| Abstract |
Nests are widespread in the animal world and aim to protect the young from predation and adverse environmental conditions while being a privileged place to assess sexual selection. These nests, modifications of the habitat in which they are built, influence the environmental conditions and likely affect the communities and ecological processes. Among the nesting species, the sea lamprey (Petromyzon marinus L.) is an anadromous, semelparous migratory fish, whose nests consist of a mound of coarse elements downstream a pit with a fine substrate. The thesis firstly describes the reproductive behaviour of the sea lamprey by studying the link between the nests and the individuals that built them. A Capture-Mark-Recapture protocol showed that males and females visited up to 10 and 7 nests respectively, and that nests could be built by either pairs or groups of up to 5 individuals, resulting in a clearly polygynandrous mating system. Data obtained during this individual monitoring was used to set up a model providing a population size estimate via a simple nest count, a model that can be easily adapted to other populations and used via an online application. Intrasexual competition and cooperative nest building, as well as the existence of potential alternative reproductive tactics, were monitored at the scale of a nest and of an entire spawning site. Video monitoring within nests showed equal individual contributions to both nest building and mating, although aggressions perpetrated by some males suggested a hierarchy. Experimental injection of eggs into recently built nests indicated that the interaction between variables related to habitat choice (current velocity) and habitat modification (slope between the lower and upper points in the nest) affected egg retention in the nest, a major aspect of egg survival. Measurements of the maintenance of river lamprey (Lampetra fluviatilis L.) eggs in a controlled environment showed a significant role of substrate size. Finally, the link between the nest and its ecosystem was described through the study of the macroinvertebrate communities occupying the different zones and several ecosystem processes. The habitat heterogeneity created by sea lamprey generated biological heterogeneity, with an increased invertebrate diversity in the nest compared to control sites. However, nutrient retention, chlorophyll accretion and litter degradation were not affected. The general objective of this thesis is thus a better understanding of a species whose ecology and place in the ecosystem remain poorly understood, although threatened in its native range while being invasive where introduced, through the use of a characteristic structure of its life cycle: the nest. |