American Eel (Anguilla rostrata) freshwater population trajectory in Canada

Abstract

Describing trends in American Eel (Anguilla rostrata) abundances has been tackled regularly at various locations across its range; however, describing a common trajectory for the whole panmictic population has been a challenge. As a step toward this broader challenge, 12 fishery-independent datasets from freshwater across the Canadian range were used to estimate a Canada-wide trajectory of American Eel relative abundance. These datasets averaged 29 years per time series (range: 14-65) over the 1952-2018 time period. Time series mostly tracked yellow eel (n = 10), with some elver (n = 1) and silver eel (n = 1). Each dataset was standardized to control for potentially confounding factors, and the standardized statistical models were used to generate a standardized time series of relative abundance. These standardized relative abundances were transformed to a common scale, and a Canada-wide trend was fit. Generally, datasets with more historical data had more negative year trends. The Canada-wide trend produced mean rates of decline of -0.021 to -0.046 per year since 1980. The model indicated a 100% likelihood that the Canadian freshwater population has declined since 1980 and a 69.2-100% likelihood that the decline was greater than 50%. Limiting data to the 2000-2018 years produced less negative trends that generally did not differ from zero. Results were robust to the data standardizations, inclusion or exclusion of individual datasets, and adjustments for the spatial distribution of data. Overall, this analysis shows that while American Eel freshwater abundance in Canada has been stable over the last two decades, significant declines preceded this time period and were not limited to the St. Lawrence basin.