Contrasting growth strategies of pond versus marine populations of nine-spined stickleback (Pungitius pungitius): a combined effect of predation and competition?

Abstract

Gigantism in isolated ponds in the absence of sympatric fish species has previously been observed in nine-spined sticklebacks (Pungitius pungitius). Patterns in sexual size dimorphism suggested that fecundity selection acting on females might be responsible for the phenomenon. However, the growth strategy behind gigantism in pond sticklebacks has not been studied yet. Here, we compared von Bertalanffy growth parameters of four independent nine-spined stickleback populations reared in a common laboratory environment: two coastal marine (typical size) and two pond (giant size) populations. We found that both pond populations had larger estimated final size than marine populations, which in turn exhibited higher intrinsic growth rates than the pond populations. Female growth strategies were more divergent among marine and pond populations than those of males. Asymptotic body size and intrinsic growth rate were strongly negatively correlated. Hence, pond versus marine populations exhibited different growth strategies along a continuum. Our data suggest that quick maturation—even with the cost of being small (low fecundity)—is favoured in marine environments. On the contrary, growth to a giant final size (high fecundity)—even if it entails extended growth period—is favoured in ponds. We suggest that the absence (ponds) versus presence (marine environment) of sympatric predatory fish species, and the consequent change in the importance of intraspecific competition are responsible for the divergence in growth strategies. The sex-dependence of the patterns further emphasizes the role of females in the body size divergence in the species. Possible alternative hypotheses are also discussed.