研究發現大白鯊喜歡固定生活及捕食場所

2009-11-23

　　一項最新研究發現，大白鯊可能喜歡獨來獨往，是不合群的動物，但海洋裡這種最可怕的食肉動物也會成群結隊的聚集在墨西哥和夏威夷之間的一個深海『水洞』，即著名的『大白鯊咖啡館』，嬉戲，交配。

　　大白鯊是最令人感興趣的動物，也是最讓人感到膽戰心驚的動物，但是科學家承認，他們對這種凶猛的食肉動物了解很少，目前他們不知道大白鯊在世界大洋裡游來游去是如何避開重重阻礙，不斷繁衍後代，繼續幸存下來的。據專家說，跟生活在開闊水域的其他鯊魚一樣，大白鯊的肉非常值錢，在亞洲，它們的鰭更是價格不菲，因此這種巨大的『獵手』面臨著滅絕的風險。

　　這項最新研究在太平洋東部發現大白鯊的一個數量驚人的新遺傳群體裡進行，到目前為止，這是迄今發現的第三個這種『進化枝』。另外兩個大白鯊群生活在遠離南非海岸和位於澳大利亞及新西蘭之間的水域裡。長期以來人們一直認為，位於食物鏈最頂層的鯊魚喜歡在公海游來游去，隨便找個地方覓食，並隨便在任何地方進行交配。但是該科研組利用衛星對鯊魚經常出沒的地方進行監控，監聽它們發出的聲音，研究它們的遺傳樣本，最後他們吃驚地發現，生活在太平洋東部的大白鯊其實非常愛家，喜歡群居生活。該科研組由斯坦福大學的巴巴拉·布洛克(Barbara Block)領導。

　　大白鯊可以生長到6米長，體重可達3噸。在長達8年的科研工作中，科學家乘船給近100頭大白鯊貼了電子標簽，並收集了更多大白鯊的組織樣本。通過研究發現，大白鯊不僅按照一定路線進行遷徙，而且它們在遷徙過程中還嚴格遵守預定時間。每年8月到12月間，大白鯊會出現在遠離加利福尼亞州中部和北部海岸的水域裡，捕食海豹和海獅。該研究結果發表在英國雜志《皇家學會學報B》裡，該研究發現，大白鯊在這個地區的首選獵場是『紅三角(red triangle)』。

　　它們從1月開始向位於夏威夷西部大約4000公裡的深水域進發，4月到7月間，會有大量大白鯊聚集在這裡。不過一些行動遲緩的大白鯊——尤其是雄性，會在途中的『大白鯊咖啡館』稍作停留，科學家猜測它們這樣做是為了與過往的雌性進行交配。該研究總結說，這些發現將對鯊魚保護工作有幫助，『大白鯊喜歡在固定場所生活和捕獵的習慣，有助於科學家精確估算出它們的數量，並有助於他們對大白鯊進行監控和管理。』

　　據這項迄今最大規模的鯊魚調查說，生活在世界開闊水域的鯊類——包括大白鯊和錘頭鯊，有三分之一瀕臨滅絕。該研究由世界自然保護聯盟(IUCN)實施，研究工作於今年初結束。由於商業金槍魚和旗魚捕撈活動經常附帶捕捉鯊魚，幾十年來已經有大量鯊類消失，其中包括藍鯊和灰鯖鯊。近段時間由於鯊魚肉價格不斷飛漲，一些漁業公司把捕捉鯊魚作為快速掙錢的副業。歐洲是對鼠鯊和白斑角鯊肉的需求增長最快的市場。每年大約有1億頭鯊魚被商業捕撈船和釣友們捕獲。據國際愛護動物基金會(IFAW)透露，僅過去10年間，幾種鯊魚的數量就下降了80%以上。

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Picture a great white shark. Do you think of the giant predator hunting deep in the ocean, hundreds of miles from the beach where you might swim with your family? Many scientists assumed that that’s how great white sharks spent most of their time, only rarely coming near the coastal areas that might bring them close to humans.

A new study, however, shows some surprising results: Pacific great white sharks swim much closer to busy beaches in California than people had believed. Several have even gone inside the San Francisco Bay, east of the Golden Gate Bridge.

But the study does not suggest that humans are in any more danger from sharks than scientists thought we were before these findings. Why? Because the sharks apparently just aren’t very interested in people.

For the study, 10 scientists tagged 179 great white sharks from 2000 to 2008, to track the animals’ movements. The tracked movements showed that the sharks spend several months a year in waters off central and northern California, sometimes passing close to beaches where swimmers and surfers gather. The sharks are drawn to these areas by prey, such as elephant seals and sea lions.

Tagging and Tracking the Sharks

Researchers lured sharks to their boat with a decoy, or fake prey, in the shape of a seal, a favorite shark meal. Then, with darts, they attached two kinds of tags to track movement: satellite tags that give a rough location, and acoustic, or sound-based, tags that provide a precise location when the animal swims near a receiver that picks up sound waves.

The scientists set up four of the receivers in areas off the California coastline. All four areas have large colonies of typical prey for the sharks—seals and sea lions. The researchers did not expect their tagged subjects to enter San Francisco Bay. They learned this by accident when the sharks set off receivers in the Bay that had been set up there by a different research team tracking salmon.

The Man-eater Myth

The researchers don’t see any cause for alarm in the Bay area or on Pacific beaches. If a “major concentration” of great white sharks has spent this much time close to the coast without humans noticing before, then it “shows us the sharks are really minding their own business,” said Salvador J. Jorgenson of Stanford University, the study’s lead author.

The study, in fact, reinforces what history has shown: Despite its reputation as a ferocious man-eater, the largest predatory fish on the planet mostly stays away from humans. Since 1952, there have been only 10 deadly shark attacks in California waters. More people die from dog bites or hitting a deer with their car than from shark attacks.

New Findings About Migration

The tags also helped the researchers learn about the migration patterns of the great whites. Many scientists had believed that sharks roam the ocean randomly. Instead, they discovered that the sharks migrate along very precise routes. Starting from the California coastline in late winter, they swim thousands of miles across the Pacific, often as far as Hawaii. One spot in the migration area has been nicknamed the “White Shark Café,” because so many sharks gather there. In late summer, they return to the California coast, sometimes to a spot within a half-mile of where they started.

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Tags reveal white sharks have neighborhoods in the North Pacific, Stanford researchers say

Salvador J. Jorgensen, Carol A. Reeb, Taylor K. Chapple, Scot Anderson, Christopher Perle, Sean R. Van Sommeran, Callaghan Fritz-Cope, Adam C. Brown, A. Peter Klimley, and Barbara A. Block

Philopatry and migration of Pacific white sharks

Proc. R. Soc. B published online before print November 4, 2009