Recently published research performed by IMARES Wageningen UR and the Vrije Universiteit Brussel shows that seawater smelt found in the Wadden Sea and North Sea hardly ever spawn in the IJsselmeer. The landlocked smelt population in this freshwater lake area is not doing very well, with the result that the numbers of fish-eating water birds have also decreased considerably. Because smelt are an important source of food for these birds, research was carried out to see if the two smelt populations were still interconnected and whether migrating smelt still have any effect on the freshwater populations.
Afsluitdijk is a barrier
Smelt, a migratory fish, used to swim freely in and out of the inland sea, the Zuiderzee: towards freshwater in order to spawn and then back to sea again. Since the Afsluitdijk dam was built, two forms of smelt have been in existence: the migratory sort that lives in the Wadden Sea and North Sea and spawns in any freshwater that can be reached, and a landlocked population which lives and spawns in two lake areas, the IJsselmeer and Markermeer. This particular freshwater population has been declining since 1990. As a result, many fish-eating water birds have also greatly decreased in numbers.
Natura 2000 site
Both lakes, being important for fish eaters such as black tern, grebe, little gull and goosander, have been designated as Natura 2000 sites. Because of the decline of smelt as an important food source for these types of birds, the question arose as to whether the two populations of smelt were still connected and whether the migratory smelt were still of any benefit to the freshwater populations. That is the reason for investigating during spawning time whether any individuals arriving in the IJsselmeer/Markermeer come from salt water. Any chance of exchange is limited to the drainage sluices that they can swim through when draining is taking place.
Research into otoliths
Growth rings, which occur in otoliths (ear bones) just as they do in tree trunks, provide information about conditions during growth. Fish that live in seawater have much higher concentrations of the strontium isotope 88Sr than freshwater fish. Together with the Vrije Universiteit Brussel, IMARES has used Laser Ablation-ICP-MS analyses to measure strontium profiles from the nucleus to the exterior of ear stones in fish. The nucleus shows the circumstances at the beginning of the fish's life and the exterior at the moment the fish was caught.
The strontium profiles in the ear stones of smelt caught in the IJsselmeer and Markermeer during spawning were analysed to see if 'seawater smelt' were present in the freshwater area. To provide a reference for 'seawater smelt', about a dozen samples from the Wadden Sea were also analysed. Out of the more than 100 otoliths investigated, not one single stone clearly indicated 'salt water'. We can therefore conclude that there is no evidence at present that seawater smelt have a significant impact on the landlocked population in the IJsselmeer area.