Laboratory experiments on the gastric evacuation of juvenile herring ( Clupea harengus L.)

Laboratory experiments were conducted on groups of 9.9 to 13 cm (mean) total length (L T) juvenile herring ( Clupea harengus L.) to describe the temperature dependency of the gastric evacuation. Experiments were conducted at 13 and 16 °C. The parameters of the general gastric evacuation model were B = 0.953 and R = 0.082 at 13 °C and B = 1.323 and R = 0.843 at 16 °C indicating an exponential evacuation. The evacuation constants of the exponential gastric evacuation model were R = 0.098 at 13 and R = 0.202 at 16 °C resulting in a temperature dependency parameter (A) of 0.241. The internal dynamics of the stomach and its storage capacity were studied by the use of stained copepods. Investigations on the functional response of herring to different food densities were described by the Michaelis-Menten equation. Our experimental design allowed us to test the hypothesis of Richter et al. [Richter, H., Lückstädt, C., Schlechtriem, C., Focken, U., Becker, K., 2002. Do the fish stomach content models currently used for daily ration estimation really work? Evidence of violation of a basic assumption concerning stomach evacuation rates. Arch. Fish. Mar. Res. 49 (3), 171–188] that fish have two evacuation modes, one evacuation constant while only evacuating food and a significant higher constant while feeding, which would question the basic assumption of most consumption models, that only use one evacuation constant for consumption estimates. The experiments of Richter et al. [Richter, H., Lückstädt, C., Schlechtriem, C., Focken, U., Becker, K., 2002. Do the fish stomach content models currently used for daily ration estimation really work? Evidence of violation of a basic assumption concerning stomach evacuation rates. Arch. Fish. Mar. Res. 49 (3), 171–188] suggested that a doubled constant would give the best match of observed feeding and estimates of consumption models. Through the results of simulations of two experiments in which we combined observed snatching rates, estimated evacuation constants and particle weights of consumed copepods, we cannot support this hypothesis. A doubled evacuation constant did not improve our simulations, on the contrary the simulations with a doubled constant gave a worse fit with our observed values. Knowledge on the gastric evacuation is a prerequisite for the evaluation of a potential top-down impact of this zooplanktivorous species on the plankton community.