Review of Jellyfish Blooms in the Mediterranean and Black Sea

In the last decade, the media have been reporting on an increasingly high number of gelatinous plankton blooms in the Mediterranean and Black Sea. Thousands of tourists are stung, fisheries are harmed and coastal industrial plants are clogged by gelatinous masses. This review provides an update on this phenomenon and illustrates how the problem is affecting societies. Overfishing may be one of the reasons behind the increase in several jellyfish populations, and the author argues for including gelatinous zooplankton in fisheries management.

This document stems from the Workshop on Algal and Jellyfish Blooms in the Mediterranean and Black Sea, organized by the General Fisheries Commission for the Mediterranean (GFCM) in Istanbul (Turkey) on 6–8 October 2010. Although the results and examples in the review are from the Mediterranean and Black Sea, the experiences gained from this region can be used to provide management advice in other areas. The Mediterranean and Black Sea have a relatively long history of negative impact from jellyfish on human activities, and long-term records exist of both native and introduced species, providing an ideal opportunity to discuss abundance trends, causes and management practices of gelatinous zooplankton.

The review covers a wide spectrum of topics: biology, ecology, causes and consequences of gelatinous bloom events (including both ecological and societal impacts), methodology, future research needs and recommendations for management. The review is packed with beautiful colour illustrations of gelatinous species by Alberto Gennari. Gelatinous zooplankton is comprised of a diverse group of organisms with jelly-like tissue that are found within almost all marine phyla. By ‘jellyfish’, the author refers to three phyla: Cnidaria (‘true jellyfish’), Ctenophora (comb jellies), and Chordata (pelagic tunicates). Fact sheets on the nine most important species causing blooms in the Mediterranean and Black Sea are included as an appendix.

The impact of gelatinous zooplankton is usually associated with negative consequences for fish populations due to predation and competition with fish larvae. However, predation by carnivorous gelatinous zooplankton may prevent the potential monopolization by successful fish species, and thereby enhance the diversity of nekton. Gelatinous zooplankton is a natural part of the pelagic food web as prey for higher-level predators. The author suggests that if gelatinous zooplankton are increasing world-wide, species of jellyfish-eating fish and reptiles will also become more abundant. Although periodic blooms of gelatinous zooplankton often have negative effects on many human activities, such as fisheries, tourism, industry and aquaculture, humans may also take advantage of jellyfish as food and for medicine and cosmetics. According to the review, jellyfish-watching may even become a future tourist attraction!

The impact of gelatinous zooplankton was demonstrated in the early 1980s when the ctenophore Mnemiopsis leidyi A. Agassiz, 1865 was introduced to the Black Sea, coinciding with a dramatic fall in the fisheries. The impact was mitigated when another ctenophore, Beroe ovata Bruguière, 1789, which feeds on Mnemiopsis, also arrived in the Black Sea, thereby re-balancing the predator–prey dynamic. Another example when an overpopulation event in the Adriatic in the 1980s of Pelagia noctiluca (Forsskål, 1775), known commonly as ‘the mauve stinger’, contributed to a series of shifts in the pelagic food web, followed by a reduction in the fisheries. Historical data, dating back to the eighteenth century, show a 12 year periodicity in the mass occurrences of this species. However, in the 1990s the blooms arrived more frequently. The author hypothesizes that the reduction in fish stocks during 1980s and 1990s in the Adriatic Sea was due to an increase in blooms of gelatinous plankton in combination with overfishing.

Jellyfish blooms are natural population phenomena that occur seasonally, but blooms may also be enhanced by anthropogenic impacts such as overfishing, ocean warming, habitat modification and alien introductions. According to the review, overfishing and global warming are the most important drivers of the increased presence of gelatinous zooplankton, globally as well as in the Mediterranean and Black Sea.

The monitoring of gelatinous zooplankton requires different techniques from those employed for crustacean plankton. Several methods applied by scientists are described, e.g. diving, counting from boats, planes, beach strandings, submersibles, hydro-acoustics, continuous plankton recorder and video cameras at fixed stations. An interesting method, recommended in the review, is the ‘Citizen Science’ method where the public assist with semi-quantitative abundance estimates for common species (‘Jellywatch’). Despite its limitations, this method has proved valuable for large-scale information on bloom events and new species records in the Mediterranean.

In order to understand the mechanisms behind blooms of gelatinous zooplankton, the review emphasizes the need for more knowledge on the biology of several species. The field of jellyfish biology is still data-poor by comparison with other zooplankton taxa. The author recommends future studies on the colonization of non-indigenous species, population genetics, trophic ecology, advective transport, biology of the benthic life-stage (polyps), and socio-economic impact.

Despite uncertainties about the causes behind outbreaks of gelatinous zooplankton in the Mediterranean, the Black Sea and elsewhere, interactions between humans and jellyfish are undoubtedly increasing as man increasingly populates the coastlines. The author urges the development of management tools in order to mitigate the effect of blooms. He advocates incorporating jellyfish research into fisheries management, and recommends an ecosystem-based management of fish stocks. This will enable managers to evaluate the functioning of the ecosystem, and understand possible causes behind unwanted changes.

According to the review, the global increase in jellyfish populations can only be mitigated by a reduction in the underlying ‘global’ drivers (e.g. eutrophication, overfishing, global warming and shipping). However, this will depend on international agreements, which, according to the author, is a less realistic approach. Several measures to prevent or cope with jellyfish blooms on a local scale are referred to, including physical destruction (of the medusa or polyp stage), bio-control agents, the design of selective fishing gear, preventing the spreading of invasive species and an increased use of jellyfish products for food and medicine.

The author recommends the development of an ‘early warning system’, which could be a very useful management tool. This would enable aquaculture, tourism and fisheries to regulate their activities based on forecast models that combine observed distributions of gelatinous blooms with physical ocean models, allowing precautionary actions prior to potential impacts of bloom events.

Overall, this document is a comprehensive, well-referenced summary of current knowledge on blooms of gelatinous zooplankton in the Mediterranean, Black Sea, and other regions. In order to make this an even better source for students and marine scientists, some relevant recent published work from the region should be added to the reference list (e.g. Bernard et al. 2011; Brotz & Pauly 2012). The author states that jellyfish are increasing globally, although this view is not generally held by the scientific community (see references in Gibbons & Richardson 2013). Regrettably, no attempts to quantify the socio-economic cost of jellyfish blooms are made, although a few attempts exist in the literature (e.g. Knowler 2005; Nastav et al. 2013). Estimates of the socio-economic costs of blooms would benefit both funding and research prioritization on gelatinous zooplankton. Also, unfortunately, there are several small mistakes, which should be corrected in a future edition, e.g. typescript mistakes, errors in the nomenclature of species names, or quoted references missing in the reference list.

In conclusion, this comprehensive review provides a framework for the inclusion of gelatinous plankton into fisheries science and the management of human activities such as tourism and coastal development. It is clear that a new type of human approach to the ecosystems is needed to confront the jellyfish blooms in the Mediterranean and Black Sea region. This booklet is a valuable contribution to the ongoing discussion on the evidence and causes behind global trends in gelatinous zooplankton, and a useful guide for scientists and managers working on gelatinous zooplankton worldwide.

Tone Falkenhaug

Senior Scientist

Institute of Marine Research, Flødevigen, His, Norway

E-mail: [email protected]

© 2014 Tone Falkenhaug