Shipping traffic, salinity and temperature shape non-native fish richness in estuaries worldwide

Non-native species threaten biodiversity conservation and ecosystem functioning. Management at early-invasion stages can prevent ecological and socioeconomic impacts, but rely on the identification of drivers of non-native species occurrence at distinct scales. Here, we identify environmental and anthropogenic correlates of non-native fish richness across estuaries worldwide. We performed model selection using proxies of colonization pressure, habitat availability and connectivity, anthropogenic disturbance and climate, to assess the primary mechanisms underlying non-native species occurrence. Latitudinal and guild-related trends in non-native occurrence were also investigated using species thermal and salinity affinities. Data retrieved from a literature review revealed 147 non-native fish species in 147 estuaries worldwide. Shipping traffic, salinity (minimum and range values) and temperature (minimum value) were the main predictors of non-native fish richness. Hotspots of non-native species were under heavy levels of shipping traffic, had higher salinity (both minimum and range values) and colder waters. We also found evidence of thermal limits to species' geographic area of introduction. Latitude of invaded estuaries was negatively correlated with species' minimum, mean , maximum thermal affinities , positively correlated with thermal affinity ranges. Most non-native species recorded in estuaries were freshwater, but their minimum salinity affinities ranged from 2 to 35 pss. Moreover, species within marine guilds were mostly stenohaline and showed affinity for minimum salinities around 20-30 pss, which may be related to the positive relationship between non-native richness and estuary's increased salinity. Our results indicate that colonization pressure, disturbance (as result of multiple shipping impacts) and habitat filtering are the primary mechanisms underlying non-native fish richness in estuaries, contributing to the development of management strategies targeting early-invasion stages. Matching climate between native and non-native ranges was particularly important for predicting introductions at the global scale, whereas local fluctuations in salinity likely drove non-native richness in response to increased habitat availability.

Automated summary

Non-native species pose a threat to biodiversity and ecosystem functioning in estuaries. This study found that shipping traffic, salinity, and temperature were the main factors influencing the occurrence of non-native fish species in estuaries globally. Hotspots of non-native species were located in areas with high shipping traffic, higher salinity, and colder waters. The study also revealed that there were thermal limits to the geographic range of introduced species and a positive relationship between non-native richness and estuary salinity. These findings contribute to understanding the mechanisms underlying non-native fish richness in estuaries and can aid in the development of management strategies.