Modelling forage fish species distribution in the Canadian Salish Sea

Forage fish are small, schooling fish that play a vital role in marine ecosystems, serving as a primary food source for seabirds, marine mammals, and commercially important species such as salmon. Despite their ecological importance, forage fish distributions remain understudied in the Canadian Salish Sea. Improved knowledge of their habitat uses and population dynamics is essential for effective conservation and management. In this study, a predictive geospatial model was developed to identify and map forage fish species “hot-spots” in the Canadian Salish sea. The model was constructed using 2,629 observations of six common forage fish species taken from Fisheries and Oceans Canada’s juvenile Pacific salmon survey, spanning 2000-2023. Discussions with a panel of local experts were then conducted to identify 18 environmental variables that were likely to influence the distributions of the forage species. Forage fish occurrence probabilities were estimated using a stacked ensemble approach which combined Neural Network, Generalized Linear Mixed Effects, and XGBoost models. The stacked ensemble achieved an average AUC of 0.73 across a 3-fold cross-validation, indicating strong overall ability to distinguish between presence and absence. Predicted presences were accurate 98% of the time (precision), and the model successfully detected 72% of all true presences (recall). Wind-driven surface current speeds, dissolved organic nitrogen (DON), and mesozooplankton biomass were found to be the most important predictor variables. A predictive map of forage fish hotspots was generated and reviewed, providing insight into the spatial distribution of the six species. Hotspots were identified around the Fraser River delta, along the southern coast of Vancouver Island, and throughout the Gulf Islands. Generally, higher probabilities were associated with inlets, such as Desolation Sound. The lowest probabilities of occurrence were associated with deep waters in the central Strait of Georgia.