African Journal of Aquatic Science 2009, 34(3): 255–259 Printed in South Africa — All rights reserved Copyright © NISC (Pty) Ltd AFRICAN JOURNAL OF AQUATIC SCIENCE ISSN 1608–5914 EISSN 1727–9364 doi: 10.2989/AJAS.2009.34.3.7.983 African Journal of Aquatic Science is co-published by NISC (Pty) Ltd and Taylor & Francis Blood lactate levels as a biomarker for angling-induced stress in tigerfish Hydrocynus vittatus from the Okavango Delta, Botswana NJ Smit 1 *, G Howatson 1,2 and R Greenfield 1 1 Centre for Aquatic Research, Department of Zoology, University of Johannesburg, Auckland Park, PO Box 524, Johannesburg 2006, South Africa 2 School of Psychology and Sport Sciences, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK * Corresponding author, e-mail: nicos@uj.ac.za Received 12 May 2009, accepted 18 August 2009 Introduction The tigerfish Hydrocynus vittatus is not only the most sought-after freshwater angling fish in Africa, but is one of the most important predatory fishes in African waters (Winemiller and Kelso-Winemiller 1994, Skelton 2001, Økland et al. 2005). Although abundant in certain areas throughout Africa, its numbers have declined in many rivers due to water extraction, pollution, obstructions such as dams and weirs, and fishing pressure (Steyn et al. 1996, Skelton 2001). This has been recognised specifically in South Africa by the Department of Environmental Affairs and Tourism who recently included tigerfish on the protected species list, together with such marine icons as the great white shark Carcharodon carcharius and the once thought to be extinct coelacanth Latimeria chalumnae (DEAT 2007). Despite the high profile of the tigerfish as a protected ecologically and economically important species, only limited published information on its biology in specific populations is available. These include data on growth, feeding and population dynamics in Lake Kariba (Matthes 1968, Kenmuir 1973); movement patterns and ecology in the upper Zambezi River and Zambezi floodplain (Winemiller and Kelso-Winemiller 1994, Økland et al. 2005); allozyme variation in and between upper Zambezi River (Namibia) and Olifants River (South Africa) populations (Kotzé et al. 1998); and accumu- lation of selected metals in the tissue and organs of the Olifants River tigerfish (du Preez and Steyn 1992) and of mercury in the Lake Kariba tigerfish (Mhlanga 2000). To date, the majority of research on tigerfish has been on populations from the Zambezi and Olifants River systems with no biological information available on the populations found in the Okavango Delta, Botswana. The Okavango Delta, specifically the panhandle region with deeper, faster flowing water, is one of the most popular areas in Africa for anglers to catch tigerfish (for the specific region, see van der Bank and Smit 2007, Figure 1). Many tourist lodges and camps in this area specifically cater for recreational anglers and are the source for valuable income for this region. While some anglers keep a portion of the fish captured (up to five per day are permitted), many fish are captured and immediately released. The game fishing industry further encourages anglers voluntarily to release fish as a way to expand recreational fishing and therefore catch-and-release angling is growing, as a proportion of total fishing, in this region. The ultimate success of catch- and-release angling, however, depends on ensuring high release survival rates by minimising injury and mortality (Bartholomew and Bohnsack 2005). Despite the importance of this principle, research examining the physiological response to angling and possible sublethal impacts thereof on freshwater game fish is only available for a few popular North American and European species, i.e. Atlantic salmon, rainbow trout and largemouth bass (Brobbel et al. 1996, Cooke et al. 2002, DuBois and Dubielzig 2004), and is completely lacking for any of Africa’s freshwater game Although critical in catch-and-release angling, no data are available on angling-induced stress in African gamefish. Blood lactate levels were used as a biomarker for angling-induced metabolic stress in tigerfish caught by angling in the Okavango Delta, Botswana. Blood was drawn and analysed for blood lactate from 66 anaesthetised fish. The landing time, handling time, body mass and total length were recorded prior to reviving and keeping the fish in aerated water for recovery before release. A strong positive relationship (r 2 = 0.607) was found between landing time and body mass, as well as significant elevations in blood lactate concentrations following rod-and-line angling, regardless of angling time. These data suggest that longer angling time significantly increases physiological stress, which may have an impact on breading success and mortality in tigerfish. Keywords: catch-and-release angling, fish, physiology, recreational fisheries management, stress