Lewis addresses the sudden increase of jellies to our shores this past year.

Many people would have noticed that this summer has been particularly bad with the frequent influxes of Pelagia noctiluca, the mauve stinger, and numbers reaching plague proportions. Cast your mind a little further back to Easter time and there were swarms of Physalia physalis, the Portuguese man o’ war, and other siphonophores littering the beaches. Whilst it is true that we have always suffered the effects of jellyfish blooms over the course of the year, it is also true that 2018 has been one of the worst years for a long time in Gibraltar. Why?

The first thing to note is that this is a global phenomenon which is indicative of our changing oceans. Warmer waters, driven by climate changes, do in fact favour jellyfish reproduction. This is partially due to an increase in food availability because of longer summer seasons which increase the chances of young Mauve stingers reaching adulthood. Already here things can begin to get a little complicated.

Plankton are made up of two groups; phytoplankton are the plants and zooplankton, which include the larval stages of most of the coastal species on our coastline. As jellyfish numbers increase then the amount of plankton being consumed also increase and consequently plankton abundance decline; here lies the rub. The zooplankton will include many species that once out of their planktonic stage actually eat jellyfish and hence the suppression of zooplankton numbers is not a great outcome.

All things should be taken with a pinch of salt, after all, a healthy ecosystem should be in balance. Typically, this is the case but climate changes are indeed tipping this delicate balance in favour of the jellies. Warmer, more acidic waters and lower dissolved oxygen levels, all associated with climate change, are bad for most marine animals, but favour jellyfish.

But jellyfish have their predators too. Turtles, tuna, swordfish, salmon, triggerfish and sunfish are all consumers of them, so why are the consumers’ respective numbers not increasing in line with the rise of jellyfish?

In the case of tuna, swordfish and salmon, the answer is glaringly obvious, as at one point or another they have ended up on our plate; overfishing. Industrialised commercial fishing techniques have been incredibly efficient at outsmarting these animals and the availability of them at restaurants throughout the year is alarming. Perhaps limiting such destructive fishing practices throughout the Mediterranean would be beneficial, but it would clearly require co-operation from many different Governments. In truth, there is probably no real will to establish and co-ordinate such an agreement anyways as different countries have opposing vested interests.

The three Mediterranean turtles are the loggerhead (Caretta caretta), the green (Chelonia mydas) and the leatherback (Dermochelys coriacea) – and they all eat jellyfish! In the case of the loggerhead and the green, they tend to eat jellies whilst they are hatchlings and juveniles but tend to consume less in adulthood. By contrast, the leatherback eats jellyfish constantly throughout its life. All three species are very important with respect to controlling jellyfish numbers and recent declines in their populations are a worrying trend with all three species presently classified as vulnerable or endangered. Within the Mediterranean, one can find green and leatherback nesting sites in the eastern basin around Greece and Cypress. The leatherback has no known nesting sites within the Med and the closest breeding spots are along the coast of the North West Atlantic. As ocean dwellers they do often visit the Mediterranean searching for food, of which recently, there has been an abundance of.

These three species have been affected greatly by plastic pollution as they often mistake plastic bags for jellyfish. On attempting to ingest the bag they can suffocate on it and this often happens with the greens and the loggerheads whilst they are sub-adults, taking them out of the food web early. Consequently, the predation pressures commonly exerted on jellyfish are removed allowing the plague populations to grow.

Unfortunately, there is a similar story with the Grey Trigger fish (Balistes capriscus) and the Sunfish (Mola mola) whose populations are similarly affected by plastic ingestion.

A final point to consider, which has been underrepresented in all discussions on this topic, is the effect of the North Atlantic Oscillation (NAO). Briefly, there is a permeant low pressure system over Iceland and a high-pressure system over the Azores. The interaction and relative positions of these systems control the strength of the westerly winds into Europe. In a positive NAO, the Azores high pushes the Atlantic storms towards Northern Europe where it dumps all the rain and keeps the South relatively dry. Inversely then, in a negative NAO year the Azores high pressure system is weaker and so the Atlantic storms hit Southern Europe, making it a wetter winter in Gibraltar but a dryer one in Northern Europe.

Some scientific studies have suggested that in the years where we suffer a Negative NAO, like 2018 has been, the subsequent summer suffers more heavily with jellyfish. An interesting observation indeed but worrying should it turn out to be correct as we are likely to see a repetition every 2nd summer. So, in the spirit of scientific investigation I would propose the following prediction: If the science is correct then summer 2019 should be a positive NAO which should mean a dryer, storm free winter and should also see less jellyfish in the summer time. Here is to hoping it turns out to be correct, or the reaction from the public could be more stinging than the jellyfish!