William Kudoja explains how sonar technology is being used to assess the quantities of targeted fish species in Lake Victoria and is helping the authorities to set reasonable catch quotas.
Twice a year, at dusk, a fleet of research vessels begins combing the depths of Lake Victoria in East Africa. The crew members gaze intently at the water surface while their boats advance at a steady, methodical pace. Although the scene bears all the hallmarks of a well organized search party, there is not a single searchlight in sight. That’s because instead of using a beam of light, the crew members are relying on beams of sound to find what they are looking for.
The crew in question is a team of scientists working for the EU-funded Lake Victoria Fisheries Research Project (LVFRP) of the Lake Victoria Fisheries Organization. They are investigating the quantity of fish stocks in the world’s second largest freshwater lake. Assessing the amount of living matter, or biomass, in a body of water covering 68,800 km² may seem like a Herculean task, but the scientists are certainly up to the challenge. Their vessel is equipped with sound navigation ranging (sonar), a technology that was initially developed and used to find submarines at great depths under the ocean.
The scientists are using sonar not only to determine the numbers of fish present in Lake Victoria, but also to identify the various species they find, and to pinpoint areas where fish stocks are concentrated or most heavily depleted. Here’s how it works: onboard the research vessel is a small sonar system, at the heart of which is a transducer, a device that converts electrical energy from a transmitter into high-frequency sound waves, or sonar signals. The sonar signals travel through the water and form an ‘acoustic beam’. When the beam hits a fish in the lake, it bounces back an echo, which is captured by the transducer. The transducer converts the echo back into electrical energy and relays it to a laptop computer. That’s when things get interesting. Coupled with position data from the vessel’s global positioning system (GPS), the computer converts the incoming echoes into a high-resolution echogram showing the exact number and location of targeted fish. What’s more, each fish species emits an echo with a unique amplitude that is identified by the computer. By separating the echoes, the researchers can calculate the biomass of different fish species.
Why go to all the trouble? The reason is that Lake Victoria’s fish stocks are at risk of being over-exploited by the three nations bordering the lake – Kenya, Tanzania and Uganda. With accurate data on the number of fish in the lake, the authorities can impose reasonable fishing quotas that will ensure sustainable fisheries management in the region. Accordingly, the LVFRP initiative is an important element of the larger fisheries management programme of the tri-state Lake Victoria Fisheries Organization, which aims to harmonize national measures for the equitable utilization of the lake’s living resources.
The project has so far carried out stock assessments for Lake Victoria’s three most important commercial fish species: the Nile perch (Lates niloticus), the Nile tilapia (Oreochromis niloticus) and a sardine-like fish known locally as the dagaa (Rastrineobola argentea). The LVFRP team members have found that the stocks of Nile perch amount to 530,000–650,000 tonnes per square kilometre, while those of Nile tilapia and dagaa each amount to approximately 1.2 million tonnes/km². From these biomass estimates, the scientists have calculated the indicative maximum sustainable yield (MSY), or the amount of fish that can be harvested each year without depleting the stocks. For the Nile perch, for example, the MSY is around 212,000 tonnes. If the region’s fishery is to remain sustainable, the harvested amount should be below the MSY so that the fish are able to spawn.
Most recently, data from the biannual acoustic surveys were combined with catch assessments and trawl surveys to indicate that Nile perch fishing activities had reached critical levels. The findings showed a significant decline in the number of fish reaching maturity, the presence of too many immature fish in catches, and low fecundity levels. In response to these worrying trends, the Lake Victoria Fisheries Organization partner states have introduced measures to ban the harvesting and processing of Nile perch within the size range 50–85 mm throughout the lake. The measures are aimed at protecting both very young fish, so that they can breed at least once, and adult fish that are about to spawn. To ensure that fishermen catch fish of the permitted size, the use of gill nets with a minimum mesh size of 127 mm has been recommended for the Nile perch and Nile tilapia fisheries. For the dagaa, the recommended mesh size is just 10 mm, to be used in designated fishing grounds only.
These and other measures directly related to the development of a long-term fisheries management plan for Lake Victoria have been welcomed by local fishing communities. The fishermen now catch bigger fish, get more money for their catch, and are assured of fish the next day. Thanks in large part to the ongoing efforts of LVFRP scientists, the sustainability of the lake’s resources, and the future prospects for local fisheries, are improving steadily.