Images: Pieterjan Verhelst, Ine Pauwels & Jan Soors ©
Like salmoninds, twaite shads (Alosa fallax) live at sea but migrate upstream in rivers to spawn. However, where salmonids are notorious to swim vast distances deep inland, twaite shads often remain in the lower reaches of rivers and even spawn in the estuaries, where tidal effect is noticeable. Due to the construction of sluices and pollution, essential spawning habitat for twaite shads became inaccessible or lost in most parts of Europe. Yet, the River Scheldt still has a well-developed estuary and due to policy, the water quality has improved substantially over the last decade. This lead to the return of a spawning population of twaite shad in Belgium in the 2010s after 100 years of absence. However, it is unclear whether the conditions for a sustainable recovery are met and hence, how successful reproduction is.
Despite being a highly sensitive fish to handling and stress, we managed to tag the species since 2015 with acoustic transmitters to get a better understanding of their migratory whereabouts in the estuary. The fish are tagged either externally or internally, and the battery of the transmitters last for at least 1 year, allowing us to observe returning shads. Various fishing techniques are applied to catch the shads in the best condition as possible, such as stow and fyke nets. Apart from telemetry, the actual spawning behaviour is being monitored as well by a network of volunteers. Twaite shad spawning typically starts in the evening and shows itself as circular water ripples, often accompanied with jumping fish and splashing. All this information provides us insight in what drives the shads to migrate and spawn, which areas they prefer and how long they stay at the spawning grounds.