Tech that captures fish sound on laptop saves rare species

A transformatory fish project in Lake Victoria that captures fish sound on a laptop has assisted in investigating fish stocks in the Lake while avoiding exploitation of the fish through overfishing.

 The project which was first launched in 2004 by the EU-funded Lake Victoria Fisheries Research Project (LVFRP) of the Lake Victoria Fisheries Organization has also been instrumental in highlighting the number fish species in the world's second largest lake.
The lake covers over 68,800 kilometers squared and trying to assess the amount of living matter in the lack would be a near impossible mission. Yet dwindling fish stocks have been a cause for concern over the years, which is how the technology has come in handy.

Dubbed sonar technology, the process involves vessels like boats used by researchers and scientists being equipped with sound navigation tools. This  technology was initially developed and used to find submarines at great depths under the ocean.
 A small sonar system is fitted into the vessel, with its primary component being 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. Together 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. Interestingly, 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. “There has been a massive over exploitation of the fish stocks in the lake owing to overfishing. Remember this lake is shared by the three East African countries and with this technology we felt we would provide accurate data to impose some fishing quota meant to control fishing and manage our fisheries,”said Morris Ondimu one of the researchers in the project.

The project has so far carried out stock assessments for Lake Victoria´s three most important commercial fish species: the Nile perch, Nile tilapia, and a sardine-like fish  commonly known as dagaa.     According to the researchers, 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.

Statistics have shown that for Nile Perch, the population in the lake is heavily threatened with significant decline in the number of fish reaching maturity and the presence of too many immature fish being caught by the fishermen.  To respond to these growing concern 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.

Written by Dominic Wandati for African Laughter

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