Prey-specific gastric evacuation of Norwegian spring spawning herring (Clupea harengus)
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Gastric evacuation rates have earlier mainly been measured based on laboratory experiments. The aim of this study is to obtain information of gastric evacuation rates when the herring are feeding in their natural environment. Gastric evacuation field study was performed on Norwegian spring spawning herring (Clupea harengus) based on two catches obtained by purs-seining in the Norwegian Sea between 1 and 4 June 2002. After capture the herring were kept in the seine for about 24 hours, and during that time usually more than 20 individuals were sampled from the catch at time intervals of approximately three hours and frozen. The herring sub-samples were taken with gillnet stretched out in the seine. Sea surface temperature was noted at the ships sensors at the time of sampling. In the laboratory the stomach contents were identified to the lowest taxonomic group, dried and weighed, and used to calculate the prey-specific evacuation rates. In general the exponential model gave the best description of the evacuation rates for all the different prey types, but the results were unequivocal. The herring tend to have rapid digestion, and after 24 h most of the stomach contents were evacuated. There were no significant (P<0.05) differences in the gastric evacuation rates between any of the prey categories, and a temperature difference of 1.9 °C had no effect on the gastric evacuation rates. Knowledge of gastric evacuation is important for the quantification of the daily food consumption of herring, and the estimation of the predatory impact of herring in the Norwegian Sea.