Saturday, August 30, 2014

Zambezi River otolith collection field trip report, July 2014

Participating  researchers: PhD students: Geraldine Taylor and Richard Peel 

This field trip is part of a larger project on the Zambezi, Kavango and Kwando rivers and Lake Liambezi 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 is undertaken primarily by two PhD students: Richard Peel and Geraldine Taylor through Rhodes University.  

The project is funded mainly 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. Permission to carry out this research in the Sikunga Channel was granted by the Sikunga Conservancy.  The field trip was carried out from the 13th to the 23th July 2014. The objective of the trip was to collect otoliths from six species of fish (the striped 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. 

In addition to the main objective of collecting otolith samples for growth rate comparisons, gillnet catches in the Sikunga Channel were compared to those obtained in 2011 to assess whether catches have improved since the channel was protected by the Sikunga Conservancy in mid 2012.  
Figure 1. An 11 year old threespot (Oreochromis andersonii) otolith (sectioned) illustrating the alternating opaque and hyaline growth zones used to estimate age. 
Fish samples were collected using multifilament and monofilament gillnets, baited longlines and by angling. Catch per unit effort data was recorded for the gillnets and longlines. Target species were measured, weighed, sex and stage of development were determined, stomach contents were examined, and otoliths were extracted. A total of 18 gillnet nights, nine longline nights and nine longline days were recorded for catch per unit effort data.  

A total of 623 otolith samples were collected from the six target species and any other cichlid species caught in the nets. This total consisted of 183 silver catfish, 210 striped robbers, 81 sharptooth catfish, 1 blunttooth catfish, 58 purpleface largemouth, and 80 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.  

Comparison of catches in the Sikunga Channel between 2011 and 2014  
Catch per unit effort in numbers and weight has increased slightly since 2011 (Table 1). The species composition of the catches remains very similar, with slightly more species present in the current catches compared to the 2011 catches. It must be noted that this increase in CPUE since 2011 in the channel has come about despite the definite decline in CPUE in the main river recorded by both the community monitors for the Community Conservation Fisheries in KAZA Project (Figure 2B) and the Ministry of Fisheries and Marine Resources (Figure 2C) (Tweddle 2014).   

Table 1 The catch per unit effort (CPUE) for 2011 and 2014 in both average number of fish caught per gillnet and average weight of fish caught per gillnet.  
Figure 2 Evidence for decline in CPUE in the Upper Zambezi River fishery taken from B) data recorded by community monitors at four stations along the Zambezi River and C) Ministry of Fisheries and Marine Resources experimental gillnet data collected from various stations along the Zambezi River.  

Peel, R. 2012. The biology and abundance of three cichlid species from the Kavango and Caprivi regions, Namibia. MSc Thesis, University of Namibia, Windhoek, Namibia.  
Tweddle, D. & Hay, C.J. 2014. Co-management of Upper Zambezi Fisheries, vital natural resources for floodplain communities. KIFI Science Forum, 12th-14th August 2014, Kamutjonga Inland Fisheries Institute, Divundu, Namibia.  

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