Additive and synergistic impacts of fishing and warming on the growth of a temperate marine fish

Fishing activity and environmental variation can both have profound impacts on the trajectory and variability of marine populations. Fishing can alter the demographic structure and functioning of populations and exert... [ view full abstract ]

Fishing activity and environmental variation can both have profound impacts on the trajectory and variability of marine populations. Fishing can alter the demographic structure and functioning of populations and exert non-random genetic selection, whilst changes in physical conditions such as temperature can directly affect aquatic animal physiological performance or indirectly affect fitness through alterations to interspecific and trophic interactions. Despite the wealth of work undertaken in marine environments on the causes of biological change, there remains a tendency to treat natural and anthropogenic drivers of marine systems in isolation or just additively. This approach overlooks the potential for synergistic or antagonistic interactions to occur. Indeed, it is increasingly becoming acknowledged that the direction and magnitude of biological responses to natural environmental variation and climate change can be mediated by other anthropogenic disturbances such as fishing, and vice versa.
Somatic growth is an ideal candidate with which to explore additive and synergistic impacts of fishing and environmental variability due to its strong biological relevance and its heightened sensitivity to natural and anthropogenic drivers. I developed 19-year growth biochronologies (1980-1999) for three south-east Australian populations (southerly range limit) of a site-attached temperate reef fish, purple wrasse (Notolabrus fucicola) using individual-based growth information naturally archived in otoliths. The south-east Australian purple wrasse commercial fishery began in the early 1990s; before this there was negligible recreational or commercial fishing. The growth of older fish was proportionally higher and that of the youngest fish proportionally lower after the onset of commercial fishing; 2-year olds grew 7.4% slower, but 5-year-olds grew 10.3% and 10-year-olds 26% faster in the latter period. These results are consistent with a density dependent response to harvesting. Average growth rates across all ages increased by 6.6%. oC-1, reflecting either a direct or indirect temperature effect. Finally, the distribution of individual thermal reaction norms significantly changed post fishing, showing that fishing and temperature can have a synergetic impact on marine populations. Understanding the relative importance of, and interaction between, natural and anthropogenic drivers in shaping marine systems provides valuable ecological and evolutionary context that is essential to sound fisheries management and species conservation.