Recent expansion of marine protected areas matches with home range of grey reef sharks

Abstract

Dramatic declines in reef shark populations have been documented worldwide in response to human activities. Marine Protected Areas (MPAs) offer a useful mechanism to protect these species and their roles in coral reef ecosystems. The effectiveness of MPAs notably relies on compliance together with sufficient size to encompass animal home range. Here, we measured home range of 147 grey reef sharks, Carcharhinus amblyrhynchos, using acoustic telemetry in New Caledonia. The distribution of home range was then compared to local MPA sizes. We report a home range of 12 km² of reef for the species with strong differences between adult males (21 km²), adult females (4.4 km²) and juveniles (6.2 km² for males, 2.7 km² for females). Whereas local historic MPA size seemed adequate to protect reef shark home range in general, these were clearly too small when considering adult males only, which is consistent with the reported failure of MPAs to protect sharks in New Caledonia. Fortunately, the recent implementation of several orders of magnitude larger MPAs in New Caledonia and abroad show that recent Indo-Pacific MPAs are now sufficiently large to protect the home ranges of this species, including males, across its geographical range. However, protection efforts are concentrated in a few regions and cannot provide adequate protection at a global scale.

Figure 1

Acoustic array, marine protected areas and shark sampling in the New Caledonian archipelago, South-Western Pacific. Red circles indicate acoustic receivers. The number of sharks sampled in the four study regions are displayed. Green contours represent no-take MPAs and pink contours represent no-entry MPAs. Map generated with QGIS Version 3.2 (https://www.qgis.org/) and shapefile data from.

Figure 2

Comparison of home range for grey reef shark adults and juveniles during and outside mating season. UD₉₅ and UD₁₀₀ values represent the surface of outer reef slope habitat encompassed by the 95th and 100th percentile of daily positions. Large dots and bars indicate group means and their bootstrapped 95% confidence intervals. Significance of difference between group means were assessed with pairwise permutation Student tests and displayed with lower case letters. Graphics generated with R package ggplot2 (https://ggplot2.tidyverse.org).

Figure 3

Marine Protected Areas (MPAs) ability to protect grey reef shark home range according to their size. (A) Distribution of home range values were used to model the ability of an MPA covering a given area of outer reef slope to cover sharks home range. This ability was modelled considering all sharks (plain lines) or adult males only (dotted lines). Black and blue lines present the ability of MPAs to cover sharks’ UD₉₅ and UD₁₀₀ respectively. New Caledonian MPA sizes are displayed with vertical dashed lines, in red for old MPAs (before 2010) and in green for recent ones. (B) Indo-Pacific MPAs from the World Database of Protected Areas (UNEP-WCMC, 2014) classified according to creation year and covered area of reef. (C) Location of Indo-Pacific MPAs. Graphics generated with R package ggplot2 (https://ggplot2.tidyverse.org) and rgdal (https://CRAN.R-project.org/package=rgdal).