Reproductive and condition status of “cold water” marine fish: new insights from a changing environment

Cold water fish species in temperate and subtropical regions are exposed to stress caused by fishing and climate change. Variations in species’ responses to these stressors are determined not only by the nature of the stressor itself and the environmental context but also by species-specific attributes. The survival of individuals is largely dependent on their health status (e.g. nutritional condition, absence of diseases, etc.) allowing the successful completion of their life cycle. On the other hand, the replenishment of populations depends on the success of the reproduction and recruitment. Faced with unfavourable environmental conditions, fish may respond with environmentally-mediated, balance of energy allocation – favouring health over reproduction, or vice versa – which can have important effects on fish population dynamics. In this context, the aim of this PhD Thesis was to generate critical knowledge on the main reproductive traits and health status of exploited fish species with an affinity for cold waters in the Gulf of Lions, Catalan Sea and Balearic Sea (NW Mediterranean) and Galician waters (Atlantic), inhabiting close to the warmer edge of their distribution area. Three case studies of “cold water species” were selected (Argentina sphyraena, Micromesistius poutassou and Merluccius merluccius) and, in each case, various biological traits – including the factors regulating early oogenesis, reproductive tactics, physical condition, parasitism and energy allocation trade-offs – were examined. In A. sphyraena, results showed differences in condition and reproductive capacity (egg quality and quantity) suggesting that this species is better adapted to the colder and more productive Gulf of Lion waters than to the Balearic Sea warmer and less productive waters. Refering to M. poutassou from the north-western Mediterranean, intensity-dependent relationships between parasitism by nematodes and fecundity, condition and spleen size were analysed: apparently individuals with higher parasitism were those showing enhanced feeding ability (e.g. better-conditioned or larger specimens), lower batch fecundity and higher spleen size index, which is linked to immune activity. In M. merluccius, temporal dynamics of oocyte development were described from the earliest oocyte stages showing relations to fish body length, environmental conditions and upwelling waters. Moreover, the results question the current definitions of fundamental terms related to the egg production estimation methods used for management assessment. Overall, the main findings support the idea that species-specific biological traits and plasticity influence population’s sensitivity and response to external stressors such as climate change and fishing. Moreover, this thesis contributes to the improvement of egg production estimation methods and to the understanding of fundamental biological mechanisms and their variability. But overall, the results are relevant not only from a theoretical point of view, but have implications for fisheries management and food safety; they support the need for a broader perspective when assessing stocks that takes the health status of fish populations into account.