Eutrophication as a result of human activity has resulted in increased algal blooms and turbidity in aquatic environments. We investigated experimentally the effect of algal turbidity on the mating system and sexual selection in the sand goby, Pomatoschistus minutus (Pallas), a marine fish with a resource-defence mating system and paternal care. Owing to male-male competition and female choice, large males can monopolize multiple mates, while some males do not achieve mating at all. We show that the number of eggs laid was the same in both turbid and clear tanks but that mating success was more evenly distributed among males in turbid than in clear water. The opportunity for sexual selection was lower in turbid conditions. In turbid conditions mating success was less skewed towards large males. Our results suggest that increased turbidity can change mating systems and decrease the opportunity for sexual selection as well as selection intensity.

No abstract

Reproductive activities are often conspicuous and can increase the risk of predation. Evidence suggests that individuals are capable of responding to predators in a risk-sensitive manner. However, most studies tend to consider only the predator-mediated responses of males and females in isolation and with little regard to differences in local environmental conditions. Here, we experimentally investigate the effects of environmental visibility (turbidity) and predation risk on reproductive decisions in the sand goby, Pomatoschistus minutus, when exposed to a visually oriented predator, the European perch, Perca fluviatilis. We found that gobies were more reluctant to spawn in the predator's presence, although larger males spawned sooner than smaller males. Interestingly, latency to spawning was unaffected by the visual environment, suggesting that gobies may be relying on non-visual cues under turbid conditions.

Human-induced eutrophication, resulting in increased algal growth and water turbidity, is an alarming problem in aquatic systems. Many studies have focused on the effects of algal turbidity on mate choice and sexual selection in fish, but little emphasis has been given to the ways it can constrain mating success. Here we experimentally investigated the effects of algal turbidity on maximum male mating success and parental care in the sand goby, Pomatoschistus minutus, a fish with a resourcedefence mating system and male parental care. For this purpose, we introduced to 1 nest-holding male 5 random-sized ripe females in either clear or in turbid water. After spawning, we observed how many mates and eggs the male received and followed his parental behaviour and egg survival for 6 days under turbid or clear water conditions. When spawning took place in clear water, the number of eggs the male received into his nest increased with the total weight of five females in his tank. However, when spawning took place in turbid water, there was no relationship between female size and the number of eggs laid, although the number of females that spawned was the same as in clear water. The results indicate that females adjust the number of eggs they lay according to water turbidity. This could explain previous findings that mating success is more evenly distributed among males in turbid than clear water conditions. Significance statementThe first responses of animals to human-induced changes in the environment are behavioural. Subtle changes in the behaviour of individuals can have profound consequences for populations and communities. Human-induced eutrophication, leading to algal blooms and water turbidity, is a major environmental problem in aquatic systems worldwide. Our results on the sand goby suggest a new mechanism by which water turbidity may affect fish mating systems and weaken sexual selection. When spawning takes place in clear water, the number of eggs accumulated in a males' nest is an increasing function of the fecundity of the females. However, when spawning in turbid water, this positive relationship between female size and egg numbers disappears. We believe this is because females do not perceive the competition from other females in turbid water and therefore invest less in present reproduction.