Sperm Density and Sperm Viability in Wild Male Fish are Influenced by Different Factors

Introduction: Sperm quality is important for male reproductive success [1], yet little is known of the factors that influence sperm quality in wild fish. Factors known to affect sperm quality in other organisms include age, inbreeding and exposure to pollutants. Feminised male roach (Rutilus rutilus) have been reported in freshwater ecosystems receiving wastewater treatment works (WWTW) effluents, resulting from exposure to endocrine disrupting chemicals (EDCs). Wild roach with feminised gonads have reduced sperm quality (sperm motility, sperm density, and fertilization success), compared with less feminised fish from less contaminated sites [2]. However, the influence other factors on sperm quality is not known. For instance, some WWTW effluents are genotoxic [3], so sperm quality might be expected to decrease with age in adult fish living in rivers receiving these effluents, resulting from increased damage to germ cells. In the current study we analysed a large dataset of variables collected from wild fish from effluent-contaminated rivers in order to identify key influences on sperm density and sperm viability. Methods: Sperm density and sperm viability were examined in 138 adult wild male roach collected from three rivers receiving WWTW effluents. These fish were collected from the River Bourne (2006) the River Nene (2007) and the River Arun (2008) (as per 1). Sperm viability (assessed using trypan blue dye exclusion) and sperm density were recorded for all fish. Gonadal status was assessed using histological techniques and the following feminised phenotypes were recorded separately for each male fish: the presence of developing eggs in the testis of males (the ‘intersex condition’) and the presence of female-like reproductive ducts (ovarian cavities). The influences of the explanatory variables on both sperm density and viability were assessed using Linear MixedEffect (LME) models, using river as a random effect to control for differences between rivers. Variables suspected of influencing sperm quality were included in the maximal models, including body length, skin roughness (due to tubercles, one of the secondary sexual characteristics of male roach), two indices of internal genetic diversity (measures of inbreeding, assessed using 15 DNA microsatellite loci), Intersex Index [based on the number of oocytes present in the testes (4)] and the presence of ovarian cavities. Results and Discussion: Based on analyses of the combined dataset from all three rivers, a range of factors were found to influence sperm quality, but the factors that were associated with sperm density and viability differed. There were significant differences in average sperm viability between fish from the three rivers (F(2,137) = 24.645, p < 0.00001) but there were no significant differences in