Long-term changes in reef fish assemblages after 40 years of no-take marine reserve protection

Marine reserves can increase the abundance and size of harvested fish species, but the indirect effects of protection on wider fish assemblages are less well understood. Better understanding how marine reserves indirectly effect reef fish is essential to interpreting changes in non-targeted taxa and informing management expectations as to the long-term effects of marine protection. We investigated how reef fish assemblages and habitats have changed within New Zealand's oldest marine reserve after 40 years of no-take protection (1978 to 2018), and how they differed inside and outside the reserve in 2018. Reef fish assemblages differed between 1978 and 2018, and between reserve and fished sites, and these effects were greatest at depths where habitats had changed. We report an overall increase in fish biomass in the reserve, but a long-term decline in total fish density. The increase in biomass under protection was largely driven by an increase in six target species. Eleven species exhibited long-term declines in the marine reserve; three likely a result of indirect effects related to habitat change and/or increased predation, while other declines likely reflect large-scale changes independent of reserve protection. Predicted indirect effects associated with large-scale habitat change were not apparent, except for an increase in one non-target herbivore species alongside increased kelp abundance. While further work is needed to confirm the mechanisms driving individual indirect effects, the results demonstrate that the long-term indirect effects of protection on reef fish are less common, more complex and more difficult to predict than direct effects.