INTERACTIONS BETWEEN NONNATIVE AMERICAN SHAD (ALOSA SAPIDISSIMA) AND FALL CHINOOK SALMON (ONCORHYNCHUS TSHAWYTSCHA) IN LOWER COLUMBIA RIVER FOOD WEBS
Haskell, Craig Andrew
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Lower Columbia River Food Webs have undergone many changes resulting from impoundment, introduction of nonnatives, and supplementation with hatchery salmon (Oncorhynchus spp.). American Shad (Alosa sapidissima) are an anadromous clupeid introduced to the West Coast in the late 1800’s with annual returns to the Columbia River Basin that are greater than salmon. Shad conveyance of marine derived nutrients results in a net input of phosphorus (P) and nitrogen (N) to the lower Columbia River. The quantity of N and P is similar to current, but far less than historical inputs by salmon. However, the overall input of nutrients by shad is small in comparison to background river-loading. Juvenile shad are abundant, feed on Daphnia, and are present near the end of the juvenile salmon seasonal outmigration in July and August. An empirical feeding study of juvenile salmon and shad indicated that salmon fed at a high rate on Daphnia in July and fed on juvenile shad in August. No juvenile shad were collected in July, but in August, shad primarily consumed cyclopoid copepods. Laboratory trials yielded a Type-II functional response curve: y = 29.858 x*(4.271 + x)-1 indicating that at Daphnia densities typical of those in the lower Columbia during August, Daphnia reductions have little effect on salmon consumption. However, during July, when Daphnia density in the Columbia River is often < 5 . L-1, consumption decreased linearly with density. Bioenergetics modeling predicted that juvenile salmon would not grow when consuming Daphnia at 20 oC or higher. Feeding on shad, a prey with higher energy density, subyearlings could grow at 22 oC, but had to consume at least 35 shad . day-1. At temperatures of 16-18 oC, subyearlings had to consume 20 shad . day-1 or 30,000 Daphnia . day-1 to grow. Bioenergetics runs using empirical data indicated that salmon were not consuming enough to grow. Shad-mediated food web effects are constrained in time and space both in their ability to influence food webs through bottom up processes such as nutrient enhancement and through top down process such as reducing zooplankton forage shared by juvenile salmon.