Abstract
An understanding of the drivers that influence community structure and functioning is essential to predict and manage the impacts of human induced stressors on the marine environment. The current thesis aimed to resolve the relative importance of natural, and anthropogenic induced, variation in kelp abundance versus fishing pressure in driving temperate reef fish community and food web structure. Reef communities were compared in an orthogonal design between marine reserves and fished areas and barren and kelp bed habitat, within the Marlborough Sounds and Fiordland.
Underwater visual surveys of the reef fish community, revealed that reef fish density varied with fishing pressure and the presence of kelp beds, with interactions between the two factors. Blue cod (Parapercis colias) displayed the biggest variation in density between reserves and fished areas, with a larger fishing effect in barren habitats. Further, the density of non-exploited fish species, Evechinus chloroticus, and macroalgae also varied with fishing pressure. The results suggest that fishing pressure is heavier in the Marlborough Sounds, compared to Fiordland. Benthic surveys of kelp abundance, as well as regional variability in the response of fish to the factor KELP, imply there is a difference in the kelp community between the two regions. Fish community structure is likely driven by an interaction between habitat quality and fishing pressure at both the local and regional scale.
Comparison of 8 reef fish species common to both regions found up to 40 % less organic matter from macroalgae supporting omnivorous species in the Marlborough Sounds, compared to Fiordland. Combining trophic position data with stratified surveys of abundance and species biomass, revealed regional differences in the contribution of macroalgae to whole fish communities in terms of density of biomass. In Fiordland over 77% of the biomass of exploited reef fishes was supported by macroalgae, compared to 31% in the Marlborough Sounds. Regional differences in trophodynamics may be explained by a lack of kelp input to the food web in the Marlborough Sounds, as supported by surveys of kelp density.
The contribution of macroalgae to cumulative fish biomass decreased with trophic level in Marlborough, while the high trophic level fish species in Fiordland were supported by an equal mix of phytoplankton and macroalgae, providing further support for the hypothesis of lower kelp input in the Marlborough Sounds. Here we find that regional differences in fish community and food web structure result in the Fiordland fish community using 2.91 times more organic matter per unit area than the community in Marlborough.
The results suggest there is likely an important interaction between differences in kelp bed habitat and changes in reef fish community structure wrought by fishing, for controlling reef fish communities. This thesis corroborates the importance of kelps as foundation species on temperate reefs and highlights the need to recognise spatial variability, and multiple environmental stressors, when designing management approaches to protect the integrity of temperate reef ecosystems.