An increasing amount of evidence shows that fishing and climate change are changing fish life-histories. Many fish now mature at younger age and smaller size and this trend is expected to continue. We use a South East Australian marine ecosystem model implemented in the Atlantis modelling framework to explore how changes in body size and maturation age of harvested species may affect their demography, species interactions and recovery potential. We found that even small decreases in fish sizes are amplified by positive feedback loops in the ecosystem and can lead to major changes in their natural mortalities. However, the magnitude and direction in predation mortality changes differed among the ‘shrinking’ species. The impact that reduced growth rates of some harvested species had on many functional groups in the ecosystem (change in biomass and diets) was comparable to the impact caused by the introduction of moderate fishing into an unfished system. Some of the ‘shrinking’ species failed to recover to the previous biomass levels even after fishing was completely stopped. The natural mortality of these species remained high, suggesting that the ecosystem had moved into a new state. Our model shows that human induced reduction in fish size will alter predation regimes, changing species interactions and strength of natural selection. The outcomes of this interplay between natural and fisheries induced selection on harvested stocks will determine the final rate of phenotypic change and is difficult to predict from single species models or experiments.
Asta Audzijonyte
Fisheries and Environmental Management Group
Department of Environmental Sciences
Viikinkaari 2, P.O. Box 65
FIN-00014 University of Helsinki