Local habitat and regional oceanographic influence on fish distribution patterns in the diminishing kelp forests across the Aleutian Archipelago

Abstract

Fish distributions can be influenced by changes in their local habitat features and regional oceanographic conditions, both of which can occur at different spatial scales. Currently, the coastal waters throughout the Aleutian Archipelago are dominated by two discrete habitats types, kelp forests and urchins barrens, both of which span known U.S. biogeographic breaks near Buldir and Samalga Passes. These passes roughly divide the western Aleutians from the central Aleutians, and the central Aleutians from the eastern Aleutians, respectively, and have been suggested as important breaks for demersal fishes and deep marine fauna due to corresponding changes in ocean conditions. This study assessed how local habitat features compare to regional oceanographic conditions in how they influence nearshore fish assemblages throughout the Aleutian Archipelago. We hypothesized that these assemblages would differ markedly between the kelp forests and urchin barrens, and that local habitat features such as forest size and the abundance of understory kelps and turf algae are more important to assemblage structure than large-scale oceanographic conditions. To explore these hypotheses, 32 kelp forests and 24 non-forested urchin barren areas occurring on 15 Aleutian islands were surveyed for fish assemblage structure and habitat characters using scuba. Within the kelp forests, both the interiors (middles) and the margins (edges) of the forests were sampled for comparison. In general, fish assemblages did not significantly differ between kelp forests and urchin barrens in the western and central Aleutian Islands. Across all three regions, we found that of 26 habitat characters considered, five (density of the stipitate kelp Agarum, the percent bottom covers of understory foliose algae and encrusting coralline algae, and the availability of gravel and bedrock) together provided the best correlation with the observed variation in the fish assemblages, but this appeared to be due to the greater influence of these in the eastern Aleutians where the abundance of Agarum is greatest. However, the relationship between these habitat characters and fish assemblage structure was weak across the archipelago and not statistically significant in any of the regions. Implications of these analyses suggest that a further reduction in kelp forests may not impact fish assemblages in the western and central Aleutians. However, this is likely an artifact of these forests already being in a diminished state, with most of the kelp forests having been replaced by urchin barrens, and thus the impacts to the fish assemblages have presumably already occurred. However, this may be different in the eastern Aleutians where kelp forests remain more abundant and where the influence of Agarum appears strongest. In addition, while nearshore fish assemblages were not significantly different on the opposing sides of Buldir Pass, they were significantly different on the opposing sides Samalga Pass, suggesting that variation in large-scale oceanographic conditions around this pass were important drivers of differences in fish assemblages. However, if ocean properties, such as temperature and salinity, change around Buldir Pass in the future, then corresponding differences in the fish assemblages may develop.