A comparison of three techniques for fluorochrome marking of juvenile Clarias gariepinus otoliths Reece Wartenberg 1 , Anthony J. Booth 1 * & Olaf L.F. Weyl 2 1 Department of Ichthyology and Fisheries Science, Rhodes University, P.O. Box 94, Grahamstown, 6140 South Africa 2 South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown, 6140 South Africa Received 5 November 2010. Accepted 20 January 2011 Intramuscular injection of the antibiotic oxytetracycline (OTC) has been the only method previously employed for chemically marking C. gariepinus otoliths for ageing studies. This study compared intramuscular injection, immersion, and dietary incorporation methods of administering OTC to determine the most effective technique. No differences in either growth or mortality were found between experimental groups while intramuscular injection of OTC was found to be superior to either mass immersion or dietary inclusion of OTC when marking Clarias gariepinus otoliths. Key words: African sharptooth catfish, oxytetracycline, fluorochrome marking. INTRODUCTION African sharptooth catfish, Clarias gariepinus (Burchell 1822), is widely distributed with a natu- ral range that extends from southern Turkey to the Orange River, South Africa (Skelton 2001). In addition to translocations within its southerly range (Cambray 2003), Cambray (2005) noted that as a result of poor aquaculture practices and intro- ductions from a number of unknown sources, C. gariepinus has now invaded South America, Europe, Asia, and Australia. Its life history charac- teristics include a fast growth rate to a maximum length of 1300 mm total length (TL) (Bruton 1976), a high fecundity, an omnivorous diet and the ability to breathe air (de Moor & Bruton 1988; Cambray 2003). Understanding the biology and population dynamics of this invader would assist in its management and possibly eradication. Information on the growth, maturity, and mor- tality of C. gariepinus are all dependent on precise and accurate age estimates. As with other teleosts, age in C. gariepinus can be determined by counting growth increments on fish hard structures such as vertebrae (Pivnicka 1974; Willoughby & Tweddle 1978), pectoral spines (Clay 1982; Quick & Bruton 1984) and otoliths (Potts et al. 2008; Richardson et al. 2009). Determining the frequency of growth zone deposition is crucial if accurate estimates of age are to be determined (Campana 2001). Mark–recapture of chemically tagged fishes is considered one of the most reliable methods for validating growth increment deposition rates (Campana 2001). This method relies on marking a hard structure, such as the otolith, using a calcium-chelating fluorescing chemical marker, that forms a visible mark on the otolith at the time of tagging. Later, growth zone deposition rates can be directly determined by correlating the time at liberty after tagging with the number of growth zones formed on the otolith distal to the chemical mark (Fig. 1). Choosing an appropriate marking method is therefore a crucial component of any age validation study. Fluorescing chemical markers, known as fluoro- chrome markers, such as oxytetracycline (OTC) (Babaluk & Craig 1990; Simon et al. 2009) and aliza- rin red S (Blom et al. 1994; Beckman & Schulz 1996) have been used to mark fish hard parts not only by injection (Lang & Buxton 1983; Weyl & Booth 2008) but also by immersion (Simon et al. 2009; Hendricks et al. 1991), and dietary inclusion (Hendricks et al. 1991; Honeyfield et al. 2006). In the past, direct injection has been the only method that has been examined to determine growth zone periodicity of C. gariepinus popula- tions directly (Weyl & Booth 2008). Unfortunately this technique is costly, too time-consuming to mass mark large numbers of fish, and is often not suitable for small fishes because of the potential physical damage caused by injection and handling. As a result, only the deposition rate of growth zones *Author for correspondence. E-mail: t.booth@ru.ac.za African Zoology 46(1): 72–77 (April 2011)