Characteristics of Egg Production of Calanus finmarchicus during U.S. GLOBEC Georges Bank process cruises, 1995-1999.

J. A. Runge, S. Plourde, P. Joly, E. G. Durbin, and B. Niehoff

Egg production rate, hatching success and adult female body size and mass of Calanus finmarchicus were measured during 10 process cruises spanning the 1995-1999 field component of the U.S. GLOBEC Georges Bank Program. The main purposes of these measurements were to: (1) estimate across-bank, seasonal and interannual variation in egg laying and hatching success, (2) estimate the extent to which food limits egg production on the bank, (3) develop a relationship between egg production rate and a preserved-female reproductive index that could be applied to the estimation of egg production in broadscale survey samples and (4) measure recruitment rate into the Calanus population for estimation of mortality rates of early life history stages. Female C. finmarchicus were producing eggs at relatively high rates in at least some regions of Georges Bank during all months between January and June. Median, monthly egg production rates ranged from 24 eggs female-1d-1 in January to 50 eggs female-1d-1 in April-June. There was a corresponding increase in female body size and clutch size between January and April-June. Mean egg diameter ranged from 142 to 149 µm among stations, and there was a small, significant (p < 0.02) seasonal trend in the linear regression on year day, from 145 µm in January to 144 µm in June. Hatching success varied between 50-95%, excluding one station measured on two successive days in the NEP in late March, when hatching success was about 30%, but there was no seasonal trend. Seasonal variation of C- and N-specific egg production rates is much less than egg production rate, indicating that some of the seasonal difference is due to changes in female body size and consequently clutch size, with which there is a significant (although with a high degree of variation) positive relationship. While some of the variation in mass-specific EPR may be attributed to body size and temperature, the data also indicate food-limited egg production at certain times and regions. The temperature-adjusted mass-specific rates fit reasonably well in an Ivlev relationship with integrated chlorophyll a (0-50 m) standing stocks measured contemporaneously in the water column. This relationship suggests that egg production is often food-limited at standing stocks < 100 mg chl a. m-2.