, while fish from Berners Bay suffered an energy deficit that was fueled primarily by metabolism of lipid and, to a lesser degree, protein. Age-0 eulachon in Berners Bay depleted more of their midwinter reserves than age-1 fish and metabolized more protein to meet energetic demands.

The contribution of protein metabolism to this winter energy deficit increased with decreasing body size, suggesting that the smallest age-0 fish were most susceptible to starvation. A shift in the size-frequency distribution of both age groups in Berners Bay during winter may have resulted from the selective mortality of the smallest fish in the population.

Although eulachon in both bays fed during winter, zooplankton were more abundant in Fritz Cove, suggesting that enhanced feeding opportunities may help to preserve lipid reserves and reduce starvation risk.

By John Hudson

Cardiac Arrhythmia Is the Primary Response of Embryonic Pacific Herring Exposed to Crude Oil during Weathering

Teleost embryos develop a syndrome characterized by edema when exposed to weathered crude oil constituents in water. Previous studies using zebrafish demonstrated that crude oil exposure causes cardiogenic edema, and that the most abundant polycyclic aromatic hydrocarbons (PAHs) in weathered crude oils (tricyclic fluorenes, dibenzothiophenes, and phenanthrenes) are cardiotoxic, causing arrhythmia through a pathway that does not require activation of the arylhydro carbon receptor (AHR).

In Pacific herring (Clupea pallasi), a species impacted by the Exxon Valdez oil spill, the developing heart is the primary target of crude oil exposure. Herring embryos exposed to the effluent of oiled gravel columns developed dose-dependent edema and irregular cardiac arrhythmia soon after the heartbeat was established. At a dose that produced cardiac dysfunction in 100% of exposed embryos, tissue levels of tricyclic PAHs were below 1 Ámol/kg, suggesting a specific, high affinity target in the heart.

These findings have implications for understanding the mechanism of tricyclic PAH cardiotoxicity, the development of biomarkers for the effects of PAH exposure in fish, and understanding the long-term impacts of oil spills and other sources of PAH pollution in aquatic environments.

By Mark Carls

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