Environmental factors important to high-latitude nearshore estuarine fish community structure

Abstract

Estuarine habitats exhibit spatial and temporal variability due to fluctuating hydrological inputs and heterogenous coastal formations. In high-latitude regions, such as the Gulf of Alaska, shifts in coastal hydrology may be further exacerbated by climate warming leading to structural responses in nearshore estuarine fish communities. To better understand relationships between physicochemical factors and fish community structure within high-latitude estuarine systems, we conducted systematic beach seine collections at multiple river mouth sites in a macro-tidal estuary located in the northern Gulf of Alaska (Kachemak Bay) during summer and fall of 2018. The effect of local physicochemical conditions (temperature, salinity, dissolved oxygen, turbidity), spatial factors (site, bay location, current type), and temporal factors (sampling date, month) were assessed in relation to community structure (taxonomic abundances). Juvenile fishes made up a vast majority of the community (99% of total catch) confirming the importance of estuaries as nursery habitats. Variability in fish community was best explained by the combined effects of site and month. Bay location (inner versus outer bay) and current type (low versus high water speeds) represented intermediate spatial factors (broader than site-level) that also had significant effects on community structure. Month and bay location were characterized by spatiotemporal gradients in dissolved oxygen and turbidity, respectively. Juvenile Pacific herring (Clupea pallasii) were associated with more turbid, less saline conditions found in the inner bay, while saffron cod (Eleginus gracilis) were associated with less turbid, more saline conditions of the outer bay. Juvenile Pacific salmon (Oncorhynchus spp.), starry flounder (Platichthys stellatus), and crescent gunnel (Pholis laeta) were associated with low current sites that were not well-defined by any measured physicochemical factor. Adult and juvenile Pacific sand lance (Ammodytes personatus) were good indicators of both the inner bay and high current sites. Temporal patterns in fish abundances confirmed the findings of prior studies in the area - juvenile salmonids appeared early, followed by juvenile gadids, and then sand lance later in the fall. Overall, variability in fish community structure was better explained by overarching spatial and temporal factors rather than by the local physicochemical conditions. These results point to broader scale coastal shifts as potential mechanisms driving changes in community structure within similar systems in this region.