Feral
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Perhaps this contributed to their decline. The industrial fishing of herring and mackerel after the Second World War must also have done so, and by the late 1950s the sport-fishing had ceased. Since then the odd fish has been taken in nets, including one, off the Irish coast, of over 1,200 pounds.
The migrations inland were no less impressive. Before they were silted up by forest clearance and the runoff from ploughing, before they were weired, impounded and polluted, the water in most of the rivers in Europe is likely to have been clear. Most rivers would also have supported runs of migratory fish on the scale of those the Europeans encountered when they first arrived in North America. There they found sturgeon, some of them eighteen feet long, moving up the rivers in such numbers that, an English visitor recorded, ‘in one day, within the space of two miles only, some gentlemen in canoes caught above six hundred . . . with hooks, which they let down to the bottom and drew up at a venture when they perceived them to rub against a fish’.6 Which river was this? The Potomac, that foul drain which runs through what is now Washington DC.7
Above the bottom-hugging sturgeon, Callum Roberts tells us, swarmed alewife and shad (migratory members of the herring family) in such numbers that there seemed to be more fish than water: in 1832 European settlers caught almost 800 million of them in the Potomac alone. In other rivers salmon were packed so densely that, an English army captain remarked, a gun could not be fired into the water without hitting some of them.
Oysters formed reefs across the bays and rivermouths that presented a hazard to shipping. The colonists, one source claims, picked twenty-pound lobsters out of the rockpools–and could think of nothing to do with them except use them as bait or feed them to the pigs.8 Fathom-long halibut were caught only for their heads and fins; the flesh was discarded as inferior to that of other species that thronged the coasts.9
There is no reason to believe that the volume of fish and shellfish would, in an undisturbed system, have been any lower in Europe. We are less aware of what went before only because humans reached Europe earlier, their later technologies were more intrusive and their harvests more intensive than those of the Native Americans. The decline of the great herds of the rivers and seas began, in many cases, long before it could be recorded in writing.
But early documents hint at what there once was. Shoals of migratory fish whose existence in Britain we have all but forgotten jammed the rivers: shad, lamprey and sturgeon, jostling with hordes of salmon and sea trout. Until the eleventh century, when the diet shifted to marine fish, probably as a result of the depletion of freshwater species, they helped to feed much of Britain. By the thirteenth century sturgeon were so rare that only the king was permitted to eat them. But the marine ecosystem, when large-scale exploitation began, must still have been close to the opulent state early travellers later encountered in the New World. Roberts reports that Viking settlements in the north of Scotland were characterized by a mass of remains of cod, pollock and ling much bigger than any caught in inshore waters there today.
Everywhere the animals that lived in the sea were both more numerous and bigger than they are today. Cod commonly reached five or six feet in length. Even the great white shark is not as great as it once was. Roberts tells us that ‘today, the maximum length of a great white shark is listed in guidebooks as 6 metres, but reports in the eighteenth- and nineteenth-century literature, too numerous and detailed to be dismissed, suggest sizes of 8 or 9 metres were not uncommon. Accounts at the time compare them in size with whales.’ Haddock were once a yard long. Plaice were the size of road atlases, turbot like tabletops. The specimens we see on the fishmonger’s slab are, for the most part, youngsters, caught before they were able to reach even a tenth of their maximum weight.
Genetic profiling of the great whales suggests that their populations, before whaling began, were higher than biologists had assumed. The larger the original population, the greater the variation within that which remains. An analysis of genetic data in the journal Science suggests that the North Atlantic alone supported around 265,000 minke whales, 360,000 fin whales and 240,000 humpbacks.10 Today the minke whales, after a severe population decline, have recovered to 149,000, the fins to 56,000 and the humpbacks to 10,000. The whales once visited all the seas of the region; by the eleventh century they were being hunted in both the English Channel and the North Sea.11
Just as on land, the ecology of the sea is more complex than scientists once assumed. The trophic cascades now being discovered in the oceans are, if anything, even more remarkable than those of the terrestrial ecosystem. Fishermen and many fisheries scientists, for example, have long assumed that if whales are removed from the southern oceans, the volume of their prey–mostly fish and krill–will rise. This argument has been used by the Japanese government to justify its continuing slaughter of these beasts.12
But recent work suggests that reducing the population of whales might have had the opposite effect. As whale numbers have declined, so have the krill:13 to just one-fifth of their volume before the 1980s.14 Their collapse, until recently, mystified observers. It now seems that the whales perform an essential role in keeping nutrients in the surface waters. If undisturbed, the plant plankton at the bottom of the foodchain sinks out of sight, beyond the photic zone (the waters in which the light is strong enough to permit plants to grow). The nutrients it contains sink with it, becoming unavailable to most lifeforms. The surface waters rapidly become depleted of essential minerals, especially iron, whose scarcity limits growth. In the summer, when plant plankton is reproducing fastest, the wind and waves drop, allowing it to sink more rapidly. The same applies to the faeces of the animals that eat it.
Even today, a study in the journal Nature calculates, the mixing power caused by movements of animals in the oceans is comparable to that of the wind, waves and tides.15 This, it says, is a conservative estimate. When whales were more abundant, the effect would have been still greater. Simply by plunging up and down through the water column, the whales help to keep plankton circulating in the surface waters. But their impacts extend far beyond that. They often feed at depth and defecate at the surface, producing great plumes of iron-rich manure that fertilize the plants in the photic zone, on which krill, fish and other animal plankton feed. One paper estimates that, before their population was reduced, whales recycled at least 12 per cent of the total iron content of the southern ocean’s surface waters.16 More whales meant more nutrient cycling, which gave rise to more plankton, producing more fish and krill.
Another study, in the Gulf of Maine, estimates that whales and seals, by defecating at the surface and recycling nutrients there, would, before they were hunted, have been responsible for releasing three times as much nitrogen into those waters as the sea absorbed directly from the atmosphere.17 Whales in the gulf typically dive to a hundred metres or more to feed, bringing back the nutrients they harvest to the surface. The volume of plant plankton has declined across most of the oceanic regions in which it has been studied over the past century. The principal reason is the rising temperature caused by man-made climate change.18 But according to the marine biologist Steve Nicol, the decline has been steepest where whales and seals have been most heavily hunted.19 The fishermen who have insisted that the predators of the species they hunt be killed might have been reducing, not enhancing, their catch.
If the production of plankton declines, so does the transport of carbon to the deep ocean. By stimulating plankton blooms through recycling iron, another study suggests, sperm whales in the southern oceans cause the removal of around 400,000 tonnes of carbon from the atmosphere every year.20 The extra plants absorb carbon dioxide, then, after being kicked around the surface waters a few times, sink into the abyss, where the carbon remains for a very long time. The whales also release around 200,000 tonnes of carbon through respiration, which means that, on balance, roughly the same amount of carbon is taken out of circulation.
When you consider that the sperm whale is just one of several species, th
e southern ocean is just one of several regions and the current number of leviathans is a fraction of what it once was, it becomes clear that whales could once have caused the sequestering of great quantities of carbon, perhaps tens of millions of tonnes every year. This is enough to make a small but significant difference to the composition of the atmosphere. Another paper maintains that during the twentieth century the whaling industry shifted over 100 million tonnes of carbon from the oceans to the atmosphere, simply by turning whales into oil and other products that were burnt or otherwise oxidized.21 Allowing whale numbers to recover could be seen as a benign form of geo-engineering.
The removal of the great sharks, which took place, on the whole, later than the destruction of the whale population, has had similarly devastating effects. Caught for their fins or accidentally by nets and lines set for other species, big sharks have vanished with astonishing speed. Off the eastern seaboard of the United States, for example, in the thirty-five years beginning in 1972, tiger sharks declined by 97 per cent, scalloped hammerheads by 98 per cent and bull sharks, dusky sharks and smooth hammerheads by 99 per cent.22 The result is an explosion of animals which no other species is big enough to eat: large rays and skates and smaller sharks. Many of them have increased tenfold or more. In Chesapeake Bay alone, for example, there are now an estimated 40 million cownose rays.
Cownose rays eat shellfish, and this population consumes some 840,000 tonnes a year–almost 3,000 times as much as the total landing of clams of all descriptions in Virginia and Maryland.23 By 2004 they had wiped out North Carolina’s scallop fishing industry and were rapidly doing the same for oysters, hard clams and softshell clams. The economic damage caused by the destruction of large sharks surely outweighs any money made by catching them.
The collapse of the cod shoals off north-eastern America has had the opposite effect. Released from their predators, commercially valuable shellfish–in this case, shrimps, crabs and lobsters–have exploded, creating a new industry as valuable as the one it replaced. These too are now being heavily exploited.24 Regardless of the economic consequences, the destruction of one of the world’s great natural spectacles–the vast spawning aggregations on the Grand Banks and other shallow seas off the Atlantic coast, and the frenzy of tuna, sharks, dolphins and whales attendant upon them–is a tragedy.
In some places where cod were abundant they have failed to return even when fishing for them has ceased. This could be because cod appear to engineer their environment, creating the conditions necessary for their survival. On the Grand Banks they preyed heavily on mackerel and herring. When most of the cod disappeared, the population of mackerel and herring boomed, with the result that the relationship was reversed. The smaller fish became major predators of cod, eating their eggs and fry before they could mature.25 The same thing has happened in the Baltic Sea, where cod eggs are eaten by herring and sprats.26
Turtles also appear to have changed the world to suit themselves. When Columbus arrived in the Caribbean, according to one study, that sea contained 33 million green turtles.27 There were similar concentrations off the east coast of Australia and in other tropical and subtropical seas: turtles all the way down. Today there are 2 million green turtles, worldwide. They largely subsisted on turtle grass, a weed which once grew on the beds of great tracts of shallow water. These were the savannahs of the sea, supporting vast herds of grazing animals: dugongs, manatees and herbivorous fish as well as unimaginable numbers of green turtles (which were, permitted to live into old age, much larger than the average size of those of today). The grazers, in turn, supported marine lions, hyenas and cheetahs: big predatory fish, mammals and in some places reptiles, namely giant saltwater crocodiles.
When the turtles were slaughtered, mostly before the nineteenth century, the remaining population could no longer keep the turtle grass cropped. As the blades grew longer, they shaded the seabed and shielded the sediments from the current. The weed, uneaten, started to age and rot, and detritus built up in the still water beneath the beds. This became a food source for parasites which then began to destroy the living grass (a process biologists call ‘turtle grass wasting disease’). Across much of the range that green turtles once occupied, the turtle grass has died off.28 This, in other words, is a similar story to that of the mammoth steppes of Beringia, which, as the grass grew longer and its detritus insulated the soil, turned to mossy tundra when the grazing animals were killed (see chap. 6).
Perhaps the most famous trophic cascade in the seas took place along the eastern rim of the Pacific, where sea otters, once widespread and abundant, were almost wiped out by both native people and fur traders. The result was the near-disappearance of the coastal ecosystem. Sea otters prey on urchins among other species. Sea urchins graze on kelp, the long and leathery seaweed that, in the right conditions, produces tall, dense growths reminiscent of terrestrial forests. These harbour a wonderful variety of fish and other creatures. When the sea otters were killed, the urchins wiped out the kelp forests, bringing down the rest of the natural system.29 In the few places in which the otters have survived and begun once more to proliferate, the kelp forests have started to return, just as the reintroduction of wolves to the Yellowstone National Park has permitted the trees to grow back. But now, in one of their remaining strongholds, the Aleutian archipelago, the sea otters are disappearing again, apparently because of another disruption of the ecosystem. Killer whales, deprived by human hunters of the seals and sealions they once preyed upon, have started eating the otters instead.30
Fishing has transformed the life of seas everywhere, to a much greater extent than most people know. As on land, no removal of an abundant species is without consequences, consequences that often ramify through the system. Take the humble oyster. I have mentioned the remarkable abundance of oysters on the eastern seaboard of the Americas that the first European adventurers encountered. It appears that similar concentrations were once found in other seas. A map made in 1883, 500 years after trawling began there, marks an area of the North Sea the size (inevitably) of Wales as oyster reef.31 Before the age of trawling and dredging, it is possible that most of the North Sea bed was encrusted with oysters, while shellfish of other species would have colonized the sediments on which oysters could not settle.
One result is that this grey sea might once have been clear. Like most two-shelled molluscs, oysters filter the seawater. They also stabilize the sediments of the seabed. Less mud would have been raised, and that which was washed into the water would quickly have been extracted again. As the great beds were smashed by fish trawlers and oyster dredgers, the sea’s filters were shut down at the same time as the crust of life was broken, releasing the mud that lay beneath. Even the Humber estuary–a mud bowl whose waters are now as murky as a hedge fund’s tax returns–was once lined with oyster reefs. On the tidal slops, Callum Roberts tells us, you can still find oyster shells ‘smoothed by more than a century of tides’. By creating, through the accumulation of cemented shells, a hard bottom onto which other oysters could attach, the shellfish, like cod and green turtles, engineered the environment that suited them. They also provided a substrate onto which many other species could attach, in turn creating habitats for yet more wildlife.
In Chesapeake Bay on the Atlantic coast of the United States there were sufficient oysters, according to one paper, to have ‘filtered the equivalent of the entire water column every 3 days’.32 As the early colonizers broke the land, much of the soil–and the nutrients it contained–began to wash into the sea. This process–called eutrophication–has been blamed for the periodic bloom of plant-like plankton, whose decay and nocturnal respiration sucks the oxygen out of the water, killing many of the animals the bay contains. This plankton contains species which poison the water, causing lethal red tides. Fascinatingly, however, despite the great dump of nutrients into the bay from around 1750 onwards, it was not until the 1930s, when the oysters had been more or less fished out, that such disasters began to occur.33 The oysters filtere
d and consumed the plankton, preventing it from blooming and from poisoning the ecosystem. The damage, from the 1930s onwards, was self-perpetuating. As the oysters were reduced to the point at which they could no longer keep the water clear, they began to suffer from a lack of oxygen and the overabundance of sediments. This made them susceptible to disease, which further reduced their number. The report describing this effect remarks that Chesapeake Bay, the Baltic, Adriatic and parts of the Gulf of Mexico, are now ‘bacterially dominated ecosystems’.34
The Black Sea also appears to have been transformed by the removal of some of its dominant species. After its predators–such as dolphins, bonito, mackerel and bluefish (Pomatomus saltatrix)–were reduced by commercial fishing, the plankton-eating fish they preyed on proliferated. The result is that animal plankton numbers crashed, which meant that plant and plant-like plankton multiplied, sometimes poisoning the water, often depleting it of oxygen.35 When the anchovies on which the predatory fish once preyed were then over-harvested, and comb jellies from the Atlantic arrived in the ballast water of ships in the 1980s and were able rapidly to occupy the depleted ecosystem, the chain of destruction came close to completion.
One of the most visible transformations has been the apparent shift from fish to jellyfish. The fishing trip I described in the second chapter was the last occasion on which I have taken even a moderate haul from the coast on which I live. I have launched my kayak dozens of times in the three years since then and not returned with more than two fish. This astonishes me in view of the abundance I encountered when I first arrived in Wales. Then, a single trip would supply as much fish as my family could eat in the season. On some occasions, in just a couple of hours, I caught as many as 150 mackerel, as well as weavers, gurnard, whiting, pollock, codling and scad. (I returned the rarer and smaller fish.) Those were thrilling moments: pulling up strings of fish amid whirling flocks of shearwaters, gannets pluming into the water beside my kayak, dolphins breaching and blowing. It was, or so it seemed, the most sustainable of all the easy means of harvesting animal protein. Now, for reasons I have not been able to identify clearly, that brief era–my first two years in Wales–has passed. I was surprised to discover that the fisheries officials and scientists I spoke to not only had no explanation for this apparent change; they had no data either. If there has been, as I suspect, a population crash, no one is studying it.