Jellyfish populations around the world have been increasing in recent years, and several very large jellyfish blooms have been reported since the early 2000s. The cause of these, and the general population increase, has remained somewhat unclear until now though. Is it simply observation bias? Cyclic population change? Warming waters? Changing currents?

But now, thanks to new research from the Institut de Recherche pour le Développement (IRD), the causes have become clear. The primary cause is overfishing, and with it the decline of many ecologically important species. Many significant predators of jellyfish, such as tuna and sea turtles, have seen their numbers plummet in recent years as a result of overfishing. And with their decline, jellyfish have begun to see their populations grow. But perhaps far more important than that decline, though, is the overfishing of small pelagic fish, such as sardines and herring, which are the main competitors of jellyfish.

“However, jellyfish are primarily taking advantage of the overfishing of small pelagic fish. Just like these cnidarians, sardines, herring, anchovies and more feed off zooplankton. Thus, they represent their main competition for food. In areas where too many of these fish are caught, they free up an ecological niche. Jellyfish now have free rein and can thrive. Furthermore, small fish eat the eggs and larvae of jellyfish. Therefore, under normal conditions, they regulate the population. In their absence, there is nothing to stop the proliferation of these gelatinous creatures.”

So, in short, overfishing is resulting in a decreased level of competition for the jellyfish, allowing their numbers to boom.

The new research was done by comparing “two ecosystems belonging to the same ocean current, the Benguela, which flows along the south of Africa. The first ecosystem is located off the coast of Namibia. Here, fish stock management measures are not very restrictive. The stocks are barely restored before fishing activities start up again. Jellyfish are currently colonising these coastal waters. The second ecosystem is located 1,000 km further south, off the coast of South Africa. Here, the opposite is true: fishing has been tightly controlled for 60 years. The jellyfish population has not increased.”

“A vicious circle is developing in affected areas. Under the water, the links in the food chain are much more flexible than on Earth: prey species can feed off their predators. As such, jellyfish devour larval fish. Their proliferation prevents the renewal of fishery resources. This invasive species in turn threatens fisheries. In Namibia, some 10 million tonnes of sardines in the 1960s made way for 12 million tonnes of jellyfish.”

The research makes it clear that a more better approach to fishing, one that takes whole ecosystems into account, is necessary if the health of the oceanic ecosystems is to be maintained. To put it a different way, management measures need to be put into practice which consider potential impacts across all levels of the trophic network. Otherwise, at some point in the not-too-distant future, the seas may be populated primarily by jellyfish, as they once were.

While there has been much anecdotal evidence of the jellyfish population boom, there is, as of now, no hard scientific data proving the increase. Data on overfishing is a different matter though…

“Overfishing has also been widely reported due to increases in the volume of fishing hauls to feed a quickly growing number of consumers. This has led to the breakdown of some sea ecosystems and several fishing industries whose catch has been greatly diminished. The extinction of many species has also been reported. According to a Food and Agriculture Organization estimate, over 70% of the world’s fish species are either fully exploited or depleted. According to the Secretary General of the 2002 World Summit on Sustainable Development, ‘Overfishing cannot continue, the depletion of fisheries poses a major threat to the food supply of millions of people.'”

“The cover story of the May 15, 2003 issue of the science journal Nature – with Dr. Ransom A. Myers, an internationally prominent fisheries biologist as the lead author – was devoted to a summary of the scientific information. The story asserted that, as compared with 1950 levels, only a remnant (in some instances, as little as 10%) of all large ocean-fish stocks are left in the seas.”

Here’s a rundown of some of the most prominent recent examples of overfishing:

– In the summer of 1992, the Northern Cod fisheries completely collapse, total biomass fell to 1% of its earlier level. “The collapse of the Northern Cod fishery marked a profound change in the ecological, economic and socio-cultural structure of Atlantic Canada. The change was expressed most acutely in Newfoundland, whose continental shelf lay under the region most heavily fished, and whose communities were nearly all of those who lost employment because of the moratorium. Considering the importance of the cod fishery to the livelihood of Canada’s coastal communities, and the Northern Cod’s initial abundance in the region, the fishery being mismanaged until it collapsed – from which to this day it has not recovered – is nothing short of shocking.” Not that shocking really…

To date, about two decades later, the fisheries there have only recovered to about 10% of the ‘original’ stock. Fisheries researchers have put the slow recovery down “to inadequate food supplies, cooling of the North Atlantic, and a poor genetic stock due to the overfishing of larger cod.”

– Sharks are rapidly going extinct — with some estimates saying that they may be extinct within only the next few decades. Over 100 million sharks are currently being killed every year, and possibly as many as 273 million according to recent research. This is largely as a result of the booming shark fin trade, being caught as by-catch, and recreational fishing. Many species of shark have seen their overall numbers and population range fall by as much as 90% in just the last 20-30 years.

– “The Peruvian coastal anchovy fisheries crashed in the 1970s after overfishing and an El Niño season largely depleted anchovies from its waters. Anchovies were a major natural resource in Peru; indeed, 1971 alone yielded 10.2 million metric tons of anchovies. However, the following five years saw the Peruvian fleet’s catch amount to only about 4 million tons.This was a major loss to Peru’s economy.”

– “The sole fisheries in the Irish Sea, the west English Channel, and other locations have become overfished to the point of virtual collapse, according to the UK government’s official Biodiversity Action Plan. The United Kingdom has created elements within this plan to attempt to restore this fishery, but the expanding global human population and the expanding demand for fish has reached a point where demand for food threatens the stability of these fisheries, if not the species’ survival.”

– “Many deep sea fish are at risk, such as orange roughy, Patagonian toothfish, and sablefish. The deep sea is almost completely dark, near freezing and has little food. Deep sea fish grow slowly because of limited food, have slow metabolisms, low reproductive rates, and many don’t reach breeding maturity for 30 to 40 years. A fillet of orange roughy at the store is probably at least 50 years old. Most deep sea fish are in international waters, where there are no legal protections. Most of these fish are caught by deep trawlers near seamounts, where they congregate because of food. Flash freezing allows the trawlers to work for days at a time, and modern fishfinders target the fish with ease.”

– “Blue walleye went extinct in the Great Lakes in the 1980s. Until the middle of the 20th century, it was a commercially valuable fish, with about a half million tonnes being landed during the period from about 1880 to the late 1950s, when the populations collapsed, apparently through a combination of overfishing, anthropogenic eutrophication, and competition with the introduced rainbow smelt.”

With regards to the causes of overfishing — beyond the simple maximization of profits, there is a large degree of mistrust of fisheries science by many fishermen. We’ll close with a quote from a fisheries scientist: “The subjective impression the fishermen get is always that there’s lots of fish – because they only go to places that still have them… fisheries scientists survey and compare entire areas, not only the productive fishing spots.”

The new research was published in the Bulletin of Marine Science.

Source: Institut de Recherche pour le Développement (IRD)

Image Credit: © IRD / L. Corsini; Purse Seine via Wikimedia Commons; Jellyfish via Flickr CC