Saturday, August 2, 2014

Kavango River otolith collection field trip report, June 2014


Participating  researchers: Geraldine Taylor and Richard Peel (PhD students)

Introduction

This field trip is part of a larger project on the Zambezi, Kavango and Kwando rivers in Namibia which aims to improve knowledge of the aquatic ecosystems supporting fisheries through studies on fish population dynamics, food webs, nutrients and biodiversity. The research will be undertaken primarily by two PhD students: Richard Peel and Geraldine Taylor through Rhodes University.
The project is funded primarily by the Southern African Science Service Centre for Climate Change and Adaptive Land Management (SASSCAL) and co-funded by the South African National Research Foundation and Nedbank Namibia’s Go Green Fund. The project is administered by the Namibia Nature foundation and involves several collaborating institutions including the South African Institute for Aquatic Biodiversity, Rhodes University, University of Namibia and the Namibian Ministry of Fisheries and Marine Resources.

The field trip was carried out from the 16th to the 27th June 2014. The objective of the trip was to collect otoliths from six species of fish (the stripped robber Brycinus lateralis, the silver catfish Schilbe intermedius, blunttooth catfish Clarias ngamensis, sharptooth catfish Clarias gariepinus, purpleface largemouth Serranochromis macrocephalus, and African pike Hepsetus cuvieri) for ageing purposes. In fishes, factors which influence metabolic rates (e.g. temperature or spawning) are reflected in the growth of calcified structures such as scales and otoliths and result in the appearance of alternating opaque and hyaline growth zones (Figure 1). When counted, these growth zones are used to estimate age, which is necessary for the determination of growth rates. The growth rates of these species will be compared between the Zambezi, Kwando and Kavango rivers to assess if previously described differences in growth rates reported by Peel (2012) for three commercially important cichlid species (threespot Oreochrmois andersonii, greenhead Oreochromis macrochir, and redbreast Coptodon rendalli), are consistent across species with different life history characteristics.
Figure 1 An 11 year old threespot (Oreochromis andersonii) otolith (sectioned) illustrating the alternating opaque and hyaline growth zones used to estimate age. 

Sampling

Fish samples were collected using multifilament and monofilament gillnets, baited longlines and by angling. Catch per unit effort data was recorded for the gillnet and longline gears. Target species were measured, weighed, sex and stage of development were determined, stomach contents were examined, and otoliths were extracted.
A total of ten gillnet nights, nine longline nights and nine longline days were collected for catch per unit effort data.
A total of 505 otolith samples were collected from the six target species and any other cichlid species caught in the nets. This total consisted of 221 silver catfish, 96 striped robbers, 102 sharptooth catfish, 36 blunttooth catfish, 21 purpleface largemouth, and 12 African pike. In addition stomach content data was collected from all of the target species, which will be used in the food web analysis of these river systems.

Figure 2: A good sharptooth catfish (Clarias gariepinus) catch from the longline. 

Figure 3: Interesting by-catch from the longline: a large nembwe (Serranochromis robustus) on the left and a silver catfish (Schilbe intermedius) on the right. 


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