Unusual patterns of jellyfish sightings reflect the state our seas

Parliamentary Yearbook reports on unusual patterns of UK jellyfish monitored by the Marine Conservation Society during 2013.  According to the charity, jellyfish populations are “important indicators of the state of our seas”.

Despite their name, jellyfish are not fish. They are invertebrates belonging to an animal group called the cnidarians.  This animal group has stinging cells which are used both to capture prey, and as a defence against predators.    Their stinging cells contain a capsule which consists of a rolled filament and a poison.  When contact is made with the surface of the jellyfish, these capsules open, ejecting the filaments into their prey, through which poison is injected.  They are perfectly camouflaged because their body is 95% water. 

The collective name for a group of jellyfish is “bloom”, “swarm”, or “smack”.  The preferred term for a large group of jellyfish that congregate in a small area associated with seasonal increase or when numbers exceed the norm is “bloom”.

Scientists do not know the ultimate causes of jellyfish blooms but believe that bloom formation is a complex process which may depend on a number of factors. This includes ocean currents, nutrients, sunshine, temperature, season, prey availability, reduced predation and oxygen concentrations. 

Over the last ten years, Britain’s Marine Conservation Society (MSC) have been responsible for monitoring sightings of marine life species - including jellyfish - in UK and Irish waters. Researchers from the MSC have reported unusual patterns of jellyfish sightings for 2013.  Compared with previous years, there were very few sightings reported until June, followed by unusually high numbers reported during the summer months.  The researchers believe this was due to an unusually cold spring, followed by an extreme summer heat wave.

According to Dr Peter Richardson, the MSC’s biodiversity programme manager, “the scarcity of jellyfish reports before June was unusual and could well be linked to the exceptionally cold spring”.

He went on to say: “As our waters warmed, sightings of jellyfish increased with moon jellyfish reported in large numbers around the UK, reports of compass and blue jellyfish in the South West, and blooms of lion’s mane jellies around North Wales and north-west England.”

Dr Richardson reports that there is evidence that numbers have been increasing around the world. At the same time, he noted the diversity of opinion as to the cause. While some scientists argue that numbers increase and decrease as part of a normal cycle every 20 years, others believe that the increases are linked to causal factors which disrupt the natural water ecosystem. 

One such factor is the pollution of our oceans from industrial and agricultural practices, resulting in a process known as ‘eutrophication’.  This occurs when water bodies receive excess chemical nutrients such as phosphorous and nitrogen which disrupt the natural ecosystem to stimulate excess plant growth.  It results in massive algal blooms, accompanied by low levels of oxygen.  Jellyfish are known to thrive in such waters.

Other causal factors described include over-harvesting of fish - which compete with jellyfish for the same kind of food - is believed to lead to jellyfish proliferation and climate change, which affects changes in water temperature.

Whatever the cause, Dr Richardson believes that we “should consider jellyfish populations as important indicators of the state of our seas” and that large increases “are telling us about the health of our seas and cannot be ignored”.

www.parliamentaryyearbook.co.uk

Email: parliamentaryyearbook@blakemedia.org