The dynamics of gastric evacuation in predatory fish: A mechanistic model of gastric evacuation – development and applications in fish and fishery biology

Predators’ food choice and capacity for food digestion and assimilation is a key element in their physiology and central for our understanding of their role in the ecosystem. The biologists have long recognized the importance of food competition and inter- and intraspecific predation for the dynamics of fish communities.

They have therefore developed multispecies models for fish stock assessment to quantify the interaction strength between predators and prey, and accurate estimates of predation rates are of central importance. The predation rates are usually estimated from information about the amount and species composition of the contents of sampled stomachs of predatory fish by use of models that describe the digestion and gastric evacuation rates of the contents.

A considerable internationally coordinated effort has therefore been invested in sampling and analyses of stomach contents. In contrast, research on gastric evacuation dynamics has been sparse, and established models to describe evacuation rates are in consequence disparate. Based on observed patterns of evacuation rate from historic data corroborated by results from follow up experiments, the mechanistic cylinder model of gastric evacuation presented in this thesis was therefore developed.

Inherent in the model are rules for the primary and interactive effects of relevant prey characters as well as the effects of predator size and temperature. The parameters of the models were estimated from evacuation rate data obtained from an extensive range of laboratory experiments on marine predatory fish, where the model proved its generic value.

Subsequent laboratory tests confirmed the ability of the model to accurately predict the course and rate of gastric evacuation in more complex scenarios of meal composition and feeding pattern relevant for predatory fish under natural conditions. Hence the development of the cylinder model constitutes a significant step towards a better understanding of the dynamics of gastric evacuation in predatory fish.

The model framework provides a versatile tool that, for the first time, accurately describes the evacuation of the individual prey from the stomach and predicts their ingestion times. Implementation of the model in fishery and ecological research should therefore significantly improve the quantification of predator-prey interaction strength in general and facilitate studies on feeding dynamics and small-scale species interactions.

The model further allows for identification of the upper limit of the assimilative capacity in wild predatory fish leading to the development of a new concept of assimilative capacity. This new concept has profound implications for parameterization of functional responses in food web models leading to a more realistic description of predator-prey interactions.