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忙しくても安心！10分でサクッと学べる長文読解（テーマ：生物）

こすもすらいふ

以下に「鮭の生態」をテーマにした、英検準1級レベルの長文を作成しました。7～8分でじっくり読める文量で、単語や文法のポイント、日本語訳、内容に関する問題と模範解答も含めています。

※この長文の単語数は 891語 です。

The Life Cycle of Salmon

Salmon are fascinating creatures known for their remarkable life cycle. These fish, found in both the Atlantic and Pacific Oceans, undergo one of nature’s most extraordinary journeys. From the moment they are born in freshwater rivers to their long migration to the ocean and eventual return to their birthplace to spawn, the life of a salmon is full of challenges and mysteries. Understanding the life cycle of salmon is crucial not only for environmental conservation but also for maintaining healthy ecosystems.

Born in Freshwater Rivers

Salmon begin their lives in freshwater rivers, where they are born as eggs. After hatching, they remain in the river for several months to a few years, depending on the species. During this time, the young salmon, known as fry, feed on small insects and other organisms. They also go through a process called smoltification, during which their bodies adjust to prepare for life in saltwater. This adaptation is essential for their survival, as it enables them to make the transition from the freshwater environment of the river to the salty ocean.

The Long Migration to the Ocean

Once they are ready, the smolts (young salmon) begin their journey downstream to the ocean. This migration is one of the most perilous parts of their life cycle. Along the way, they must avoid predators such as birds, larger fish, and even humans. For some species, the journey can be hundreds or even thousands of kilometers long, taking them through various environments, including estuaries, where rivers meet the sea. Estuaries are crucial as they provide a safe space for salmon to gradually adjust to saltwater before fully entering the ocean.

Life in the Ocean

After reaching the ocean, salmon spend several years growing and maturing. In the vastness of the ocean, they continue to face dangers from predators such as seals, whales, and larger fish. However, the ocean also provides them with an abundance of food, including smaller fish, plankton, and shrimp. This diet allows them to grow rapidly and prepare for the next stage of their journey.

One of the remarkable traits of salmon is their ability to navigate across vast distances in the ocean. Scientists are still studying how salmon manage to find their way back to the very rivers where they were born. Some researchers believe that salmon use a combination of the Earth’s magnetic field and their sense of smell to navigate. This incredible ability ensures that they can complete their life cycle.

The Return to the River and Spawning

After spending several years in the ocean, salmon instinctively know it is time to return to their birthplace to spawn. The journey back is equally challenging. They swim upstream, often against strong currents, waterfalls, and other obstacles. During this time, they stop eating and focus solely on reaching their spawning grounds. The energy they stored during their time in the ocean sustains them on this final journey.

Upon reaching the river where they were born, salmon begin the process of spawning. The female salmon lays her eggs in gravel beds, while the male fertilizes them. Once this process is complete, both the male and female salmon typically die. Their bodies, however, provide essential nutrients to the river ecosystem, supporting other species such as birds and small mammals.

Environmental Challenges

Salmon face numerous environmental challenges throughout their life cycle. Dams, pollution, and habitat destruction have made it increasingly difficult for salmon to survive. Dams, in particular, block the migration routes of salmon, preventing them from reaching their spawning grounds. Conservation efforts, such as building fish ladders that help salmon bypass dams, have been implemented to mitigate these problems. Additionally, pollution from agriculture and industry can contaminate the rivers where salmon spawn, threatening the health of both the fish and the ecosystem.

Climate change is another significant factor affecting salmon populations. Rising water temperatures can disrupt the delicate balance of their freshwater habitats, making it harder for eggs to survive. In some regions, efforts are being made to protect these habitats by restoring rivers and reducing human impact.

The life cycle of salmon is not only a testament to their resilience but also a reminder of the interconnectedness of ecosystems. Protecting salmon means protecting the health of rivers, oceans, and countless other species that rely on these environments.

Vocabulary and Grammar Points:

  1. Spawn (産卵する) – To produce eggs in water (used especially for fish).

    • Example: "Salmon return to their birthplace to spawn."

  2. Fry (稚魚) – A young, recently hatched fish.

    • Example: "The fry remain in the river for several months before migrating."

  3. Smoltification (スモルト化) – The process by which young salmon adapt to saltwater.

    • Example: "During smoltification, salmon prepare for life in the ocean."

  4. Navigate (航行する、進路を見つける) – To find one’s way, often over long distances.

    • Example: "Salmon navigate back to the rivers where they were born."

  5. Estuary (河口) – The area where a river meets the sea.

    • Example: "Estuaries provide a safe space for salmon to adjust to saltwater."

Questions :

  1. What is smoltification, and why is it important for salmon?

  2. How do estuaries help salmon during their migration?

  3. What challenges do salmon face during their journey to the ocean?

  4. How do salmon find their way back to their birthplace after spending years in the ocean?

  5. What environmental challenges are threatening the survival of salmon populations?

Sample Answers (模範解答):

  1. Smoltification is the process by which young salmon adapt to saltwater. It is important because it allows them to survive the transition from freshwater to the ocean.

  2. Estuaries provide a safe environment where salmon can adjust to saltwater before fully entering the ocean.

  3. Salmon face predators such as birds, larger fish, and humans, as well as long distances and changes in water conditions.

  4. Scientists believe that salmon use a combination of the Earth's magnetic field and their sense of smell to navigate back to their birthplace.

  5. Environmental challenges include dams blocking migration routes, pollution affecting rivers, and rising water temperatures due to climate change.

Japanese Translation (日本語訳):

サケは、その驚くべき生態で知られる魅力的な生物です。大西洋や太平洋に生息するこれらの魚は、自然界で最も驚異的な旅を行います。淡水の川で生まれ、海へと移動し、最終的には産卵のために生まれ故郷に戻るサケの生涯は、多くの困難と神秘に満ちています。サケの生態を理解することは、環境保護や健全な生態系の維持にとって重要です。

淡水の川で生まれる

サケは、淡水の川で卵としてその生涯を始めます。孵化後、数ヶ月から数年の間、川で成長します。この間、稚魚（fry）と呼ばれる若いサケは、小さな昆虫や他の小さな生物を食べて成長します。彼らはまた、スモルト化（smoltification）と呼ばれる過程を経て、海水で生きるための身体的な準備をします。この適応がなければ、サケは淡水から海水への移行を生き延びることはできません。

海への長い旅

準備が整うと、稚魚は川を下り、海へと旅を始めます。この移動は、サケの生涯で最も危険な部分の一つです。彼らは鳥や大型魚、人間などの捕食者を避けなければなりません。種によっては、この旅は数百キロメートルから数千キロメートルにも及ぶことがあり、河口（estuary）などのさまざまな環境を通り抜けます。河口は、川と海が交わる場所で、サケが徐々に海水に慣れるための安全な場所です。

海での生活

海に到達すると、サケは数年間そこで成長し成熟します。海では、アザラシやクジラ、大型魚などの捕食者に直面する一方で、小魚やプランクトン、エビなどの豊富な餌を得ることができます。この食事により、サケは急速に成長し、次の旅に備えます。

サケの驚くべき特徴の一つは、広大な海の中でナビゲートする能力です。科学者たちは、サケがどのようにして自分が生まれた川を見つけるのかをまだ研究しています。いくつかの研究者は、サケが地球の磁場や嗅覚を使って進路を見つけていると考えています。この驚異的な能力は、彼らが生態を全うできるための重要な要素です。

川への帰還と産卵

数年間海で過ごした後、サケは本能的に産卵のために生まれ故郷に戻る時が来たことを知ります。帰還の旅は同様に困難です。彼らは強い流れや滝などの障害物に逆らって泳ぎます。この間、サケは食事をせず、産卵場に到達することだけに集中します。海で蓄えたエネルギーが、この最後の旅を支えます。

生まれた川に到着すると、メスのサケは砂利の中に卵を産み、オスはそれを受精させます。この産卵が終わると、オスもメスも通常は死にます。しかし、その遺体は川の生態系にとって非常に重要な栄養素となり、鳥や小動物などの他の生物を支えます。

環境問題

サケはその生涯を通じて多くの環境問題に直面します。ダム、汚染、そして生息地の破壊は、サケが生き残るのをますます困難にしています。特にダムは、サケが産卵場に戻るための移動経路を遮断し、彼らの繁殖を妨げています。これに対処するため、ダムに魚道を設置するなどの保全対策が進められています。さらに、農業や工業からの汚染は、サケが産卵する川を汚染し、魚や生態系の健康を脅かしています。

気候変動もサケの生存に影響を与える重要な要因です。水温の上昇は、サケの卵が生き残るために必要な淡水環境のバランスを崩す可能性があります。いくつかの地域では、川を回復し、人間の影響を減らすための取り組みが進められています。

サケの生態は、その強靭さを示すと同時に、生態系がどれほど密接に結びついているかを思い起こさせます。サケを保護することは、川、海、そしてそれらに依存する無数の生物の健康を守ることを意味します。


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