Commercial whaling reduced several species to a small fraction of their original numbers. The recovery since is one of the better-documented conservation experiments, and the results vary enormously by species.
The recoveries
Several populations have increased substantially since protection, with some approaching pre-exploitation estimates.
Humpback populations in several regions have shown sustained growth over decades, sufficient for downlisting.
Which demonstrates that populations reduced to very low numbers can recover when the pressure is removed, and it is among the strongest evidence available for that proposition.
The non-recoveries
Others have not recovered despite decades of protection.
North Atlantic right whales remain critically endangered, with mortality from ship strikes and fishing gear entanglement exceeding what the population can sustain.
Which identifies the cause precisely — protection from hunting does not address other human-caused mortality.
Several populations of blue and other large whales remain at small fractions of historical numbers.
Entanglement
Fishing gear, particularly vertical lines connecting seabed traps to surface buoys.
Which whales encounter and become wrapped in, causing drowning, injury, infection and chronic energetic cost from dragging gear.
Documented entanglement rates in some populations are very high, with a majority of individuals showing scarring.
Ropeless gear, using acoustic release rather than permanent vertical lines, has been developed and tested, with cost and operational obstacles to adoption.
Ship strikes
Collisions with vessels, which are frequently fatal for the whale and unnoticed by the ship.
Speed restrictions in high-risk areas reduce both collision probability and lethality, with evidence supporting both effects.
Routing changes moving shipping lanes away from concentrations have been implemented in several regions with documented reductions.
Compliance with voluntary measures has been consistently poor, which is why mandatory measures have followed.
Noise
Ocean noise from shipping, seismic survey and military sonar interferes with whale communication and navigation.
Which is documented to cause behavioural change, displacement and, in specific cases involving intense sonar, strandings.
Ambient ocean noise has increased substantially with shipping traffic, and quieter vessel designs exist and are not required.
The reduction in shipping during pandemic restrictions produced measurable reductions in noise and in whale stress indicators, which was an inadvertent experiment.
The ecological role
Whales affect ocean systems in ways that have become better understood.
Nutrient transport, moving nutrients from depth to surface and between regions.
Carbon, through their biomass and through effects on plankton productivity.
Which has produced valuations of whales in carbon terms that have been used in policy arguments, and the estimates carry substantial uncertainty.
Whaling now
Commercial whaling continues under objection and reservation by a small number of countries, and under indigenous subsistence provisions elsewhere.
Volumes are far below historical levels, and the argument continues over both the conservation and the ethical positions.
What the case shows
Removing a single pressure can produce dramatic recovery.
Where multiple pressures operate, removing one is insufficient.
And the pressures that now limit recovery — fishing gear, shipping, noise — are addressable with known technical solutions that have not been implemented at scale.
Monitoring methods
Photographic identification from individually distinctive markings, particularly tail flukes and dorsal fins.
Which allows mark-recapture estimation and tracking of individuals over decades.
Acoustic monitoring detects vocalisations, which works at long range and in poor visibility.
Genetic sampling from skin biopsies establishes population structure and relatedness.
Aerial and vessel surveys along transects provide abundance estimates.
Together these have produced far better data than existed when protection began.
Population structure
Species are divided into populations that mix little, which matters for management.
Which means a species can be globally secure while a specific population is critically endangered, and several are.
Genetic analysis has revised understanding of these boundaries substantially.
Strandings
Provide data unobtainable otherwise — diet, health, contaminant loads, cause of death.
Which is why stranding networks exist, and reports from members of the public are how most strandings are detected.
Mass strandings of certain species have been linked to specific sonar activity in several investigated cases.
Whale watching
A substantial industry that has replaced hunting economically in several places.
Which provides a direct economic argument for live whales and produces its own disturbance.
Guidelines on approach distance, vessel numbers and engine handling exist, with compliance varying.
Studies have documented behavioural responses to vessel presence, with effects on resting and feeding.
Contaminants
Long-lived predators accumulate persistent pollutants through the food chain.
Concentrations in some populations exceed thresholds associated with reproductive impairment.
Which affects populations that are otherwise protected, and the compounds involved were banned decades ago and persist.
Prey availability
Some populations are limited by food rather than by direct mortality.
Which links whale recovery to fisheries management for their prey species, and it is a connection that management frameworks handle poorly since they are administered separately.
Reporting sightings
Public sighting reports contribute to distribution data and to stranding response.
Which is why reporting schemes exist in most coastal countries, and photographs with date and location are particularly valuable.