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Skipper of the SCH 81 sails on board research vessel Tridens

On 3 June 2013, Sjaak van der Zwan, skipper of the SCH 81, left for Cork (Ireland), where he reported for duty on the Dutch research vessel Tridens operated by the marine research institute Imares Wageningen University. The 73 metre-long vessel set sail from Cork for an 11-day scientific expedition on the Atlantic Ocean, the purpose of which was to investigate the fish eggs of the mackerel and horsemackerel. One remarkable discovery made by the expedition, that has since been revealed, is that scad spawn at night, while mackerel do so during the day.
From Cork, the Tridens headed for the fish grounds, carrying on board a cargo of ‘torpedoes’ for collecting plankton. The vessel sailed a grid pattern, and released a torpedo equipped with a sensor every 25 kilometres. In an oblique (V-shaped) track, the entire water column was fished, from the surface to 5 metres above the seabed. During the entire process, the torpedo measured its precise depth and the temperature of the water. The torpedo is effectively a rocket-shaped, steel rack containing a net with a mesh of just 280 micrometres, used for sieving the plankton from the water. Using a flow gauge, the quantity of water filtered during the process is determined. Based on that water volume, it is possible to calculate how many eggs are floating in each square metre of the water column. Once on board, the fish eggs of the mackerel and scad were separated from the other plankton, and counted.
As part of the study, as well as the fish eggs, adult mackerel and scad were caught using a pelagic fishing net. On board the Tridens, the roe were removed from the fish, and once back on shore, the number of egg cells per fish and per fish variety were counted. Each individual fish can have between one hundred thousand and one million individual eggs.
The data from this entire study will be sent to the International Council for the Exploration of the Sea (ICES) that coordinates European research. ICES also advises the fishery ministers in Europe on the number of fish available for commercial fishery. The annual fish quotas (TAC) are based on this research data. 

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