Ontogenetic, Temporal, and Spatial Variation of Feeding Niche in an Unexploited
Stable isotope ratios of carbon and nitrogen were measured in an unex-ploited 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...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.564.2563 http://nsgl.gso.uri.edu/aku/akuw06005/akuw06005_part7.pdf |
| Summary: | Stable isotope ratios of carbon and nitrogen were measured in an unex-ploited 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 attribut-able 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 com-petition 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 252 Kline—Variation of Feeding Niche 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 subsi-dies as well as food chain length. |
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