Mr Hartvig Christie1, Dr Eli Rinde1, Dr Partrik Kraufvelin2
1Norwegian Institute For Water Research, NIVA, Oslo, Norway, 2SLU Sweden, Øregrund, Sweden
After overfishing of the top predator coastal cod, its prey organisms like mesopredatory fish and crabs are expected to increase. In 12 large rocky shore mesocosms (13 m3 each), after establishment of diverse Fucacean dominated ecosystems, replicated manipulations of small fish (goldsinny wrasse, Ctenolabrus rupestris) and green crab (Carcinus maenas) densities, each mesopredator species, both in combination, and fish combined with nutrient addition were performed during the summer seasons of 2018, 2019, and 2020. The crabs mainly affected mussels and barnacles and had minor (but not significant) effect on macroalgal distribution, both alone and in combination with wrasses. The wrasses affected the algal distribution by enhancing some filamentous algae species at the expense of the larger brown Fucus species. Additionally, high densities of wrasses enhanced the effect of eutrophication. However, the effects of the wrasses on the macroalgal community varied between years and were related to the feeding preferences of the fish (found by gut content analysis). The effects were minor when the wrasse diet was dominated by filtering animals like mussels, and more pronounced when it consisted of macrofauna grazers like amphipods, isopods and gastropods. The experiments illustrate different pathways of top-down effects to take place.
Presentation Slides – Christie Hartvig
Biography:
Hartvig Christie is a marine biologist and works as a senior researcher (emeritus) at Norwegian Institute for Water Research. He has more than 35 years experience from ecological studies of kelp forests, seaweeds, and seagrass beds. Main activities include natural ecosystem functioning and effects of anthropogenic factors. The experiences cover field work including diving, and experiments in aquariums and mesocosms.
