Condition of fish in catches of pelagic fisheries: Post capture fish condition within several pelagic fisheries

Pelagic fisheries in European waters target small pelagic fish such as herring, mackerel, horse mackerel, blue whiting and silver smelt. The Dutch pelagic fishery is performed with pelagic freezer trawlers. Catches are pumped from the cod-end on board into the Refrigerated Seawater (RSW) tanks. From the RSW tanks fish are fed into the on-board fish processing factory where fish are sorted, frozen and packed. To gain insight in at what point fish die in catch-processing of pelagic freezer trawlers, the condition of fish sampled from the catch-processing directly after the fish-water separators was assessed during five research trips on four different vessels in the period December 2018 – December 2020. Condition of individual fish was assessed by scoring the impairment of a set of nine reflexes. Fish condition was expressed as RAMP score, calculated from the reflex scores (lower RAMP score represents better condition). Per haul the counts of living fish were expressed as percentage of the total number of fish sampled to determine the percentage of fish that was alive at the sampling point (the fish-water separator just before entering the RSW tanks). For all tested species the %Alive ranged widely among trips and hauls. Weighted mean %Alive and mean RAMP scores of fish that were alive when scored, were 81.2% and 0.28 for horse mackerel (799 fish, 3 trips, 34 hauls), 22.6% and 0.47 for mackerel (450 fish, 2 trips, 10 hauls), 56.0% and 0.41 for summer herring (113 fish, 1 trip, 11 hauls), 82.6% and 0.42 for winter herring (1150 fish, 2 trips, 14 hauls) and 68.7% and 0.40 for Atlanto-Scandian herring (385 fish, 1 trip, 14 hauls). Variation in %Alive and fish condition may be caused by haul duration, catch weight and the time the cod-end is behind the vessel before the pumping of fish on board starts. Dissolved oxygen concentration, pH and temperature were measured in the water in RSW tanks during one trip for horse mackerel (7 hauls) and herring (2 hauls). Oxygen levels in RSW tanks quickly decline upon introduction of the fish, probably due to oxygen consumption by fish.