Sexual Dimorphism in Burbot

Sexual dimorphism arises in response to differing selective pressures on the sexes.  The burbot (Lota lota) is a common boreal species that is not overtly sexually dimorphic, but exhibits a unique reproductive ecology that may lead to subtle sexual differences.  We examined a variety of morphological and reproductive characteristics in burbot sampled from a mid-sized (1100 ha) northern lake over one full year to assess both seasonal and sex-based variation.  Spawning occurred in early February.  Seasonal variation tended to be more pronounced in females than in males for many of the traits examined.  Males and females did not differ significantly in growth rate, pectoral or pelvic fin lengths, swim bladder mass, or liver lipid concentration.  However, males had significantly higher body condition (somatic mass at length), larger gas glands, and smaller livers (expressed as HSI) than females.  Males also had significantly larger gonads (expressed as GSI) than females, opposite to the pattern observed in most other boreal shield fishes.  Burbot do exhibit significant sexual dimorphism and the relatively high gonadal investment of the males suggests that sperm competition may be intense in this species.