Marine reserves and selective fishing shape mating behaviour, secondary sexual trait and growth in European lobster

Abstract

Well-functioning mating systems are perceived as vital for population growth rate and resilience to environmental change, but we know surprisingly little about the interplay between human-induced mortality and the mating systems of exploited marine species. Fishing and hunting can reduce densities, shift sex ratios and often targets individuals with the largest body size or with the most conspicuous characters (e.g. horns and claw). These traits tend to be sexually selected traits important for the outcome of competitive interactions and mate attraction. Thus, human harvest probably has profound, but not straightforward, effects on mating patterns and the strength and dynamics of sexual selection in many species. Marine protected areas (MPAs) are implemented worldwide with the aim of restoring exploited species and ecosystem functioning and they should also have the ability to conserve species’ mating patterns and secondary sexual traits. In this thesis, I have empirically examined several aspects of the mating system in wild European lobster (Homarus gammarus) and how it responds to intensive fisheries selection, but also whether MPAs can rescue a species’ natural mating patterns and secondary sexual traits targeted by the fishery. I have used both genetic analysis methods and morphological data from three lobster reserves on the Norwegian Skagerrak coastline and adjacent area open to fishing as control, and also explored the typical female fertilization pattern in United Kingdom (UK). In paper I, I sampled DNA from males and eggbearing females in one of the lobster reserves and fished area across multiple years and compared paternity data for any differences in mating behaviour. The results confirmed sizeassortative mating, with females’ preference for males with a body size larger than their own, however, the size difference within each pair were much larger in the reserve than in the fished area (22.5% compared to 6.4% in the fished area). Two cases of multiple paternity were also found in the fished area. Male size (body and claws) strongly influenced male mating success, but only in the reserve as selection differentials on these traits were not significant in the fished area. Lastly, estimation of sexual selection gradients on male traits found the selection to be acting strongest on relative claw size (claw size relative to body size), rather than on absolute claw and body size. In paper II, I joined a paternity study in a fished region of the coast of UK where we found no incidence of multiple paternity on egg-bearing females and concluded single paternity to be the common fertilization pattern. In paper III, I show that legal-sized male lobsters have larger relative claw sizes inside MPAs compared to same sized in fished areas (up to 8.4% larger). There were no differences between areas for females, which have smaller claws than males. This study is possibly the first to document the usefulness of MPAs in preserving a trait under strong sexual selection and under ongoing harvest selection in the same study system. In paper IV, I also found support for a positive MPA effect on body growth for legal-sized lobsters and most clearly for moult increment of females. It suggests that this may be caused by the catchability of lobsters in the trap fishery to be positively correlated with high growth rate and that lobster undergo intensive and selective fishing pressure against faster growing individuals. In sum, the findings in this thesis suggest that (1) selective fishing has the ability to weaken sexual selection with potential consequences for rates of fisheries induced evolution, (2) that MPAs can be highly effective in preserving sex-specific variation and phenotypic diversity and (3) that even small-scale MPAs can help maintaining the scope for sexual selection in a sedentary species with complex mating behaviour and life-histories such as the lobster.