Whale watching operators in US waters work under approach rules that specify how close a vessel may come. The distance is the visible part of a broader set of behavioral requirements.
Disturbance is a legal category
Federal law protecting marine mammals prohibits harassment, defined to include acts that disrupt behavioral patterns such as feeding, breathing, nursing or migration.
That definition does not require injury. A vessel that repeatedly causes a whale to change course or cut a dive short can meet the standard.
Fixed approach distances exist because a bright-line number is enforceable in a way that a behavioral judgment made at sea is not.
The cost to the animal is energetic
Large whales feed intensively in short seasons and must bank enough energy to migrate and, for females, to produce milk through months without feeding.
An interrupted feeding dive is not merely a lost meal. Diving and surfacing are expensive, and repeated interruptions raise cost while lowering intake.
Nursing pairs are the most sensitive. Calves surface frequently and cannot follow a mother who moves off to avoid vessels, so displacement separates them.
Noise travels further than the boat
Sound carries efficiently underwater, so a vessel affects a much larger area than its hull occupies. Engine noise overlaps the frequencies many whales use to communicate.
Whales respond by calling louder, repeating calls or falling silent, which reduces the range at which they can coordinate. The effect persists as long as the noise does.
This is why rules often specify engine handling, requiring vessels to slow well before the limit and to avoid shifting gears repeatedly nearby.
Approach geometry matters as much as range
Guidance generally requires parallel approach from the side and slightly behind, never head-on and never across the path of travel.
Positioning ahead forces the animal to alter course or dive. Approaching from behind at speed replicates the geometry of a pursuing predator.
Multiple boats create a problem no single skipper causes, since vessels spread around an animal leave it no unobstructed direction to travel.
Self-regulation fills the gap
Operators in busy ports commonly adopt voluntary codes stricter than the legal minimum, coordinating by radio to limit how many vessels are present.
The commercial logic is direct. A fleet that pushes animals out of a reliable area destroys the product it sells, so restraint is self-interested.
Enforcement against the occasional recreational boater is far harder, and in most areas that is where the majority of close approaches now originate.