Fur seal activity moderates the effects of an Australian marine sanctuary on temperate reef fish

Offshore islands are often preferentially selected for marine sanctuaries (no take areas) over inshore reefs on populated coasts because they support relatively unique marine communities and critical habitat for protected marine wildlife and are often less accessible to people. To evaluate whether marine sanctuaries around an offshore island with a large fur seal colony performed differently to inshore areas, we compared fish assemblages at 36 reef sites within marine sanctuaries and fished areas around an offshore Island and within inshore areas of the Batemans Marine Park in temperate southeastern Australia. In each site, we quantified fish assemblage structure using 3–4 replicate baited remote underwater video deployments. We also evaluated the role of fur seal activity, sea surface temperature and wave exposure in structuring fish assemblages on reefs inside and outside marine sanctuaries. Of these variables, only fur seal activity on shallow island reefs (∼10 m deep) was significantly related to fish assemblage structure with a negative relationship to fish richness and abundance. The fish assemblages in marine sanctuaries differed significantly from fished areas on deep reefs around Montague Island (30–40 m deep) and on shallow inshore reefs (∼10 m deep), with 119% and 45% more fish and 49% and 19% more species of fishes in marine sanctuaries in the two areas, respectively. There were also significantly more kyphosids, labrids, southern maori wrasse (Ophthalmolepis lineolatus) and silver sweep (Scorpis lineolata) in marine sanctuaries than in fished areas on deep reefs around Montague Island. Although there was, on average, 34% more fish on shallow reefs inside than outside marine sanctuaries around Montague Island, there were no significant differences in fish assemblages when the influence of fur seal activity was taken into account. Given that fur seals numbers are increasing and they eat reef fishes, marine sanctuaries on shallow reefs adjacent to seal colonies may not achieve some of the positive benefits associated with increased fish biomass (e.g. increased kelp cover) to same level as sanctuaries more distantly located. We suggest, therefore, that careful consideration of long-term conservation goals (e.g. conservation of fur seals or local enhancement of fish diversity) is needed before preferentially locating marine sanctuaries adjacent to fur seal colonies.