Gastric evacuation in horse mackerel. I. The effects of meal size, temperature and predator weight

Gastric evacuation experiments were performed on horse mackerel Trachurus trachurus. A nearly full matrix experimental design with respect to the variables predator weight (< 10-400 g) meal size (up to 7.8% body weight) and temperature (10-20° C) was covered with 0-group smelt Osmerus eperlanus as prey. A general evacuation model without meal size as a variable was fitted to the data on wet weights as well as on dry weights by means of non-linear regression technique. Two methods of data transformation, relative data and square root transformation, were applied to improve variance homogeneity. The most reliable model fit was achieved on dry weight data applying the square root transformation technique: S_t=[S₀^0·31 - 0·109t W^0·248 e^0·032T]^3·23 where S_t=stomach content (g wet weight) at time t after ingestion, S₀=the initial meal size, W=predator (g wet weight), and T= temperature. The estimated coefficient of the exponential temperature function, δ=0·032, corresponds to a Q₁₀ value of 1·4 which is outstandingly low in comparison with results on other species. However additional experiments to determine maximum daily food rations indicated that appetite in contrast to gastric evacuation is strongly temperature dependent.