UBC Theses and Dissertations
Predation by juvenile salmonids on harpacticoid copepods in a shallow subtidal seagrass bed : effects on copepod community structure and dynamics Webb, Donald G.
The hypothesis that predation by juvenile chum (Oncorhynchus keta (Walbaum)) and pink (Oncorhynchus gorbuscha (Walbaum)) salmon controls the abundance of harpacticoid copepods inhabiting a shallow subtidal seagrass (Zostera marina L.) bed on Roberts Bank, British Columbia, was tested. Previous studies of the impact of juvenile salmonid predation on harpacticoid communities have yielded contradictory results and have been based on indirect and weak evidence. The hypothesis was tested in two ways: by comparison of patterns of harpacticoid mortality with patterns of salmonid consumption and by the response of the harpacticoid community in controlled field experiments in which epibenthic predators/disturbers were excluded from portions of the seagrass bed. Samples of Zostera leaves and underlying sediment were collected at approximately biweekly intervals from late January to early July in 1986 and 1987 for the estimation of harpacticoid copepod abundance at the study site. Collections for estimation of juvenile salmonid abundance and gut contents during their period of residence in the area were made concurrently at a low tide refuge adjacent to the seagrass bed. Juvenile chum and pink salmon were found mainly to consume three harpacticoid copepod species (>75% by number in gut contents in both years): Harpacticus uniremis Kroyer, Tisbe cf. furcata (Baird) and Zaus aurelii Poppe. These copepod species were found primarily as epiphytes on Zostera leaves and demonstrated distinct periods of abundance in this habitat. Using a simple and robust deterministic, compartmental population dynamic model, mortality of adults and potential adults of these three species between sampling dates was estimated. In comparison with an index of consumption estimated for the salmonid population, temporal patterns of copepod mortality generally did not correspond to temporal patterns of salmonid consumption. This lack of correspondence indicates salmonid consumption did not cause the observed declines in Harpacticus uniremis, Tisbe cf. furcata and Zaus aurelii abundance during the sampling season in either year. Controlled exclusion cage (7 mm mesh) experiments were conducted within the Zostera marina bed from late March-early April to mid June in both 1986 and 1987, the period of major juvenile salmonid occurrence in the study area in both years. Exclusion of large epibenthic predators/disturbers appeared to have little effect on the harpacticoid copepod community inhabiting either seagrass leaves or the sediment. Treatment controls were adequate in design. Abundance of Harpacticus uniremis, Tisbe cf. furcata and Zaus aurelii generally did not increase in the exclusion treatment relative to the control and shifts in species abundance and composition of the harpacticoid community did not occur. It appears that juvenile salmonids and large epibenthic predators/disturbers in general, have little impact on the dynamics of harpacticoid copepod populations at this study site.
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