Annual cycle in female Three-spined sticklebacks (Gasterosteus aculeatus L.) from an upland and lowland population

The annual cycles in female Three-spined sticklebacks from an upland (275 m) and a lowland (25 m) population were compared. The average length and weight of the lowland females were greater, but in both populations growth was checked in winter and again in the breeding season. Both populations also showed a winter decline in condition followed by a sharp increase in spring associated with the maturation of the ovaries. The hepato-somatic index of the females also increased during the spring, but the dry matter content of the liver declined during the spring and early summer. Lowland females had larger livers than the upland fish. In both populations average gonadosomatic index increased steadily throughout the winter, then abruptly in spring. The lowland population bred from May to early August, but the upland population bred only in May.
The energy content of the carcases of females was lowest in winter and again during the breeding season whereas the ash content showed maxima at these two periods. Lipid and glycogen analyses of the carcase, liver and ovaries also suggested that winter and the breeding season were periods of depletion from the carcase and liver, but not from the ovaries.
The observations indicate that the ovaries are to some extent insulated from fluctuations in resource availability. This supports experimental studies which indicate that egg production in the female stickleback can be subsidised by depletion from the carcase and liver when necessary.