1. Otoliths – an invaluable tool for unraveling fish population dynamics Capacity-building and application in a large floodplain wetland system, the Okavango Delta, Botswana Glenn Wilson 1, Belda Mosepele 2 & Keta Mosepele 3 1 Ecosystem Management, University of New England, Armidale, NSW 2351 Australia; glenn.wilson@une.edu.au 2 BIOKAVANGO Project, Harry Oppenheimer Okavango Research Centre, P/Bag, University of Botswana 285, Maun, Botswana 3 Harry Oppenheimer Okavango Research Centre, P/Bag, University of Botswana 285, Maun, Botswana B ackground and O bjectives All fish have structures in their cranium called otoliths, that provide a sense of balance and orientation in the water column. Otoliths grow in concentric layers called growth increments — age and microchemical data from these daily or annual otolith features can provide a wealth of information on the population dynamics of freshwater fish. Until now, the appropriate techniques have been virtually untested in the Okavango Delta and local training in these methods and their application has been unavailable. In March 2009, as part of the BIOKAVANGO aim of capacity building for biodiversity sustainability in the Delta, an 8-day training course in otolith methods was run at the Harry Oppenheimer Okavango Research Centre (HOORC), Maun, Botswana. The objectives of this were (1) to provide research and fisheries management personnel with foundational skills in otolith processing and analyses, and (2) to establish a program of ongoing training and research collaboration between HOORC and the University of New England. L aboratory M ethods Larvae and juveniles – Up to an age of 5-6 months, these lifehistory stages often deposit daily rings in their otoliths. These are either removed by pinning the fish out in a wax dish (photo a) under a quality dissecting microscope (b) or by softening their head tissues in bleach, upon which the otoliths fall out of the cranium. One or more lapillae or sagittae are mounted in Crystalbond 509 resin (c) on a microscope slide, and any spare otoliths stored in a microculture tray cell (d). Crystalbond is melted onto the slide on a laboratory hotplate (e). The otoliths often need some grinding or polishing to clarify the increments for counting, using lapping film (f). Lastly, the rings are counted under a binocular microscope at around 400x magnification. Older stages – Post-juvenile stages of long-lived species deposit annual growth increments in their otoliths. However, these otoliths are usually too large to process effectively using the above methods. These are instead set in epoxy resin (g) in a mold (h) and sectioned using a low-speed saw (i). The sectioned otoliths are then mounted on microscope slides using Crystalbond (d) and polished with ‘wet and dry’ sandpaper and lapping film (f). Counts require either a dissecting or binocular microscope, depending on the size of the increments and the otolith. F ield M ethods Seine (drag) nets (top-left), drift nets (bottom-left) and multi-panel gill nets (right) are likely to be some of the most useful techniques for sampling fish in the Okavango Delta for age-structure analyses. However, these all need to be deployed with the presence of crocodiles, elephants and hippos in mind, as well as your specific research questions and objectives. Small fish can be frozen or preserved in ethanol in the field and processed in the laboratory. Larger fish are processed in the field and their length, weight, sex etc details recorded and their otoliths removed. (a) Lapillus from a juvenile fish showing daily growth increments. (b) Whole and sectioned (c) sagitta showing the overall shape, nucleus and annual growth increments. C apacity B uilding The first course in fish ageing methods for the BIOKAVANGO project was conducted at HOORC in March 2009. It comprised four days of lectures and field and laboratory sessions, with a four-day field trip to the HOORC Nxaraga field camp to sample fish for subsequent otolith analyses. Approximately 14 staff and students from HOORC, the BIOKAVANGO Project, and the Botswanan Fisheries Division took part. Otoliths were extracted from a range of species in the field, to commence an ongoing training and research collaboration between Australia and Botswana. A pplications in the O kavango D elta The following are the primary information needs and applications for otolith data in the Okavango Delta: 1. Growth and longevity. Basic descriptions of growth patterns and longevity are unavailable for virtually all Okavango fishes. 2. Year-class strength and flooding. Climate change, upstream abstraction and interannual climate variability all have the potential to alter annual Delta flooding patterns. Otolith data can assist in linking the importance of flooding and fish production through age-structure analyses to help forecast response to future conditions. 3. Which part of the flood pulse is most critical for spawning? From daily age estimates, hatch or spawning dates can be back-calculated and their timing matched to variables such as hydrology or climate. 4. Where are fish spawning? Otolith microchemistry may help discriminate between main-channel and floodplain origin of fish larvae. abcdefghiabcdefghi