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The Puzzle of the Precocious Southern Fishes (SSAWRN)
22 November 2010
When Kondja Amutenya was growing up in northern Namibia, he was instructed early and often how important it would be to get a good education. His father, who worked in a diamond mine in the south, was virtually never home with his family. Kondja learned first-hand about another kind of life he did not care for – livestock herding. During every vacation from school, his mother dispatched him from his village of Uukuuvu-Onemanya to help his cousins look after the cattle, goats, donkeys, and other livestock at the family’s remote cattle post. Just to get to the post was a 70-kilometer walk, he recalls. “I would get up early in the morning and just walk without stopping until I got there.”
Against these experiences was the consistent and supportive voice of his mother, a teacher, who told Kondja and his three sisters that this was the kind of life they could expect if they did not study hard in school. She never bullied them, he recalls, but simply encouraged them, bringing home books for them to read whenever she could. He listened carefully, studying hard enough to get into high school in the regional capital of Oshakati, near the Angolan border, where the northern campus of the University of Namibia (UNAM) is now being developed. And his sisters listened as well: Today, one is a geologist, another an environmental scientist and the third a medical student in Pretoria, South Africa.
Kondja, after earning a BS in natural resources at UNAM, found a job at the National Marine Institute Research Center, in Swakopmund. This beach resort, founded in 1892 as the main harbor of then-German South-West Africa, is adjacent to the Namib Desert, one of the oldest and driest in the world. Tourists come to see the nearby sand dunes, some of which are nearly 1,000 feet tall. Kondja came to learn about fisheries.
When a friend studying in Stellenbosch, South Africa, told him about RISE, he contacted Denis Hughes, leader of the SSAWRN network. On the basis of his university performance and experience with fisheries, he was accepted as an MPhil student at the Okavango Research Institute (ORI) of the University of Botswana. At the ORI, near the northern city of Maun, he was set to work with fellow student Nqobizitha Siziba on a puzzling size difference between tilapia of the same ages from the northern and southern parts of the Okavango Delta. “This phenomenon had been observed earlier,” he said, “but they didn’t know how prevalent it was. When we started, we could see that a little fish down here that was six or seven inches long would have the same gonad development as a fish of about 13 inches up north. We needed to know if the northern fish was older, or what was going on.”
Since he couldn’t tell the ages of these fish by length, Kondja set to work learning a more accurate technique of aging that uses the tiny otoliths, or “earstones,” which are hard calcium carbonate structures located directly behind the brain. Otoliths have been found to exhibit growth rings similar to those of trees as the fish grows. A fish grows more slowly in the winter, forming a darker, denser ring; in the summer, when the fish grows faster, a clearer ring is formed. Kondja’s results showed that the small fish in the south are indeed the same age as their much larger cousins in the north, and just as mature sexually. The current hypothesis to explain this discrepancy is that the southern fish, which depend on the temporary seasonal flood water to grow and mature, must do so rapidly, whereas the northern fish, which live in permanent flood channels, have more time to mature.
Kondja works with his faculty advisors, Wellington Masamba and Keto Mosephele, on other features of the fishery as well, such as the impact of water quality on growth, including dissolved oxygen, pH, and temperature. Fisheries issues are highly important – and often contentious in the Delta. Some of these fishes, especially the three-spot and red-breasted tilapias, are important sources of livelihood for 40 to 50 commercial gill-netters. Also, some 3000 women and young girls fish for small fish with baskets, traps, and simple rods and lines. Finally, the recreational fishermen who stay at the tourist lodges pursue the larger game fish, mostly tiger fish and large-mouth tilapia.
Because the commercial fishermen have been accused of overfishing the Delta and causing fish stocks to decline, the ORI has attempted to investigate this claim. A recent study concluded that the more significant catch fluctuations are caused not by overfishing but by drought, spraying for tsetse flies, and burning of riverine vegetation for cattle grazing. It found that the current total catch of about 1000 tons of fish a year could be at least doubled with no detrimental effect. Another important conclusion is that compared to other African wetland systems, the density of fish in the Okavango is low because of the clean, low-nutrient water that feeds the Delta from the undeveloped highlands of Angola.
Meanwhile, Kondja, perhaps remembering his father’s hard work in the diamond mines of Namibia, is eager to continue his hydrology studies on another front as well. He wants to apply what he has learned to test the effects of diamond mining on groundwater quality back in his native Namibia. Since most of the people of Botswana and Namibia depend on bore holes sunk into the sand, the quality of this stored water is crucial to livelihoods – but easily jeopardized by surface activities.