European lobsters with mild, none, and severe shell disease (left to right). Photo: Andrew Rowley / Swansea University

By Charlotte Eve Davies
28 March 2016

(The Conversation) – Global climate change is altering the world’s oceans in many ways. Some impacts have received wide coverage, such as shrinking Arctic sea ice, rising sea levels, and ocean warming. However, as the oceans warm, marine scientists are observing other forms of damage.

My research focuses on diseases in marine ecosystems. Humans, animals, and plants are all susceptible to diseases caused by bacteria, viruses, parasites, and fungi. Marine diseases, however, are an emerging field.

Infectious agents have the potential to alter ocean life in many ways. Some threaten our food security by attacking important commercial species, such as salmon. Others, such as bacteria in oysters, may directly harm human health. Still others damage valuable marine ecosystems – most notably coral reefs.

To anticipate these potential problems, we need a better understanding of marine diseases and how climate change affects their emergence and spread.

Recent studies show that for some marine species diseases are spreading and increasing. Climate change may also promote the spread of infectious agents in oceans. Notably, warming water temperatures can expand these agents' ranges and introduce diseases to areas where they were previously unknown.

Many diseases of marine species are secondary opportunist infections that take advantage when a host organism is stressed by other conditions, such as changes in pH, salinity or temperature. A bacterium that is dormant (and therefore noninfective) at a certain temperature may thrive at a slightly higher temperature.

One well-documented example is the emergence of epizootic shell disease (ESD) in American lobsters. This disease, thought to be caused by bacteria, is characterized by lesions that penetrate inward from a lobster’s shell surface towards the inner flesh, making infected lobsters unmarketable. ESD can also kill lobsters by making it difficult for them to shed their shells in order to grow.

In the 1990s, following almost a decade of above-normal summer temperatures, ESD affected so many lobsters that the Atlantic States Marine Fisheries Commission declared that the Southern New England fishery (Connecticut, Massachusetts, New York and Rhode Island) was in collapse and recommended closing it. Fishery models that incorporated shell disease offered convincing evidence that ESD was a major factor in the decline of the stock. This episode underscores the importance of considering marine diseases in stock assessments and fishery management.

Now there are concerns that ESD will continue to spread north to Maine’s US$465.9 million lobster fishery. In 2015 the Gulf of Maine showed record high abundances of lobster, making it one of the most productive fisheries in the world.

However, sea surface temperatures in the Gulf of Maine have increased faster than 99 percent of the global ocean over the past decade, warming three times faster than the global average. Since temperature is a primary factor in the spread of this disease, observers fear that it could have devastating effects on Maine’s lobster fishery. [more]

Is global warming causing marine diseases to spread?

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