Ecological, behavioural and molecular aspects enabling invasive round goby translocation via boats

Aquatic invasive species are one of the biggest threats to biodiversity worldwide. Especially in freshwater ecosystems, the introduction of non-native species has led to many detrimental ecological and economic effects. Invasive fish belong to the taxa that exert the most severe impacts on aquatic ecosystems, and globally caused the decline or extinction of native species through predation or competition. Understanding and preventing ways of translocation is crucial to avoid further introductions of invasive fish. The round goby (Neogobius melanostomus) is one of the most notorious invasive fish species in European and North American waters. A commonly proposed way of translocation for round gobies is that they lay eggs on boat hulls, which can then be transported within or across water bodies. For example, translocation via recreational boats could help round gobies to spread into the ecologically and economically valuable lakes of Switzerland, which cannot be reached via ballast water transport or active dispersal. However, it is unclear how the bottom-dwelling round goby establishes contact with boats on the water surface. In this study, we investigate potential mechanisms of round goby translocation via recreational boats and derive measures to prevent it by determining which life stages act as propagules, how vector contact is established, and which mechanisms contribute to a successful translocation.
A key behavioural aspect enabling egg deposition on boat hulls is vertical habitat use. Round gobies readily use vertical harbor walls as habitat and use them as beachheads to reach boat hulls, potentially enabling egg deposition there. While practical evidence for round goby eggs on boat hulls is still missing, high breeding frequencies in spawning traps close to the surface confirms that nesting there is a plausible option. Round gobies using vertical habitat are larger and more generalist than those using the bottom substrate during the reproductive season. This differential habitat use by a more competitive subset of the population could help to explain the successful translocation history of round gobies. Another aspect that helps round goby eggs to survive adverse conditions during overland transport is their desiccation tolerance of up to 48 hours. Molecular mechanisms including an enhanced response to oxidative stress, adjustment in protein metabolism and changes in developmental processes contribute to the survival of prolonged desiccation.
The confirmed presence of a round goby larva inside a boat motor shows that there is a second possible mode of translocation via recreational boats. Round goby larvae drift in the water column at night, making their uptake via cooling water intake possible. A compilation of anecdotal reports of fish and other macrofauna inside of boat motor cooling systems from boating forums and the observation of regular presence of boats active after sunset confirms that this mode of translocation can accelerate in-water spread of invasive fish.
Understanding translocation mechanisms of invasive species is crucial for the prevention of their spread. The studies included in this work demonstrate the different ways in which recreational boats can contribute to the introduction of invasive fish into new water bodies and reveal mechanisms that enable vector contact and survival of transport. The results elucidate novel aspects of why certain species are successful invaders and can improve future risk models. The translocation of eggs laid on boat hulls and larvae in cooling water systems can be mitigated by different measures like adjustment of mooring conditions, thorough cleaning of potential nest sites on boats, drying of boats over several days, or emptying and flushing cooling systems before every transport. Applied in management campaigns and communicated to relevant stakeholders, the proposed measures can therefore improve the biosecurity of un-invaded waters.