A new study in Movement Ecology describes how migratory fish passed through the Haringvliet Sluices before the introduction of Kierbeheer (the partial opening of the Haringvliet Sluices to allow limited saltwater intrusion and facilitate fish migration). The analysis, based on data from tagged fish collected between 1996 and 2018, provides an important reference point for follow-up studies on the effects of Kierbeheer on fish migration.
Wageningen University & Research PhD candidate Melanie Meijer zu Schlochtern analysed data from 1,751 tagged Atlantic salmon, sea trout and sea lamprey. By combining this data with information on water levels, environmental conditions and sluice management, she investigated which conditions determined passage through the Haringvliet Sluices.
Limited passage opportunities
The results show that less than one-third of migratory fish actually enter the Rhine-Meuse delta, and that these fish were only able to successfully pass through the Haringvliet Sluices at limited time windows. Conditions appeared to be particularly favourable towards the end of a discharge period. Almost three-quarters of all successful passages took place during this period. At that point, the difference in water level between the sea and the river becomes smaller and flow velocities through the sluices decrease. The results also show that river discharge is important in attracting migratory fish for upstream migration.
Based on these findings, the authors recommend keeping the sluices open for longer periods at low flow velocities, thereby increasing opportunities for migratory fish to pass.
The study also showed that several migratory fish, all of which had been captured and released on the seaward side of the Haringvliet Sluices, did not reach the Rhine and Meuse via the Haringvliet but instead used the alternative route through the Port of Rotterdam.
Differences in passage behaviour between fish species
In addition to the general pattern of passage towards the end of discharge periods, the study also found clear differences between the three fish species. Atlantic salmon proved to be the most selective in the conditions under which the sluices were passed. The species only passed through the sluices within a relatively narrow window of suitable conditions. Almost all salmon and sea trout passed through the sluices during daylight hours, suggesting that vision may play an important role for these species when overcoming barriers. Sea trout showed a much more variable pattern than salmon. This species passed under a wider range of conditions and took, on average, longer to enter the river after being released near the Haringvliet Sluices. Sea lamprey followed yet another strategy. This species migrated exclusively during a limited part of the year (April-May) and also used the fish locks in the sluice pillars when the Haringvliet Sluices were closed. These differences demonstrate that measures to improve fish migration do not automatically have the same effect for all species.
The importance of long-term data
"We had already suspected that the Haringvliet Sluices form a barrier to migratory fish, but this long-term study is the first to provide scientific evidence based on data collected over several decades," says WUR PhD candidate Melanie Meijer zu Schlochtern. "Our results show that species such as salmon, sea trout and sea lamprey struggle to pass through the sluices and that favourable conditions with low flow velocities occur only rarely. Fish could therefore benefit from longer periods of these favourable conditions, such as under the current Kierbeheer, in which the sluices remain open for longer when water flows into the Haringvliet. This study also highlights the importance of long-term monitoring and collaboration in understanding migration patterns. The first fish were already being tracked in 1996, at a time when many people did not yet have a computer at home. By combining data collected over so many years and under a wide range of conditions, a robust picture emerges of how migratory fish cope with these barriers."
First step in a larger study
This study focused on the situation before the introduction of Kierbeheer and was carried out in collaboration with ATKB and funded by Rijkswaterstaat and the Ministry of Agriculture, Fisheries, Food Security and Nature (LVVN). In follow-up studies, Meijer zu Schlochtern is investigating, using recently collected field data, how migratory fish respond to the adjusted sluice management and what effects Kierbeheer has on fish migration.
Relevance for migratory fish worldwide
Migratory fish are under pressure worldwide because dams, sluices and other barriers disrupt their migration routes. At the same time, estuaries and deltas are increasingly being modified to protect coastal areas from flooding and sea-level rise. As a result, knowledge of fish migration at such barriers is becoming increasingly important.
