Calanus finmarchicus egg production and food availability in the Faroe–Shetland Channel and northern North Sea: October–March

Physiological characteristics (egg production, lipid content and gonad development stage) of Calanus finmarchicus were examined on five cruises in the north-east Atlantic carried out between the months of October to March in the years 1993–95 and related to phytoplankton food availability. Appreciable egg production rates were only recorded in October and March. Published values for maximum water volume filtered daily by individual females and the standing stock of phytoplankton (estimated from chlorophyll concentration) are used to argue that ingestion of phytoplankton carbon alone was not sufficient to support the egg production rates recorded in March. Data are presented that indicate C. finmarchicus is fuelling egg production through use of internal lipids. We argue that this species exhibits a life history strategy whereby females release a limited number of eggs to the water column upon returning to the surface waters following diapause, i.e. prior to the onset of the spring bloom. This pre-spring bloom egg production will comprise only a small component of the annual Calanus egg production in shelf waters. Nevertheless, given that the recruitment of Calanus to their feeding grounds over the shelf is a function of transport of the copepods from off-shelf to shelf regions via ocean currents, a burst of egg production in the period immediately following diapause and prior to the population's transport to shelf waters, would increase the chances of large numbers of the population reaching shelf regions. Thus, even a limited pre-spring bloom burst of egg production may have important consequences for recruitment in this species.