| Summary: | In fish, reproductive competition among males may take two forms: (1) monopolization of females through courtship or aggression towards other males and (2) sperm competition among spermatozoa from several males to fertilize eggs from one female. This thesis tests whether body size is associated with male and female reproductive success in Atlantic cod, 'Gadus morhua'. Both forms of competition exist in Atlantic cod. The significance of body size in male reproductive success should depend on which form of competition is the prevalent way in which males gain access to eggs. The effect of sire size on the quality of semen was assessed by sperm competition experiments. Pre-mixed semen from pairs of genetically marked males was used to fertilize eggs from one female. Fertilization success was not affected by male body size. Male condition factor (K) and spermatocrit were positively associated with fertilization success. Spermatocrit was a linear function of spermatozoa density. Spermatocrit varied among males and increased during the spawning season. Relative fertilization success of males varied with different females, suggesting female choice at the gamete level. A mate competition experiment tested the association between male reproductive success and body size. Pairs of males, differing in body size, and genotype at marker loci, spawned with single females in tanks during one reproductive season. Most egg batches were fertilized by both males and the proportion of larvae sired by each male varied significantly among batches within tanks. Small males were more successful with small females, but no association between male size and success existed with large females. Male reproductive success (estimated number of larvae sired) was not associated with body size, K, or reproductive allotment (body weight lost during spawning), and was marginally associated with male relative activity. Female reproductive success was best predicted by reproductive allotment, which was correlated with female size. Fecundity was ...
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