Since the mid 1980s fish health problems and mortalities due to the exposure to plankton pest species, such as jellyfish swarms and blooms of certain algae, are well known to the aquaculture industry and their insurers.
Plankton species are mostly microscopic organisms that cannot swim against prevailing currents and consequently are carried around as the seas move. However, plankton range in size from bacteria to large jellyfish and form the largest part of marine food webs including larvae of bottom living species and fish. The focus here is on the jellyfish species, mainly planktonic stages of cnidaria .
A recent example of damage to marine fish farming would be the November 2007 mauve stinger jellyfish ( Pelagia nocticula) influx, which wiped out 100,000 salmon at Glenarm, Northern Ireland's only salmon farm, and caused fish damage and mortalities at some Scottish sites (e.g. the "Bloody Bay" site at Mull whe
re many fish died). Swarms were also reported along the west coast to Ullapool and around the Orkney Isles. Pelagia is an oceanic species that has no bottom living phase and develops directly and rapidly from egg to adult medusae. Pelagia is one of several jellyfish that is common in offshore temperate waters and in the Mediterranean Sea. In recent years and in earlier outbreaks, in the 1980s and 1990s, Pelagia swarms have caused serious public health concerns, disrupted tourist amenity and industry and have resulted in large economic costs and losses. Pelagia is a scyphomedusae as are the Lion's mane ( Cyanea) and jellyfish Moon ( Aurelia) species, more common in UK waters.
Given the burgeoning growth of the aquaculture industry since the 1980s, it is perhaps surprising that there is relatively poor data available to quantify and assess the extent of the problems caused by jellyfish, or to advise on or mitigate their effects. Much of the information relates to extreme incidents where concerns prompted industry anxiety or public notice.
Probably the majority of incidents are not recorded as jellyfish damage; indeed many available records assign mortalities to "plankton" or "gill damage", with jellyfish or algae plankton not mentioned specifically. Also it is often hard to sample and identify plankton species as causes except when they are massively abundant. Large jellyfish swarms are obvious, while smaller species such as the narcomedusa Solmaris or the
siphonophore Muggiaea, are only a few millimetres in size and transparent. Others are unusual looking and fragile. The siphonophore Apolemia uvaria looks like fuzzy grey string, and can measure from a few centimetres to metres in length. These species and many others are now known dangers, with the worst swarms able to wipe out cages of fish over a tide or two.
Fish farmers experience jellyfish as a complex problem set that cause economic losses associated with;
- direct losses of fish mortalities and disposals,
- reduced growth on or after exposures to harmful plankton,
- increased operational costs and costs of mitigation actions,
- production losses in emergency slaughtering, and reduced prices,
- some increases in insurance premium.
Farmers have developed strategies and cage designs to avoid or reduce these problems, such as sites for cages being kept out of tidal eddies or such, watch keeping and warning each other of incidents, deploying mesh screens, suspending feeding, increasing oxygen by aeration etc. If direct losses are larger than a pre-set limit, compensation is usually given by insurance companies. Smaller losses are covered by the fish farm owners and these often may be unreported. The limited data available to scientists in Marine Scotland Science, from their industry and science colleagues in Scotland, Ireland, Norway and elsewhere, allows some assessment of the relative extent of plankton induced mortalities.
A summary of these data show that; of the specific plankton caused incidents reported to Marine Scotland Science between 1999 and 2005 from around Scotland, including Shetland; out of 4.7 million fish lost (9.5 thousand tonnes) around 60% of deaths by weight and numbers were due to jellyfish. Using recorded data from a particular set of farms in the Scottish region of Skye and the Outer Hebrides during 2002-2005; mortalities caused by jellyfish or plankton accounted for more than 10% of total recorded losses in terms of fish numbers, and 17% of the fish biomass lost. Other recorded losses include diseases, sea lice, transfers and storms etc. Jellyfish caused mortalities were mostly quite isolated and hard to predict. Coastal species may form local swarms and they sometimes occur in massive incidents from July to November, these most often involving oceanic species carried in to coastal waters. Changes in ocean climate may result in changes in the frequency and extent of such swarms appearing. There are jellyfish and harmful algae species present though much of the year - some species are coastal and some are oceanic, many are swarm or bloom forming.
Given that algal and/or jellyfish populations naturally fluctuate over the seasons and between years, although they may occur nearly all year round, then even when they are not obvious or abundant, it is highly likely that there are ongoing small scale and chronic problems that are often not noticed or recorded. Indications of jellyfish damage include behaviours such as jumping, gasping, collision with cage walls, and symptoms may include bleeding and necrosis in the sometimes pale gills with excess mucous or the presence of or protozoa (e.g. Trichodina sp.), eye damage, darker patches or stripes like burn marks on the fishes' sides and on the gill covers. Usually in severe cases death is caused by suffocation due to damage to respiratory exchange surfaces and high oxygen demand when stressed. Following the primary insult other disease organisms (e.g. Piscirickettsia) and parasites (e.g. Paramoeba) may become established and contribute to the overall mortality figures. A "grading-effect" sometimes occurs, where most or all large fish in a sea-cage are killed and most or all of the smaller fish survive. Effects appear to be long-term and fish can show residual effects (i.e. reduced feeding/growth, altered behaviour) for up to a month post-exposure.
Marine Scotland Science are keen to apply their research skills, to increase their understanding of the problems. They welcome any information on such incidents the industry can provide.
More information about jellyfish biology.