Spring Floods and Fish Migration and Populations

By Tys Theijsmeijer, Senior Director of Ecosystem Stewardship Programs and Policy

Spring is the most intense month of activity at the Cootes Paradise Fishway as it’s spring spawning season for most of the fish species of the lake. To accommodate this period of fish migration, extra operating days occur in the April operation schedule. The Cootes Paradise Fishway, a HHRAP project managed by RBG, enters its 29^th year of operation and I have witnessed all of them firsthand. Early years of hope included both a positive transformation of marsh health, but also a big boast to the fish numbers, however since that time fish populations have suffered numerous setbacks while two Eurasian species have thrived – Goldfish and Rudd.

The Cootes Paradise Marsh – Spencer Creek system is a young fish producing supersized system and among the biggest on Lake Ontario. While the marsh is certainly transforming back to its historical self, the fishery rebirth has been an up and down process caused by new fish diseases and various sewage and contaminant spills. In addition the same time the urban runoff area continues to grow adding new water quality challenges. A new published collaborative report on local fish populations is now available for an in-depth dive into the story, and brief summary overview video is now available courtesy of the HHRAP website, presented by one of the reports lead authors.

The 2024 Fishway fish total count was the third lowest of the past 20 years, although still several times the first year, with the lower number perhaps the last echo of the 2018 Chedoke Creek sewage spill that killed much aquatic life. We look forward with new hope for 2025, supported by recent years young fish abundance. This March has so far been positive with the first species in, Rainbow Trout, passing through in numbers higher than recent years. Equally important is the Common Carp population ongoing decline, but with this sewage spill survivors population still exceeding 10,000 large fish in the bay.

What about the water level? If you’re a fish, the question is: how high or low will the lake’s water level be in 2025? Currently, the lake’s water level is trending notably below average. With most of the snowpack already melted, we should have a good sense of water levels by the beginning of April. For the fish, this melted snowpack has filled the rivers with a normal spring flood and flow. If sustained, these conditions should support successful spawning for river-spawning species.