Ontogenetic, temporal, and spatial variation of feeding niche in an unexploited population of walleye pollock (Theragra chalcogramma)

Ontogenetic, temporal, and spatial variation of feeding niche in an unexploited population of walleye pollock (Theragra chalcogramma)

T.C. Kline Jr.

Ontogenetic, temporal, and spatial variation of feeding niche in an unexploited population of walleye pollock (Theragra chalcogramma)This is part of Resiliency of Gadid Stocks to Fishing and Climate Change
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Description

Stable isotope ratios of carbon and nitrogen were measured in an unexploited population of walleye pollock (Theragra chalcogramma) from
Prince William Sound, Alaska (PWS) as a metric of feeding niche. Stable isotope data of pollock were compared with each other and with stable isotope data of age-0 herring (Clupea pallasii). There were significant
species, location, and time period differences in synoptically sampled age-0 pollock and herring. Age-0 pollock were generally 13C-enriched relative to herring even as carbon isotope values varied with time. A positive relationship between 13C content and length of pollock from Zaikof Bay, PWS, was consistent with a pattern of low 13C being attributable to food subsidies, most likely oceanic zooplankton.

The carbon isotope difference between age-0 pollock and age-0 herring increased when 13C decreased. This coincided with a relative increase in age-0 pollock food chain length compared to age-0 herring. This is posited to reflect reduced overlap in food supply and thus competition
during a period of inferred high food subsidies. Subsidies are
further posited to vary according to hypothesized oceanic zooplankton population cycles.

A systematic increase in nitrogen isotope values with respect to size suggested an average ontogenetic food chain length increase of about 1.3 trophic levels over a pollock's life span after age-0. Trophic level variability was consistent with facultative planktivory for all adult sized pollock, including those with lengths over 400 mm, above which there was a sharp trophic level increase. The ability for pollock to maintain a relatively low trophic level for most of their life is hypothesized to enhance the species’ ability to sequester energy. Stable isotopes may provide a useful metric for detecting trophic change in terms of subsidies as well as food chain length.

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