Due to the changing climate, young herring arrive in the Wadden Sea earlier and earlier in spring. That is shown in a new publication by NIOZ ecologists Mark Rademaker, Myron Peck, and Anieke van Leeuwen in Global Change Biology. "The fact that we were able to demonstrate this was only due to very consistently - for more than 60 years - and continuously sampling the fish every spring and every fall with exactly the same fyke [net] every time," Rademaker says. "Recognizing this kind of change requires extreme precision and endurance." Since 1960, NIOZ, Royal Netherlands Institute for Sea Research, has been measuring the number and species of fish that swim in the Marsdiep, between Den Helder and Texel, day in and day out, using a standard fyke, in spring and fall. These measurements show that the peak of the number of young herring swimming into the Wadden Sea since 1982 comes at least two weeks earlier now. "Such a calculation is difficult with a species of fish that swims in large schools," Rademaker says. "One day, there may be only ten herring, while the next, there are suddenly ten thousand fish swimming by. So, if you were to take a measurement just one day or the other accidentally, you would get a completely different picture." According to Rademaker, the solution to that problem lies in extremely consistent measurement, almost to the square meter. "Only by carrying out measurements in the same place over and over again, and almost continuously, year after year, can you reliably reveal changes in the long term." Rademaker says, "When I projected that frequency from other research programs onto the data from the NIOZ fyke, picking out a few random measurement days, the changes in the timing of the herring did not show up." The study of the rhythm of young herring in the Wadden Sea will also be one of the chapters in the dissertation that Rademaker will defend next month at Wageningen University. Just as he was able to... |