Egg and faecal pellet production of Calanus finmarchicusfemales from controlled mesocosms and in situ populations:influence of age and feeding history

Abstract

Egg and faecal pellet production of Calanus finmarchicus females were investigated individually under superabundant food conditions at 5 and 10°C. The females came from (1) in situ populations in the Raunefjord (southern Norway) in March and May 1997 and (2) cohorts reared in 2 mesocosms, one with high (H) and one with low (L) food concentrations. The egg production rates of newly moulted females coming from mesocosms increase from zero to maximal rates over 10 to 15 d at 10 and 5°C, respectively. The delay in reaching maximal fecundity rates depends on time required for gonad maturation, which is related to the use of storage lipid and also to food supply for ending the maturation. The faecal pellet production rates also increase with time, suggesting that fecundity is governed by ingestion. Finally, we suggest a conceptual model of different events between moulting and optimal spawning activity in newly moulted females in superabundant food conditions. The fecundity, egestion and lipid metabolism of wild females (caught in March and May) were studied in the same way. The results are compared to observations of young females from mesocosms and the influence of seasonal origin of females on the studied processes is assessed. Data from experiments run with females from Mesocosms L and H permit us to discuss the effect of feeding history on fecundity and egestion. Although females from Mesocosm L show shorter prosome lengths than those from Mesocosm H, fecundity is not significantly different. However, females from Mesocosm L have a much higher production of faecal pellets than females from Mesocosm H, suggesting that the previous food limitation during copepodite development in Mesocosm L induces higher maximal ingestion rates of these females than the previous well-fed females (Mesocosm H). A few assumptions to explain these different patterns are proposed.