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This study was funded by the Chevron-operated Wheatstone Project's State Environmental Offsets Program and administered by the Department of Biodiversity, Conservation and Attractions. The Wheatstone Project is a joint venture between Australian subsidiaries of Chevron, Kuwait Foreign Petroleum Exploration Company (KUFPEC), Woodside Petroleum Limited and Kyushu Electric Power Company, together with PE Wheatstone Pty Ltd (part owned by TEPCO). Funding support was also received from the Gorgon Barrow Island Net Conservation Benefits Fund, which is administered by the Department of Biodiversity, Conservation and Attractions. Thank you to Helen Chedzey and Mark Broomhall from the Remote Sensing and Satellite Research Group at Curtin University for generating and providing the remotely sensed total suspended solids data for this study.

Natural fluctuations in the abundance and length of targeted fish are often disrupted by acute environmental changes and anthropogenic impacts, particularly fishing pressure. Long-term assessments of targeted fish populations inside and outside areas closed to fishing are often necessary to elucidate these effects, yet few of these studies extend over long time periods. We assessed trends in the abundance and length of six targeted fish species in areas open and closed to fishing on seven occasions spanning a 9-year period (2005–2010 and 2013) at the Houtman Abrolhos Islands, Western Australia. Shallow (8–12 m) and deep (22–26 m) coral-dominated reef sites were sampled across four geographically separated island groups using baited remote underwater stereo-video (stereo-BRUV). Between 2005 and 2010, populations of Lethrinus miniatus, Lethrinus nebulosus, Plectropomus leopardus, and Chrysophrys auratus became increasingly dominated by larger individuals, potentially indicative of an ageing population. Between 2010 and 2013, however, there was a significant increase in the proportion of smaller L. miniatus, L. nebulosus, and P. leopardus in both open and closed areas, reflecting increased recruitment perhaps due to changing environmental conditions associated with a marine heat wave anomaly. This recruitment pulse was not observed for the other species in this study (Chr. auratus, Choerodon rubescens, and Glaucosoma hebraicum). Lethrinus miniatus, L. nebulosus, Chr. auratus, and P. leopardus were larger in closed areas relative to open areas; however, they were not more abundant. These complex responses to protection also varied across sampling years for certain species (e.g., P. leopardus). Monitoring changes over the long-term in areas open and closed to fishing provides a sound basis for separating environmental variability from that associated with fishing mortality, which is crucial for optimising fisheries management.

Areas closed to fishing are advocated as both fisheries management and biodiversity conservation tools. However, few studies investigate the responses of suites of both target and non-target fish species within an assemblage, which is an important consideration for ecosystem-based fisheries management approaches. Diver-operated stereo-video was used to assess the abundance and length of coral reef fish across multiple areas both open and closed to fishing at the Houtman Abrolhos Islands, Western Australia. After taking into consideration spatial differences in benthic habitat, the composition of fish assemblages was found to differ between open and closed areas. The target species, Plectropomus leopardus, was approximately two times more abundant in closed areas. Furthermore, 51 % of P. leopardus were larger than the minimum legal length (MLL) for retention in closed areas compared with only 1.8 % in areas open to fishing. Another target species, Choerodon rubescens was surveyed in greater abundance at sizes larger than the MLL in closed areas (64 % >400 mm) in comparison with areas open to fishing (36 %). A number of non-target species were also larger in closed areas (e.g., Kyphosus cornelii, Scarus schlegeli). In contrast, several non-targeted prey species were more abundant in open areas (e.g., Pomacentrus milleri was six times more abundant in open areas). Our results document complex responses of target and non-target species in closed areas at the Houtman Abrolhos Islands.

Diel variation is known to alter the composition and structure of reef fish assemblages. What is unknown is how nocturnal fish assemblages respond to closed fishery area management. Diurnal and nocturnal reef fish assemblages at the Houtman Abrolhos Islands, Western Australia, were studied using baited remote underwater stereo-video systems (stereo-BRUVs). Surveys were conducted during the day and at night (using blue illumination) from three sites inside and three sites outside a closed fishery area (CFA). The relative abundance of 116 fish species from 41 families was recorded. Significant changes were observed in fish assemblage structure from day to night (driven by high dispersion at night) and in areas open versus closed to fishing (driven by increased abundance within the CFA). The effect of protection from fishing was consistent for both diurnal and nocturnal fish assemblages. Closer examination of six targeted fish species showed that their response to diel changes and the absence of fishing pressure varied from species to species. The targeted fishes Pagrus auratus and Glaucosoma hebraicum were sampled during the day and at night with both species responding positively to protection from fishing. Results suggest that the inclusion of diurnal and nocturnal fish assemblage data will provide an improved ability to assess the effectiveness of closed fishery area management.