Overview

The problem

Jellyplankton outbreaks (massive occurrences of medusae and other gelatinous planktonic organisms) are an increasing threat for some key economic activities in coastal areas, such as fisheries, aquaculture and recreation.

Project objectives

• identify and quantify key factors regulating the abundance and succession of jellyplankton species in European waters
• better understand the role of jellyplankton in marine ecosystems
• develop models able to forecast the likelihood of jellyplankton outbreaks
• estimate the socio-economic effects of jellyplankton outbreaks in European coastal areas

The work

Through a broad joint effort among 10 partners we will define the basic biological and ecological factors that govern reproduction, growth, and survival for a number of different species, commonly occurring in high abundance. Five different habitats are appointed as model environments, all characterised by their seasonal or permanent mass occurrence of jellyplankton, but otherwise quite divergent. These study sites are investigated with the aim of defining and quantifying the importance of each target species, and especially its ability to affect fish production, and are backed up by other field- and experimental studies. The results are put into a historical perspective by reviewing previous documentation of fisheries and jellyplankton biomass. Literature and new data will be used in a mathematical model, to define the competitive ability between the targeted jellyplankton species and visual predators like fish, and this will help in understanding in which biological and environmental conditions one or the other of the two types of competitors will dominate. We will also design and use a particle-tracking model that will improve the possibility of giving prognoses for mass-occurrence due to advective transport. As a more specific approach towards the aquaculture industry we will study acute and chronic effects on fish of jellyfish stings. The main findings of relevance to the society will be reported and recommendations of actions being made.

The study sites

Lurefjorden in Norway is known for its mass occurrence of the coronate scyphomedusa Periphylla periphylla. This species is not a targeted one in the EUROGEL project, but the unique possibility of studying which environmental and trophic factors cause a situation with a jellyfish as the stationary dominant top predator, makes this fjord an ideal model site. The fjord lacks mesopelagic fishes whereas these are common in neighbouring fjords. The seaward entrance is 200 m wide and 20 m deep, the length is 20 km, and the broadest section is 2 km wide. The main basin is 440 m deep and the fjord volume is around 14 km3. The temperature varies seasonally in the surface water between 2-5°C in mid winter to 15-19°C in summer. The salinity varies between 26-33 psu in the surface water. Both temperature and salinity is stable over the year below 100 m, with average values of 6.1°C and 33.2 psu, respectively.

Sandsound Voe in Shetland, is a shallow (max 24 m) channel some 2.5 km long and 1 km wide, connected to the sea at its southern end, and to an inner basin to the North. The inner part is 2.5 km long and 1.6 km wide. The physical environment of these water bodies is poorly studied. Several salmon-, sea trout-, and mussel farms are established in the area. Severe jellyfish outbreaks are common here and have drastically affected the salmon and sea trout farming in these areas. 

Limfjorden in Denmark has an area of 1575 km2, a maximum depth of 10 m, opens in the West to the North Sea and in the East to the Kattegat. The salinity ranges from near-freshwater conditions in some inland sections to > 33 psu near the North Sea. At present, there is an extensive fishery on the blue mussel (Mytilus edulis), which is commonly seen in increasing abundance during eutrophication. Fishery on edible fish is hardly existent, and fish are almost exclusively caught for industrial production. Species caught today are predominantly sprat (Sprattus sprattus) and herring (Clupea harengus). Plaice, flounder and cod, were earlier basis for an important fishery, but cannot be caught commercially any more (see fig.). Concurrent with the decrease of these species, scavengers (shore crabs, Carcinus maenas), invertebrate filter feeders (Mytilus) and pelagic invertebrates (Aurelia aurita and hydromedusae) have dramatically increased in abundance. A recent study has shown that occurrence of jellyplankton in the fjord is negatively correlated to the observed densities of pelagic fish eggs and larvae of early-spawning fish (flounder, plaice, and cod). We hypothesise that pelagic invertebrate predators play a critical role in preventing the recovery of commercially usable fish stocks in this eutrophicated sound. We expect particularly the recruitment of cod and flatfish to be limited by high ingestion rates of Aurelia, Obelia and Rathkea.

Catches of edible fish in Limfjorden, Denmark, between 1981 and 1996, as calculated from landing statistics.

The Mar Menor lagoon in Spain is separated from the Mediterranean Sea by a 20 km long sandy barrier with three shallow inlets. It covers an area of 135 km2, the average depth is 3.6 m, and salinity ranges from 43 to 47 psu. The lagoon has traditionally supported important fisheries (grey mullet, sea bream and prawns), but the stocks have decreased considerably. Medusae have always been present, but the two dominant species today, the large-sized scyphomedusae Cotylorhiza tuberculata and Rhizostoma pulmo, were not recorded until about ten years ago. They developed the first massive outburst in 1993, and this has been recurrent every summer up to date. Anthropogenic activities such as salt production, dumping of mining waste, boat sport, human settlements, and intensive construction work due to increased tourism, have been responsible of drastic changes in the water circulation. These activities have altered the water properties, the sediment composition, and both the benthic and pelagic communities. The high input of nutrients in the lagoon (2000 tonnes N and 60 tonnes P annually) from the irrigated lands, watersheds, groundwater drainage, and salt production is changing the originally oligotrophic water through progressing eutrophication.