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作者：吉姆•里卡扥維奇，薩斯•蘇克曼
魚苗放流計畫會傷害所繁殖的魚種，這個觀念好像和我們直覺想的不太一樣。畢竟，如果問題只是魚太少了，那為什麼不多繁殖一點呢？這個理由和鮭魚系統中相關元素微妙的牽聯有關。
最早的水產種苗繁殖場只是簡單的魚卵孵化中心，孵化後將小魚苗放流至溪流中，希望能減低野生魚卵在溪流底部碎石中孵化所造成的死亡率來增加魚的族群。但在1920年後，籌畫者開始餵食魚苗，將它們養成手指般大小才將它們放回溪流中。但小魚所吃的食物－魚內臟雜碎、馬肉、內臟、廢棄的豬肉、牛肉是很不經濟的，而且甚至會在擁擠的飼養環境中傳播疾病。
直到1960年，隨著顆粒狀魚飼料的出現，水產種苗繁殖場才成功地將大量的魚養成比手指還大的尺寸，甚或是兩歲的階段，這個時候魚要開始適應鹹水，為它們的成魚階段作準備。
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營運中的水產種苗繁殖場, 1998
雖然繁殖場的繁殖復育技術日漸成熟，魚苗放流計畫實際上會降低他們所希望增加之野生族群的活力。其中一個問題是根源於「環境負載力」，也就是一條特定溪流所能涵養魚隻的最大數目。當人工繁殖的魚群加入百萬隻小魚游回大海的旅程時，那些人工繁殖的魚苗會在回到大海的過程中和野生的魚競爭。這時，顯而易見的，沒有足夠的食物來滋養這些小魚，對人工繁殖魚群及野生族群來說都是傷害。
當人工繁殖魚苗成年後和野生鮭魚交配後，它們的子代會比純種野生的子代孱弱。每一條河及其支流都有其特定的魚種生存於其中，天擇在一代代中成就了最適合其特定環境的魚種，適合在此迴游及生育，將它們的基因傳給下一代。例如有些魚只要從大海迴游進溪流幾哩就幾乎準備交配，而牠們注定在內陸育卵的表親在這個時候還沒有交配的準備，要等到幾個月牠們重回淡水後才會開始準備產卵。此外，野生的魚通常在其原生的溪流中會對寄生蟲及疾病有抵抗的能力。
有時候，基因流會意外的有所交流，如人工繁殖的魚迴游產卵時，在另一條溪流迷路了。在其他許多例子中，管理人將手指般大小的魚由一條河移殖至另一條河，或者是，經由人工繁殖所產出的鮭魚和野生原產的魚交配，而因此降低它們對當地的適應力。舉例來說，原產的銀鱒對出現在奧勒岡州Nehalem河沿岸特定的寄生蟲具有抵抗力。而由其他沒有這種寄生蟲的河流而來之年輕、人工繁殖的銀鱒在轉殖至Nehalem河支流幾年，之後，當那裡的成魚被回收，會發現它們的子代對寄生蟲的抵抗力還不及野生種，但又比那些移殖的人工繁殖魚群還不易感染寄生蟲。研究人員的結論是，群體的混合會造成族群對寄生蟲的抵抗力降低。在其他的例子中，科學家們發現人工繁殖的銀鱒所產的卵減少，且比野生魚所產的卵還不容易存活，理由是什麼呢？顯而易見的，人工繁殖的魚較早產卵，並不像野生的魚可以配合溪流環境的時機。
最後，雖然魚苗放流計畫產生了更多的個體，假設這些成魚成為商業性漁業的主要目標的話-他們通常在由許多河流匯集的海洋中，或是在如哥倫比亞河這種較大河系的下游流域，捕捉由各條河而來的混合群體-比較弱的野生群體則會被過度捕捉。立法者制訂規則，允許民眾捕捉三分之二的，被認為是「較強壯」的流放魚群中，但這些規定其實也造成較弱的（野生）群體的三分之二被捕捉，而這較弱的族群會因這大量的損失而元氣大傷。目前，洄游至哥倫比亞河產卵期的魚群之中，有80%為魚苗放流計畫的結晶。為了大多數鮭魚所實行的管理方法，根本不符合另外20%野生種的需要。
這令人困擾的魚苗放流計畫，將魚群看成一部大機器中可以任意交換的零件。最聰明的預防方式正如資源保育專家阿爾多•里奧波德所建議的─保存所有的族群（而不是限制捕捉其中一部份的族群）。漠視這項預防，只會讓這複雜的系統不平衡，並將鮭魚置於險境。
作者吉姆•里卡扥維奇是生物學家，29年以來，他投身於鮭魚管理及研究。他最近的著作《Salmon
Without Rivers》說明了太平洋鮭魚危機的根源及演化。另一位作者薩斯•蘇克曼，是Ecotrust的巡迴記者，報導有關社區、經濟及環境，到沿岸的溫帶雨林之議題，並活躍於加州西北部馬托河（Mattole
River）流域的復育工作。（編按：《Salmon Without Rivers》的英文書評可參考
http://www.tidepool.org/BOOKS/
salmonwo2.cfm）
其他資料，改編自Salmon Nation的網站文章：
Behind That Farmed Salmon Steak;
A Natural History of the Pacific Salmon;
Recalling Celilo;
Visualizing Salmon Nation: Maps
全文與圖片詳見：http://www.ecotrust.org/publications/hat
cheries.html
版權歸屬Environment News Service(ENS)，環境資訊協會(楊璧如
譯，黃向文 審校)
中英對照全文：http://e-info.org.tw/issue/animal/2001
/is-animal01101701.htm |
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by Jim Lichatowich and Seth Zuckerman
It might seem counter-intuitive that hatcheries could
actually harm the species being propagated. After all, if the problem is
too few fish, why not simply hatch some more? The reason lies in the
subtle connections between elements of the salmon system.
The earliest hatcheries were simply egg-incubating
stations that released tiny fry into the streams, hoping to increase
fish populations by reducing the mortality of eggs incubated in
stream-bottom gravels. By the second decade of the twentieth century,
managers began to feed the fry and raise them to fingerling size before
turning them loose. But the diet they received - a mixture of fish
offal, horse meat, tripe, and condemned pork and beef - was ineffective,
and even spread disease through the tightly packed schools of fish.
It wasn't until 1960, with the advent of pelletized feed
made from fishmeal, that hatcheries had significant success in raising
large numbers of fish to large fingerling size or even to the smolting
stage, when the fish begin to adapt to salt water for their adult lives.
Hatcheries in operation, 1998
Even as they became more proficient at raising juvenile
fish, the hatcheries were actually undermining the vitality of the wild
stocks of salmon that they hoped to supplement. One problem was rooted
in "carrying capacity," the maximum number of fish that a
particular stream can support. As hatcheries infused rivers with
millions of fingerlings for their journeys to the sea, the hatchery fry
came to compete with the wild fish traveling oceanward at the same time.
At times, there simply wasn't enough food to nourish all of the young
fish, to the detriment of both wild and hatchery stocks.
When hatchery-bred fish return as adults and interbreed
with wild salmon, they produce offspring that are less hardy than their
purely wild counterparts. Each river and tributary has a distinct strain
of fish, the product of generations of natural selection in which the
fish that best fit a particular environment were the most likely to
return and reproduce, passing on their genes to the next generation. For
instance, fish whose spawning journey takes them just a few miles from
the ocean enter the river nearly ready to mate, while their cousins
destined for spawning grounds far inland will not be ready to lay their
eggs until months after they re-enter fresh water. In addition, wild
fish often possess resistance to the parasites and diseases of their
native streams.
Sometimes the genetic wires get crossed accidentally, as
when hatchery fish stray into another stream upon returning to spawn. In
many other cases, managers transplanted fingerlings from one river
system to another. Either way, the hatchery-bred salmon mated with wild
native fish and diluted their local adaptations. For example, native
coho salmon possess resistance to a particular parasite that is present
in coastal Oregon's Nehalem River. Young hatchery coho from another
river, where the parasite isn't prevalent, were planted in a tributary
of the Nehalem for several years. Later, when adults were collected
there, their offspring proved less resistant to the parasite than the
wild stocks but not as susceptible as the hatchery transplants.
Researchers concluded that the mixing of the stocks had reduced the
population's resistance to the parasite. In another case, scientists
found that eggs laid by coho of hatchery descent that spawned in the
wild were less likely to survive than the eggs of wild fish. The reason?
Apparently, the hatchery fish were spawning earlier in the year, which
was not as well matched to the conditions of that river as the native
salmon's timing.
Finally, even if the hatchery program yields many
adults, if those adult fish are targeted by a commercial fishery that
captures fish from a combination of stocks - in the ocean where fish
from many rivers mingle together, or in the lower reaches of a large
river system like the Columbia, for example - the weaker wild stocks
will be overharvested. Regulators will devise rules that will allow,
say, two-thirds of the stronger stocks to be taken. But those rules will
result in a two-thirds harvest of the weaker stocks, whose population
will be harmed by such a large loss. Currently, some 80 percent of the
adult spawners returning to the Columbia River are the product of
hatcheries. Regulations that serve the majority of salmon are unlikely
to match the needs of the wild 20 percent.
An indiscriminate hatchery program treats fish like
interchangeable parts in a large machine. The first precaution of
intelligent tinkering, counseled conservationist Aldo Leopold, is to
save all the parts. Neglecting this precaution, we have unbalanced an
intricate system and placed the salmon in danger.
Jim Lichatowich is a biologist who has spent
twenty-nine years in salmon management and research. His recent book
describes the roots and evolution of the Pacific salmon crisis. Seth
Zuckerman serves as Ecotrust's circuit rider, reporting on issues of
community, economy, and environment throughout the coastal temperate
rain forest, and is active in watershed restoration on the Mattole River
in northwestern California.
Additional materials adapted from Salmon Nation appear
on the following pages:
Behind That Farmed Salmon Steak
A Natural History of the Pacific Salmon
Recalling CeliloVisualizing
Salmon Nation: Maps
http://www.ecotrust.org/publications/hat
cheries.html |
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