Levels of liver and plasma lipids of the burbot Lota lota were highest before reproduction and decreased during and after spawning. Triacylglycerols accounted for 95% of the liver lipids. High concentrations of leptin-immunoreactive peptide were present in the burbot liver, suggesting that it may be secreted by this organ.

Variation of metabolic heat dissipation between quiescent and active spermatozoa of Salmo salar m. sebago was investigated by direct calorimetry. The technique offers some promise to monitor energy metabolism of teleost sperm. The Fisheries Society of the British IslesThe physiology of fish spermatozoa, especially topics related to motility have received considerable recent attention. As Lahnsteiner & Patzer (1998) summarize, duration of motility, patterns of motility, the mechanisms of initiation and inhibition of motility have all been described well in many fish species. In contrast, only little information is available on the metabolic activity of fish spermatozoa although direct quantification of metabolic costs involved in the quiescent and active phases are important bioenergetic parameters. This deficiency probably reflects the methodological difficulties arising from the requirements of rapid analysis under interchanging test media and metabolic phases.This paper describes an alternative physiological technique, direct calorimetry, to study whether and to what extent metabolic activity of fish spermatozoa is changed during the motility initiation/inhibition. Spermatozoa of landlocked salmon Salmo salar m. sebago Girard were used as a model. The eight mature salmon (50-70 g) were obtained from Saimaa Fisheries and Aquaculture at Enonkoski in early November 1998. The milt samples were collected by hand-stripping and used immediately for calorimetric measurements and determination of spermatocrit by centrifugation (12 000 rpm; 5 min). The spermatocrit was 36-43%, which gives a sperm density of 1·9-2·6 10 10 cells ml 1 as estimated by a spermatocritbased linear regression model (Piironen, 1985).Heat dissipation measurements were performed with a flow-through 3·5 cm 3 stainless steel perfusion assembly of an isothermal microcalorimeter (TAM 2277, Thermometric). The calorimeter was calibrated at 100 W, which allowed the detection of a heat output of 120 W maximum.The milt sample of 75 l (with density around 1·6 10 9 cells) was put into the ampoule, which was then lowered gently into the calorimeter. After an hour of aerobic incubation at 7 C, the heat output of inactive spermatozoa was measurable and was then monitored for 2-3 h. For initiation of the metabolic activity, the milt was diluted with one of the two activation solutions. The first solution consisted of 0·9% (w/v) NaCl buffered to ‡Author to whom correspondence should be addressed.