American eel (Anguilla rostrata) prey consumption in the Upper St. Lawrence River, the Ottawa River and Eastern Lake Ontario: Stomach Contents in a Declining Population
In their peak abundance, the American eel, Anguilla rostrata, comprised 50% of the nearshore fish assemblage in the upper St. Lawrence River and Eastern Lake Ontario. Since the 1980s the population has faced rapid decline and is now classified as an endangered species in Ontario. This study used 1513 eels collected from Eastern Lake Ontario, the St. Lawrence River and the Ottawa River between 2008 and 2016, 880 of which had stomach contents that were the focus of this stomach content analysis. Variables affecting ingestion were assessed through statistical analysis and descriptive summaries. The main prey types ingested by eels were fish, crayfish and invertebrates. In the spring, 60% of stomachs had contents, 67% in the summer and 37% in the fall. The strongest seasonal correlation was observed in the increased abundance of crayfish in the summer, the season in which they do not moult. The composition of prey types in stocked eels and non-stocked eels was very similar. Stomachs of stocked eels were three times as full as those from non-stocked eels. This difference was attributed to variation caused by location. Stomach fullness varied based on location, the greatest influence was observed in the Ottawa River. The composition of the prey types remained constant throughout locations. Eels caught in hoop-nets had increased stomach fullness compared to those caught in electrofishing surveys. The length of eel had the most predominate influence on prey ingestion. Small eels consumed mainly invertebrates, transitioning with increasing size to feed primarily on crayfish and eels greater than 400mm in length fed most heavily on fish. The stomach contents demonstrated unique ecological interactions that were not previously observed. There are benefits associated with ecological services that a higher population of eels provide, positively impacts the ecological integrity of this system. The results of this study provided a clear understanding of American eel feeding ecology that can be used as a historical reference for long term monitoring.