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Modelling fish bioenergetics, behaviour, predator-prey interactions in environmental gradients

Constraints from bioenergetics have repeatedly been used to explain behaviour, for example how optimal foraging maximizes energy intake or how habitat selection balances the trade-off between survival and growth. In aquatic environments, however, bioenergetics is complicated by the fact that oxygen availability may vary and even reach harmfully low concentrations; in addition, the respiration cost is also higher than in air. In aquatic habitats, the energy budget of an animal is therefore paralleled by an oxygen budget, which may change behavioural trade-offs fundamentally. We developed a state-dependent model for fish bioenergetics that includes the flow of both energy and oxygen through the organism. The oxygen for metabolism comes from aerobic respiration or anaerobic lactate build-up, and increased lactate reduces swimming ability and thus the probability that the individual can escape predators.

The environment�s oxygen concentration puts a constraint on maximum aerobic respiration; other constraints are imposed by encounters with predators and prey given the ambient light conditions, flow through the digestive tract, and the capacity to store lactate. Hypoxic water layers are often safer from predators but leave little oxygen for swimming (predator escapement and feeding) and digestion (growth and avoiding starvation). We investigated the emerging trade-offs as individuals have to balance oxygen flow, energy flow, predation risk, and starvation risk in a vertically structured water column. By using models, we show how hypoxic layers may i) decrease mortality by serving as a refuge from predators or ii) increase mortality and reduce growth by increasing exposure to predators.

Aquatic environments are typically characterised by strong vertical gradients of light, oxygen content, temperature, prey, and predator densities. In the Oslo fjord, sprat (Sprattus sprattus) actively utilise vertical gradients of light and oxygen saturation to maximise survival during their over wintering period. The deep layers are hypoxic and provide safety because predators are scarce.

By remaining in hypoxic habitats, however, sprat build up an oxygen debt that must be repaid at some point. In addition, oxygen-demanding activities such as foraging, digestion, growth, or escape-reactions come at the cost of increased predation risk, since sprat are forced to seek oxygen in water layers where predators are present. We developed predictions on optimal habitat use and foraging activity of fish faced with this trade-off using a state-dependent bioenergetics model accounting for the flow of both oxygen and energy in an organism.

Sprat stay in hypoxic layers during the day, and seek oxygen-rich waters during night when encounters with predators are rarer. We also explore interactions between internal physiological constraints and external environmental factors such as prey- and predator density and vertical profiles of hypoxia, light, and temperature. Behavioural strategies depend strongly on the distribution of oxygen in the vertical habitat. This illustrates the need to integrate internal physiology with external ecology to fully understand both individual behavioural and population level responses of fish under environmental stress.

Reported by

University of Bergen
Dept of Biology, Pbo 7800
5020 Bergen
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