Book Excerpt: A Temporary Refuge（一時的拒絶）から抜粋
by Lee Spencer
illustrations by Cathy Eliot
Wild summer steelhead, an anadromous variety of rainbow trout, have a complex life cycle that passes through at least seven life stages. For their first two years, the majority of these juvenile fish make use of their natal stream from its headwaters to ultimately reach the ocean, where they spend an additional two years far out beyond the coastal zones in the North Pacific. Because of the productive richness of the marine environment, steelhead experience an immense increase in size during this pelagic portion of their existence. The final chapters of this steelhead saga involve their return from the ocean on a spawning run, which will eventually take them back to the particular headwater reach that held them in its gravels as fertilized eggs approximately four years before. While some summer steelhead will survive to spawn again, most of them die as a consequence of their first spawning run, during which they do not, other than incidentally, feed.
野生の夏のスティールヘッド（ニジマスの遡河性魚種）は少なくとも7つのステージを通るという複雑なライフサイクルを持ってる。最初の2年間は、稚魚の大半は最終的には大洋に辿り着くために、川の上流部の生まれた小川を利用している。アメリカの北西部では海岸から離れた処で、更に２年間過ごしている。スティールヘッドは遠洋で生活している間、海洋の豊かな環境により、そのサイズは素晴らしく増大する。スティールヘッドの苦労話の最後の章は産卵のために海洋からの回帰についてで､約４年前に砂利の中に受精した卵を埋めた川の上流部に、最終的に戻ってくることだ。いくつかのサマー・スティールヘッドは再度産卵するために生きているが、そのほとんどは最初の遡上の結果として死ぬ。その間、偶然餌を採る以外に餌を採らない。
Buried in the gravels of a streambed by their mothers, summer steelhead eggs take six to eight weeks to hatch their tiny fish. The time from fertilization to hatching depends on the temperature of the water working its way through the streambed, delivering oxygen to the eggs. When they have hatched, salmon are called alevins and they continue to live in the pebble and cobble substrate beneath the bed of the stream. Once free of the egg membrane, alevins can live on about half the oxygen necessary to maintain the developing embryo within the egg. Alevins carry a bulbous red yolk sac that makes up about half their body mass, and they leave the egg with a fully developed balance organ in their inner ear. This allows the young fish to assume an upright horizontal posture in the gravel. Immediately, alevins begin to disperse through the gravel interstices, moving away from their nest.
メスによって産卵床の砂利に卵が埋められると、サマー・スティールヘッドの卵は仔魚の孵化まで６〜８週間掛かる。受精から孵化までの期間は、酸素を卵に運びながら、川底まで流れる水流の温度に依存している。孵化すると、鮭はalevins(仔魚、しぎょ)と呼ばれ、川底近くの小石や中石の環境に生息しつずける。魚卵の皮膜から出ると、仔魚は魚卵内の胎児の成長を維持するために必要な､約半分の酸素で生きる。仔魚はおおよそ体の半分にもなる球根状の赤卵黄嚢を付けていて、内耳内の十分に発達した平衡感覚器を持って、卵を置き去りにする。これで仔魚は砂利の中で、まっすぐな水平姿勢が取れるようになる。直ぐに、仔魚は砂利の隙間の中で分散し、住処から散らばり出す。
These first movements, slow and pinball-like, initially carry the alevins down through the gravels and then upstream, facing into the current. At the start of this, their first journey, they are repelled by light.
これらの最初の移動は、ゆっくりとピンボールのようで、水流に向かいながら、まず川底へ、そして上流に仔魚を運んで行く。この最初の旅の時点では、仔魚は光を嫌う。
Floods of sufficient size to shift those portions of the streambed gravels that contain these early steelhead life stages are one of the primary dangers they face. A second danger is the deposition of fine sediments upstream, which can block the current that is supplying oxygen to the eggs.
スティールヘッドの初期成長段階で使われる川底の砂利を移動させるほどの洪水は、彼らが直面する一番大きな危険の1つだ。次に大きな危険は上流の細かな沈殿物の沈殿で、魚卵に酸素を供給する水流をブロックしてしまう。
The juvenile steelhead, after two to three weeks, are slimmer, having absorbed the yolk initially distending their bellies, and they are now attracted to light, which brings them wriggling up just beneath the surface of the streambed. By this time the young fish have developed protective coloration and markings that will serve as camouflage. With the fall of night, they will swim to the surface to gulp air for their air bladder. This air allows them to control their buoyancy and maneuver within the dramatic push and pull of a free-flowing stream.
生まれて2，3週間後のスティールヘッドの稚魚は痩せていて、腹部の膨らんだ卵黄から養分を吸収している。そして、仔魚は光に引き寄せられ、川床表面近くまでよじ登ってくる。この段階では幼魚はもう体に保護色をまとい、カモフラージュ用のマークを持つようになる。夕暮れに、魚の浮き袋に空気を貯めて、表面に泳ぎ出す。この空気で浮力をコントロールできるようになり、川の流れへ劇的な出入りを行うことができるようになる。
Buy Lee Spencer’s “A Temporary Refuge” from Patagonia
With their emergence from the gravel—leaving the streambed behind as before, when they had left the protective membranes of their eggs—the alevin life stage ends, and the young steelhead are called fry. They enter into the free-flowing stream environment in general proximity to their eggs. On the middle and upper portions of the main creek, we regularly begin to see newly emerged fry around the third week of May. The steelhead fry are a bit under an inch long and have large dark blotches—parr marks—evenly spaced along the sides of the young fish, as well as a pattern of spotting, effectively transparent fins, and variations in skin tone between the back, sides, and belly. They hide them in the variegated environment of freshwater stream edges.
河床から離れ、砂利から抜け出すと、
Bear in mind that the creek in late spring carries much more water than it will in a month or two. This seasonal stream bank may well be especially complex, providing myriad opportunities for the new fry to find places where the current eddies and is otherwise slowed by vegetation, pebbles and cobbles, and partially buried sticks and chunks of bark. If there are no protected stream-edge reaches nearby, the fry are pushed along with the flow until they find such protected water. In the case of a pronounced flood—a relative rarity after the end of April—the fry can be swept some distance, and large numbers of them may perish.
The fry face into the currents of the creek, looking at what drifts downstream past them, a behavior known as drift feeding—learning what is edible and, by mouthing fir needles, bark chips, rare feathers, dogwood sepals, and so on, what is not. Other experiences involve competition with other fry and with other fish species for food and for territory. The young fish are also learning what predators look like, whether kingfishers or dippers, sculpins, western pond turtles, crayfish, or large diving beetles, to name only a few creatures attentive to the fry. Disturbed or spooked, the fry will take shelter under or behind items in the complex creek-margin habitat. In the absence of any other cover, I have seen these fish swim down to the gravels of the creek bed and disappear into them with a flit of their bodies.
As fry grow bigger, they gradually forage in stronger currents and deeper water. Within a few weeks, new mediodorsal parr marks will appear on top of their backs in front of their dorsal fins. By late spring I have counted hundreds of steelhead fry in less than fifty yards of the creek.
By the shorter and generally wetter days of November, the steelhead fry have grown to roughly three inches long. Between early winter and early spring, most of the fry move downstream to the North Umpqua River, where they will spend their second year in this larger and much more complex fluvial habitat. Once the juvenile steelhead have entered their second year, they are called parr.
During the previous alevin and fry life stages, some imprinting on aspects of their natal stream and gravels, perhaps olfactory in nature, must occur with steelhead and other Pacific salmon. The literature states that an adult salmon can distinguish by smell the species, sex, sexual readiness, population, and whether another fish had the same parents it did.1 It strikes me that the only part of the life cycle of a wild summer steelhead that would allow the imprinting of what a sibling would smell like would be either the fertilized egg, the alevin, or the early part of the fry stage—perhaps all of them together.
Generally, with the spring that ends their second year, steelhead parr metamorphose into smolts, the life stage that readies them for their life in the ocean. The transition from parr to smolt occurs when the juvenile steelhead born in the main creek average five-and-a-quarter inches long.2 This transformation is energy intensive and, prior to this metamorphosis, the pre-smolts begin storing fats and carbohydrates to help fuel the process.
In preparation for the marine environment, the young steelhead’s parr marks begin to fade while at the same time guanine is deposited on the skin beneath the scales, giving the juvenile fish uniformly blue-green backs, silver sides, and white bellies. Whether seen from below, from above, or from the side, the colors effectively camouflage the fish in the more uniform, open-water ocean ecosystem. The fins darken with melanin, the jaws of the smolts grow backward-oriented, needle—like teeth, the better to grip their prey with—and the wrist, or peduncle, of the tail also lengthens.
Other changes modify the gills and kidneys so that the young fish can excrete excess salts, where before these salts were retained during their lives in the freshwater stream. Young steelhead hemoglobin changes too, in order to adapt to the lower amount of oxygen in salt water. The literature states that steelhead smolts imprint on a portion of the path they will need to follow home. Smolts also begin to grow small magnetite crystals in the forward portions of their brain, undoubtedly to assist the maturing steelhead with orientation in this new effectively shoreless and bottomless saltwater world. To me it seems quite likely that this new sensitivity to the alignment of the planet’s magnetic field is an important part of the imprinting.
Behavioral changes occur with smolting too. From being solitary creatures of territories that live their lives facing into whatever currents they encounter, steelhead smolts begin to gather in groups and to purposely swim downstream with the currents in a concerted migration to the ocean.
That brief review gives an idea of what is generally going on out of sight, or in sight, in the main creek and the pool. Summer steelhead (spawning adults and repeat spawners) arriving from the sea understandably exert a strong attraction on those males that remain in the stream. The fresh-run summer steelhead still have, on balance, their ocean colors, colors that camouflage them in the marine environment and don’t do so bad in the blue-green snowmelt-colored flows. Male steelhead that are searching for spawning opportunities are easy to differentiate from the fresh-run steelhead because they have completed their metamorphosis and exhibit secondary sexual characteristics, of which the most obvious are the elongated snout, generally darker colors, and the red gill plates and lateral stripes.
Most wild summer steelhead enter freshwater in May and June, with a secondary return in late August and early September. The earliest return to the pool is between late May and middle June. How long the summer steelhead remain in the pool is dependent on the weather, with the final exodus most often being sometime in the first two weeks of December. Wild summer steelhead’s spawning peak is in February and March.
One May afternoon, my friend Dan Callaghan and Sis and I watched two spawning steelhead males patrol up and down the pool under the current seam that carried most of the drifting debris. These fish were rising and taking things at least once a minute. Often it seemed to be plant debris that was taken, whereas the drowned dark flying ants, which were common on that May day, were being passed up. I don’t know if the ants would have been passed up had they been struggling. Certainly, movement is a clear hint that what a fish is looking at is alive and, thus, probably nutritious.
Besides a small number of freshly entered wild summer steelhead that may be appearing in the pool for varying amounts of time, and spawning steelhead that also appear to come and go, and the eggs and alevins in the gravels, there are also steelhead carcasses present in the creek, in various stages of decomposition. The nutrients freed up by the rotting of these carcasses are of unimaginable importance to the local ecology of the stream.
I use the term unimaginable because we do not know what has been lost with the disappearance of the millions of tons of parental carcasses that used to build up in North Pacific streams. They are gone, probably never to return during our time as an industrial species on this planet.
The parental carcasses increase the nutrients available to the next generations of steelhead. They do so by increasing the amount of food that will be available to a host of stream-dwelling and near-stream-dwelling organisms, many of which serve as food for steelhead fry. Usually, parental carcasses have already been rendered by the time their alevins leave the gravels and take up life in the stream as fry. This increase in nutrients also decreases the likelihood of competition between juvenile steelhead for available resources and between steelhead and other juvenile salmon species and with other types of fish altogether. My good friend Carroll Kirk told me that he could tell where he had put a salmon in his oat field because of the more robust growth in that location for several years afterward.
Not surprisingly, all life in the basins of Pacific Northwest rivers and creeks relies to some degree on the bodies of these salmon and the other anadromous fish, part of a one-way movement of marine nutrients to headwater areas of creeks and rivers. Studies have shown that salmon carcasses deliver many times the amount of nutrients from the ocean to the stream than was initially delivered from the stream to the ocean by juvenile fish. More to the point, the migrating salmon delivers most of its nutrients to a discrete natal portion of a single drainage basin.
Notes:
Brown and Brown 1996, in Quinn 2005.
Apparently somewhat smaller than your average steelhead smolt for the region.
Apparently somewhat smaller than your average steelhead smolt for the region.
