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

Abstract

Aim Understanding 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. Location Comoros, Madagascar, Mozambique and Tanzania, Western Indian Ocean (WIO). Methods We 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. Results Canonical 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 Conclusions Spatial 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.

Contrasting patterns in the abundance of fish communities targeted by fishers on two coral reefs in southern Mozambique

“…Carpenter et al 1981). In contrast, Samoilys et al (2019) found no effect of habitat variables on fish assemblage in the WIO region; rather, geomorphology, exposure and fishing pressure drove patterns. Indeed, these factors probably contributed to the 14% of site variation in fish abundance in our models, after accounting for the influence of habitat type and depth.…”

Coral reef fish communities, diversity, and their fisheries and biodiversity status in East Africa

“…Similarly, some of the geographic gradients are likely to reflect the distribution of people and urban−rural gradients interacting with the extent of shallow water and management restrictions . Higher spatial structure reported among fish communities at finer taxonomic levels does not necessarily conflict with the weaker structure found in my family-level analysis , Samoilys et al 2019.…”

Diversity of Coral Reef Fishes in the Western Indian Ocean: Implications for Conservation

“…Reefs of the WIO are intensely fished in many locations [38,55,56], climate induced coral bleaching continues to degrade reefs [57] and both these impacts threaten some reef fish species [9] which could undermine studies examining patterns in species richness. However, fishing effects are largely seen in declines in biomass and fish size data [51,58,59], so with the species presence/absence data in the current study fishing effects will only manifest in zero values. Five species completely absent from Comoros and in the top 12 ranking species in the SIMPER analyses are species typically taken in WIO artisanal fisheries [38,60].…”

“…The three islands of the Union of Comoros are small, oceanic and recent volcanic islands [50] isolated from other large coral reef systems: ~250 km from Mozambique and ~450 km from Madagascar. The total coastline length and reef area in Comoros are ~70-90% smaller than mainland east Africa or Madagascar [51]. These biogeographic factors therefore likely explain the naturally lower species richness and different assemblages in Comoros.…”