Sperm competition and fertilization success in Atlantic cod (Gadus morhua): effect of sire size and condition factor on gamete quality

Sperm competition experiments were conducted to test the null hypothesis th at sperm quality is not affected by male body size in Atlantic cod (Gadus m orhua). Genetic markers (allozymes) were used to determine the proportion o f larvae sired by pairs of males when their sperm was combined with eggs of a single female simultaneously. Significant differences in fertilization s uccess between males were not explained by differences in body size. Fertil ization success was positively associated with male condition factor (K) an d with spermatozoa density in each male's semen when equal volumes of semen from each male were used. Male K was positively associated with male ferti lization success when the volume of semen used from each male was adjusted to add approximately equal numbers of spermatozoa from each male. The relat ive fertilization success of males varied depending on which female was the egg donor, suggesting that female "choice" at the gamete level may be occu rring in cod.