The consequences of complex habitat loss for the New Zealand blue cod, Parapercis colias
Climate driven threats are predicted to decrease the complexity of biogenic habitats. Within temperate coastal marine environments, we know that complex macroalgal beds support more complex communities through the provision of microhabitats and refuges. Macroalgal habitats have potential interacting benefits and costs for predators, as increased macroalgal biomass supports higher richness and diversity of prey species, but prey within these habitats might be more difficult to catch. An important New Zealand fishery species, the blue cod (Parapercis colias), is a large bodied temperate reef fish found exclusively throughout the coastal waters of New Zealand. Its dependence on subtidal coastal reef environments mean that it is important to understand how a loss of complex macroalgal habitats might alter the way that blue cod forage, and how the trade-off between prey abundance and availability will affect its abundance and productivity. This thesis aims to understand the influence of complex macroalgal habitats on P. colias prey availability and behaviour, on the foraging success of P. colias, and ultimately on P. colias population dynamics. Experiments were conducted using choice chambers to evaluate whether two alternate P. colias prey, Forsterygion lapillum and Heterozius rotundifrons, showed a preference for complex habitats with and without predation risk. Both species preferred complex habitats in the absence of predation cues, but F. lapillum showed a more consistent preference for complexity in response to predation risk. A mesocosm experiment was used to investigate whether the consumption rate and functional response of P. colias differs for these two prey types in the presence and absence of habitat complexity. Results indicated that the mobile fish prey, F. lapillum benefitted from the refuges provided by complexity and suffered lower consumption rates, whereas the sedentary crab, H. rotundifrons did not. Finally, using a simple population model, the trade-off between prey abundance and predation success on the population dynamics of P. colias with and without habitat complexity was explored. Models showed that scenarios with complex macroalgal habitats generally support more predators, and faster population growth rates than scenarios lacking habitat complexity. However, scenarios with complex habitats were predicted to be more sensitive to fishing pressure and have the potential to be more vulnerable to overexploitation. These results highlight the importance of understanding how habitat complexity mediates relationships between commercially important fishery species and their prey, in order to understand how habitat loss may alter their foraging success and population dynamics.