By Mark Kinver
Science and nature reporter, BBC News
An ban on hunting humpback whales is credited for the species' recovery
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."
RED LIST DEFINITIONS
Extinct - Surveys suggest last known individual has died
Critically Endangered - Extreme high risk of extinction - this means some Critically Endangered species are also tagged Possibly Extinct
Endangered - Species at very high risk of extinction
Vulnerable - Species at high risk of extinction
Near Threatened - May soon move into above categories
Least Concern - Species is widespread and abundant
Data Deficient - not enough data to assess
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."
Hunting for answers
The findings are likely to impact on the current stand-off between pro- and anti-whaling nations over the merits of a global moratorium on commercial whaling.
Pro-hunting nations could use the assessment to argue that the humpback whales' recovery means that the mammals could now be caught sustainably.
Whereas anti-whaling countries will argue that the recovery is the result of the global ban, and that any form of hunting will again lead to a dangerous decline in numbers.
"It is a political question, and it will be answered in a political form," Dr Reeves observed.
"I think it is really important to stress that the Red List is not about whether a given species can be exploited.
"It's simply about looking at the status of these animals and giving people some idea where they are now in relation to where they used to be."
Despite the improvement in the status of the two large species, the Red List showed that nearly a quarter of the 86 species assessed were considered to be at risk of extinction, nine of which were listed as Endangered or Critically Endangered.
Dr Reeves said the vaquita, a porpoise in the Gulf of California, Mexico, was identified as the one that was mostly likely to become extinct in the coming years.
Only about 150 individuals are believed to remain in the wild, yet an estimated 15% of the dwindling population is believed to drown in fishing nets each year.
Elsewhere, the baiji (Lipotes vexillifer) has been listed as Critically Endangered, Possibly Extinct, after an extensive survey of the creatures' entire range in China's Yangtze River last year failed to detect a single surviving specimen.
Its demise was blamed on an unsustainable number being caught in fishermen's nets as by-catch.
"The trouble is that the nets that cause the most trouble, the entangling nets that are unselective in what they catch, are still being used," Dr Reeves told BBC News.
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."