The direct and indirect effects of marine reserve protection on reef fish assemblages

Abstract

Marine reserves are established to protect marine ecosystems from fishing, allowing harvested fishes to recover: these are the “direct effects” of protection. However, changes may also occur among species not typically exposed to fishing mortality, via “indirect effects”. Most marine reserve studies have focused on direct effects, and the prevalence and relative importance of indirect effects is poorly understood. Understanding both direct and indirect effects is key to understanding how fishing affects wider fish assemblages. To investigate the direct and indirect effects of marine reserve protection on reef fish assemblages I examined monitoring data from four northeastern New Zealand marine reserves. This long-term analysis also provided a wider assessment of how reef fish have changed over the last 2-4 decades in this region. Across all reserves examined, the large predatory sparid Chrysophrys auratus responded strongly to protection. This response was hypothesised to drive indirect effects on other species, as a result of increased predation, competition, or predator-induced habitat changes. Long-term changes in reef fish assemblages in New Zealand’s oldest marine reserve at Leigh suggested that along with direct effects on six targeted species, snapper recovery had indirect effects on three small-bodied non-target fishes and one herbivorous fish. Comparisons across three reserves revealed that the potential indirect effects identified in Leigh were not ubiquitous. While direct effects on target species like snapper and cheilodactylid Cheilodactylus spectabilis were largely predictable, potential indirect effects were more location-specific and difficult to differentiate from the effects of environmental gradients. At an offshore island marine reserve, with a more speciose temperate-subtropical fish fauna, potential indirect effects existed for just 3 wrasses, despite dramatic direct effects of protection on snapper. There was no evidence of tropicalisation of fish assemblages at this offshore island marine reserve over the last two decades. This study demonstrated that while protected fish assemblages are distinct, this is mostly driven by the recovery of harvested species. Potential indirect effects of long-term protection were identified, but were rarely consistent across multiple reserves. Trophic links between reef fishes appear weak, and the role of indirect effects is likely overwhelmed by the direct effects of fishing.