MEPS 299:33-44 (2005)  -  doi:10.3354/meps299033

Columbia River plume fronts. II. Distribution, abundance, and feeding ecology of juvenile salmon

Alex De Robertis1,6,*, Cheryl A. Morgan2, Robert A. Schabetsberger2,3, Richard W. Zabel4, Richard D. Brodeur1, Robert L. Emmett1, Carolyn M. Knight2,5, Gregory K. Krutzikowsky2, Edmundo Casillas4

1NOAA Fisheries, Northwest Fisheries Science Center, Hatfield Marine Science Center, Newport, Oregon 97365, USA
2Oregon State University, Hatfield Marine Science Center, Cooperative Institute for Marine Resources Studies, 2030 S. Marine Science Drive, Newport, Oregon 97365, USA
3Zoological Institute, University of Salzburg, Hellbrunnerstr. 34, 5020 Salzburg, Austria
4NOAA Fisheries, Northwest Fisheries Science Center, 2725 Montlake Boulevard East, Seattle, Washington 98112-2097, USA
5Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester, Dorset DT2 8ZD, UK
6Present address: Alaska Fisheries Science Center, 7600 Sand Point Way NE, Seattle, Washington 98115, USA

ABSTRACT: Well-defined fronts develop at the seaward edge of riverine plumes where suspended materials and planktonic organisms are concentrated by convergent water flows. Riverine plume fronts have been hypothesized to be favorable fish habitats because they can lead to localized prey aggregations. We examined the spatial distribution of juvenile Pacific salmonids Oncorhynchus spp. in and around plankton-rich frontal regions of the Columbia River plume to test the hypothesis that juvenile salmonids aggregate at riverine plume fronts to feed. Juvenile salmonids tended to be abundant in the frontal and plume regions compared to the more marine shelf waters, but this pattern differed among species and was not consistent across the 2 study years. Stomach fullness tended to be higher in the more marine shelf waters than either the front or plume areas, which does not support the hypothesis that salmonids consistently ingest more prey at frontal regions. Many prey organisms were disproportionately abundant at these fronts, but salmon stomach-content analysis did not reveal higher stomach contents at fronts or identify prey groups indicative of feeding in the frontal areas. Although our results indicate that the Columbia River plume influences the distributions of juvenile salmon, our observations do not support the hypothesis that juvenile salmonids congregate to feed at fronts at the leading edge of the Columbia River plume. The short persistence time of these fronts may prevent juvenile salmon from exploiting these food-rich, but ephemeral, features.

KEY WORDS: Frontal regions · Juvenile salmon · Distribution · Feeding habits · Columbia River plume

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