EFFECTS OF ALTERNATIVE MALE MATING STRATEGIES ON CHARACTERISTICS OF SPERM PRODUCTION IN THE ATLANTIC SALMON (SALMO-SALAR) - THEORETICAL ANDEMPIRICAL INVESTIGATIONS

Atlantic salmon (Salmo salar) males mature as either tiny precocious p arr before seaward migration, or as older and larger anadromous males. Anadromous males dominate the spawning redds and aggressively defend females against parr intrusions. Parr gain fertilizations by sneaking in to ejaculate while anadromous males and females spawn. Such differe nces in mating advantage generate asymmetries in risk of sperm competi tion between the male strategies. Theoretical sperm competition models predict that males typically mating in disfavoured roles (here, the p arr strategy) should be selected to offset this disadvantage by invest ing more into spermatogenesis to achieve fertilization success. First, we present a theoretical model which analyses gametic expenditure for salmon parr and anadromous male reproductive strategies. We then use the natural variance in mating pattern within this species to compare empirically how males invest in spermatogenesis. A range of reproducti ve traits were measured for both male strategies. Absolutely, anadromo us males have larger testes and produce greater numbers of sperm than parr males. However, results show that parr invest relatively more hea vily into total spermatogenesis, and have a larger gonosomatic index t han anadromous males. Relative to body size, parr produced greater num bers of sperm and volumes of stripped ejaculate. There was no differen ce in sperm length between the two male strategies. However, more sper m were motile in parr ejaculates, and these sperm lived longer than an adromous male sperm. Our findings may explain how male parr, under ele vated risks of sperm competition and occupying a disfavoured mating ro le (parr weigh only 0.15% of the average body mass of anadromous males ) achieve disproportionately high fertilization success.