Impact of an Invasive Crayfish (Faxionus limosus) Introduction on a Freshwater Mussel Population in Cape Breton Highlands National Park.

Abstract

Freshwater ecosystems are often disproportionately impacted by invasive species due to increased connectivity and exploitation by humans. Biological invasions negatively influence ecosystems, but also threaten species which may provision ecosystem services. Freshwater mussels (order Unionida) are pivotal components of aquatic ecosystems, linking trophic levels and moderating water quality through filtration. However, freshwater mussels are globally imperiled, with many populations facing extirpation. Within Cape Breton Highlands National Park (CBHNP), invasive spinycheek crayfish have become established in Freshwater Lake, a historically biodiverse lake with a population of Pyganodon mussels. Crayfish—often aggressive invaders—negatively influence health and functionality of freshwater ecosystems as they infiltrate various trophic levels of aquatic food webs. This project’s goal was to assess spinycheek crayfish influence on Freshwater Lake’s ecosystem using Pyganodon as a case study. Over time, living Pyganodon have become harder to detect within Freshwater Lake. During 2008 surveys, 79% of mussels detected were living, compared to only 41% in 2021 (p < 0.001). 2024 midden surveys showed that Pyganodon in Freshwater Lake were generally smaller than those in a healthy (i.e., lacking invasive crayfish) freshwater ecosystem, at 7.55 cm versus 8.24 cm (p < 0.001). Smaller Pyganodon may be preferentially targeted by crayfish in Freshwater Lake, which is supported by a lack of evidence of recruitment within the living Pyganodon population, but multiple individuals ≤ 5 years of age within middens. Pyganodon recruitment within Freshwater Lake may also be limited by an apparent decrease in fish-host availability, with finfish CPUE decreasing from 54.94 in 2008 to 13.54 in 2018. Environmental parameters such as temperature showed high variability since the spinycheek crayfish introduction, potentially influencing their behaviours and abundance. Further research should ascertain potential impacts of spinycheek crayfish on Freshwater Lake and its Pyganodon population under potential management regimes, including lack thereof.