6 minute read
FISH INNER EAR
NOT JUST A TALL TALE
Thousands of visitors come to Namibia annually with the sole purpose of relaxing at the seaside with a fishing rod. Maybe one of these fishers told you a tall tale recently: for example, that they caught a fish that can speak, just like the golden fish of the famous Pushkin poem. Well, that was a blatant lie. But it is a scientific fact that fish can hear. And that the ear bone in the fish’s inner ear holds the key to exactly where the selfsame fisher will fish most successfully in future.
THE BLACK BOX OF FISHERY SCIENCE
One of Namibia’s experienced marine biologists, Dr Margit Wilhelm, lectures at the University of Namibia’s Department of Fisheries and Aquatic Sciences. She explains: “Fish do not have ears, but they can hear and feel pressure with an inner ear bone; a hard stony structure located directly behind the brain of bony fishes. And it has rings on it much like a tree. Scientists refer to the otoliths as “the black box” of fishery science because it records information and it keeps it forever.” As the saying goes, some facts are mind-blowing and sometimes stranger than fiction. A small fish in the southern waters of the Atlantic Ocean, home of the cold Benguela current, has something very specific in common with Namibia’s oldest tree in the heat of Outapi in Owamboland. The science of dendrochronology – which dates events through the study of tree rings – is well known and enables scientists to date trees like our massive Baobabs, believed to be a staggering 800 years old. The science of sclerochronology applies the same principles to fish.
Dr Wilhelm explains: “The width of the increment for a tree especially – everyone can picture that – provides telltale signs. If it was formed in the rainy season of a good year, the tree will grow lots and form a wide ring or increment, for example, and when examined later that will point to a very good rainy season. And in bad years the ring will be really narrow. This enables you to literally go back in time and assign a particular calendar year or season to each ring. With this you can look back very far in time, especially considering that trees can get hundreds of years old.”
According to Dr Wilhelm, the same principle can be applied to fish. Counting the annual growth rings on the otoliths is a common technique in estimating the age of fish. She gained valuable experience when she worked in Texas after completing her doctoral degree studies. There she learned from Dr Bryan Black, one of the world’s foremost experts in the field.
ADAPTING TO A WARMER OCEAN IN THE TIME OF CLIMATE CHANGE
In the long term, scientists can use the data gained through this “tree-ring-technique” to determine how fish will respond to temperature in the future. This is extremely important in the context of rising ocean temperatures due to climate change. Dr Wilhelm explains: “The ocean is a very big heat sink – so, as the land temperatures are heating up the ocean is also heating. Consequently, climate also affects the fish stocks.”
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
