Super Swarms

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Are there plenty more fish in the sea?

Essay by Jamie Watts : Photographs by Michael AW

The Ocean is our key habitat.  Dominating the prime food webs in the oceans by the quadrillion, for hundreds of millions of years have been, and still are, the fishes.

Some animals just fit into their world considerably better than the rest.  They seem to have carved themselves into their particular ecological niches perfectly compared to all the other, dizzying varieties of body and behaviour plans that evolution’s tinkering has produced.  These super-animals succeed on spectacular scales. 

An alien visitor assessing life on Earth in the last few decades could not fail to notice humans and our agricultural animals dominating the terrestrial world.  But only about a third of the surface is terrestrial, and human dominance of this portion of the Earth is an extremely recent phenomenon.  Our observant alien, looking at animal life on this planet going back millions of years, would probably understand the oceans to be our key habitat.  And dominating the prime food webs in the oceans by the quadrillion, for hundreds of millions of years have been, and still are, the fishes.

Of the nearly forty thousand species of fishes, which make up nearly two-thirds of all vertebrate species, just twenty or so have fine-tuned their ecological success formulas to a whole different level.  They out-feed, out-grow and out-breed not just all the other fishes competing in the vast oceans, but also the squids, the plankton and everything else in the two-thirds of the planet covered by sea.  They have expanded their ranges and their populations over vast sections of the world’s waters, and dominated their habitats.  These few fish species gather and concentrate the majority of Earth’s marine food.  In turn, their vast schools attract almost all of the oceans’ predators to feed where these fishes spawn or overwinter.

We humans have been hitting some of the stocks of these super-fishes hard over the last few decades, and in a couple of cases for much longer.  They have been a magnificent resource. The Vikings, the Basques and Japanese fishermen followed them across oceans centuries ago, and they have provided our coastal civilisations with dependable nourishment for even longer.  We now have a pretty clear understanding of the many ways we have over-fished their stocks in the last half-century.  Catches for most of the world’s fisheries peaked in the mid 1990s, and since then, despite our best efforts to manage them, the subsidies and technology associated with industrial-scale fishing have steadily eroded our wild catches and the biomasses of most of these stocks by a quarter or more.

Yet, like boxers up against the ropes, many of these super-species have shown incredible resilience, amazing capacities to survive, and have rallied to thrive in Earth’s richer seas even against the onslaught of our fishing fleets.  A handful of these fishes even today maintain stocks in the multi-millions of tonnes. So what are these million-tonne fishes, and where (and why) do they swarm?

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The enigmatic sardine and its kin; Almost every diver knows about South Africa’s sardine run, but many may not realise that this spectacular event involves only a small proportion of the South African sardine stock, which is in itself not the greatest stock of this magnificently successful species.

The sardine, also called the pilchard, Sardinops sagax, is a clupeid, a simply built fish closely related to a handful of other massively successful fish species; the herrings and anchovies.  This group of schooling, silvery, shallow-breeding fishes has been an ecological force for tens of millions of years, feeding by filtering small, nutrient-dense copepods from the richer seas of the world with comb-like gill structures called gill rakers.  It’s a simple ecological formula, and it works superbly well; a quarter of the world’s fish catch comes from a handful of these closely related species.

The 150-gramme, 20-odd centimetre sardine prefers the richest, cool, subtropical continental shelves, areas that may be 15 Celsius in winter and warming up to 20 Celsius in summer.  This species is one of the most heavily caught fish in history.  In the mid 1980s, the richest fishing grounds for the Peruvian and Japanese stocks each maintained (for a brief period) catches of around five million tonnes a year. Catches from the slightly smaller South African stock peaked in the late ‘60s, and the Californian and Australian stocks are a bit smaller.  Even the heavily fished stocks of today still make up millions of tonnes worldwide.

From Western Sahara and Morocco, through the Med, up the west coasts of Europe to Norway and across to Iceland, the sardines’ close cousin, the Mediterranean sardine or European pilchard, Sardina pilchardus, takes over, again supporting a million-tonne a year fishery.

The big, offshore, cold-water cousins of the sardines are – or at least were – an even greater ecological success.  For centuries, the rich arc of water, packed with food, from the North Sea to Iceland, past Greenland to Newfoundland, has fed a rich ecosystem and supported generations of fishermen.  The gorgeous footage and photographs coming out from northern Norway of humpback whales and orcas feeding on the herring, Clupea harengus, documents the comeback of this remarkable fish after the stock collapse in the late ‘60s.

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The rich, cold-water shelves of Peru support immense stocks of another small cousin to the herring and sardines.  Having the perhaps unenviable distinction of supporting the largest single species fish catch in history at 13 million tonnes in a single season (just before the stock collapsed), the cigar-sized Peruvian anchoveta, Engraulis ringens has made a small rebound, and still makes up nearly two-thirds of the fish biomass along much of the rich Peruvian continental shelf.  These days mostly fished for oily fishmeal for salmon farms, the sheer abundance of these fish once supported not only an immense fishery, but also indirectly a lucrative seabird guano fertilizer industry.  The opposite corner of the Pacific has similarly rich seas, and the Japanese anchovy, Engraulis japonicas, is the ecological equivalent there.

The big fish that launched the Vikings; a rather different group of fishes dominates the shallow seafloors of the richest cool seas as predators of a variety of crustaceans, molluscs and fishes.  For well over a thousand years, one fish stock stands apart for nurturing the growth as a seafaring people, then the spread of the Vikings as they headed further out to sea, first as fishermen and traders, then as raiders.  The northeast Atlantic Arctic stock of Atlantic cod Gadus morhua is also remarkable in having never been overfished to near collapse.  While the cod stocks across the Atlantic infamously crashed in the ‘80s, and smaller, more southerly populations eroded, the Lofotens-Barents Sea cod have held strong.  This is all the more impressive for a big, predatory fish that sits somewhat high up in the food web, normally the most susceptible niche for overfishing.

The cod’s smaller, north Pacific cousin has thrived even more spectacularly in the ultra-rich, cold continental shelf waters of the Bering Sea.  If you’ve ever eaten a ‘crab stick’, you’ve probably had Alaska pollack Gadus chalcogrammus, the most heavily fished species on Earth for the last couple of decades, and the second most heavily fished species ever right after the Peruvian anchoveta.  Another cousin, the slender polar cod Boreogadus saida, a little smaller again, has specialized in feeding on the rich under-ice fauna around the edges of the Arctic ice, in turn providing food for millions of seals and seabirds as well as narwhals in these semi-frozen seas.

Silver darts of the cold seas; A handful of unrelated, but oddly similar small, sleek, silvery fishes swarm along the edges of Earth’s richest shelves.  Around the Arctic and subarctic, particularly on the Atlantic side, the capelin, Mallotus villosus attracts humpback whales and other predators to its vast spawning aggregations.  In the slightly less frigid waters from central Japan to the Kuril Islands, the Japanese saury Cololabis saira takes over.  Saury are particularly rich, oily and calorie-dense, and form concentrated swarms to the point where the whales feeding in Japanese seas actually prefer them to krill.

The shelves of Antarctica, covered by sea ice for much of the year, provide habitat for another small, slender silvery fish that lives much of its life at a little below zero Celsius. The long-lived Antarctic silverfish, Pleuragramma antarctica, is sought-after by emperor penguins and the vast numbers of ice-dwelling Antarctic seals.

Further north, where Antarctic waters run up along the west coast of South America, the slightly chunkier Chilean jack mackerel, Trachurus murphyi swarms inshore.  Still the world’s most successful jack, these oily fishes have been depleted somewhat, like several other oily fishes, to provide food for farmed salmon.

Tunas; Tropical ocean desert rats; All of these fishes thrive in cool, nutrient rich seas.  But the vast majority of the world’s oceans are far sparser in food resources; nutritionally, these warm seas are true deserts.  One group of fishes is athletic enough to thrive here, and they alone dominate these food webs.  Tunas’ sleek, superbly streamlined, heavily-muscled bodies and hydrofoil tails allow them to travel more efficiently, farther and faster than any other group of fishes.  They expend three-quarters of their entire energy intake on swimming, but because they are so hydrodynamic and powerful, they can cover enough ground to find enough food to thrive in these vast, sparse seas.  A fisheries scientist once described skipjack tuna, Katsuwonus pelamis to me as ‘the rats of the sea’.  These 10-kilo powerhouses are spartan travelers and tough survivors – good at being able to eke out a living, find their food, grow fast, reproduce young and thrive in what is effectively an ocean desert.  Their range covers well over half of Earth’s entire ocean area.  Skipjacks have been hit hard by our insatiable appetite for them in the last few decades, but are still hanging in there.

The food of the skipjack, the small bullet and frigate tunas, Auxis sp, are even more abundant around tropical coasts.  The true mackerels belong to the same family as the tunas, and some species are almost as successful as the skipjacks, albeit needing somewhat richer foraging grounds.  The most successful mackerel, the Pacific chub mackerel, Scomber japonicus thrives in shallow waters almost the entire length and breadth of the Pacific Rim.

The cold, dark vastness of the mesopelagic realm; These near-surface fishes occupy the most biologically dense and competitive zones of the world’s seas, where sunlight allows the production of new life.  Yet this is not where most of the world’s marine life resides.  Biological matter tends to sink, and it sinks into the thick slab of ocean from about 100 to 1200 metres’ depth – the world’s largest ecosystem, the mesopelagic zone.  This zone is dark, cold and in many places low in oxygen, yet offers a refuge for animals that can tolerate these conditions.  And it is vast.  The mesopelagic fishes that shelter here during the day and come closer to the surface at night, outnumber and outweigh all the world’s near-surface fishes together, several times over.

About two-thirds of these midwater fishes – hundreds of millions of tonnes - are the finger-sized, ten-gram lanternfishes, or myctophids.  Never pursued to much of an extent by human fishermen, probably due to their small size, mushy texture and unusual and not particularly tasty waxy oils, lanternfishes are nonetheless perhaps the greatest calorie-dense food resource on Earth for large animals.  They are preferred as prey by animals as varied as tropical dolphins, king and emperor penguins, and jumbo and giant squids.  Most species come out of deep water closer to the surface at night to feed, and in certain areas like the northwest Indian Ocean, the Galapagos / Costa Rica Dome area and the Southern Ocean, form immense biomasses.  In some areas, other types of mesopelagic fishes are locally even more abundant than lanternfishes.  Offshore off northern Chile and Peru, the tiny Panama lightfish Vinciguerria lucetia dominates the slopes of the outer continental shelf.

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The non-fish fish swarms; the fishes have not managed to dominate everywhere; a couple of other animal species have challenged these super-fishes in the sheer scale of the success of their swarms.  Probably due largely to our overfishing of large predatory fishes and sharks, which used to prey on the young squid, populations of the large and aggressive jumbo squid Dosidicus gigas have grown to become probably the most successful squid on earth, challenging the fishes for dominance in the seas off the western side of the Americas, from Chile almost to Alaska, and offshore to the Costa Rica Dome.

The greatest swarms of all, though, belong to neither fish nor squid.  A single family of crustaceans has lifted off the seafloor, taken the free-swarming lifestyle and perfected it.  There are over 80 species of krill, but a handful of the larger species make up the greatest animal biomasses in the world’s oceans, outnumbering even the mesopelagic fishes.  The most successful is the Antarctic krill, Euphausia superba.  Over 150 million tonnes of krill stream from the Antarctic Peninsula across the Scotia Sea, and the total extent of this species all around the Antarctic is two to three times this amount.  It is the only wild animal species left that outweighs humankind.  Although some time around 2020, we’ll overtake them.

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