Reproduction of Calanus glacialis in the Lurefjord (western Norway):: indication for temperature-induced female dormancy

The reproductive activity of Calanus glacialis was studied during the winter-spring transition 1996 in the Lurefjord, western Norway, focussing on the impact of phytoplankton development and temperature. Depth-stratified samples were taken at weekly intervals to determine female abundance, vertical distribution and gonad development stage. Single live females were incubated to measure in situ egg production rate, clutch size and spawning frequency. These field observations were combined with feeding and starvation experiments to study the importance of food supply in greater detail. The depth distribution of the females changed during the investigation period. On February 28, most females were located between the surface and 100 m. During March, the majority of the females inhabited the upper 50 m, and by mid-April, most females had migrated to depths below 100 m. The reproductive activity of C. glacialis was uncoupled to the phytoplankton bloom. Females were already mature at the beginning of March and egg production began. Maximum reproduction was recorded on March 28 before the beginning of the spring bloom; thereafter, gonad maturity and egg production rates declined rapidly before the end of the bloom, and by April 11 the majority of females had migrated to depths >100 m. No faecal pellets were produced when these females were offered food, and gonads became equally depleted in females starving or feeding for 5 d, indicating a deep physiological change such as dormancy. The descent of the females to lower depths and the arrest of their reproductive activity were apparently related to a temperature increase in the surface layer, similar to earlier observations in the White Sea.