A temporal comparison of nearshore fish communities in Hamilton Harbour and the Bay of Quinte areas of concern (1988–2021)

Lake Ontario and, in particular, areas across the western basin have been subjected to intense anthropogenic stressors. In response, Areas of Concern (AOC) were designated to aid in rehabilitation efforts to reduce nutrient loading and improve aquatic habitat. Here we use fish community metrics (e.g., Index of Biotic Integrity (IBI)) as well as species occurrence and catch modelling to compare trends in fish population condition over 30 years between Hamilton Harbour (HH) and the Bay of Quinte (BoQ). We found clear spatial differences within HH and limited evidence for improvement in nearshore fish populations. The few exceptions were declines in non-native species catch and richness and total offshore species catch, driven primarily by declines in common carp (Cyprinus carpio) and alewife (Alosa pseudoharengus). Similar declines in non-native species richness were evident in the BoQ, but IBI and fish population metrics, including proportion of piscivore biomass and native species catch and richness, were always greater in BoQ. In contrast, several metrics and species in HH showed recent declines (following 2012) that rolled back improvements observed in the early 2000 s. Such HH-specific changes suggest that local (e.g., ongoing anthropogenic disturbance, novel invasive species, and minimal changes in habitat supply) rather than regional factors are limiting recovery of the fish community.