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Fish stocking is a ubiquitous management measure in freshwater fisheries but it can induce important ecological impacts on communities and recipient ecosystems. Habitat enhancement might limit the ecological impacts of fish stocking by directly supporting biodiversity and helping to maintain ecosystem functioning. In the present, we used a mesocosm experiment to assess whether habitat enhancement, through the addition of coarse woody habitat (CWH) in the form of thin branches bundled or within an iron cage, can limit the impact of stocking of a predatory fish, juvenile largemouth bass Micropterus nigricans, on prey communities and ecosystem functioning. Results demonstrated that, prior to stocking, there was overall no significant effect of habitat enhancement on zooplankton communities and a strong effect on benthic macroinvertebrate communities that was associated with a decrease in the decomposition rate of organic matter in mesocosms containing caged-CWH. We also found that the stocking of juvenile largemouth bass significantly affected the structure of zooplankton communities while we did not observe significant effects on macroinvertebrate communities and ecosystem functioning. This effect on zooplankton community structure was compensated by habitat enhancement, notably when using caged-CWH. This study showed that habitat enhancement alone may not be sufficient to compensate the potential effects of predatory fish stocking, and that the choice of materials used to enhance habitats is fundamental in driving the efficiency.

Niche divergence resulting from coevolution is commonly believed to favour coexistence among competing species; however, recent investigations have demonstrated that an unexpected niche convergence can occur when native and non- native species coexist. Yet, our understanding of the ontogenetic characteristics of this niche convergence remains limited. In the present study, we quantified the stable isotope niche of native brown trout (Salmo trutta) in allopatry and sympatry with non-native brook trout (Salvelinusfontinalis) across four age classes. Our results demonstrated that brown trout displayed a stable isotope niche closer to brook trout in sympatry than in allopatry, which was likely driven by an increased consumption of terrestrial invertebrates by sympatric brown trout. Stable isotope niche overlap was the strongest for young-of-the- year individuals and the intensity of overlap between sympatric native brown trout and non-native brook trout decreased during ontogeny. These findings indicate that niche convergence between the species occur at the earliest age class of the native species and are maintained across ontogeny. La divergence de niche est une mecanisme qui favorise la coexistence des especes mais des travaux recents ont demontre qu'une convergence de niche pouvait se produire lorsque des especes natives et non-natives coexistaient. Ace jour, notre connaissance de la mise en place de cette convergence au cours de l'ontogenie reste neanmoins limitee. Dans cette etude, nous avons quantifie la niche isotopique de quatre classes d'age d'une espece native, la truite commune (Salmo trutta), vivant en allopatrie et en sympatrie avec une espece non-native, l'omble de fontaine (Salvelinusfontinalis). Nos resultats demontrent que la truite commune presente une niche isotopique plus semblable a celle des ombles de fontaine en sympatrie par rapport a sa niche isotopique en allopatrie et que cela est probablement cause par une plus forte consommation d'invertebres terrestres par les truites communes en sympatrie. Le chevauchement des niches isotopiques est le plus important pour les juveniles de l'annee, et l'intensite du chevauchement entre truites communes et ombles de fontaine en sympatrie diminue au cours de l'ontogenese. Ces resultats indiquent que la convergence de niches se produit des la toute premiere classe d'age de l'espece native et se poursuit durant toute la vie des individus., Introduction Biological invasions can provide new insights into our understanding of species coexistence because co-evolutionary history is lacking in novel communities resulting from human introductions of non-native species (Cadotte et [...]

Relationships between native and invasive species can modify trophic interactions in food webs and the diet of native species, leading to substantial changes in their trophic niches. We quantified the stable isotope niche of native amphibians (two species of tadpoles and two species of newts) and the invasive red swamp crayfish (Procambarus clarkii) in 18 ponds of an area invaded for more than 30 years. We tested whether crayfish presence and abundance explained variation in the size and position of the amphibians' stable isotope niches compared with proxies of pond resource availability and competition levels. Agile frog tadpoles (Rana dalmatina) had consistently low trophic positions, while tree frog tadpoles' niches (Hyla arborea) showed signs of an opportunistic diet. Newts (palmate newt (Lissotriton helveticus) and marbled newt (Triturus marmoratus)) had high trophic positions consistent with a predatory diet. Crayfish showed a high level of trophic variability, but their trophic niche never overlapped with the trophic niche of amphibians. Amphibian niche size and position were associated with amphibian density and pond canopy cover rather than with crayfish presence or abundance. This study suggests limited changes in amphibian diets in the presence of red swamp crayfish in ponds compared with results from experimental studies, suggesting that complex environmental conditions and the long time since invasion might limit trophic interactions between these native and invasive species. [ABSTRACT FROM AUTHOR]

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 deserve further attention. Dans les cours d'eau europeens, les truites brunes (Salmo trutta) indigenes se nourrissent principalement de proies aquatiques, mais consomment une plus forte proportion de proies terrestres quand elles sont en sympatrie avec des ombles de fontaine (Salvelinus fontinalis) non indigenes. Il s'agit d'un rare exemple de convergence de regimes alimentaires qui pourrait etre associee a des changements de l'activite nycthemerale et du motif de regroupement par les truites brunes sympatriques. Nous avons enregistre l'activite et les emplacements de truites brunes de deux origines et pour deux modes de concurrence (allopatrie et sympatrie, quatre combinaisons) placees dans des enclos doubles en riviere pendant 29 jours pour verifier ces hypotheses. Les truites brunes d'origine sympatrique ou placees en sympatrie etaient plus diurnes et regroupees que celles d'origine allopatrique ou placees en allopatrie. Des changements de l'activite nycthemerale des truites brunes mises dans un nouveau mode de concurrence se sont produits graduellement au fil de l'etude. Ainsi, les truites brunes presentent une forte souplesse comportementale en reaction au concurrent non indigene et peuvent reprendre un comportement allopatrique quand les ombles de fontaine sont retires du systeme. Ces ajustements comportementaux pourraient avoir des effets insoupconnes sur les reseaux trophiques et le fonctionnement d'ecosystemes, ce qui merite plus d'attention., Introduction Biological invasions have important ecological and evolutionary consequences on native biodiversity (Strayer et al. 2006; Ehrenfeld 2010; Lowry et al. 2013). One of the most common and pervasive effects [...]

Freshwater fishes are widely stocked to enhance existing stocks and create new fishery opportunities, but quantification of ecological effects of stocking on recipient communities is still limited. Here, we quantified recent stocking practices in geographically close gravel pit lakes with contrasting history and management, and measured effects of stocking on fish community structure (taxonomic and functional diversity and body‐size structure). Between 2011 and 2017, 50% of managers stocked fish into gravel pit lakes at least once. However, stocking density (1.6 kg.year−1.ha−1 to 907 kg.year−1.ha−1), stocking diversity, and species stocked (from rainbow trout‐dominated to cyprinid‐dominated) were highly variable. Stocking intensity and choice of stocked species were primarily driven by management objectives and lake size. Stocking intensity was associated with changes in the recipient fish community, with an increase in taxonomic and functional richness and nonlinear changes in community size spectrum. Our findings demonstrate that recent stocking practices can modulate ecological dynamics of fish communities in gravel pit lakes with important consequences on functional characteristics that should be incorporated into management practices. [ABSTRACT FROM AUTHOR]

Clarifying the mechanisms associated with the coexistence of invasive species is important to understand the overall impact of multiple invasions on recipient communities. Here we examined whether divergence or convergence in dietary niche occurred when invasive Lepomis gibbosus and Australoheros facetus coexist in Iberian streams. We used stomach content analyses to determine dietary niche composition, width, and overlap in allopatric and sympatric counterparts in the Lower Guadiana throughout the dry-season. The variations in dietary niche between pumpkinseed and the cichlid were consistent with predictions derived from the niche divergence hypothesis. Although there were no changes in the use of plant material from allopatry to sympatry in either species, sympatric pumpkinseed and the cichlid displayed marked shifts in the use of animal prey and a decrease in niche width relative to allopatric counterparts. Moreover, sympatric pumpkinseed and cichlid showed similar niche width but differed significantly in plant and animal prey use. Taken together these results suggest that divergence in dietary niches may play a role in mediating coexistence of multiple invaders in Iberian streams.