Efficacy of exclusion periods for mitigating harm to freshwater fishes moving into a Great Lakes coastal wetland

Exclusion periods (i.e., periods when activities are not permitted near or in-water) are among the most commonly applied harm mitigation measures for freshwater fish and fish habitat. We used a 25 year dataset collected at a wetland entrance in Lake Ontario, Canada to assess the efficacy of coolwater (15 March – 31 May) and warmwater (01 May – 15 July) exclusion periods. When both are applied, all 16 evaluated fishes are well protected (i.e., >70% of their spawning run is protected); however, when only one period is applied protection declined, particularly for early-arriving warmwater fishes. If activities continued into the first few weeks of an exclusion period, risk increased for early arriving fishes. Interannual variation in peak arrival timing was high and the development of models that forecast arrival timing based on within-year conditions (e.g., water temperatures) is recommended. Such models would inform risk-based within-year decision making on whether activities can continue into an exclusion period. Overall, results support the continued application of exclusion periods as an effective measure for mitigating harm to fish and fish habitat.