The influence of fluctuating ramping rates on the diets of small-bodied fish species of boreal rivers

The response of aquatic organisms to flow regulation at hydro facilities has become a key issue for the development of sustainable management practices intending to reduce the impact in regulated rivers. In previous studies, unrestricted ramping rates have been found to have a negative effect on invertebrate communities and shortened the length of the food web. The decrease in the length of the food web was inferred from a significant decrease in the difference between macroinvertebrates and fish delta N-13 signatures, equivalent to the loss of the one trophic level. A before-after-control-impact (BACI) design was applied to determine the possible effects of ramping rate restrictions on the diet of small-bodied fishes in a boreal river in northern Ontario. Stomach contents were identified to the taxonomic level of order. Using these data, abundance, diversity and taxa composition were calculated to yield an understanding of the alterations that occurred in the food web, and how fish species diets may have changed to compensate for the shift in the invertebrate trophic level. It was found that unrestricted ramping rates were associated with an increase in invertebrate abundance within the stomach contents. There was no effect from the change to unlimited ramping for both stomach content EPT abundance and species diversity, as both rivers followed a similar trend over time. After unrestricted ramping rates, a greater frequency of baseline taxa and a lower frequency of predatory macroinvertebrates were found within the altered river stomach contents compared to what was expected. This demonstrates that the diets of small-bodied fishes have changed to compensate for the alterations to the food web due to unrestricted ramping rates, and implies that ramping rates should be taken into consideration in the regulation of operating regimes on altered rivers.