Age and size-related differences in reproductive success and reproductive costs in threespine sticklebacks (Gasterosteus aculeatus)

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Abstract
In an anadromous population of threespine sticklebacks (Gasterosteus aculeatus), only a minority of 1-year-old (1 +) males enter the spawning grounds to reproduce. The majority of males on the spawning grounds are 2-year-old (2+) fish. We compared reproductive success and weight loss during reproductive activities in 1 + and 2 + males in order to assess the costs of reproducing at 1 + in G. aculeatus. Results of laboratory experiments showed that the smallest 1 + males (58± 2 mm), although sexually mature, were excluded from breeding by large 2+ males (74 ± 1 mm). The bigger 1 + males (62 ± 1 mm) were able to compete for nest sites with the 2+ fish slightly larger than themselves (68 ± 1 mm). In the laboratory, 1 + breeders lost more mass than 1 -I- nonbreeders over a 36-day experimental period. Two-year-old breeders lost the same mass as 2+ nonbreeders. All breeders died in higher numbers than nonbreeders during the experimental period. In the field, the percentage of nesting males did not differ between the two age classes during the first and the second reproductive periods but was significantly lower for 2 + fish in the third reproductive period of the season. The mean number of eggs obtained by 1 + and 2 + males was not significantly different. Those 1 + males that migrated to the breeding grounds achieved relatively high reproductive success even though laboratory results showed that nesting success depended on size.

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