Interspecific association of brown trout (Salmo trutta) with non-native brook trout (Salvelinus fontinalis) at the fry stage

The introduction of non-native brook trout (Salvelinus fontinalis) in Europe has led to displacement and decreasing populations of native brown trout (Salmo trutta). Some studies have found that brown trout shift to a diet niche similar to brook trout when the two species live in sympatry, which conflicts with the competitive exclusion principle. A change in feeding niche may be a sign of early interspecific association and social learning, leading to behavioral changes. As a first step to address this possibility, it is essential to assess the interspecific association between the species during the early ontogenetic life stages. In this study, we therefore assess whether juvenile brown trout associate with non-native juvenile brook trout to the same extent as with conspecifics by setting up two experiments: (i) a binomial choice test allowing visual and chemical cues to estimate the species specificity of group preference, and (ii) an association test without physical barriers to estimate the degree of association of a focal brown trout with a group of either conspecifics or heterospecifics. In experiment (1), we found that focal juvenile brown trout preferred to associate with the stimuli groups and did not discriminate either against conspecific or heterospecific groups. Furthermore, more active individuals showed stronger preference for the stimuli group than less active ones, regardless of species. In experiment (2), we found that brook trout groups had a tighter group structure than brown trout groups, and that focal brown trout showed stronger association with brook trout than with brown trout. These results indicate that brown trout may associate with brook trout at an early life stage, which would allow for interspecific social learning to occur. Future studies should look closer into causes and consequences of interspecific association and social learning, including potential effects on the phenotype selection in brown trout populations

Behavioural adjustment of fish to temporal variation in fishing pressure affects catchability: an experiment with angled trout

In passive fisheries, such as angling, the fishing success depends on the ultimate decision of a fish to ingest the bait, based on an individual’s internal state, previous experience and threat perception. Fish surviving capture by anglers are known to be less vulnerable, and catch rates usually quickly decline with increasing fishing effort. Previous theoretical models have thus suggested fishing closures as a means to recover responsiveness of fish to angling gear and maintain catch rates, yet empirical support remains limited. In a controlled replicated pond experiment, we evaluated the effects of temporal variation in fishing pressure on catch rates of rainbow trout (Oncorhynchus mykiss) by simulating short term fishing closures. These fishing closures increased catch rates and population-level catchability, by reducing threat perception at the population level and allowing released individuals to return to a vulnerable state. Our experimental results show that periodic fishing closures benefit catch-rates but at the risk of aggravating the likelihood of overharvesting

Socially induced stress and behavioural inhibition in response to angling exposure in rainbow trout

It is well known that fish can learn to avoid angling gear after experiencing a catch‐and‐release event, that is, after a private hooking experience. However, the possible importance of social information cues and their influence on an individual's vulnerability to angling remains largely unexplored, that is, social experience of a conspecific capture. The effects of private and social experience of hooking on the stress response of fish and subsequent catch rates were examined. Hatchery‐reared rainbow trout, Oncorhynchus mykiss (Walbaum), were implanted with heart rate loggers and experimentally subjected to private or social experience of hooking. Private and social experience of angling induced an increased heart rate in fish compared with naïve control fish. While private experience of hooking explained most of the reduced vulnerability to capture, no clear evidence was found that social experience of hooking affected angling vulnerability in fish that had never been hooked before. While both private and social experiences of angling constitute significant physiological stressors for rainbow trout, only the private experience reduces an individual's vulnerability to angling and in turn affecting population‐level catchability

Interspecific Interactions between Native Brown Trout and Invasive Brook Trout: Insights into Behaviour and Morphology

The introduction of non-native species represents a global threat to ecosystems and biodiversity. In Europe, the introduction of the invasive brook trout (Salvelinus fontinalis) has led to species displacement and local extinction of native brown trout (Salmo trutta) populations. Additionally, several studies have documented a convergence in feeding niche, where brown trout in sympatry with brook trout utilise terrestrial insects to a larger extent. This conflicts with the competitive exclusion principle, as competition should increase divergence between species. In this thesis, I examine behavioural interactions between invasive brook trout and native brown trout at various life stages, and investigate the convergence in feeding niche and its possible effect on morphology and development. As the juvenile stage constitutes a major bottleneck for salmonid populations, we conducted two experiments assessing the association and territoriality between juvenile brown trout and brook trout, and the influence of inter- and intracohort competition between the species. The results showed that brown trout do not discriminate against either conspecific or heterospecific groups, and that brook trout had a tighter group structure than brown trout groups. Additionally, juvenile brown trout were competitively inferior against brook trout when contesting a territory and took longer to feed and spend more time further away in presence of an adult brook trout. Moreover, as terrestrial insects are more common during the day, we investigated whether the converging feeding niche in sympatry could be explained by differences in diel activity between sympatric and allopatric brown trout. Indeed, sympatric brown trout was more active during the day than allopatric brown trout and showed a stronger association towards other individuals in sympatry. Compared to allopatric brown trout, sympatric brown trout also had a head morphology more typical for drift feeding, suggesting an adaptation to forage on terrestrial insects. Furthermore, terrestrial and aquatic insects differ in relative content of certain omega-3 fatty acids, vital for development of neural tissues. Thus, we examined the relative contribution of aquatic and terrestrial prey in the diet and if this could affect brain volume. Here, we found that brain volume was negatively correlated with higher consumption of omega-3 deprived terrestrial prey. Collectively, the results show that invasive brook trout have a major impact on native brown trout at an early life-stage and that the change in feeding niche may affect development of neural tissues in brown trout. As most studies have focused on the direct competition between brown trout and brook trout, future studies should focus on direct and indirect effects on other species as well as ecosystem effects. Additionally, a management plan to eradicate or reduce brook trout populations in key ecosystems should be implemented in Sweden

Invasive brook trout disrupts the diel activity and aggregation patterns of native brown trout

In European streams, native brown trout (Salmo trutta) feed primarily on aquatic prey but consume a higher proportion of terrestrial prey in sympatry with non-native brook trout (Salvelinus fontinalis). This is a rare example of diet convergence that may be associated with changes in diel activity or aggregation pattern by brown trout in sympatry. We recorded the activity and positions of brown trout from two origins and in two competition modes (allopatry versus sympatry, four combinations) placed in replicated stream enclosures for 29 days to test these hypotheses. Brown trout originating from or placed in sympatry were more diurnal and aggregated than those originating from or placed in allopatry. Changes in the diel activity of brown trout placed in a novel competition mode occurred progressively throughout the study. Thus, brown trout show strong behavioral flexibility in response to the non-native competitor and can revert to allopatric behavior when brook trout is removed from the system. These behavioral adjustments may have unsuspected effects on food webs and ecosystem functioning, which deserves further attention.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Hook Avoidance Induced by Private and Social Learning in Common Carp

It is essential for fish to respond appropriately when faced with a threatening situation. Accordingly, fish are able to reduce predation risk through learning. In addition to privately learned experiences, fish can acquire social information about a threat by observing the response of conspecifics and use such public information to adapt future behavior through learning. It is unclear if social learning can also influence the behavioral response of fish when faced with human-induced threats in the form of angling. Using an experimental approach in the laboratory, we examined the influence of private (i.e., direct experience of hooking) and social information on angling vulnerability in Common Carp Cyprinus carpio—a species regularly exposed to catch-and-release angling. Compared with control groups, individuals with direct or social experience of catch-and-release angling expressed significantly elevated hook avoidance behavior during a short-term vulnerability assessment hours after a catc