Partitioning no-take marine reserve (NTMR) and benthic habitat effects on density of small and large-bodied tropical wrasses.

Garry R. Russ,Brock J. Bergseth,Justin R. Rizzari,Jake R. Lowe,Angel C. Alcala,Heather M. Patterson

doi:10.1371/journal.pone.0188515

Abstract

No-take marine reserves (NTMRs) are increasingly implemented for fisheries management and biodiversity conservation. Yet, assessing NTMR effectiveness depends on partitioning the effects of NTMR protection and benthic habitat on protected species. Such partitioning is often difficult, since most studies lack well-designed sampling programs (i.e. Before-After-Control-Impact-Pair designs) spanning long-term time scales. Spanning 31 years, this study quantifies the effects of NTMR protection and changes to benthic habitat on the density of tropical wrasses (F. Labridae) at Sumilon and Apo Islands, Philippines. Five species of wrasse were studied: two species of large-bodied (40–50 cm TL) Hemigymnus that were vulnerable to fishing, and three species of small-bodied (10–25 cm TL) Thalassoma and Cirrhilabrus that were not targeted by fishing. NTMR protection had no measurable effect on wrasse density, irrespective of species or body size, over 20 (Sumilon) and 31 (Apo) years of protection. However, the density of wrasses was often affected strongly by benthic cover. Hemigymnus spp. had a positive association with hard coral cover, while Thalassoma spp. and Cirrhilabrus spp. had strong positive associations with cover of rubble and dead substratum. These associations were most apparent after environmental disturbances (typhoons, coral bleaching, crown of thorns starfish (COTS) outbreaks, use of explosives and drive nets) reduced live hard coral cover and increased cover of rubble, dead substratum and sand. Disturbances that reduced hard coral cover often reduced the density of Hemigymnus spp. and increased the density of Thalassoma spp. and Cirrhilabrus spp. rapidly (1–2 years). As hard coral recovered, density of Hemigymnus spp. often increased while density of Thalassoma spp. and Cirrhilabrus spp. often decreased, often on scales of 5–10 years. This study demonstrates that wrasse population density was influenced more by changes to benthic cover than by protection from fishing.

Highlights

Coral reef ecosystems are under increasing threat from both natural and anthropogenic environmental disturbances[1,2,3], and appear vulnerable to global warming [2, 3], increased frequency and intensity of storms [4, 5], declining water quality [4, 6] and overfishing [7]
This study demonstrates the importance of benthic habitat as a determinant of wrasse density
Three key findings stand out: 1) Densities of large-bodied wrasses targeted by fishing were not affected by No-take marine reserves (NTMRs) protection; 2) benthic composition drove densities of wrasses in the longterm; 3) benthic composition drove assemblage structure of wrasses

Summary

Introduction

Coral reef ecosystems are under increasing threat from both natural and anthropogenic environmental disturbances[1,2,3], and appear vulnerable to global warming [2, 3], increased frequency and intensity of storms [4, 5], declining water quality [4, 6] and overfishing [7] These disturbances often modify processes and structure in the benthic habitat, with subsequent effects on reef fish populations [8,9,10]. Parrotfish and goatfish that often feed over dead surfaces (sand, rubble, dead coral), commonly increase in density following shifts in benthic cover from live coral to dead surfaces [11, 16, 17] These changes in reef fish density can be sustained for years to decades, but eventually fish populations return to pre-disturbance states as hard coral recovers [8,9,10]. While some reef fish increase in local density due to disturbance events, it is currently accepted that environmental disturbances to the benthos affect density of more reef fish groups negatively than positively [11, 14, 18]

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