Insights for No Net Loss Policy in Practice: An empirical study of juvenile Pacific salmon use of offset habitats in the Fraser River Estuary

Abstract

The Fraser River Estuary (FRE) provides critical habitat for juvenile Pacific salmon (Oncorhynchus spp.), offering shelter, transition zones, and nutritional resources during migration. To ensure no net loss of fish habitat (NNL), s. 35(2) Fisheries Act authorizations for the harmful alteration, destruction, or disruption of fish habitat (HADD) require that habitat loss be balanced via actions like offsetting (R.S.C., 1985, c. F-14). Few studies have quantitatively analyzed fish use of created offset habitats in the FRE, meaning the effectiveness of these marshes in supporting Pacific salmon remains uncertain. This study evaluated juvenile salmon use of offset and reference habitats in the FRE during the 2023 outmigration period. From April to July, 9,636 juvenile salmon were captured at 23 marsh sites (13 offset and 10 reference). Generalized Additive Models (GAMs) assessed salmon abundance in relation to site type (offset vs. reference) and Julian day.

Results showed reference sites supported 3.05 times more salmon per sampling occurrence (p < 0.001). Seasonal peaks and salmon species diversity were also greater in reference sites. These results were reflected at the species-specific level, where reference sites hosted over three times more chum (O. keta), Chinook (O. tshawytscha), and sockeye salmon (O. nerka) than offset sites. Findings suggest that offset habitats fail to meet the expected long-term productivity standards, raising ecological, social, and economic concerns. This misalignment with NNL policy underscores the need for enhanced monitoring, improved offset design, and adherence to the mitigation hierarchy to ensure fish habitat protection policies achieve their intended ecological outcomes.