Male-biased sexual size dimorphism in the nest building corkwing wrasse (Symphodus melops): Implications for a size regulated fishery

Abstract

Size selective harvesting can also be selective on sex in species displaying sexual size dimorphism (SSD). This has potential consequences for mating systems and population dynamics. Here, we assessed spatial variation in SSD and body size in eight Norwegian populations of corkwing wrasse (Symphodus melops), a species where males either mature as nesting or sneaker males. Corkwing wrasse is increasingly harvested in a size-selective fishery which supplies the Norwegian salmon aquaculture industry with cleaner fish to reduce sea-lice infestations. In our study, mature nesting males had larger body sizes than females and sneaker males, and the size difference was significantly larger in the four northernmost populations. Contrasting life history traits in two of the populations (a southern and a northern), we found that the larger SSD in the north was because of nesting males delaying maturation and growing faster relative to females and sneaker males. Mature northern nesting males also had smaller gonads at smaller sizes relative to their southern counterparts, indicating a trade-off between reproduction and somatic growth in males. Applying the current minimum legal size limit for commercial fishing in Norway (12 cm) would have failed to protect any mature nesting males in five out of the eight populations. Moreover, the findings of more male-biased SSD and female-biased sex ratios in the northern populations imply that there is larger potential for sex-selective harvesting in these populations. To avoid fisheries-induced changes in sex ratios, we advocate for spatially structured management units and to refine the current size-regulations using either sex-specific minimum size limits or a slot limit, also protecting the larger fish and therefore more nesting males.