Intensive genetic assessment of the mating system and reproductive success in a semi-closed population of the mottled sculpin, Cottus bairdi

Most genetic surveys of parentage in nature sample only a small fraction of the breeding population. Here we apply microsatellite markers to deduce the genetic mating system and assess the reproductive success of females and males in an extensively collected, semi-closed stream population of the mottled sculpin fish, Cottus bairdi. In this species, males guard nest rocks where females deposit the eggs for fertilization. The potential exists for both males and females to mate with multiple partners and for males to provide parental care to genetically unrelated offspring. Four hundred and fifty-five adults and subadults, as well as 1259 offspring from 23 nests, were genotyped at five polymorphic microsatellite loci. Multilocus maternal genotypes, deduced via genetic analyses of embryos, were reconstructed for more than 90% of the analysed nests, thus allowing both male and female reproductive success to be estimated accurately. There was no genetic evidence for cuckoldry, but one nest probably represents a takeover event. Successful males spawned with a mean of 2.8 partners, whereas each female apparently deposited her entire clutch of eggs in a single nest (mean fecundity = 66 eggs/female). On average, genetically deduced sires and dams were captured 1.6 and 9.3 metres from their respective nests, indicating little movement by breeders during the spawning season. Based on a 'genetic mark-recapture' estimate, the total number of potentially breeding adults (c. 570) was an order-of-magnitude larger than genetically based estimates of the effective number of breeders (c. 54). In addition, significantly fewer eggs per female were deposited in single than in multidam nests. Not only were perceived high-quality males spawning with multiple partners, but they were receiving more eggs from each female.