Declining abundance of coral reef fish in a World-Heritage-listed marine park

Abstract

One of the most robust metrics for assessing the effectiveness of protected areas is the temporal trend in the abundance of the species they are designed to protect. We surveyed coral-reef fish and living hard coral in and adjacent to a sanctuary zone (SZ: where all forms of fishing are prohibited) in the World Heritage-listed Ningaloo Marine Park during a 10-year period. There were generally more individuals and greater biomass of many fish taxa (especially emperors and parrotfish) in the SZ than the adjacent recreation zone (RZ: where recreational fishing is allowed) — so log response ratios of abundance were usually positive in each year. However, despite this, there was an overall decrease in both SZ and RZ in absolute abundance of some taxa by up to 22% per year, including taxa that are explicitly targeted (emperors) by fishers and taxa that are neither targeted nor frequently captured (most wrasses and butterflyfish). A concomitant decline in the abundance (measured as percentage cover) of living hard coral of 1–7% per year is a plausible explanation for the declining abundance of butterflyfish, but declines in emperors might be more plausibly due to fishing. Our study highlights that information on temporal trends in absolute abundance is needed to assess whether the goals of protected areas are being met: in our study, patterns in absolute abundance across ten years of surveys revealed trends that simple ratios of abundance did not.

Oceanography and Marine Biology

“…Babcock et al (2008) compared results from surveys in 2006-07 with those of previous surveys (Ayling & Ayling 1987, Westera et al 2003 and found lower abundance of lethrinids, suggesting that their abundance has declined over time. Ten years of surveys by Vanderklift et al (2019) support this, finding parallel declines inside and outside the Mandu Sanctuary Zone. The abundance of Labridae (wrasses) and Chaetodontidae (butterflyfish) have also declined, while other families, including parrotfish and surgeonfish, do not appear to have changed.…”

“…However, the abundance of corals in some sheltered locations south of Point Cloates has declined steadily since 2011 (Holmes et al 2017), and localised decreases in coral cover have also occurred north of Point Cloates (e.g. Vanderklift et al 2019). Cyclones might have caused some mortality, but the declines are coincident with major warming events and seem most likely to be caused by water temperatures exceeding thermal thresholds.…”

“…In parts of the western Indian Ocean, sea urchins such as Echinometra mathaei have become abundant, probably because the abundance of predatory fish that eat them has been reduced by unsustainable rates of fishing (McClanahan 1995(McClanahan , 2008. At Ningaloo, there is no obvious correlation between the abundance of E. mathaei and the abundance of its predators (Babcock et al 2008), but the abundance of lethrinids (one of the predators of E. mathaei) is decreasing (Vanderklift et al 2019).…”

The status of current and tentative marine natural World Heritage areas

“…Recent research uncovered potential shortfalls in the terrestrial natural WHA estate, documenting an average 63% increase in human pressures (Allan et al 2017), potentially compromising the outstanding universal value of several sites. Increasing human pressures have also been documented within marine natural WHAs (mnWHA), including a 22% decline in coral reef fish abundance per year in the Ningaloo Marine Park WHA (Vanderklift et al 2019) and a loss of around 50% of coral within the Great Barrier Reef WHA (De’ath et al 2012). The human impacts driving these declines, however, have yet to be quantified across the global mnWHA estate.…”