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Effects of primary production and other factors on the size and abundance of juvenile coho salmon in artificial off-channel habitat Decker, Andrew Scott

Abstract

In British Columbia and the Pacific Northwest, construction of off-channel habitat including artificial ponds and groundwater-fed channels, figures prominently in restoration efforts intended to benefit coho salmon (Oncorhynchus kisutch). To examine how juvenile coho production was effected by autotrophic productivity in these systems, I estimated the average size and abundance of juvenile coho during late summer and early spring over two years in three pairs of physically similar channels and one pair of ponds with one member of each pair having relatively high periphyton biomass and the other member having low periphyton biomass. Differences in total alkalinity between paired sites suggested that nutrient availability was a strong determinate of periphyton biomass on artificial substrate. However, periphyton biomass did not appear limited by phosphorus alone; phosphorus was relatively abundant in the groundwater channels, and as a result, nitrogen and possibly other nutrients were also important. Average coho fry size and biomass per unit area were greater by late summer in the periphyton-rich sites, but fry density was not. Physical habitat variability also failed to explain high variation in coho densities among the groundwater channels. Biomass and taxonomic composition of benthic invertebrates also appeared to be related to periphyton biomass, but a strong interaction between invertebrate abundance or distribution and current velocity was a confounding factor. In the subsequent spring, pre-smolt coho were still larger in the periphyton-rich site in the majority of cases. However, water temperature during the winter had a much greater effect on average pre-smolt size than growth rate the previous summer. In the groundwater channels, consumption of chum salmon (O. keta) eggs, fry, and carcasses may have also contributed to high coho growth rates during the winter. The abundance of spring pre-smolts in the channels and ponds was not related to either periphyton biomass or average fry size the previous summer. Favorable growing conditions during the winter may have lessened size-dependent overwinter mortality, but the benefit of nutrient addition to coho production in off-channel habitat remains uncertain. Pre-smolt abundance in the off-channel sites was strongly influenced by small differences in water depth and velocity in winter and by fry density the previous summer. Physical habitat differences between the channels and ponds also influenced abundance: pre-smolts densities were consistently higher in the deeper, more structurally complex ponds. Large, density-dependent declines in coho abundance in the relatively warm and hydrologically stable groundwater channels from September to March contradicts traditional emphasis on starvation and adverse physical conditions (e.g., freshets) as primary sources of coho winter mortality. My results suggest that other factors such as predation or energetics warrant attention.

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