Disentangling drivers of the abundance of coral reef fishes in the Western Indian Ocean.

Melita A Samoilys,Kennedy Osuka,Andrew Halford

doi:10.1002/ece3.5044

Abstract

AimUnderstanding the drivers of the structure of coral reef fish assemblages is vital for their future conservation. Quantifying the separate roles of natural drivers from the increasing influence of anthropogenic factors, such as fishing and climate change, is a key component of this understanding. It follows that the intrinsic role of historical biogeographical and geomorphological factors must be accounted for when trying to understand the effects of contemporary disturbances such as fishing.LocationComoros, Madagascar, Mozambique and Tanzania, Western Indian Ocean (WIO).MethodsWe modeled patterns in the density and biomass of an assemblage of reef‐associated fish species from 11 families, and their association with 16 biophysical variables.ResultsCanonical analysis of principal coordinates revealed strong country affiliations of reef fish assemblages and distance‐based linear modeling confirmed geographic location and reef geomorphology were the most significant correlates, explaining 32% of the observed variation in fish assemblage structure. Another 6%–8% of variation was explained by productivity gradients (chl_a), and reef exposure or slope. Where spatial effects were not significant between mainland continental locations, fishing effects became evident explaining 6% of the variation in data. No correlation with live coral was detected. Only 37 species, predominantly lower trophic level taxa, were significant in explaining differences in assemblages between sites.Main ConclusionsSpatial and geomorphological histories remain a major influence on the structure of reef fish assemblages in the WIO. Reef geomorphology was closely linked to standing biomass, with “ocean‐exposed” fringing reefs supporting high average biomass of ~1,000 kg/ha, while “lagoon‐exposed fringing” reefs and “inner seas patch complex” reefs yielded substantially less at ~500kg/ha. Further, the results indicate the influence of benthic communities on fish assemblages is scale dependent. Such insights will be pivotal for managers seeking to balance long‐term sustainability of artisanal reef fisheries with conservation of coral reef systems.

Highlights

Biodiversity is declining globally as a result of direct human impacts including overexploitation of natural resources which in turn threatens ecosystem functioning (Butchart et al, 2010; Mora et al, 2011)
Our results suggest that the productivity of reefs in the Western Indian Ocean (WIO) in terms of fish biomass depends on their geomorphology, exposure, and nutrient levels
We suggest the biomass of Epinephelinae, Lutjanidae, Lethrinidae, and Haemulidae may be a useful metric, as these families contain widely exploited target fishery species in coral reef fisheries (Samoilys & Carlos, 2000) and have been regularly surveyed in UVC surveys in the WIO over the last 20 years (Obura, et al, 2017)

Summary

| INTRODUCTION

Biodiversity is declining globally as a result of direct human impacts including overexploitation of natural resources which in turn threatens ecosystem functioning (Butchart et al, 2010; Mora et al, 2011). Coral reefs are one of the most biodiverse ecosystems on the planet and of great importance for livelihoods and economies (Hoegh‐ Guldberg et al, 2007; Moberg & Folke, 1999) These issues come together in the Western Indian Ocean (WIO) where coral reefs are associated with developing countries with artisanal fisheries of high socioeconomic value for poor coastal communities, though in many areas fisheries management measures are inadequate (Samoilys, Osuka, Maina, & Obura, 2017; Walmsley, Purvis, & Ninnes, 2006; Wells, Samoilys, Makoloweka, & Kalombo, 2010). The Western Indian Ocean (WIO) biogeographic region (Spalding et al, 2007) represents a region of highly variable coral reefs (Sheppard, 2000), within an oceanic context of the South Equatorial Current and the East African Coastal Current (Schott, Xie, & McCreary, 2009) This provided an ideal study area for enabling the collation of 16 explanatory variables to explore drivers of the structure of coral reef fish assemblages

| METHODS

Acanthurus nigrofuscus 4

Findings

| CONCLUSIONS