The Energy Budget of a Local Jellyfish Proliferation: Periphylla periphylla in The Trondheimsfjord, Norway

Trondheimsfjorden is one of approximately one dozen Norwegian fjords where the crown jellyfish, Periphylla periphylla (Scyphozoa, Coronatae) has had massive blooms. It has established large local populations during the last two decades. The population in Trondheimsfjorden is mainly established in three innermost basins. The growth and proliferation have been under close monitoring by regular research vessel cruises done by Trondheim Biological Station (TBS), Department of Biology, Norwegian University of Science and Technology (NTNU). The Periphylla population seems to have reached a local carrying capacity year 2007. Simultaneously with the increase in the jelly population, local artisanal fisheries for codfishes have suffered from reduced catches. The reduction in catches is caused by clogged nets, fish quality reduction due to burn marks, and longer working hours caused by longer travelling distances and cleaning of nets. Together these factors have resulted in a negative economic development, and reduced the number of fishermen. This study estimates the economic consequences of the Periphylla bloom in the Trondheimsfjord. A simple ecologic model, public available fisheries statistics from the Norwegian Directorate of Fisheries, and data provided by Yajie Liu, have been applied. The model is based on calculations and estimations of the energy budget (measured in units of carbon), for the growth and maintenance of the current Periphylla population. The calculations include estimates of biomass and density, respiration rates, carbon demand and size distribution of Periphylla in the entire fjord. The economic and ecologic impacts have focused on the inner fjord basins: beyond the shallow sills at Tautra. The results show a total biomass of Periphylla of 101 466 tons, an average density of 1.83gm-3, average production of 2.01 gCm-2year-1, and an average carbon turnover rate of 0.0175 per day. The size distribution of the jellies is quite different between the four basins. The distribution are also changing on year to year basis within the basins. The yearly average possible codfish production, which could have been produced with this carbon budget, is estimated at approximately 70 tons. After the year 2007 when the jelly population assumingly reached the local carrying capacity, this would sum up to an average amount of 600 000 900 000 NOK per year, for the fisheries. For the entire period 2007-2015 the total loss is estimated to be 900 000-950 0000 NOK