What do male tench, Tinca tinca, advertise with morphological ornaments?

In contrast to females, tench Tinca tinca (L.) males have large pelvic fins with a thickened and bent second fin ray. Males also produce notable ventral protuberances during breeding, but the function of these male ornaments is not known. Using wild-caught fish, we found that both the size of pelvic fins and ventral protuberance were dependent on body mass/length ratio but not necessarily on condition. Plasma testosterone concentration and relative gonad mass were positively correlated with condition factor. Plasma testosterone concentration was not related to measures of non-specific immune function, but correlated positively with the size of pelvic fins corrected for body length. However, the studied male ornaments were not related to the measures of immune defence or to the load of Diplostomum spp. We did not find evidence for male–male dominance or female preference for large male characteristics within the natural variation in these traits. Thus, our study suggests that also other mechanisms than sexual selection on good genes might contribute to the maintenance of sexual dimorphism in tench.