Mitigation as conservation: evaluating freshwater mussel translocations in Canada

Freshwater mussels (Unionidae) are among the most imperiled animal groups globally, with declines driven by habitat loss, pollution, and invasive species. In many regions, including Canada, mitigation translocations (i.e., the relocation of mussels ahead of in-water construction) are used to prevent immediate harm, particularly where threatened species occur. However, their effectiveness beyond the short-term remains poorly evaluated. We synthesized data from 11 translocation events in southern Ontario rivers, comparing displaced mussels from the prescribed search area, resident populations at relocation sites, and mussels from control sites across standard monitoring intervals (initial, 1-month, 1-year, and 2-years). The proportion of threatened species remained stable over time, with no evidence of disproportionate declines. Detection of tagged mussels declined rapidly within the first month across all groups, followed by more gradual decreases; observed mortality remained low, including limited first-month mortality in displaced (0.74%) and resident (0.23%) populations and none at control sites, consistent with handling effects. Community composition shifted significantly between initial and 2-year surveys across all groups, including controls, indicating assemblage change was not unique to relocated populations. These findings support mitigation translocation in reducing acute construction-related mortality while underscoring the need for improved long-term monitoring, standardized reporting, habitat assessment, and community involvement to strengthen evaluation and long-term recovery outcomes.