Otolith of a 6-year old female Merluccius paradoxus (deep-water hake) caught in 1997.
Kabeljou (kob) is an inshore species that visiting anglers love. “This species is most affected because inshore temperatures in shallow waters heat up the most, especially in Angola and northern Namibia. Because the water is warming, kabeljou already moved southwards from Angolan to Namibian waters.” The ultimate aim of the research is not to cater for those who fish for fun, but to predict big-picture future trends for sustainable utilisation of Namibia’s rich fishing grounds, the so-called offshore resources (for example sardine). To this end, Dr Wilhelm spends hours behind her microscope where she uses photographs to measure the increments on the otoliths in detail. Interestingly, while modern-day technology enables this process, old methods still remain relevant today. There is no need to keep the ear bone in alcohol, ethanol or formalin to access information. Much like bone, otoliths stay dry for hundreds of years. A simple envelope will do for storing them. Developed countries like Norway have stored hundreds of years of information related to fish age and growth in this way to help them plan and manage their resources better. CATCHING UP ON LOST DATA Namibia has much catching up to. At the time of independence, fish resources had dwindled due to overexploitation and lack of conservation and protection. The Ministry of Fisheries is trying to reverse this trend. Namibia’s fish waters were frequented by boats from countries all over the world since the 1960s due to the abundance of species like sardine (in Namibia better known as pilchard), anchovy, hake and horse mackerel (maasbanker). Due to the policy of open access fishing, the nets of foreign fishing boats often left with the very key to the future management of our own resources: otolith bones. “It is only since independence
that we keep proper otolith archives in Namibia”, says Dr Wilhelm. Data was lost in spite of efforts by organisations like the International Commission of South East Atlantic Fisheries, who tried to manage the fish stock. As a result, many Namibian scientists have been trying to obtain access to “lost data” (held by other countries) for years. Dr Wilhelm made an unexpected breakthrough last year when she located a laboratory in Poland (Department of Fisheries Resources, National Marine Fisheries Research Institute) with a wealth of information. “Boats from Poland also fished in Namibian waters and in their collections kept accurate records of sardine, horse mackerel and hake species together with their otolith bones from the 1960s onwards.” This new collaboration will enable Dr Wilhelm (and her PhD student, Faye Brinkman) to date some species back to the sixties to establish if their growth rates per year have changed with the changing ocean temperatures and the heavy fishing. They are starting with this exciting research this year. THE DIFFERENCE BETWEEN TRACKING THE HISTORY OF TREES AND FISH Namibia’s most famous Baobab has been standing in the very same place for a staggering 800 years. In contrast, fish move over far distances and are exposed to many different temperatures throughout their lifetime or even in just a few days. They do not leave any tracks, but that does not make tracking their history impossible. If years later you find an ear bone on the seashore, or on the bottom of the ocean or in scats of other animals, it will enable you to identify the species it once belonged to. The shape and size of the otolith is specific to a species. Dr Wilhelm agrees that this makes her work challenging. She obtained interesting data related to hake (the deep-water species) during her post-doctoral research in Texas and subsequently at the University of Namibia. She found that otolith growth of deep-water hake is best at cold ocean temperatures in winter. Cold winter temperatures go with increased wind and food supply. The wind ultimately brings up nutrients from the bottom (this is called upwelling) and increases the food of hake. With the expected decrease in wind and upwelling, and with ocean temperatures heating up because of climate change, hake growth is also expected to be negatively affected. Says Dr Wilhelm, “Sustainable harvesting does not mean you should not eat fish at all, but it advocates that we should harvest fish responsibly. The more we know about how fish and their growth rates change with time, the better we can predict the future. We are ultimately responsible for our resources.” Long live the black box of fishery science! Linda de Jager
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