The North Atlantic Marine Mammal Commission


Upcoming events:


Frequently asked questions

1. Question: Do surveys count all the whales in the survey area?

Answer: No they don’t. The survey design usually allows only a small proportion of the total survey area to be viewed by observers. Therefore they are likely to see only a similar proportion of the total number of whales in the area. Total abundance is estimated using the density of animals on the transects.


2. Question: It is easier to see whales that are close to the observer than those that are farther away. How do you account for that in estimating abundance?

Answer: Most modern surveys use a method called “Line Transect”. In such a survey, the observers measure or estimate the perpendicular distance to every group of whales they see. As expected, in most cases more whales are seen nearby than farther away, simply because it is more difficult to see things that are far away. But some species of whales are large and easy to see, whereas others are small and hard to spot. For example, a fin whale is much easier to see, and can be seen from farther away, than a minke whale. As a result, the distribution of perpendicular distances is different for each species. Analysts use this distribution to estimate the "effective strip width" for each species. The effective strip width corresponds to the width of the strip that is actually surveyed by the observers on the ship or plane. As you would expect, effective strip width tends to be narrow for species like minke whales that are small and hard to see, and wider for species like fin whales which are large and easier to see from a distance.

Effective Strip Width

Strip1 cropped

The effective strip width (dashed lines) is estimated for each species based on the frequency distribution of perpendicular distances to sightings.

Strip 2 for web

Effective strip width for minke (small) and fin (large) whales. Fin whales can be seen from farther away than minke whales.



3. Question: Observers are not perfect, and even the best observer is going to miss seeing some whales. How do you account for whales that are missed by observers?

Answer: The line transect method assumes that all animals close to the transect are seen by observers, but human observers do miss whales, even when they are nearby. This problem is usually addressed by having two separate teams of observers operating from the same ship or plane. The two teams usually operate independently, meaning they don’t communicate with each other about sightings. Often the teams see the same whales, but sometimes one team records a sighting that is missed by the other. The proportion of missed sightings for each team, along with the distance data gathered by the teams, enables the analyst to estimate the proportion of visible whales that were missed during the survey. This is used to correct the estimate of abundance.


4. Question: Some whales stay underwater for long periods of time, and would not be visible to observers when the ship or aircraft passes by. How do you account for these “missing” whales?

Answer: This is indeed a very difficult problem for whale surveys. Whales that are diving when the ship or plane passes by are invisible to observers and cannot be counted. This problem is dealt with in two ways:

  • For species that don’t stay under water for long periods of time, like minke whales and harbour porpoises, the issue can be addressed by having two sets of observers with different viewing fields. On a ship survey, there might be one set of observers using powerful binoculars to search far ahead of the ship, and another set of observers using only their eyes to search relatively nearby. On an aerial survey, the plane might circle around after a whale is sighted and survey the same part of the transect again: a so-called “racetrack” survey. The sightings of these two sets of observers (or one set of observers two times in the case of racetrack) are compared and sightings of the same group of whales (duplicate sightings) identified. The analyst uses these data to estimate the proportion of whales that would have been missed if there had been only one set of observers searching close to the ship, and this is used to correct the estimate of abundance.
  • An alternate approach uses information on the diving habits of the target species, such as the proportion of time the whale spends at or near the surface, and the frequency of dives. These data are usually obtained in external studies, often involving the application of satellite or radio tags which record and transmit information on the activities of the animal. This information is combined with data from the survey to estimate the probability that an animal would have been visible (or not) during the passage of the ship or plane, and this is used to correct the estimate of abundance.


5. Question: Don’t ships and airplanes scare the whales away?

Answer: This is more of an issue for ships than for aircraft, because aircraft move so quickly that a whale does not have time to flee even if it does hear the plane. Whales can have differing responses to ships. For example, some species of dolphins are attracted to ships and approach them, even surfing on the ship’s bow wave. Other species do tend to leave the area when they sense the approach of a ship. Still others, such as fin whales, do not seem to react to ships much at all. Either type of responsive movement – aversive (fleeing) or attractive (approaching) – can be problematic because it can lead to underestimation of density with aversive movement or overestimation with attractive movement. The problem is addressed in the same way as that of diving whales, by having two sets of observers with different viewing fields. In this case, the distribution of perpendicular distances for each team is compared to determine if responsive movement has occurred. For example, if attractive movement is occurring, the team using naked eyes only will tend to see a greater proportion of animals close to the transect, because the whales are approaching the ship after being sighted by the team using binoculars. In some cases, the team using binoculars actually tracks individual pods of whales to determine how they respond to the ship. The analyst uses these data to adjust the estimate of effective strip width to account for the responsive movement observed.


6. Question: Whales move around, and could move between transect lines during the survey. How do you know you are not counting the same whales again and again?

Answer: It could happen that the same group of whales is counted twice during a survey. But it is assumed that the whales move in such a way that they are equally likely to be not counted at all. For example a whale might move near to a transect immediately after the ship or plane has passed. Therefore the movement of whales, if it is random, should have no overall effect on the number of whales seen. Whale movement can be an issue if a survey is conducted during a time when the whales are migrating. In such a case, the survey proceeds in a direction opposite to the movement of the whales, so that the survey does not count the same animals multiple times.