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Population estimation implies considering the biology of the species, but also the constraints of logistic aspects such as cost. While common methods based on individual counts can provide precise estimates, they require an extensive sampling effort. An alternative to these methods is using cues linked to the species abundance. In that case, producing absolute estimates requires assessing the relationship between the individuals and these cues. In this paper, we propose a model based on data on spawning behaviour and Approximate Bayesian Computation to estimate the number of sea lamprey spawners using nest counts data. By counting the daily number of occupied nests and using parameters from a behavioural study, we set up a model simulating a spawning season and returning a population estimate by comparison with field data. Our model gives realistic estimates and we discuss the parameters on which to prioritize data collection with a sensitivity analysis, and show that halving the sample size provides a still satisfactory accuracy. We made an easily parametrizable application to run the model for any people interested in sea lamprey population estimation, and believe this framework to be a good way to increase data collection for both endangered and invasive sea lamprey.

Human activities enhance the atmospheric C[O.sub.2] concentration and modify climatic patterns, which could change the growth and quality of plant materials, as well as their subsequent decomposition, thus altering stream ecosystem functioning. These effects could be modulated by interactions between plant species differing in biological traits and competitive capacity. We cultivated the forb Trifolium pratense and the grass Agrostis capillaris under different C[O.sub.2] concentration (ambient or elevated), water availability (control or drought), and competition (monoculture or mixture). The material thus grown was conditioned in a stream for microbial colonization, and its subsequent decomposition was measured in laboratory microcosms. Elevated C[O.sub.2] reduced the quality of T. pratense but not that of A. capillaris. Water shortage limited plant quality but did not interact with C[O.sub.2] concentration. Interspecific competition only affected nitrogen concentration in A. capillaris. Elevated C[O.sub.2] did not affect decomposition rate, and A. capillaris, the species richer in nutrients, decomposed more slowly. Our results showed that the decomposition rates of plant materials grown under elevated C[O.sub.2] are difficult to predict due to species-specific responses and interactions with other factors. Les activites humaines accroissent les concentrations atmospheriques de C[O.sub.2] et modifient le climat, ce qui peut influencer la croissance et la qualite du materiel vegetal, ainsi que sa decomposition subsequente, alterant ainsi le fonctionnement des ecosystemes lotiques. Ces effets pourraient etre modules par les interactions entre des especes de plantes presentant des caracteres biologiques et capacites concurrentielles differents. Nous avons cultive l'herbe non gramineenne Trifolium pratense et la graminee Agrostis capillaris dans differentes conditions de concentration de C[O.sub.2] (ambiante ou elevee), de disponibilite d'eau (temoin ou secheresse) et de concurrence (monoculture ou melange). Le materiel ainsi produit a ete conditionne dans un cours d'eau pour produire une colonisation microbienne, et sa decomposition subsequente a ete mesuree dans des microcosmes en laboratoire. Une concentration elevee de C[O.sub.2] entrainait une reduction de la qualite de T. pratense, mais non d'A. capillaris. La penurie d'eau limitait la qualite des plantes, mais n'interagissait pas avec la concentration de C[O.sub.2]. La concurrence entre especes avait une incidence sur la concentration d'azote dans A. capillaris. Une concentration elevee de C[O.sub.2] n'avait pas d'incidence sur le taux de decomposition, et A. capillaris, l'espece la plus riche en nutriments, se decomposait plus lentement. Nos resultats demontrent que les taux de decomposition de materiels vegetaux cultives a des concentrations de C[O.sub.2] elevees sont difficiles a predire en raison de reactions qui varient selon l'espece et d'interactions avec d'autres facteurs. [Traduit par la Redaction], Introduction Future scenarios of global environmental change forecast a doubling of the atmospheric C[O.sub.2] concentration by the end of the 21st century (Intergovernmental Panel on Climate Change (IPCC) 2013). Increased [...]