Abstract
Male three‐spined stickleback Gasterosteus aculeatus have a fixed amount of sperm during the breeding season because spermatogenesis is inhibited at this time. A method was developed to estimate ejaculate size in situ by removing the sperm from the male's nest. The reliability of the method was tested using known numbers of sperm. In their first mating, males ejaculated 11·64 × 106 sperm (median), representing c. 5% of the male's sperm store (median 27·88 × 107 sperm). The amount of sperm in the testes was significantly reduced in males that had mated several times (median 8·09 × 107). Additionally, ejaculate size was smaller in these experienced males (median 8·79 × 105). Heavier and larger fish invested absolutely and relatively more sperm in a mating than did lighter and smaller fish. Ejaculate size did not correlate with the mass of the egg clutch.
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