Читать книгу Life of a Chalkstream - Simon Cooper - Страница 10
ОглавлениеSPAWNING AND THE CYCLE OF LIFE
AFTER THE FRENETIC activity of summer I miss my riverside companions on a winter dawn morning. No reed-chewing water voles suspiciously eyeing my progress along the riverbank, plopping for safety under the water if I come too close. No dew-laden spider webs strung between the purple loosestrife, glinting in the rising sun, as an eager arachnid crabs with intent across the translucent filament harvesting the victims of the night. Even the rabbits have gone, and as for the lolloping hares, no chance of any of those until spring. But even if it is all quiet along the banks, in the ever-clear water of the chalkstream the game is on to create the next generation of trout and salmon.
Trout and salmon are often spoken of in the same breath, but they are in many respects as close to each other in genetic terms as a horse is to a zebra. For a fly-fisherman they define what you are on a river. As salmon and trout are two distinct breeds, so are the men that fish for them. Not to announce which you are, even though you might fish for both, is like saying you support the Manchester football team. United or City? Salmon or trout? Both are equally tribal.
For fish whose subsequent lives will diverge so totally they begin life in the same gravel beds, of the same rivers, at precisely the same time of year. In lives that will span five to seven years some brown trout will travel no more than a few hundred yards from their birthplace, whereas the salmon has a round trip of some 4,000 miles to complete its life cycle. While we may think of a salmon as a river fish, in fact the greatest proportion of its life is spent at sea. These salmon are Atlantic salmon – Salmo salar. Defined as anadromous, their natural habitat is the sea, but they must return to the river of their birth to spawn. The eggs are laid in a river and that first year of life, as they grow from fry to parr and then smolt, is all spent in fresh water. But no chalkstream could ever provide enough food for a salmon to grow to maturity, so at a year old, measuring no more than 6 inches long, they head for the ocean and the food-rich waters off Greenland. It is an epic journey that begins and ends in a stream no more than 15 yards wide and a few feet deep.
The spawning grounds created by salmon and trout in the gravel riverbed are known as redds, and the sight of the first redds, be it in October or November, is something of a red-letter day for us chalkstream watchers. Indeed, redd-spotting becomes something of an obsession from around October time. I say ‘around’ because rivers don’t obey the Gregorian calendar. Like the snowdrops in your garden that appear in January one year and February the next, the creatures of the river adapt their habits according to what’s happening around them, which is in turn dictated by the climate. And not only the weather of now; the effects of a dry summer or harsh winter for instance, may linger many months or years to come.
I’ll get excited text messages from river keepers: Seen a redd today. First of the year!!!!!!!!!!! . It is exciting because amid the gloom of late autumn and the winding down of a fishing season, it is a small ray of hope for things to come, however distant. It is also proof that as a river keeper you are doing something right. Your river is so damn perfect that fish want to breed in it. How good is that?
Walking beside the river, you will find the redds are easy to spot once you know what you are looking for: pale lozenge-shaped indentations on the river bottom, with a mound of gravel at the downstream end. Brushed clean of silt and debris, they shine out like lights compared with the surrounding gravel. Sometimes there is just one, sometimes a cluster, but it is the size that immediately marks out the difference between a trout and a salmon redd: the former about the size of a snowshoe, the latter a good-sized door mat. And there is more latent intent about the salmon redd; it will be dug deeper, down to the hard base beneath the gravel. Random gravel stones from the digging will be scattered far and wide across the riverbed. The mound of stones at the end will be much higher and more pronounced. Redds are, of course, made by the fish themselves to harness the flow of well-oxygenated water through the loose gravel to incubate their eggs, and oftentimes the hen will lay in more than one redd. Laying in a single redd is quite literally putting your eggs in one basket, and that basic instinct to perpetuate the species drives the hen to hedge her bets by laying in a series of redds, maybe with other hens. But we have to track back in time to appreciate how and why we have arrived at this point.
From my daily walks up and down the river the progress of the trout from an everyday Salmo trutta to a body quivering as if electrocuted whilst he releases his milt over the eggs is far easier to track than that of Salmo salar, the Atlantic salmon. It is in September that I start to see the first signs of spawning in the trout, who start to change in appearance in the weeks before they start the actual process of cutting redds and spawning. Suddenly that headlong pursuit of every item of food to feed on in preparation for the winter ahead slackens off. The fish are just as active, but not for food. Somewhere in their fishy brain the search for food is replaced by the search for a mate. The change sweeps over their body and suddenly that golden-brown complexion is replaced by a fierce red blush along both flanks. The males sprout a vicious-looking hook – a kype – on their jaw. The kype is largely for show, but it does make an otherwise innocuous-looking trout look like someone you would not want to mess with.
During this time the salmon are absent from this river, still making their way along the English Channel from the Atlantic to pick up the scent of their birth river somewhere on the south coast. How salmon navigate the entire journey to the far side of the Atlantic to the waters off Greenland and back again remains something of a mystery. The position of the sun, the stars and the gravitational pull of the earth are all cited as guides, but it is certain that the final leg of the journey is determined by smell.
Salmon never look to me like creatures that depend on smell for survival – their incredible ability to leap huge waterfalls or swim unceasingly for months on end seem more important – but smell is the thing. Early on in their lives they imprint the odour of their birth river onto a hormone that is secreted in the thyroid gland; it stays with them for evermore. Their hormonal library of smells is highly selective; only the ones that really matter make it onto the data bank. Likewise they will log the odour of their brothers and sisters in the river, picking up their scent in later years when the shoals are travelling across the ocean.
By the time our salmon sniffs the first scent of home, he or she has surmounted incredible odds to make it thus far. Of those 5,000 eggs laid three years ago in the River Evitt, our salar is probably the sole survivor, or at best one of two. And the dangers are far from over. Ravenous seals are gathering for an autumn feast and the drift nets in the estuary are laid in wait. It is the misfortune of salmon that they make such good eating, though it should be of no surprise. They are super-fit and have spent the past two to three years in the beautifully clean water of the Greenland Sea eating nothing but squid, shrimp, crustaceans, small cod and mackerel.
As far back as medieval times salmon has commanded a premium price, so the ever-resourceful coastal communities around Britain developed the highly efficient drift net to capture the salmon returning from the sea. There are all manner of types of drift netting, each of which has evolved for the particular locality, but the principle holds good for them all: wait for the tide to go out and then set your nets in such a way that they intercept the salmon travelling towards the estuary bottleneck on the inbound tide.
Travelling around the coastline of Britain you will see all sorts of weird and wonderful nets rigged up to capture salmon, though they are becoming fewer. Declining runs of salmon, fierce campaigning by conservation groups to have the nets removed and the harsh demands of a truly hard and difficult job are all contributing to the decline.
The simplest form of drift netting is a long net, anything from 30 yards to a few hundred, and 5 to 10 feet deep, that is slung across the tide, supported by floats along the upper edge. In shallow water it will be held in place at each end by a man holding a pole; in deeper water by boats. As the tide races through, the salmon follow, to get caught in the mesh of the net. The netsmen gather the ends of the net into a circle, capturing the salmon by hand as the circle gets smaller.
Not surprisingly this fast, efficient method is the one most favoured by poachers, but it is the fixed nets that are more typical of traditional salmon netting in the estuaries: wooden posts supporting nets that face the incoming tide. Sometimes the nets will be shaped like giant boxes as large as a van, 10 to 15 feet above the beach level, open on one side. Other times they are funnel-shaped. Or most simply, nets loosely slung between posts like a garden fence. One way or another they are doing the same job of entangling the salmon, which become more trapped the more they struggle. As the tide ebbs some of the fish will escape, but once the nets are exposed to the air the salmon don’t have long to last and the netsmen will appear to complete the harvest.
Fortunately for our salmon heading for the Evitt, these dangers are slight. The seal population along the south coast is sparse compared with say the northwest of Scotland and the estuary netting is now less common than it once was. Ahead is the brackish water of the estuary and beyond that the purity of the chalkstream water, although this change from seawater to fresh will quickly change our salmon’s physiology as the body cells, body and organs adapt for the months to come. Such problems do not trouble our brown trout, whose sole mission at this point is to find a mate. No swimming over thousands of miles for him. His potential partner may be one of the other trout that have lived within a few hundred yards of him for all their lives. There never seems to me any great logic or grand plan to the way trout choose their partners. There will be a bit of swimming around, occasionally another male will sidle up beside a paired female to be promptly chased away, but on the whole it all seems to happen at random. Or that is how it looks to me with my bank-down view, but scientists think there is more to it than this, and that even fish have that ‘eyes across a crowded room’ moment when the right mate comes into view. Nobody knows exactly what is going on in and around those redds, but somehow skin colour, conformation, size, pheromones or possibly a mix of these and other factors combine to make their choice a complex matter.
You would think that the trout at Gavelwood would get used to me – after all, they must see me just about every day. But they never seem to. The only time they are oblivious to my presence is at spawning. Every other time they will bolt for cover if I disturb them. But at spawning I can stand on the bank almost directly above them and wave my arms about like a lunatic and they will carry on with their business. If I am in the river I can damn nearly step over them in my waders as they fin away one side or another to let me pass, to return to the same spot in my wake. The creation of the redd and the spawning to follow is all-consuming, but in their enthusiasm for each other the trout also forget about their most dangerous predator, the otter.
Otters are an ever-present but rarely glimpsed part of the Gavelwood family, about whom I have mixed feelings. On the one hand I should celebrate their existence, having come back in great numbers from the brink of extinction. But on the other they are one of the best fish-eating machines invented by Mother Nature. In ten months out of twelve it will be rare for me to see an otter, though most days I will be able to tell they have passed through. They are nocturnal creatures, using the river as their highway, travelling as much as 20 or 30 miles in a night. A paw print in a muddy bank, crushed grass where they slide into the river, and spraints, or dung, are all telltale signs, as is the corpse of a fish. In winter the latter will be barely recognizable as a fish: a few bits of fin, skin and scale in an area of flattened grass where the otter will have settled down to eat. If it is, or was, a gravid female trout there will be a scattering of eggs to add to the mix of body parts, which is sad in its own way, though they will rarely go to waste as the moorhens and water voles, or maybe a passing fox, are more than happy to feast on this unexpected bounty.
Every time I see the dog otter – I think we must have just the one – I am struck by his size and lithe movement. He is around twenty pounds in weight; put in context that is about twice the weight of a domestic cat. And like a cat he is incredibly supple; he doesn’t dive or jump into the river, he pours himself in, barely making a ripple. Once in the water, despite his bulk, he is hard to spot, but I’ll be able to track his underwater progress by the surge he creates on the surface. Some yards downstream he will pop his head out of the water, swivel around to check he has put sufficient distance between us and continue on his way at a more leisurely pace.
It is that bulk that makes the otter a deadly predator, because the bulk requires constant nutrition. It is reckoned that an adult male has to consume 10 per cent of his body weight each day to survive the winter. That is a two-pound fish, which is a big fish for the Evitt. More realistically we are talking about a whole bunch of smaller fish from trout, grayling and eels right down to the tiniest, like bullheads. Of course my otter’s diet is not pescatarian – frogs, crayfish, birds and water voles are all fair game – but in winter there are meagre pickings, so a careless spawning trout is a tempting prospect.
Come the summer things are very different; food abounds and so do the otters, which often become my evening companions when I stay late to fish the evening rise. I always hear them before I see them. Otters have this high-pitched ‘eek’ noise that they ping across the meadows like sonar to keep in touch with each other; it is the mother’s way of tracking the pups. As night falls the parents seem perfectly content to let the young ones range all over Gavelwood. I can become caught in this crossfire of constant eeking, and it is not a noise you have to strain to hear. It is incredibly insistent and frequent, though the frequency is a good indicator of how well things are going. I reckon that a contented otter eeks every thirty seconds; if one becomes distressed the frequency escalates until it becomes almost continuous. And it lasts all night or until the family move on to another part of the river.
These long, light summer nights are part of the growing-up phase for the pups, when the parents bring them out from the birthing holt to learn how to explore and hunt. Strangely, unlike most other species that become fiercely protective of their young, otters are more playful. They will often swim and hunt together in the river, just keeping a weather eye out for me. Otters are pretty well the top of the food chain and they regard humans as more of an oddity than a threat. However, lower down the chain the poor fish truly suffer.
Two fit adult otters, plus three or four ravenous, growing pups, seem to be the usual summer contingent that I will see in the twilight and on into the early hours of the morning. The solitary otter I see in the winter is a stealthy hunter, but in the summer the pack instinct takes over. Pike Pool, about halfway down the main river, is our deepest part of the Evitt and is the favourite place for the family to gather for a hunting lesson. The pool, which starts when the river makes an abrupt 90-degree turn, goes down to about 15 feet, constantly eroded by the water as it hits the opposite bank and swirls in back eddies before the gradient reasserts the natural order of things and the water heads downstream as it should. Along the bank stands a line of alder trees and the roots grow down into the water. Beneath the roots is a huge undercut, the perfect refuge for the fish and eels, or so they think.
The family will not so much hunt in a pack, but they do hunt collectively – rapidly diving and surfacing across the pool with their wet, brown fur glinting in the moonlight. They pause for just a moment to catch their breath before diving again. One can only imagine the massive panic in the fish community as they flee for safety in the dark recesses under the tree roots. And safe it is from every predator other than the otter. For herons and cormorants the fish are protected once out of sight. For mink they are too deep. Pike usually give up after a single attack. But otters are persistent. Once they have the fish cornered, they will dive and dive again. As the hunt becomes more frantic and the effort greater, they will emit a sharp cough when they surface to grab a breath. Inevitably they succeed and the victorious otters will slither out of the water onto the base of the tree to start devouring their catch. They sit back on their haunches, holding the fish in front of them using the sharp claws of their webbed feet for purchase, and then tear at the body, starting with the head. It is violent and fast. From the other side of the river I can hear the flesh being torn apart. Strangely they are not competitive about the catch; they wait their turn. When one has had enough he or she will lay what is left down for another to pick it up.
I have never yet seen the otters catch a salmon; maybe they are too big or simply swim away fast rather than hide. Trout are the most common, eels not far behind, and grayling the most prized – in winter they devour every last morsel of the latter. In the summer part-eaten fish or eels, too big for the otter pups to finish, are common. With the eels the head seems to be the only bit they like to eat; decapitated eels are a common sight in the morning dew. I usually kick them back into the river for the crayfish. I used to throw the part-eaten fish into the field – dead fish on the riverbed can look alarming to visitors – but since I have discovered that otters are partial to a five-day-old, decomposed trout I also kick them back in on the grounds that it might save the life of another fish.
It is something of a fallacy that trout love the fastest water in a section of a river to live out their lives; in fact almost the reverse is true. The older and bigger a trout becomes, the more he or she gravitates to the deeper, slower parts, so autumn is the only time we get to have a good look at the long-term residents who are the brood stock for the next generation. If you are a tiny little juvenile trout the fast, shallow water is a great place to grow up because you have the place to yourself. For the bigger trout the effort of holding station in the riffles, the fast-flowing shallow water that separates the pools, is too much for any possible rewards and the risk from predators like herons very high. But for the little, tiny trout even a good-sized pebble will provide shelter from the flow whilst waiting for a tasty nymph to come tumbling by. Predators? Well, when you are small it is all about the lesser of evils. Yes, you could be plucked from the stream by a kingfisher, but in truth your greatest danger lies from the very adult trout that probably spawned you. The one thing all fish love to eat is other fish.
The trout I hoped would gather on the gravel beds in North Stream would be fast developers to do so at three years; four is more common and it is the females who first seek out the ideal patch to set up the nursery. It is true that fish often head upstream to spawn to seek out the purest water and best laying gravel, but unlike say Pacific salmon that congregate in the uppermost point of a river in a giant, swirling pink mass, brown trout are smarter than that. Quite frankly they travel only as far as they need to travel, be it a metre or a mile, which is why I had high hopes for our newly restored stream. Brown trout are eminently practical when it comes to spawning; if they have to travel 20 miles upstream to find the perfect place and mate they will do it, but if both are within a few yards, why bother? I was hoping North Stream would be that place, the breeding ground for the trout that inhabited Gavelwood already. The main river was fine, but the stream would be better with more places for redds and a better nursery for the eggs once hatched. From my point of view, it was all about making it easy for the female, because creating a redd is tough work. She positions herself over the chosen spot and then with flicks of the tail or a sideways movement of the body gradually dislodges a few pieces of gravel at a time. With thousands of movements, executed thousands of times over a period of days, gradually an indentation is cut in the gravel of the riverbed. Some of the stones get carried away on the current, but others gradually pile up in a mound at the downstream end of the cut. This mound, seemingly an unimportant by-product of the excavation, will in fact be vitally important when the females come to lay their eggs. But for now our female has to seek out the right location for her redd. The main river is just too fast in most places, as no sooner will she start to dig a hole than the rapid flow will scour it flat again, and even if she succeeded, when it comes to mating the eggs would be whipped away in the current before fertilization had had a chance to take place. So in the search for the ideal spot I am hoping that the trout moving upstream will turn right into the relative calm of North Stream to check it out.
Every action in a river causes some sort of reaction, so digging up the riverbed, however well intentioned, causes all sorts of commotion for other river creatures, and in this particular case the tiny ones. The gravel riverbed is home to millions of invertebrates, animals like snails, bloodworms, nymphs and shrimps, which thrive in the constant temperature of the chalkstream water. While 10ºC might be a very cold bath for humans, for this group it is perfect. And if they thrive, so do the creatures that eat them, namely the fish. Fish are opportunists. Unlike people they don’t have a routine that tells them it will be lunch at such and such a time. If food comes along they eat it and the moment that the redd cutting begins I will see the yearlings – fish under twelve months old – gathering below the cutting area to start hoovering up the unfortunate invertebrates, who can only drift helpless on the current until they either get caught in some weed, float down to the bottom or get swallowed. It must also be said that the yearlings, or parr, are not just there for the food; as eager adolescents they are standing by to add their bit to the spawning process. These ‘sneakers’ as they are called will slip between the adults at the crucial moment. Whether they contribute much in a normal year is debatable, but nature brings them to sexual maturity early as a back-up plan. In a bad year, maybe caused by low water or some other natural disaster that prevents enough males making it to the redds, there will at least be someone there to complete the job.
Fish are not beyond digging into the gravel themselves to find food. Watch a grayling in a river and you will see him go tail up, push his snout down into the gravel and with a puff of silt around his head suck up a shrimp. But why go to all that effort when a redd-making trout does the work for you? This is a winter feast that will only be bettered by the trout eggs themselves. And in the hot summer days, when anglers start to feel the heat and the fish get lazy, there are opportunities for both to capitalize on the dislodged food sites. At four or five spots across Gavelwood water meadows I have places where the cattle can either wade across the river or get into it to drink. As your average bovine drinks around seven gallons a day, maybe twice as much in hot weather, that is a lot of getting in and out of the river. And every time they do it stirs up the riverbed, uprooting the inhabitants. Trout get to know this, so they wait downstream, only moving out from the shade when the muddied water gives them notice of food to come. I do the same, and a well-cast shrimp imitation as the clouded water starts to clear will often turn a dead afternoon into a successful one.
The gravel of North Stream was abundant, but the decades of neglect had left it rock-hard, without the winter floods to break up the surface and sweep away the silt that had formed a crust. Within a week of reopening the Stream the worst of the silt and mud had been washed away to reveal plenty of potential spawning grounds, but when I tested them out the reality was depressing. Jabbing a garden fork into random sections of the riverbed I was mostly rewarded with a bruised hand. The tines would barely penetrate more than an inch or two. This was bad news. If I could not break through with a steel fork then the trout would find the same and keep moving on upstream to abandon North Stream. There were two options – do nothing or intervene.
Do nothing is not so bad if you don’t mind waiting for years. Gradually, nature, in the form of exceptionally heavy winter flows, would break up the surface into the loose gravel that a trout might easily dislodge. But I didn’t feel inclined to wait for years, so intervention, in the form of gravel-blasting, was the remedy. Gravel-blasting is not the nuclear solution it might at first sound. You take a high-pressure water pump with a steel probe on the end, stand yourself in the river, press the probe down into the gravel to a depth of about 6 inches and then wait while the water from the pump does the work, washing away the decades of silt that was binding together the gravel stones. When the water starts to run clear, you pull the probe out and push it back into the gravel a foot or so away. For the first ten minutes this is a fun job, but after a while the novelty palls. It is effective, however, and when you stand on the bank to admire your handiwork there is always a certain amount of satisfaction – the riverbed looks like a freshly plumped pillow and the gravel will positively glisten.
While the trout are starting to weigh up the options of North Stream, our salmon pick up the pace as the scent of the home river gets stronger. Past Land’s End they start to hug the coastline, the beaches of Cornwall then Devon almost in sight. Gradually the pack thins out as one by one they peel off for rivers like the Dart, Exe and Camel. For the remainder the chalk cliffs of the Jurassic Coast are the marker that these salmon are heading for the chalkstream rivers of Dorset, Wiltshire and Hampshire. Yet the salmon might be less eager to make the transition from salt water to fresh if they knew that the change signals the end for most of them: nineteen out of every twenty salmon are certain to be dead within a few months. The odds are much worse for the males than females, but they know nothing of this, so the urge to procreate impels them forward.
Why do they die? From the very moment our salmon enters the fresh water he or she stops feeding. And this cessation is absolute. Not a single calorie of nutrition will be consumed until the salmon returns to the sea or more probably dies. During the time of its life when food is most needed there is none. Day after day the fish swims upstream against the current, navigating weirs, dams and obstacles whilst the body adapts to the change from salt to fresh water, losing weight and condition. In the confines of the river a new raft of predators awaits; otters, pike, herons, cormorants and even fishermen line up for a piece of the action. These are not good odds, and at the head of the river the body-sapping ritual of mating will deliver the death blow to nearly all who make it that far. At Gavelwood we are about 35 miles up from the coast, more or less two-thirds of the way up the river system. For a salmon, a chalkstream like our Evitt is an easy run: no massive waterfalls to leap over or fierce currents to swim against. The greatest point of difficulty is Middle Mill, 5 miles up from the sea. At one time this was the biggest flour-grinding mill in the county, capturing the entire river flow to drive two enormous waterwheels. Below the mill races is the mill pool, a huge expanse of swirling water, which is the first place the salmon rest up on their run inland.
