Natural mortality is a poorly known aspect of fisheries biology, despite its importance in stock assessments and population analysis. Of the many potential sources of mortality and morbidity in fishes, the effect of internal parasites is perhaps the least studied. Intestinal parasites may inhibit nutrient uptake as well as stimulate an inflammatory response in fish. Intestinal parasites of several tropical pelagic elasmobranchs, including silky and night sharks (Carcharhinus falciformis and C. signatus), the pelagic stingray (Pteroplatytrygon violacea), and mesopelagic fishes including sailfin lancetfish (Alepisaurus ferox), oilfish (Ruvettus pretiosus), snake mackerel (Gempylus serpens), escolar (Lepidocybium flavobrunneum), and Atlantic pomfret (Brama brama) are described from the South Atlantic Bight in the western North Atlantic. Parasites recovered include cestodes, trematodes, acanthocephalans and nematodes. Total gastrointestinal parasite loads were compared against the size (both length and weight) of the host, showing no correlation in pelagic elasmobranchs and a small correlation in mesopelagic teleosts. Sex and parasite loads were also compared for elasmobranchs showing a weak correlation. Capture seasons were also compared, and other results of this research showed that the parasites in this study are not host-species specific. The observed parasite classes and total helminth loads are the first described for these five mesopelagic fishes and will serve as a baseline for further studies. Future research is suggested to ascertain if commercially valuable, co-occurring, co-existing pelagic fishes may also be at risk for similar intestinal parasite fauna and burdens.

Beaucoup de parasites à cycle complexe ont développé la capacité d’altérer le phénotype de leurs hôtes. Il est reconnu que ces modifications d’apparence ou de comportement sont responsables d’une augmentation de la probabilité de prédation de l’hôte intermédiaire par l’hôte définitif. Ce phénomène de manipulation parasitaire peut avoir de nombreuses conséquences à l’échelle de l’écosystème, modifiant les interactions entre les populations d’hôtes et bouleversant leur rôle écologique. Cependant, les parasites manipulateurs sont peu étudiés sur le plan écologique, et l’effet de l’environnement sur l’interaction entre ces parasites et leurs hôtes, notamment en termes de manipulation, est encore largement inconnu. Au cours de cette thèse, j’ai étudié l’effet de l’environnement sur l’interaction entre les gammares, des crustacés amphipodes de grande importance écologique dans les rivières, et leurs parasites acanthocéphales. Mes travaux ont montré que les conditions de ressources alimentaires et de température subies par les hôtes durant le développement des parasites influençaient plusieurs paramètres de l’infection mais n’affectaient pas la manipulation en termes d’utilisation de refuges. Toutefois, tandis que la géotaxie ne dépendait pas non plus de la température, l’impact des parasites sur la phototaxie des gammares était plus fort à haute température. De plus, mes travaux ont montré que l’impact conjoint de la température et des parasites pouvait modifier le rôle détritivore des gammares. Dans un contexte de changements globaux, ces travaux permettent de mieux comprendre l’importance de l’impact des parasites sur leurs hôtes et en prévoir les conséquences écologiques. / Many parasites with complex life cycle have developed the ability to alter the phenotype of their hosts. It is recognized that such changes in appearance and behavior are responsible for an increase in the probability of predation of their intermediate hosts by their definitive hosts. This phenomenon of parasite manipulation can have numerous consequences at the scale of the ecosystem, modifying the interactions between host populations and altering their ecological role. However, manipulative parasites received little attention from an ecological point of view. Thus, the effect of the environment on the interaction between these parasites and their hosts, in particular in terms of manipulation, is largely unknown. In this thesis, I studied the effect of the environment on the interaction between gammarids – ecologically important crustacean amphipods in rivers – and their acanthocephalan parasites. My experiments showed that the conditions of food resources and temperature experienced by gammarids during the development of their parasites influenced several infection parameters, but did not affect behavioral manipulation in terms of use of refuges. Nevertheless, while the geotaxis was not either effected by temperature, the impact of parasites on gammarids phototaxis was stronger at high temperature. Moreover, my studies showed that the cumulative effect of temperature and parasitism could alter the shredder role of gammarids. In a context of global changes, this work provides a better understanding of the importance of the impact of parasites on their hosts, allowing to make previsions on their subsequent ecological consequences.