The new generation of Calanus finmarchicus: estimating population recruitment from egg production rates and gonad stage analysis off northern Norway
Calanus finmarchicus populations provide a number of important ecosystem services, including a commercial value in their own right. They link phytoplankton production to the higher trophic levels of the marine food chain and drive recruitment success of commercial fish species. Dense aggregations of Calanus have been reported regularly by the commercial fishing industry, which has recently recognized the commercial value of Calanus oil as a dietary supplement due to the health benefits of its high omega-3 fatty acid content. Advanced knowledge about the occurrence, density and timing of Calanus populations along the Norwegian coast are thus crucial to understand and predict the dynamics of these copepods and their availability for planktivorous predators and Calanus fisheries. This project contributes new knowledge on the timing of reproduction and recruitment of early offspring in large Calanus patches off the north Norwegian coast to facilitate predictions of potential shifts in recruitment phenology with climate warming.
Data on the temporal and spatial reproductive patterns of the important marine copepod Calanus finmarchicus have been collected along the northern Norwegian coast in 2017 and 2018. To measure reproductive output in Calanus we apply two methods: 1) live incubations of females during cruises onboard RV Helmer Hanssen and 2) estimates of potential spawning rates from gonad stage analysis in preserved females. C. finmarchicus females can spawn >100 eggs in individual clutches, and often one or more clutches per day during the peak reproductive period with high food supply. In late April 2017, strong variations in in-situ egg production rates of Calanus were observed, with no obvious geographical (shelf to offshore) or temporal pattern (over two weeks). Additional gonad analysis from selected stations will provide clues about the reproductive state of the Calanus population as a whole. The gonads of copepods undergo distinct morphological changes during the development of the oocytes and their increase in number. In Calanus spp. young oocytes are transparent and are located in the ovary and the upper part of the diverticula which extend throughout the entire female. With maturation, i.e. vitellogensis and growth, the oocytes become brownish and are then found in the lower parts of the diverticula. Just prior to spawning, the oocytes expand in size and their nuclei dissolve. Based on these morphological changes, we can identify females that are ready to spawn in preserved sample material, and estimate the number of eggs that would have been released during the next egg release event. This way, a higher number of females can be analyzed than with live incubations alone, and from stations where such incubations are not available. The resulting data will complement stock size analyses and Calanus population stage indexing, in addition to estimates of the reproductive output for later recruitment to the population, and thus secondary production.