GONAD

Understanding Calanus Reproduction in North Norway

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Calanus finmarchicus:
providing a number important ecosystem services

Copepods link levels of the marine food chain. They feed on phytoplankton - the lowest trophic level on the marine food chain - and are eaten by juvenile fish, including commercially important species.

As a primary food source for predatory fish, Calanus play a role in how many fish survive to adulthood. Termed ‘recruitment success,’ the survival of juvenile fish will affect the population numbers of commercial fisheries stock.

Recently, the fishing industry has recognized the commercial value of Calanus oil as a dietary supplement due to the health benefits of its high omega-3 fatty acid content.

Dense aggregations of Calanus have been reported regularly by the commercial fishing industry. Advanced knowledge about the occurrence, density, and timing of Calanus populations along the Norwegian coast are crucial to understand and predict the dynamics of these copepods, their availability for planktivorous predators, and for Calanus fisheries.

The GONAD project contributes new knowledge on the timing of reproduction and recruitment of early offspring in the Calanus patches off the north Norwegian coast to facilitate predictions of potential shifts in recruitment phenology with climate warming.

The study - collection methods

Data on egg production is needed to estimate population numbers of Calanus. Females can spawn more than 100 eggs in individual clutches, and often one or more clutches per day during the peak reproductive period with high food supply.

Data on the temporal and spatial reproductive patterns of Calanus finmarchicus have been collected along the northern Norwegian coast in 2017 and 2018.

We use two methods to measure reproductive output in Calanus:

live incubations of females during cruises onboard the Research Vessel Helmer Hanssen
estimates of potential spawning rates from gonad stage analysis in preserved females

Additional gonad analysis from selected stations will provide clues about the reproductive state of the Calanus population, which may unlock important information. For example, 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).

Gonads - what researchers look for

To identify female Calanus that are ready to spawn, researchers at GONAD look at preserved samples for morphological changes. Examining preserved samples allows researchers to analyze a higher number of individuals than possible with live incubations, and from stations where live incubations are not possible.

The gonads of female copepods undergo distinct morphological changes – changes in shape and colour – during oocytedevelopment. Initially, oocytes are transparent and are found in the ovary and upper part of the diverticula. During maturation, yolk formation causes the oocytes to turn brown in a process known as vitellogensis. Now, the oocytes are in the lower part of the diverticula. When it is time to spawn, the oocytes swell and their nuclei dissolve.

By looking at a female copepod under a microscope, researchers can note oocyte location, colour, size, and number. Such data allow scientists to identify how many females were ready to spawn, and the number of oocytes per females, and therefore estimate the number of eggs that would have been released during the next spawning event.

The data collected on numbers of Calanus eggs is used to:

complement stock size analyses and
Calanus population stage indexing
estimates of the reproductive output for later recruitment to the population, and thus secondary production

Gonad development stages of Calanus spp. AD = anterior diverticulum, PD = posterior diverticulum, Ov = Ovar (from Niehoff 1996). The panels present gonads of increasing maturity (G1-4)