Ontogenetic, temporal, and spatial variation of feeding niche in an unexploited population of walleye pollock (Theragra chalcogramma)
T.C. Kline Jr.
- Pub. no.: AK-SG-08-01n
- Year: 2008
- Price: $2.75
- DOI: 10.4027/rgsfcc.2008.14
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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.