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The invertebrate connection : tracing the value of food subsidies from fishless headwaters to downstream fish populations Reiss, Aya Elaine


The role of ecosystem subsidies from fishless, headwater streams for downstream fish populations was investigated through multiple approaches in the coastal temperate rainforest of British Columbia. Terrestrial invertebrate samples were collected in two streams of differing canopy (broadleaf and coniferous) on a weekly basis to assess seasonal (spring to fall) terrestrial inputs to headwater streams. Thirty-four fishless headwater streams were also sampled for both drift and terrestrial input of invertebrates over two seasons (spring and summer) and two canopy types (broadleaf and coniferous) to quantify the magnitude of upstream subsidies at different times of year and from headwaters with different land-use histories. Potential fish growth response to headwater subsidies was examined both in a feeding experiment and through bioenergetics modeling. Juvenile cutthroat trout in experimental channels were fed one of seven diets (0-9% of their biomass) to determine if biomass accumulation was proportional to food availability. Bioenergetics modeling was used to perform a sensitivity analysis on parameters influencing fish growth and to validate experimentally-derived growth-response curves. Terrestrial invertebrate inputs increased significantly from May- September and a significant increase in total invertebrate export was observed from invertebrates in the drift from May-July. Quantification of subsidies revealed that streams running through broadleaf canopies exported greater, but not significant, numbers (mean = 1.73 individuals/m³) and biomass (mean = 1.37 mg/m³) of invertebrates than those adjacent to coniferous ones (mean = 1.49 individuals/m³ and 1.30 mg/m³ ). Drift abundance was made up of roughly 83% aquatic and 17% terrestrial invertebrates while biomass was composed of 51% aquatic and 49% terrestrial invertebrates. One-year-old cutthroat trout demonstrated significantly increased relative growth rates with greater food abundance. Sensitivity analysis of model parameters identified that variation in the ratio of terrestrial :aquatic prey caused the greatest change in consumer growth output response at high prey densities. Using the experimentally-derived fish growth response curve, measured headwater invertebrate drift densities, and drift densities reported in local fish-bearing streams, a 12-fold increase in relative growth rate of downstream consumers exposed to headwater resources was predicted. Findings from this study highlight the potentially important contributions of food resources from fishless headwater streams for downstream consumers.

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