Estimation de la capacité biotique du gaspareau (Alosa pseudoharengus) dans le bassin hydrographique de la rivière Skutik (Sainte-Croix) en fonction de l'habitat

Abstract

The anadromous alewife (Alosa pseudoharengus) of the Skutik (St. Croix) River watershed have been negatively affected by dams, overfishing, and pollution over the last several hundred years. The population of alewives decreased to as low as 900 adults observed migrating upstream at Milltown Dam near the head of tide in 2002. The productive capacity of alewife in the Skutik watershed has been estimated several different ways for different areas of the watershed, leading to ambiguity in the total carrying capacity of the watershed for alewife. The objectives here are to estimate alewife nursery area in the Skutik watershed and to provide a median estimate and range of carrying capacity. Alewife nursery area is estimated from shapefiles of waterbodies from the Canadian National Hydrographic Network and cross-referenced for accuracy against satellite imagery. In total, 423.957 km2 (104,762 acres) of nursery area were categorized as accessible or potentially accessible, and 13.77 km2 (3,403 acres) were categorized as artificially inaccessible, for a combined total of 438.218 km2 (108,286 acres). A median estimate and range of carrying capacity (51.4 mt/km2 , with an 80% confidence interval of 30.0 mt/km2 to 88.1 mt/km2 ) and spawning stock biomass in the absence of anthropogenic effects (SSB0) (48.7 mt/km2 , with an 80% confidence interval of 28.4 mt/km2 to 83.4 mt/km2 ) were applied to the total nursery area estimate for a carrying capacity of 22,533 mt and an SSB0 of 21,338 mt. These estimates of carrying capacity consider the size the population could grow to in the absence of anthropogenic effects, contrasting with historical estimates of productive capacity that are based on exploited populations. The estimates of carrying capacity and SSB0 here are based on estimated nursery area rather than population specific data. Should sufficient population-specific data become available, carrying capacity and reference points for the Skutik alewife population should be estimated and replace the values presented here.