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Stable isotope analyses offer a useful means for quantifying ecological niche dimensions, though few studies have examined isotopic response of an ecological community with respect to resource gradients such as fluctuations in prey availability. Stable carbon and nitrogen isotopes were measured for Lake Michigan salmonines and their prey collected from 2014 to 2016. Bayesian ellipse and mixing model analyses were used to quantify isotopic niche characteristics and diets, respectively, among species and years. During the 3-year study period, abundance and size structure of preferred alewife (Alosa pseudoharengus) prey changed substantially and offered an opportunity to explore predator isotopic niche response and diet shifts along a prey resource gradient. Results suggested increased reliance on alewives, especially small alewives, over the study period and were consistent with greater availability of this prey. However, differential use of alewife size classes and alternative prey sources by salmonine predators was apparent, which suggested possible resource partitioning. Characterization of ecological niche overlap using stable isotopes likely requires consideration of shared resource availability as well as specific prey and habitat preferences. Si les analyses d'isotopes stables constituent une approche utile pour quantifier les dimensions de niches ecologiques, peu d'etudes se sont penchees sur la reaction isotopique d'une communaute ecologique en fonction de gradients de ressources tels que les fluctuations de la disponibilite de proies. Des isotopes stables du carbone et de l'azote ont ete mesures pour des salmonines du lac Michigan et leurs proies, preleves de 2014 a 2016. Des analyses d'ellipses bayesiennes et de modeles de melange ont ete utilisees pour quantifier les caracteristiques des niches isotopiques et les regimes alimentaires, respectivement, des differentes especes et d'une annee a l'autre. Durant la periode de trois ans qu'a dure l'etude, l'abondance et la structure de tailles de la proie de predilection, le gaspareau (Alosa pseudoharengus), ont considerablement change, ce qui offre l'occasion d'examiner la reaction des niches isotopiques et les changements des regimes alimentaires des predateurs le long d'un gradient de ressources de proies. Les resultats indiqueraient l'augmentation du recours aux gaspareaux, particulierement les petits gaspareaux, durant la periode d'etude et concordent avec une disponibilite accrue de ces proies. Des variations de l'utilisation de gaspareaux de differentes classes de tailles et d'autres sources de proies par les predateurs salmonines ressortent toutefois, qui indiqueraient un possible partage differentiel des ressources. La caracterisation du chevauchement des niches ecologiques a l'aide d'isotopes stables necessite probablement la prise en consideration de la disponibilite des ressources partagees, ainsi que des preferences en matiere de proies et d'habitats des differentes especes. [Traduit par la Redaction], Introduction A growing number of studies have attempted to use stable isotope analyses to quantify multiple dimensions of an organism's ecological niche (e.g., Kornis et al. 2020; Layman et al. [...]

The Northern Sunfish (Lepomis peltastes Cope, 1870) is threatened in New York state, USA, but this was not the case before 1940 when the NY Biological Survey documented the species at scattered, specialized habitats in six watersheds in the central and western parts of the state. After 1940 the historic populations could not be detected, but a new population was discovered in 1974 in lower Tonawanda Creek and the nearby Erie Canal. Northern Sunfish, and a few of their hybrids with other Lepomis species, were caught at these locations during irregular sampling through 2009, but no Northern Sunfish were caught after 2009. The objectives of our study were to: (1) Determine the extent of Northern Sunfish hybridization with other Lepomis species, and (2) Evaluate how well identifications of Lepomis species and their hybrids agreed among field keys, morphometric measurements and meristic counts, and genetic methods. In 2013, we collected Northern Sunfish (descended from fish captured in lower Tonawanda Creek from 2006-2009) from NY State Department of Environmental Conservation rearing ponds, plus wild Green Sunfish (L. cyanellus Rafinesque, 1819), Pumpkinseed (L. gibbosus Linnaeus, 1758), Bluegill (L. macrochirus Rafinesque, 1819), and suspected Lepomis hybrids from lower Tonawanda Creek. Ultimately, 91 fish were identified using field keys, morphometric-meristic analysis, and mtDNA and nuclear DNA analysis. Assuming genetic analysis provided accurate identification, we found 7 Bluegill × Northern Sunfish, 8 Bluegill × Pumpkinseed, 13 Bluegill × Green Sunfish, and 3 Green Sunfish × Pumpkinseed hybrids in our sample (female parent listed second in these crosses). Keyed and morphometric-meristic identifications did not differ in accuracy and averaged 81% of genetic identification accuracy. After Northern Sunfish stocking (not in our study area) and sampling from 2008 to 2018 in several watersheds with appropriate habitat and no recaptures after 2014, we conclude that the Northern Sunfish is extirpated in western New York state. While populations of Northern Sunfish (Lepomis peltastes) existed in several New York state watersheds before 1940, only one, discovered in 1974, persisted in small areas of lower Tonawanda Creek and the nearby Erie Canal. Despite high effort, no Northern Sunfish were captured after 2009 in the places they had occupied since 1974. Because many Northern Sunfish, all descended from wild fish in lower Tonawanda Creek and the Erie Canal, exist in two New York state hatchery ponds, we recommend attempting restoration in their former habitat by stocking. [ABSTRACT FROM AUTHOR]

Despite long-term efforts to restore lake trout (Salvelinus namaycush) populations in the Great Lakes, they continue to experience insufficient recruitment and rely on hatchery programs to sustain stocks. As lake trout reproductive success has been linked to diets, spatial heterogeneity in diet compositions is of interest. To assess spatial components of adult lake trout diets, we analyzed stomach contents and fatty acid profiles of dorsal muscle collected throughout Lake Michigan and along Lake Huron's Michigan shoreline. Lake trout from Lake Huron were generally larger in both length and mass than those from Lake Michigan. However, lake trout from Lake Michigan varied more in size based on depth of capture with smaller fish being caught more in deeper set nets. Fatty acids and stomach contents indicated that alewife (Alosa pseudoharengus) were consumed more in western Lake Michigan in contrast with round goby (Neogobius melanostomus) along the eastern shoreline. Conversely, in Lake Huron, lake trout primarily consumed rainbow smelt (Osmerus mordax). These results indicate that diet compositions of lake trout populations are relatively plastic and offer new insights into within-basin heterogeneity of Great Lakes food webs. Malgre des efforts a long terme visant a retablir les populations de touladis (Salvelinus namaycush) dans les Grands Lacs, cette espece continue de connaitre un recrutement insuffisant et de dependre des programmes d'ecloserie pour en soutenir les stocks. Comme le succes de reproduction des touladis a ete relie au regime alimentaire, l'heterogeneite spatiale de la composition des regimes est un sujet d'interet. Pour evaluer les aspects spatiaux des regimes alimentaires des touladis adultes, nous avons analyse les contenus stomacaux et les profils d'acides gras de muscles dorsaux preleves a la grandeur du lac Michigan et le long de la rive du lac Huron situee au Michigan. Les touladis du lac Huron etaient generalement plus grands tant sur le plan de la longueur que du poids que ceux du lac Michigan. Cependant, la taille des touladis du lac Michigan variait davantage en fonction de la profondeur de capture, les poissons plus petits etant captures en plus grands nombres dans les filets tendus a plus grande profondeur. Les acides gras et les contenus stomacaux indiquent que les gaspareaux (Alosa pseudoharengus) etaient consommes davantage dans la partie ouest du lac Michigan, alors que le long de la rive est, il s'agit plutot des gobies a taches noires (Neogobius melanostomus). En revanche, dans le lac Huron, les touladis consommaient principalement des eperlans arc-enciel (Osmerus mordax). Ces resultats indiquent que la composition des regimes alimentaires des populations de touladis est relativement plastique et ils fournissent de nouveaux renseignements sur l'heterogeneite intra-bassin des reseaux trophiques des Grands Lacs. [Traduit par la Redaction], Introduction Lake trout (Salvelinus namaycush) is a species of special concern throughout the Laurentian Great Lakes. Lake trout were once highly abundant, with annual harvests in Lake Michigan ranging from [...]

Accurate diet estimation has long been a challenging issue for researchers investigating predators because of constraints associated with stomach content analyses. Fatty acid signature analysis offers an alternative avenue to study long-term diet trends in consumers. Despite the wealth of experiments involving fatty acids of fish and their diets, few have evaluated quantitative fatty acid signature analysis (QFASA) with fish consumers. To this end, we fed juvenile lake trout (Salvelinus namaycush), round goby (Neogobius melanostomus), and yellow perch (Perca flavescens) various invertebrate species and back-classified each predator to its respective prey using only fatty acids. Estimates were highly accurate when metabolism of diets was natively accounted for by using fatty acid profiles of predators fed known diets as the 'prey library'. While highly accurate results were obtained, accounting for each predator-prey relationship limits the use of QFASA to predators that consume a limited number of species. We call for specific knowledge as to how fatty acid profiles reflect each predator-prey interaction before attempting to use fatty acids to quantify a consumer's diet. Only after incorporating such data will QFASA provide an accurate view of individual's diets when stomach content data are not available or are invalid. L'estimation exacte des regimes alimentaires a longtemps constitue un defi pour les chercheurs s'interessant aux predateurs, en raison de contraintes associees a l'analyse du contenu stomacal. L'analyse de la signature des acides gras offre une nouvelle avenue pour l'etude des tendances d'alimentation a long terme chez les consommateurs. Bien que de nombreuses experiences aient porte sur les acides gras de poissons et l'alimentation de ces derniers, peu d'entre elles ont evalue la valeur de l'analyse quantitative de la signature des acides gras (AQSAG) pour l'etude des poissons consommateurs. A cette fin, nous avons nourri des touladis (Salvelinus namaycush), gobies a taches noires (Neogobius melanostomus) et perchaudes (Percaflavescens) juveniles de diverses especes d'invertebres et associe retrospectivement chacun de ces predateurs a sa proie respective en n'utilisant que les acides gras. L'exactitude des estimations ainsi obtenues etait tres grande quand le metabolisme des regimes alimentaires etait integre de maniere native en utilisant les profils d'acides gras de predateurs nourris de regimes connus constituant une ' bibliotheque de proies '. Si des resultats tres exacts sont obtenus, l'integration de toutes les relations predateur-proie limite l'utilisation de l'AQSAG aux predateurs qui consomment un nombre limite d'especes. Nous postulons qu'une connaissance ciblee de l'incidence de chaque interaction predateur-proie sur les profils d'acides gras est necessaire avant d'essayer d'utiliser les acides gras pour quantifier le regime alimentaire d'un consommateur. Seulement apres integration adequate de telles donnees sera-t-il possible d'obtenir un portrait exact du regime alimentaire d'un individu grace a l'AQSAG quand des donnees sur le contenu stomacal ne sont pas disponibles ou sont invalides. [Traduit par la Redaction], Introduction Accurate quantitative diet estimation of aquatic predators remains a pervasive challenge to resource managers. Stomach flushing or dissection have provided information on interspecific competition (Jacobs et al. 2010), intraspecific [...]

Lipid concentration and fatty acid composition of common prey species or taxonomic groups from four distinct regions of Lake Michigan were quantified (n = 894). We used a combination of parametric and nonparametric statistics to assess the differences in fatty acid signatures (FAS) among species and to evaluate intraspecies variation relative to interspecies variation in FAS. Discriminant function analysis performed on 13 species or taxa groups using the 18 most abundant fatty acids revealed clear separation among taxa, with overall classification success reaching 89%. Species were readily distinguished based on their overall fatty acid profile in spite of intraspecies variation (temporal, regional, and size-related). Among species sampled, pelagic and benthic clusters were formed based on the degree of fatty acid profile similarity. In alewife (Alosa pseudoharengus) and round goby (Neogobius melanostomus), fatty acid compositions differed with fish size, sampling location, and temporal variation; however, the magnitude of these differences was small relative to differences between species. Our results demonstrate the utility of fatty acid signatures in studies of food webs in large freshwater ecosystems. This study is also a necessary first step toward development of mechanistic research that investigates the effects of variation in fatty acids within the prey base on top predators. Nous avons mesure les concentrations de lipides et la composition en acides gras des especes ou groupes taxonomiques communs de proies (n = 894) dans quatre regions distinctes du lac Michigan. Une combinaison de methodes statistiques parametriques et non parametriques nous ont servi a determiner les differences dans les signatures d'acides gras (FAS) entre les especes et d'evaluer la variation des FAS au sein des especes relativement a la variation entre les especes. Une analyse discriminante faite sur 13 especes ou groupes taxonomiques avec les 18 acides gras les plus abondants presente une nette separation entre les taxons avec un succes global de la classification de 89 %. Les especes peuvent etre clairement separees d'apres leur profil global d' acides gras malgre la variation intraspecifique (temporelle, regionale et reliee a la taille). Parmi les especes echantillonnees, il se forme des regroupements pelagique et benthique d'apres le degre de similarite de leurs profils d'acides gras. Chez le gaspareau (Alosa pseudoharengus) et le gobie a taches noires (Neogobius melanostomus), la composition en acides gras varie en fonction de la taille du poisson, du site d' echantillonnage et de la variation temporelle, mais l' importance de ces differences est faible par rapport aux differences entre les especes. Notre etude demontre l'utilite des signatures d' acides gras dans les etudes de reseaux alimentaires dans les grands ecosystemes d' eau douce. Notre travail est aussi un premier pas essentiel dans la mise au point d'une recherche mecaniste qui etudie les effets de la variation des acides gras dans le contingent de proies sur les predateurs superieurs. [Traduit par la Redaction], Introduction Lake Michigan's food web has been, and is, undergoing major changes because of various anthropogenic stressors. Successful establishment of invasive species can exert both top-down and bottom-up effects with [...]

In vitro studies of brush-border intestinal transport of dipeptides and cytoplasmic hydrolysis in fish suggest that these processes could be key mechanisms in the absorption and utilization of nutrients for growth. However, in vivo experimentation to study the nutritional importance of these processes was needed. We compared three dietary formulations based on free, peptide and protein sources of amino acids. Our results were the first to show that a synthetic dipeptide (PP)-based diet could support growth in the early stages of ontogenesis of a teleost fish, rainbow trout (Oncorhynchus mykiss), whereas a free amino acid (FAA)-based diet failed. We found that fish fed an FAA-based diet had an increased rate of ammonia excretion [1.78 [+ or -] 0.19 mmol N[H.sub.3]-N/(kg body wt*h)], compared with fish fed a PP-based diet [1.25 [+ or -] 0.07 mmol N[H.sub.3]-N/(kg body wt*h)], suggesting that deamination is involved in the metabolism of dietary FAA. Teleost fish are known to obtain a high proportion of total energy from protein, compared with higher vertebrates. However, we found that feeding trout alevins a PP-based diet increased postprandial oxygen consumption for 2 to 24 h, whereas other treatments decreased 24-h postprandial metabolism. This may indicate that peptide metabolism is less efficient than protein metabolism. Juvenile rainbow trout differed from alevins in their response to FAA- and PP-based diets. These observations strongly suggest that intestinal dietary peptide transport and hydrolysis could support protein synthesis and growth in vertebrates that respond poorly to FAA-based diets. We conclude that nutrient administration may be improved by manipulating dietary peptide composition and peptide/protein ratios, leading to better utilization of synthetic peptides, with nutritional and therapeutic implications for all vertebrates. KEY WORDS: * peptides * free amino acids * rainbow trout