The energetics of fish growth

Ecologists have long been interested in the causes of differences in trophic structures in marine systems.  A new study by Diego Barneche of the Centre for Geometric Biology and Andrew Allen of Macquarie University, that has been looking at the theoretical underpinning of the growth patterns in fish, has found that the ‘cost of growth’ is paramount to determining how energy moves between levels in the food chain.

Diego and Andrew were particularly interested in using a model framework that allowed for the inclusion of this previously neglected parameter – the cost of growth – which is the amount of energy that must be expended in respiration to produce a fixed quantity of biomass.  The researchers used existing datasets to investigate the relationship between the energy allocated to growth at different developmental stages, temperatures, levels of activity and position in the food chain.

They have demonstrated the ‘cost of growth’ limits the possible structure of food webs because it has an effect on the efficiency of energy transfer between the different levels of the food chain. For example the model suggested that fish higher up the food chain had to expend more energy to produce a fixed quantity of biomass.

This figure shows the modelling outputs relating the energy needed to gain a unit of biomass to the developmental stage (ontogenetic stage) of prey at the time of predation.

Energy transfer is more efficient if the prey are young and sedentary and recognizing that the efficiency of energy transfer changes not only with the developmental stage of the prey but also with temperature, position in the food chain and activity levels is an important step forward.

The model demonstrated that top heavy food web structures were ecologically and energetically unlikely (red area in accompanying figure) because the energy needed to gain a unit of biomass would have to be very low meaning that energy transfer between levels would have to be extremely efficient. While this could be achieved by exclusive predation on offspring which would provide a high-efficiency prey resource (young, sedentary prey requiring little movement of predators), it is a short term solution.

Understanding the energetics of growth is of immense importance, particularly in the field of fisheries science.  Knowing how long wild fish stocks take to achieve maturity and how much food they need to do so, is crucial for establishing sustainable yields.

This research is published in Ecology Letters.