Environmental effects related to the local absence of exotic fish

Given the extent of biological invasions in industrialized countries, our understanding of the determinants of overall patterns of biological invasions could gain most from consideration of why exotic species are absent from some areas, rather than from distribution patterns of exotic species. Fish communities were sampled at 381 sites representing 221 rivers in the Adour-Garonne stream system (116 000 km², SW France). Very few rivers were not colonized by exotic fish species, however, on a local basis, only 33% of the sampling sites hosted exotics. Using General Linear Modelling, we found that patterns of exotic fish (occurrence, number of species, proportion within assemblage) responded to both land-use and physical variables, whereas patterns of native fish only responded to the local meso-scale characteristics of each stream reach from headwaters to mouth. All fish communities were susceptible to invasion regardless of native species richness, and higher native species richness did not decrease the opportunity for establishment by exotic species. The likelihood that exotic fish are absent primarily increased with elevation and with lower human influence upon the land cover, while human-impacted landscapes (agricultural and urban areas) were more likely to host exotic fish and higher numbers of exotic species. In light of urban and agricultural development, our ability to detect responses of exotic species to landscape alterations using a combination of simple physical and land cover variables exemplifies a cost-effective technique for assessing areas at greater invasion risk in large stream systems

Dietary breadth and trophic position of introduced European catfish Silurus glanis in the River Tarn (Garonne River basin), southwest France

Although being a widely introduced and successfully established species, the European catfish Silurus glanis L. (the world’s third largest freshwater fish) remains poorly studied in its introduced areas. Here we studied the trophic ecology of non-native European catfish in a large river system in south-western France using stomach content and stable isotope analyses (SIA). We used fin samples for SIA of catfish and hence tested the validity of using fin tissue as a proxy for muscle in SIA. The mean δ15N and δ13C values analysed from fin tissues did not differ from those analysed from muscle tissue and reflected strong and consistent relationships (r2 = 0.95 for carbon and r2 = 0.98 for nitrogen). The δ15N values varied almost 5‰ among the analysed catfish individuals, while δ13C values varied >5‰. Total length of these catfish ranged from 200 to 2240 mm and was correlated with δ15N and especially with δ13C values. Although catfish length and δ15N values were positively correlated, the mean trophic positions of catfish increased only slightly from smaller individuals to larger ones (4.3 to 4.7). However, larger catfish were considerably 13C-enriched in their δ13C values compared to smaller individuals and had up to 4‰ higher δ13C values than their expected aquatic prey. This might indicate frequent consumption of mammals and/or non-aquatic birds by the larger sized individuals, which were found in the catfish stomachs

Local extinction and colonisation in native and exotic fish in relation to changes in land use

Distribution patterns of many native and exotic fish species are well documented, yet little is known about the temporal dynamics of native and exotic diversity in relation to changes in land use. We hypothesised that colonisation rates would be higher for exotic fish species and that extinction rates would be higher for native species in large stream systems. We also predicted that cold-water species would be more impacted than thermally tolerant species. To test these hypotheses, we used generalised linear mixed models to compare changes in native and exotic fish species richness over 10 years in a French drainage basin subjected to landscape alterations. Exotic fish were more susceptible to local extinction than the native ones. Extinction was greater among cold-tolerant species and at higher elevations. Colonisation by exotic species was higher at lower elevations. Although a decade of expanding urbanisation affected fish colonisation, agricultural lands experienced higher extinction rates. In the context of global changes in land use and population pressure, our study suggests that the temporal dynamics of fish diversity are driven by landscape alterations as well as by the thermal tolerance of species

Freezing and chemical preservatives alter the stable isotope values of carbon and nitrogen of the Asiatic clam (Corbicula fluminea)

We tested the impacts of most common sample preservation methods used for aquatic sample materials on the stable isotope ratios of carbon and nitrogen in clams, a typical baseline indicator organism for many aquatic food web studies utilising stable isotope analysis (SIA). In addition to common chemical preservatives ethanol and formalin, we also assessed the potential impacts of freezing on δ¹³C and δ¹⁵N values and compared the preserved samples against freshly dried and analysed samples. All preservation methods, including freezing, had significant impacts on δ¹³C and δ¹⁵N values and the effects in general were greater on the carbon isotope values (1.3-2.2% difference) than on the nitrogen isotope values (0.9-1.0% difference). However, the impacts produced by the preservation were rather consistent within each method during the whole 1 year experiment allowing these to be accounted for, if clams are intended for use in retrospective stable isotope studies

Contribution of anadromous fish to the diet of European catfish in a large river system

Many anadromous fish species, when migrating from the sea to spawn in fresh waters, can potentially be a valuable prey for larger predatory fish, thereby efficiently linking these two ecosystems. Here, we assess the contribution of anadromous fish to the diet of European catfish (Silurus glanis) in a large river system (Garonne, southwestern France) using stable isotope analysis and allis shad (Alosa alosa) as an example of anadromous fish. Allis shad caught in the Garonne had a very distinct marine delta(13)C value, over 8 per thousand higher after lipid extraction compared to the mean delta(13)C value of all other potential freshwater prey fish. The delta(13)C values of European catfish varied considerably between these two extremes and some individuals were clearly specializing on freshwater prey, whereas others specialized on anadromous fish. The mean contribution of anadromous fish to the entire European catfish population was estimated to be between 53% and 65%, depending on the fractionation factor used for delta(13)C

A global perspective on the trophic geography of sharks

Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale compilations of stable isotope data combined with biogeochemical modelling generate hypotheses regarding animal behaviours that can be tested with other methodological approaches.This research was conducted as part of C.S.B.’s Ph.D dissertation, which was funded by the University of Southampton and NERC (NE/L50161X/1), and through a NERC Grant-in-Kind from the Life Sciences Mass Spectrometry Facility (LSMSF; EK267-03/16). We thank A. Bates, D. Sims, F. Neat, R. McGill and J. Newton for their analytical contributions and comments on the manuscripts.Peer reviewe

Les poissons herbivores dans l'écosystème récifal des Antilles

Le problème de la dégradation des récifs coralliens est actuellement au centre de nombreuses recherches en écologie. Le présent travail apporte des résultats sur le rôle des poissons herbivores dans la dynamique des récifs et a permis d'évaluer leur importance pour la resilience des communautés benthiques récifales. Le peuplement de poissons herbivores a été étudié en plongée à l'aide de relevés visuels sur les platiers et pentes externes de Guadeloupe. Les données recueillies ont été analysées par le calcul de descripteurs synthétiques de la biodiversité, par des méthodes d'ordination et de groupement et à l'aide d'autocorrélogrammes. L'étude temporelle a montré que le peuplement de poissons herbivores varie au long de l'année sans suivre de modèle saisonnier et que des facteurs écologiques comme la houle, le vent ou la marée influencent le peuplement. L'étude de la variabilité spatiale à montré l'existence d'une structuration du peuplement de poissons herbivores en fonction de la profondeur, avec un peuplement de platier distinct de celui des pentes externes. Les variables environnementales influençant la répartition des poissons herbivores sont les peuplements algaux, la couverture corallienne, la profondeur et la protection des récifs. L'étude de la consommation en algues des poissons herbivores sur les récifs a révélé que ceux ci ont du mal à réguler la croissance des algues lorsqu'ils sont surpêchés. L'attraction des espèces envers certains faciès algaux (gazon algal,débris coralliens,macroalgues brunes et vertes calcifiées),évaluée par comptage des coups de dents données par des poissons herbivores, à montré que ceux-ci évitent les macroalgues brunes.POINTE A PITRE-BU (971202101) / SudocSudocFranceF

Boreal and Lusitanian species display trophic niche variation in temperate waters

Abstract Climate change has non‐linear impacts on species distributions and abundance that have cascading effects on ecosystem structure and function. Among them are shifts in trophic interactions within communities. Sites found at the interface between two or more biogeographical regions, where species with diverse thermal preferenda are assembled, are areas of strong interest to study the impact of climate change on communities' interactions. This study examined variation in trophic structure in the Celtic Sea, a temperate environment that hosts a mixture of cold‐affiliated Boreal species and warm‐affiliated Lusitanian species. Using carbon and nitrogen stable isotope ratios, trophic niche area, width, and position were investigated for 10 abundant and commercially important demersal fish species across space and time. In general, the niches of Boreal species appear to be contracting while those of Lusitanian species expand, although there are some fluctuations among species. These results provide evidence that trophic niches can undergo rapid modifications over short time periods (study duration: 2014–2021) and that this process may be conditioned by species thermal preferenda. Boreal species displayed spatial variation in trophic niche width and seem to be facing increased competition with Lusitanian species for food resources. These findings underscore the need to utilize indicators related to species trophic ecology to track the ecosystem alterations induced by climate change. Such indicators could reveal that the vulnerability of temperate ecosystems is currently being underestimated

Gut content metabarcoding reveals potential importance of fisheries discards consumption in marine fauna

peer reviewedFisheries discards have become a source of concern for the perennation of marine resources. To reduce discards, the European Union adopted a Landing Obligation under the reform of its Common Fisheries Policy. However, food web consequences of reducing discards remain uncertain since their degree and pathway of reintegration are understudied. We used multi-marker DNA metabarcoding of gut contents and an ecological network approach to quantify marine fauna reliance on discarded fish and functional importance of discard consumers in coastal fishing grounds. We show that potential discard consumption is widespread across fish and invertebrates, but particularly important for decapods, which were also pinpointed as functionally important. Potential discard consumption may represent up to 66% of all interactions involving fish prey in the reconstructed network. We highlight that discard reliance may be more important than previously assessed in some fishing areas and support functionally important taxa. While reducing discarding remains a conservation priority, it is crucial to understand discards reintegration in marine food webs to anticipate changes in the context of an ecosystem approach to fisheries management

Coupling gut content DNA metabarcoding and stable isotope analysis to reveal potential importance of fisheries discards consumption in marine fauna

peer reviewedFishing is considered one of the most important threat to marine ecosystems worldwide. One aspect of fisheries activities, that could significantly impact food web structure and ecosystem functioning, is fisheries discards (i.e. animals caught, but returned to the sea, dead or alive). The European Union adopted a discard ban under the reform of its Common Fisheries Policy. However, fishing has impacted marine food webs since ancient times, potentially supporting some species through discards consumption, and the environmental effects of reducing fishing discards remain uncertain since their degree and pathway of reintegration into marine food webs are understudied. We aimed to quantify the reliance of marine fauna on discarded fish at a community-wide scale in the Bay of Biscay (France) and the potential impact of discards reintegration on food web structure by using a combination of gut content metabarcoding and stable isotope analysis. Our results suggest that discarded fish consumption is widespread and potentially supporting functionally important taxa, reflecting in trophic position and bentho-pelagic reliance of some species. Our study highlights that dependence on discards by marine fauna may be more important than previously assessed and that, while reducing fisheries discards remains a conservation priority, it may trigger unanticipated effects potentially affecting entire communities