Abstracts (first author)
Small fish, big fish: modelling ecological consequences of life-history evolution in harvested stocks
An increasing amount of evidence shows that fishing and climate change are changing fish life-histories. Empirically determined rates of change are usually within the range of 0.5-3% per year and they are positively correlated with fishing intensity, suggesting that some of the change is due to fisheries-induced evolution. Many fish now mature at younger age and smaller size and this trend is expected to continue. We used the marine ecosystem model Atlantis to explore how a slow (<0.1% per year) but continuous decrease in size-at-age of five harvested SE Australian fish 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. For some species, e.g. tiger flathead, a 4% decrease in length-at-age over 50 years resulted in a 50% increase in predation mortality. However, the magnitude and direction in predation mortality changes differed among shrinking species. The impact that shrinking of five species had on many functional groups in the ecosystem (change in biomass and diets) was similar and 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 currently being explored in the ecosystem model that allows for the dynamic evolution of most species.