THE LIFE HISTORY AND ECOLOGY OF RIVERINE SOCKEYE SALMON (ONCORHYNCHUS NERKA) IN A LARGE FLOODPLAIN RIVER
The productivity and life history diversity of salmonids is linked to the complexity of habitat in large floodplain rivers. Dynamic floodplain processes are driven by flow, sediment transport, cut and fill alluviation, woody plant succession, and ecosystem engineers, notably beavers, that create dynamic and biophysically complex off-channel rearing and spawning habitats. Sockeye salmon (Oncorhynchus nerka) display some of the most variable life history traits of all Pacific salmon, using lake and river rearing strategies throughout rivers of the northern Pacific Rim. However, sockeye management has primarily focused on the lake-type life history with minimal regard to the importance of the riverine form. In the Kwethluk River, Alaska, we identified both the lake (lake-type) and river (riverine) driven life histories. To elucidate the importance of lateral floodplain habitats to this species, we compared lake and riverine spawning habitat attributes, determined the densities, seasonal changes in length and weight, and diet of juveniles. We also quantified the amount of off-channel habitats that were available for spawning and rearing. We found that floodplain and lake spawning habitats had similar hydrological, physical, water chemistry attributes. Lake-type spawning occurred along the alluvial fans of the lake shoreline and in the outlet channel, whereas, riverine sockeye spawned exclusively in off-channel spring brooks and side channels. But, all spawning was located in areas dominated by upwelling ground water. We found that juvenile sockeye reared in spring brooks, beaver ponds, and backwaters of the river flood plain and that these habitats types made up over 70% of the available off-channel habitat. The average density of floodplain rearing juveniles in the fall was between .02 to .54 fish „ª m-2 and a total of 165, 711 juvenile sockeye were rearing in the floodplain study reach. The mean length of flood plain rearing juveniles in the fall of 2006 ranged from 57 to 66 mm and was similar to the mean length of ocean migrating smolts (68 mm) the following spring. The diet of juveniles rearing in off-channel habitats was dominated by zooplankton, but juveniles fed on a variety of invertebrate prey, while the diet of lake-type juveniles was dominated by zooplankton. We concluded that riverine sockeye take advantage of the complexity of large river systems and that floodplain habitats are key spawning and rearing habitats for this species. The riverine life history type may be an important part of large river systems throughout the Pacific Rim.
© Copyright 2008 Tyler Huntley Tappenbeck