By Nelisiwe Khusi
May 25, 2024, is World Fish Migration Day, a global celebration to raise awareness of the importance of free-flowing rivers and migratory fish. The theme for 2024 is “Free Flow.”
The Importance of Fish Migration
Fish migration is a natural phenomenon where fish move from one habitat to another, often covering vast distances daily, monthly, or annually, to complete their life cycle, mate, feed, or find suitable living conditions. Migratory fish play a critical role in the health of aquatic ecosystems by linking different habitats, which helps distribute nutrients and energy. Unfortunately, harmful activities such as dam construction, pollution, and overfishing have severely impacted fish migration routes. World Fish Migration Day aims to raise awareness about these issues and promote the restoration and conservation of migratory pathways to ensure the survival of these vital species.
Migratory Fish Species within the Gouritz Cluster Biosphere Reserve
Here are some remarkable examples of migratory fish and their incredible journeys within our region:
African Longfin Eel (Anguilla mossambica):
Freshwater eels are elongated fish with snakelike bodies. The African Longfin Eel can grow to 1.2 meter in length and weigh up to 6 kilograms. Juvenile eels, known as glass eels, enter estuaries and rivers at night and migrate upstream, sometimes reaching man-made dams or weirs. In freshwater, they transform into sub-adults called elvers and continue migrating until they reach about 250-300 mm in length. At this stage, they settle in one area until they mature. Once mature, they return to the sea to breed. This species is comfortable in both salt and freshwater (Skelton, 2001; WRC, 2021). Pictures of Longfin Eels on iNaturalist: African Longfin Eel
African Shortfin Eel (Anguilla bicolor):
The African Shortfin Eel can grow to 800 mm. Adult eels breed at sea, and young glass eels enter the river at night via the estuary. In freshwater, they change into sub-adults known as elvers and continue their migration upstream. At about 300 mm in length, they stop migrating and remain in the same area until fully mature when they return to the sea to breed (Skelton, 2001). Picture of Shortfin Eel on iNaturalist: African Shortfin Eel
African Mottled eel (Anguilla bengalensis):
African Mottled Eel can grow up to 1.45 meters in total length. Adult eels breed at sea, and the young glass eels enter the river at night via estuaries. In freshwater, they change into sub-adults known as elvers and continue their migration upstream. At around 300 mm in length, they stop migrating and remain in the same area until fully mature, when they return to the sea to breed (Skelton, 2001). Picture of Mottled Eelon iNaturalist: African Mottled Eel
Giant Mottled eel (Anguilla marmorata):
Giant Mottled Eel can grow up to 1.85 meters in total length. Adult eels breed at sea, and the young glass eels enter the river at night via estuaries. In freshwater, they change into sub-adults known as elvers and continue their migration upstream. At around 300 mm in length, they stop migrating and remain in the same area until fully mature, when they return to the sea to breed (Skelton, 2001). Pictures of Giant Mottled Eel on iNaturalist: Giant Mottled Eel
Freshwater mullet (Pseudomyxus capensis):
Endemic to South Africa, the freshwater mullet breeds at sea. Juveniles move into estuaries and rivers in late winter or early spring. Males remain in freshwater for up to 4 years, while females for up to 7 years. Before breeding, they return to estuaries. They feed on benthic algae and small invertebrates over muddy bottoms (Skelton, 2001). Their populations have declined due to obstruction by dams and weirs. Pictures of Freshwater Mulleton iNaturalist: Freshwater Mullet
Bull Sharks (Carcharhinus leucas):
Also known as Zambezi sharks, bull sharks are large, stocky sharks common worldwide in warm coastal waters and rivers. They can switch between marine and freshwater habitats. Bull sharks migrate to estuaries and rivers to give birth, away from larger predators. In 2009, a four-meter bull shark was captured 5.5 km up the Breede River in the Southern Cape, marking the species’ new global size record. They thrive in low-saline water and are found in several major rivers, including the Zambezi, Umzimvubu, and Breede Rivers in South Africa (McCord and Lamberth, 2009). Pictures of Bull Sharks on iNaturalist: Bull Sharks
Celebrating and Protecting Migratory Fish
World Fish Migration Day features various activities and events worldwide, including river clean-ups, educational workshops, film screenings, and the construction of fish passages to help fish bypass obstacles like dams. These efforts aim to restore migratory routes and educate the public on the importance of free-flowing rivers and healthy fish populations.
How Can You Help?
- Participate in Local Events: Join or organize events in your community to support fish migration.
- Advocate for Policy Change: Support legislation to protect migratory fish habitats and remove barriers like outdated dams.
- Educate Others: Spread the word about the importance of fish migration and the challenges these species face.
World Fish Migration Day is more than just a celebration; it’s a call to action. By understanding and supporting the incredible journeys of migratory fish, we can help ensure that these species continue to thrive. Their migrations are not just spectacular natural events but essential for the health of our planet’s rivers and oceans. So, let’s celebrate these aquatic travellers and work together to protect their pathways for future generations. Visit the World Fish Migration Foundation to learn more about fish migration and ways to raise awareness of the importance of free-flowing rivers and migratory fish species Link
References
McCord, ME and Lamberth, SJ. 2009. Catching and tracking the world’s largest Zambezi (bull) shark Carcharhinus leucas in the Breede Estuary, South Africa: the first 43 hours. African Journal of Marine Science 31 (1): 107-111.
Skelton, P. 2001. A Complete Guide to the Freshwater Fishes of Southern Africa Struik Publishers, Cape Town, South Africa
WRC. 2021. FRESHWATER CONSERVATION: Race on to save SA’s unique eel species before they slip into extinction. [Internet]. Available from: https://www.wrc.org.za/?mdocs-file=62529. [Accessed: May 24 2024 ].