Sampling fish larvae with large pumps; quantitative and qualitative comparisons with traditional gear. In: The propagation of cod Gadus morhua L.: an international symposium, Arendal, 14 - 17 June 1983
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Different types of submersible pump systems, ranging in capacity from 3 to 60 m^3/min, have been tested during the period 1977-83 to collect fish eggs and larvae from discrete depths in the Lofoten area. Comparisons have been made with regard to the quality and quantity of eggs and larvae sampled with different plankton nets. Almost without exception the pump samples contain more cod eggs and larvae per m^3 than vertical net hauls, indicating a significantly lower filtration efficiency than the 100% used in the calculations. The quality of larvae sampled by vertically hauled nets was very good. The quality of the larvae depends upon the filtration pressure and time from capture to fixation of the larvae. Thus larvae sampled by pumps with high water velocity and the high speed sampler Gulf-III had a high percentage of damaged larvae. Due to the time lag from capture to fixation, most of the larvae collected with the MOCNESS were also in a bad condition. A large filtration system, 60 m^3/min, based on a submersible 80 cm slow-rotating propeller, is described. Comparisons with MOCNESS for quantitative reasons and to the vertical net for qualitative reasons showed good agreement.