Crayfish threats to salmonids: A synthesis of current knowledge and research priorities

Large-scale assessments of threats to salmonids (i.e., salmon, trout, whitefish, and other relatives) have increased in recent decades; yet the role of crayfish—both native and nonnative—has received comparatively limited attention, and existing knowledge remains fragmented across the literature. This study addresses this gap by synthesizing current evidence of the negative effects of crayfish on salmonids across ontogenetic stages and identifying critical knowledge gaps to guide future research. The state of published knowledge regarding the impact of crayfish on salmonids is relatively disparate across taxonomies, geographies, and environmental contexts. Our review revealed 35 articles (comprising 60 crayfish-salmonid study outcomes) published over the last half century, involving 10 crayfish species (77% studying P. leniusculus, Faxonius propinquus or Faxonius virilis ) and 12 salmonid species (54% studying Salvelinus namaycush or Salmo trutta ). Studies have been predominantly laboratory (46%) and field (49%) experiments, conducted in northern and northwestern Europe and the Great Lakes region of North America. Half of the study outcomes examined the salmonid egg stage (51%), alevin (8%), fry (10%), juveniles (14%), adults (12%), and post-mortem carcasses (5%). Salmonids experience substantial mortality during early life stages, particularly from egg deposition through fry emergence. Salmonid eggs are vulnerable to predation by both native and nonnative crayfish, which act as active foragers as well as scavengers. Studies reported consumption rates of 0.1 to 9.3 eggs per individual per day, with rates varying with factors such as crayfish species, size, egg burial depth, egg density, substrate composition, and temperature. Although laboratory studies have provided valuable insights, their applicability to natural systems remains limited. Key knowledge gaps persist, particularly regarding the functional feeding responses of crayfish (i.e., egg consumption as a function of availability) and the interactions between environmental and biological factors that shape per capita effects on salmonids. Crayfish–salmonid interactions extend to fry and juvenile stages through both direct predation and competition for shelter and invertebrate prey, potentially constraining salmonid growth. Despite these plausible mechanisms, empirical studies are very scarce. Evidence for effects on adult salmonids is even more limited: correlative studies yield mixed results, and the role of crayfish in nutrient cycling via carcass scavenging remains poorly understood. By illuminating the complex, context-dependent nature of crayfish–salmonid interactions, we offer 10 priority research directions to inform future management and conservation efforts.