Storspiggens (Gasterosteus aculeatus) påverkan på abborryngel (Perca fluviatilis) via storleksberoende predation

The costal populations of perch (Perca fluviatilis) in some parts of the Baltic Sea have been in decline for about two decades. Recruitment failure in the early larval stages has been put forward as a possible cause and the decline has also been suggested to coincide with increases in three-pined stickleback (Gasterosteus aculeatus) densities. The aim of this study was to study the effects of growth and survival of newly hatched perch larvae in the presence of the three-pined stickleback, and if possible determine the main mechanism behind any negative effects the perch may suffer under such conditions. Using large scale experimental ponds as a controlled habitat, an experiment was conducted where perch larvae were being exposed to sticklebacks under four different stages of their development. Results showed that the three- spine stickleback can have a strong negative effect on the survival of young perch. This effect was strongest in the earliest stage of perch development and decreased as they grew bigger. The zooplankton densities didn't differ between the controls and stickleback treatments, suggesting that the young perch didn't suffer from food limitation. Instead predation was identified as the main mechanism behind the high mortality. The results of this study highlight the potential danger of the observed patterns of decreasing predator populations in conjunction with increasing populations of smaller prey species in the Baltic Sea.