The abundance, diversity and distribution of small fishes in mangrove and non-mangrove estuaries in warm temperate South Africa

Steven Mcgregor,Nadine A Strydom

doi:10.3989/scimar.04744.31a

Abstract

Mangroves in tropical and subtropical regions have been well documented in terms of the advantages they provide and their role in structuring ichthyofaunal assemblages, but little is known about their warm temperate counterparts. The study aimed to investigate the importance of warm temperate mangroves by comparing the abundance, diversity and distribution of small fishes in mangrove and non-mangrove estuaries in warm temperate South Africa. A 50x2 m (12-mm mesh) seine net was used over three summer seasons to sample small fishes in the Gonubie, Qora, Nahoon and Xhora estuaries (the latter two being mangrove estuaries). Fish abundance and diversity showed little variation among estuaries, despite the presence of mangroves. Estuaries in warm temperate areas are not only at the edge of mangrove distribution, but also offer alternative habitats which lend similar advantages to fish survival. It appears that warm temperate ichthyofauna have not yet evolved a dependence on mangrove systems in terms of the food, refuge and other ecological services they provide. Understanding the function of habitats and their value in enhancing fish survival in estuarine nursery areas is essential for fish conservation.

Highlights

Mangroves are distributed throughout the tropical regions of the globe, where their distribution is restricted to the 20°C winter seawater isotherm in both the southern and northern hemispheres (Duke 1993)
The aim of the study was to investigate catches of juvenile and small adult fishes during the peak summer recruitment period in mangrove and non-mangrove estuaries to determine whether differences in catches exist, and whether mangrove presence lends an advantage to fish survival in warm temperate South Africa
There was no significant difference in the catch per unit effort (CPUE) of fishes within both the estuarine and marine guilds among habitat types (Fig. 4)

Summary

Introduction

Mangroves are distributed throughout the tropical regions of the globe, where their distribution is restricted to the 20°C winter seawater isotherm in both the southern and northern hemispheres (Duke 1993). McGregor and N.A Strydom temperate climatic regions (Hogarth 2015) These warm, temperate mangrove stands typically contain fewer species (sometimes only a single species) than stands in the tropics (Hogarth 2015). Kristensen et al (2008) noted that detritus from mangroves and benthic microalgae are typically the greatest contributors to autochthonous carbon sources in tropical estuarine ecosystems. This high nutrient input, coupled with the refuge and habitat which mangrove forests provide, lends an explanation for the high abundance and diversity of fishes associated with mangrove estuaries (Laegdsgaard and Johnson 2001, Blaber 2007)

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