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The big problem for endangered orcas? Inbreeding
People have taken many steps in recent decades to help the Pacific Northwest’s endangered killer whales, which have long suffered from starvation, pollution and the legacy of having many of their number captured for display in marine parks.
They’ve breached dikes and removed dams to create wetland habitat for Chinook salmon, the orcas’ most important food. They’ve limited commercial fishing to try to ensure prey for the whales. They’ve made boats slow down and keep farther away from the animals to reduce their stress and to quiet the waters so they can better hunt.
So far, those efforts have had limited success, and research published Monday in the journal Nature Ecology and Evolution suggests why: The whales are so inbred that they are dying younger and their population is not recovering. Female killer whales take about 20 years to reach peak fertility, and the females may not be living long enough to ensure the growth of their population.
While that news sounds grim for the revered orcas — known as the “southern resident” killer whales — it also underscores the urgency of conservation efforts, said Kim Parsons, a geneticist with the National Oceanic and Atmospheric Administration’s NOAA Fisheries who co-authored the study. The population is not necessarily doomed, she said. “It’s not often inbreeding itself that will result in a shortened lifespan or kill an individual,” Parsons said. “It’s really that inbreeding makes these individuals more vulnerable to disease or environmental factors. We can support the population by supporting the environment and giving them the best chance possible.”
The struggles of the charismatic population of orcas that frequent the waters between Washington state and the Canadian province of British Columbia have been well documented — including in 2018, when one grieving mother carried her stillborn calf for 17 days in an apparent effort to mourn or revive it.
The southern resident population comprises three clans of whales known as the J, K and L pods. They are socially distinct and even communicate differently from other orca populations, including the nearby northern residents, which are listed as threatened and which primarily range from Vancouver Island up to southeast Alaska.
While the southern residents’ range overlaps with other populations of killer whales, they haven’t regularly interbred in 30 generations, the researchers said.
In the 1960s and 1970s, dozens of Pacific Northwest whales were caught for display in marine theme parks. The whale-capture industry argued that there were many orcas in the sea, and that some could be sustainably caught.
注释：
Inbreed: v
表示"近亲繁殖的"，means "produced by inbreeding"，如： Jeth : What? That we're all inbred or that we're all uneducated? 杰斯：哪些？你是说我们都近亲通婚还是我们都没读什么书？
breach: v
表示" 违反；"，means "break"，如： He never do the things breach the benefits of group. 他不做任何违背集体利益的事。
dike: n
表示" 堤坝"，means "a barrier constructed to contain the flow of water or to keep out the sea"，如： Fortunately, the flood did not break the dike. 还好，这场大水没有把堤坝冲坏。
orca： n
表示" 逆戟鲸"，如： The orca told me our Savior is coming very soon. 逆戟鲸告诉我，我们的救世主很快会来到。
grim： adj
表示"可怕的"，means " severe"，如： Her face wore a grim, set look. 她脸上显出严厉、 木然的神情
revere: v
表示"尊敬；敬畏"，means " feel or show great respect for"，如： Students revere the old professor. 学生尊敬这位老教授。
underscore: v
表示"强调"，means "give extra weight to"，如： Let me underscore two of many reasons for this concern. 值得担忧的原因很多，让我着重说明其中两点。
geneticist: n
表示" 遗传学家"，means "a biologist who specializes in genetics"，如： A geneticist is a specialist in genetics.遗传学家是遗传方面的专家。
charismatic: adj
表示" 有魅力的"，means "possessing an extraordinary ability to attract"，如： Charismatic leaders are supposed to have rough edges. 人们认为有魅力的领导者都不修边幅。
stillborn: adj
表示"死产的；流产的"，means "failing to accomplish an intended result"，如： It turned out to be a stillborn plan. 结果该项计划流产。
revive: v
表示" 复生"，means "bring back to consciousness, strength, health or an earlier state"，如： We tried to revive him but without avail. 我们试图使他苏醒,但没有成功。
clan: n
表示" 氏族"，means " group of families"，如： They still kept traces of the clan society. 他们还保留了氏族社会的痕迹。
interbreed: v
表示"杂交繁殖"，means "breed animals or plants using parents of different races and varieties"，如： Wolves and dogs can interbreed and produce offspring. 狼和狗能够异种交配而且繁育后代。
sustainably: adv
表示"可持续地"，means "capable of being sustained"，如： Can Corals Be Harvested Sustainably? 珊瑚可持续采集吗?
中文简要说明：
北美洲西岸濒临灭绝的「南方居留型虎鲸」（Southern Resident Killer Whales）群被外媒形容为非常「古怪」，它们不像其他虎鲸群那样悠游全世界，不论保育人员如何复育，它们仍濒临绝种，过去50年来只增加2只。最新研究发现，这群杀人鲸群数量无法增加的关键可能在于近亲繁殖。
英国《新科学家》（New Scientist）将栖息在美国西岸华盛顿州以北至加拿大卑诗省（British Columbia）之间海域的「南方居留型虎鲸」描述为「古怪」，原因之一在于它们不像其他虎鲸群那样迁徙全世界，它们也只吃鱼，不像其他同种猎捕海狮等哺乳类。
最令科学家不解的是，不像其他虎鲸群复育有成，「南方居留型虎鲸」数量持续递减。1970年代，商业捕捞及海水污染导致全球虎鲸数量下滑，但随着栖地保护、禁止捕捞等保育措施实施，其他虎鲸群数量都已逐年增加，唯独「南方居留型虎鲸」数量下滑。 例如与「南方居留型虎鲸」栖地重迭、生活在加拿大温哥华岛以北海域的「北方居留型」（northern residents）虎鲸，在保育行动之下，从1974年的122只上升至2018年的逾300只，这群虎鲸同样只吃鱼，不猎捕海狮等哺乳类。
相较之下，南方居留型虎鲸群1974年有71只，1996年最多时有97只，如今只剩下73只，换句话说，近50年来总量只增加2只。「南方居留型虎鲸」群一共由J、K、L三个不同家族组成。
复育不成原因：近亲繁殖
刊载在《自然生态学与进化》（Nature Ecology & Evolution）期刊的文章指出，「南方居留型虎鲸」数量无法恢复，主要原因可能在于近亲繁殖，导致雌虎鲸在达到生育巅峰前就死亡，由于雌虎鲸寿命不够长，无法确保鲸群茁壮。一般雌虎鲸约到20岁才会达生育巅峰。
带领这项研究的美国国家海洋暨大气总署（National Oceanic and Atmospheric Administration, NOAA）遗传学家卡多斯（Marty Kardos）形容南方居留型虎鲸是少数例外，他与团队为100只现存、已死亡的南方居留型虎鲸进行基因定序，以了解这个族群的遗传多样性，基因组愈相近就代表近亲繁殖的程度愈高。
团队与另外50只其他虎鲸群的基因组比较后发现，「南方居留型虎鲸」的遗传多样性最低，近亲繁殖程度最高。团队也检视虎鲸基因组与寿命的关联，发现近亲繁殖的动物寿命较短，他们推测，南方居留型雌虎鲸在达到20岁的生育巅峰前就死亡，由于雌虎鲸寿命不够长，无法确保鲸群茁壮。
不过这不代表南方居留型虎鲸群注定灭亡，研究共同撰写人美国国家海洋暨大气总署遗传学家帕森丝（Kim Parsons）表示，并不是近亲繁殖本身导致虎鲸寿命较短，而是因为近亲繁殖，导致它们更容易因为疾病、环境因素死亡，因此现在的保育重点应该是持续保护它们的栖息环境。
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