The latest global assessment of cetaceans shows that the marine mammals throughout the world's oceans have experienced mixed fortunes.
The IUCN Red List of Threatened Species reveals that some large species, like humpbacks, have seen numbers increase.
However, it warns that smaller species, including river dolphins, have declined as a result of human actions.
The IUCN added that it was unable to assess more than half of the world's cetaceans because of a lack of data.
"It shows that if you protect these animals then they can recover," said Randall Reeves, chairman of Cetacean Specialist Group for the IUCN, the global conservation body.
"I'm encouraged by the fact that several of the large whale species that had been in trouble for a long time have shown steady increases over recent decades."
The latest assessment has down-listed the humpback whale (Megaptera novaeangliae) from Vulnerable to Least Concern.
The southern right whale (Eubalaena australis) was another species that was deemed no longer at risk of extinction.
Both animals had recorded increased numbers across much of their range, primarily because they had been protected from commercial whaling, Dr Reeves explained.
"Humpbacks have really shown an ability to recover strongly from extremely intensive hunting," he added.
However, he warned that recoveries needed to be measured over a timescale of decades.
"It takes a while for conservationists to build up the confidence that it is a real signal of recovery and not a short-term variation."
Although fishing nets remain the main threat to coastal and river species, Dr Reeves said that other threats were emerging.
I think we are just seeing the start of the effects of climate change in the oceans. Another big problem is noise [from sonar], which we don't fully understand yet."
Studies have shown that the distribution of many whales is changing as the oceans warm.
Conservationists fear that the change in behaviour could result in the mammals being exposed to new diseases, and inter-species competition for food.
Military sonar is also deemed as a particular threat for deep-diving beaked whales and melon-headed whales.
Despite a growing awareness of the range of threats facing the marine mammals, increased shipping and fishing were still claiming a growing number of victims.
"If you take the North Atlantic right whale," Dr Reeves said, "preventing deliberate harm of that species has been ongoing for nearly a century and has been pretty effective.
"But the numbers that are killed in ship strikes and become entangled in lines from lobster and crab traps are preventing the population, which is not very big in the first place, recovering.
"How you stop or reduce that is a real challenge."
While species such as the humpback whale have been studied in considerable detail over the years, many other species were still a relative mystery.
Conservationists warned that the outlook for the world's cetaceans could be worse than thought because more than half of the species on the Red List were classified as Data Deficient.
"The Data Deficient category is pretty large for cetaceans, probably surprisingly large to many people," Dr Reeves said.
"It is the species that live far offshore, that are found in less developed parts of the world that, in many cases, we don't have a clue about how many there are.
"In many ways, the Red List has a value in showing where there are holes in our knowledge that need to be filled quickly."
mixed 混雜的 各種族混合的
Species 種類 人類
warn警告;告誡;提醒 預先通知 發出警告;發出預告
unable 不能的,不會的 無能力的,不能勝任的;沒有辦法的
assess 確定 對...進行估價,評價
recover重新獲得;重新找到 恢復;使恢復原狀 挽回,彌補
decades 十 十年
whale 鯨 捕鯨
primarily 首先;起初,原來 首要地,主要地;根本上
intensive 加強的;密集的 精深的;透徹的
measured\ 測量;計量 程度;限度;分寸[U]
build up增進 使增大
confidence 自信,信心,把握 信賴,信任
variation 變化;變動;變化的程度[C][U] 差別;差異 變異;變種
net 網;網狀物;網狀製品 得到,使得到
climate 氣候[ 風氣,趨勢;氣氛
behaviour 【英】=behavior 行為,舉止;態度 (機器等的)運轉狀態,性能 (事物的)反應,變化;作用
Military 軍事的;軍用的 陸軍的
claim要求 權利;要求權 提出要求
prevent 防止,預防 阻止;制止;妨礙
deliberate adj深思熟慮的,慎重的,謹慎的 故意的,蓄意的 vi仔細考慮,思考
entangled 捲入的;陷入的 被纏住的
lobster 大螯蝦;龍蝦 [C]大螯蝦肉;龍蝦肉[U] 捕龍蝦
crab 蟹[C];蟹肉[U] 捉蟹,捕蟹
trap陷阱,羅網,捕捉器 設陷阱捕捉 traps隨身物品
relative 相對的,比較的 與...有關係的,相關的
clue (解決疑案,問題等的)線索,跡象,提示 (故事等的)情節 為...提供線索