Jacks and jills: alternative life-history phenotypes and skewed sex ratio in anadromous Threespine Stickleback (Gasterosteus aculeatus)

Background: Anadromous Threespine Stickleback (Gasterosteus aculeatus) often have bimodal size-frequency distributions, suggesting alternative life-history phenotypes. Breeding at small size may decrease attractiveness to mates, competitiveness in territorial disputes, and fecundity, but early breeding may outweigh the fitness costs of small size if survival to reproduction is low or iteroparity is possible. Hypotheses: (1) Two size-frequency modes exist in spawning, anadromous, male and female Threespine Stickleback, and there is sexual dimorphism in the frequencies of smaller size-class fish. (2) Smaller size-class anadromous Threespine Stickleback are younger than larger size class fish. (3) Variation across years and populations in frequencies of smaller size-class fish and body size exist and is positively correlated with a proxy for prey abundance on the feeding grounds. (4) The sex ratio of anadromous, spawning individuals is 1:1 across years and populations. Methods: We measured standard length (SL) and determined sex of anadromous Threespine Stickleback from Rabbit Slough in the Cook Inlet Basin, Alaska, between 1992 and 2015 and from additional sites around Cook Inlet and the Kenai Peninsula, Alaska, in 2014 and 2015. We compared sex ratio, mean SL, and size-class frequencies among years from Rabbit Slough and among four sites in 2014 and three sites in 2015. We determined ages of smaller and larger size-classes using otoliths from subsamples collected in 2010 and 2012 from Rabbit Slough. Results: At least two size modes occurred most years in both sexes in Rabbit Slough, in three of four populations sampled in 2014, and in all populations sampled in 2015. Smaller size-class fish were younger, indicating alternative life-history phenotypes in anadromous Threespine Stickleback. The proportion of females consistently exceeded that of males in Rabbit Slough and two of three other sites, but males were more frequent than females among smaller size class fish. Both the frequency of smaller size-class fish and mean SL varied among years in Rabbit Slough and among populations during 2014 and 2015. Prey abundance in the marine feeding grounds was not correlated with the frequency of smaller size-class fish but was negatively correlated with mean SL for females and for smaller size-class males across years in Rabbit Slough.