Neither more food nor better food – still fish biomass increases
To increase the biomass of fish, contemporary ecological theory predicts that either the amount of food or the quality of the food has to increase. In a recent publication in Nature Communications researchers of Umeå University, Sweden, and the Institute for Biodiversity and Ecosystem Dynamics (IBED, University of Amsterdam) report that the fish biomass under identical food supply and food quality doubled by a change in the fractions of the total food supply that was channeled to juvenile and adult fish, respectively. The results have major implications for the exploitation (harvest) of fish populations and the coexistence of predatory fish and their prey.
Quantity vs. nutrition
To increase the biomass of a population, contemporary ecological theory says that it has to be supplied with more energy either through an increase in the total amount of food supplied or through supplying food that is more nutritious. When the amount and the energy content of the food are kept constant, a biomass increase can only be achieved by changing to another species that uses the supplied food more effectively. In contrast, a collaborative study by Dr Birte Reichstein and Prof. Lennart Persson at Umeå University (Sweden) and Prof. André de Roos at the Institute for Biodiversity and Ecosystem Dynamics (University of Amsterdam) now provides experimental evidence that the biomass of the Least Killifish (Heterandria formosa) increases even when the amount and the energy content of the food supply are kept constant.
Food for (thought on) growth and reproduction
All fish populations contain individuals of different sizes, as do most other species’ populations on Earth (e.g. insects and amphibians). Least Killifish females can grow up to a length of 41 mm and give birth to free swimming, 5-6 mm long, off-spring. How effectively an individual can use a certain amount of food for growth and reproduction depends on its size. In the case of the Least Killifish we know that small individuals are more efficient than large individuals. This difference in efficiency leads to that biomass production depends on how the food supply is channeled between individuals of different sizes. Switching from an equal distribution of food between small and large individuals to a distribution where the less efficient large individuals received two thirds of the food doubled the fish biomass.
The results have major implications for the exploitation of natural resources (e.g. fisheries) and under what conditions predator and prey fish can coexist. A predator can only persist if its prey is abundant enough. This means that compared to an equally distributed food supply a predator species that preys on the Least Killifish can invade at a lower food supply to the prey fish when a larger proportion of the total food supply is available to large individuals than to small individuals.
Reichstein, B., Persson, L., De Roos, A.M., Ontogenetic asymmetry modulates population biomass production and response to harvest. Nature Communications, doi: 10.1038/ncomms7441 (2015).