Diana Sharpe, a Ph.D. student at McGill University in Montreal, received a Young Explorers Grant from National Geographic to investigate a tiny fish in Africa’s Lake Victoria that’s under tremendous pressure from humans. Here she describes her challenging work on the lake.
Question: Lake Victoria, the world’s largest tropical lake, has been Africa’s most important source of inland fisheries production; what is happening to the lake’s fish today?
Over the past century, Lake Victoria has undergone massive, fundamental ecological changes, driven primarily by human activities. Overfishing during the first half of the 20th century led to the collapse of almost all of the native fisheries, which in turn motivated the introduction of a series of foreign food fishes, including the predatory Nile perch, Lates niloticus. This invasive predator, in conjunction with habitat degradation and continued over-fishing, drove hundreds of native fish species to extinction in the mid-1980s, in what has been referred to as the worst mass extinction of vertebrates in modern times.
The past two decades have brought about yet more changes, and unexpected reversals. The Nile perch, which formed the basis of an extremely productive and lucrative export-oriented fishery in the 1990s, is now on the decline, likely due to over-exploitation. A tiny native cyprinid fish (minnow) called mukene (Rastrineobola argentea), has flourished alongside the invader and is now emerging as a major commercial fishery.
Q: Lake Victoria was the scene of one of the worst mass extinctions of vertebrates in modern times. How did this happen?
Lake Victoria is famous for its rich fish assemblage, which previously included over 500 species of endemic haplochromine cichlids. However, in the 1980s, several hundred of these cichlids disappeared completely from the lake, and many other non-cichlid fish species, such as lungfish, some catfishes and native tilapias, suffered dramatic declines. Ecologists believe that three main factors contributed to these declines: Fishing pressure, introduction of several non-indigenous fish species to the lake in the 1960s, and pollution and nutrient run-off from the growing human population in the Lake Victoria basin.
Q: What fish are the local people eating these days, and in what ways are the people affecting the health of the lake’s fish populations?
In general, fish is the cheapest and most important source of animal protein in Uganda, especially as rising global food prices in recent years have put other types of meat out of reach for many Ugandans. The most recent available data suggest that mukene is now the most important commercial fish species in Uganda, followed by Nile perch and Nile tilapia. These three species are also the most widely consumed nationally. Mukene is one of the least expensive types of fish on the market (about half the cost of Nile perch or Nile tilapia), and so it has become increasingly popular, especially among low-income households.
The very limited data that we have suggests that the mukene stock is stable — if not increasing — but there is still some cause for concern. In particular, the work that we have done suggests that fishing pressure may be imposing extremely high mortality on mukene populations and may be driving detrimental changes in important reproductive traits like size at maturation. We do not yet have a clear understanding of what the sustainable level of exploitation for this stock is— and whether we may have already surpassed that point.
Q: Your hypothesis was that introduction of the Nile perch and intensification of commercial fishing have driven significant changes in the reproductive biology of the mukene over the past few decades. What did your research show and how did you go about it?
Mukene in Lake Victoria are now experiencing the dual pressure of predation from the introduced Nile perch and fishing pressure from a rapidly developing commercial fishery, in addition to other changes in the lake ecology that may alter the quality of their habitat. We wanted to test whether fishing pressure and Nile perch predation might be driving any changes in the reproductive biology of mukene. To answer this question, we looked at variation in key reproductive traits across mukene populations from 10 lakes in Uganda that differed in the intensity of Nile predation and fishing pressure. We focused much of our sampling efforts on the on the remote, swamp-locked lakes near Lake Kyoga (a large, shallow lake about 150km downstream of Lake Victoria), many of which lack Nile perch and mukene fisheries. As relatively pristine ecosystems, these lakes provide a rare glimpse into the past.
We found that mukene from unperturbed lakes (those lacking the introduced Nile perch and commercial mukene fisheries) were dramatically different from those found in perturbed lakes (lakes with introduced Nile perch and/or commercial mukene fisheries). Specifically, we found that mukene from perturbed lakes had significantly reduced body sizes, smaller size at maturation, and increased fecundity. Our findings raise some concerns for the sustainability of the mukene fishery, especially given that reductions in body size and size at maturity can lower the reproductive capacity of fish populations and can also decrease the yield of fisheries. Furthermore, rapid changes in reproductive traits may signal that fishing mortality is extremely high and have sometimes preceded drastic population declines, such as the historical collapse of Canada’s Atlantic cod stocks.
Q: How did you approach your work on the lakes? Were you working with local scientists and others?
All of my work was done in close collaboration with Ugandan fisheries scientists at the National Fisheries Resources Research Institute of Uganda (NaFIRRI), building on partnerships developed by my Ph.D. supervisor at McGill (Dr. Lauren Chapman), who has worked in Uganda in collaboration with NaFIRRI and others for almost 20 years.
It was important to us to involve local community members in our sampling as much as possible. Whenever we first arrived at a fishing community, we met with the local authorities to explain our objectives, obtain permission to sample in the lake and negotiate the terms for hiring local fishermen as part-time field assistants. We typically hired three to five fishermen at each lake, who rented us their canoes for a day or two and helped us with the sampling.
Working in such remote locations was fascinating and rewarding but also challenging at times, and I am grateful for the help of a wonderful, enthusiastic field team of Canadian and Ugandan students and scientists, without whom this expedition would not have been possible.
Q: What should the next steps be to ensure the future of fish in the Victoria Lake basin, both for biological diversity and to meet the protein needs of the millions of people in the region? You report that Uganda’s population is expected to triple by the year 2050.
Yes, the UN estimates that Uganda’s population will triple, topping 90 million, by 2050. In fact, the region immediately surrounding Lake Victoria has the highest population growth rate on the African continent. Meeting the basic needs of this growing population while sustaining the biodiversity and fisheries of the lake will be a major challenge.
As a first step, more research is certainly needed to be able to understand the dynamics of this complex and rapidly changing ecosystem and to guide management decisions in the future. One of the major ongoing initiatives of the Ugandan government is to include local fishing communities in fisheries management through locally run Beach Management Units. This system of co-management is still in its infancy, so we have yet to see how effective it will be. Another suggestion being advanced by Ugandan fisheries scientists is to expand aquaculture, which would improve food security while also taking some pressure off of the wild fish stocks of Lake Victoria.
In terms of the mukene fishery specifically, one relatively straight-forward management solution is to encourage the transition from a near-shore to offshore fishery. Research by a Ugandan fisheries student has already shown that this would simultaneously increase yields of mukene while reducing bycatch of other fish species, such as juvenile Nile perch and cichlids.