Counting animals and estimating distance are two difficult tasks at sea. Counting whales and dolphins can be a daunting task especially when they are feeding or diving. Most marine mammals spend more than two-thirds to 90% of their time underwater which makes counting impossible without eyes underwater. Sighting records are known to be very subjective to the observer. Just ask the people around you on your next sighting and see what answers you get! The short video clip above shows that what you see at the surface may only be a small percentage of the animals underwater.
It is best to give a range in cases of uncertainty. Usually we count the maximum number we see with certainty at the surface at any given time (lower end of range) and with experience may add a factor of say one-third or two-thirds (upper end of range) depending on the species and its behaviour. Alternatively, if we get a minimum count of say 100, then we may just say 100+.
Keep in mind that a blue whale that dives may re-surface elsewhere 8-20 minutes later and should not be added to the minimum count. If uncertain the individual is added to the maximum count. At the end of the sighting period a best estimate is recorded, subjective to the observer.
Even experts make errors in counting. For example, during an initial study of a pod of humpback dolphins feeding the count was initially recorded as 4, 5 and 6 on three different days, each with a sighting duration of over 2 hours. Later, photographic evidence showed that there were five dolphins in all three sightings.
One benign method used by researchers to count whales & dolphins is photo identification of the flukes, dorsal fin and/or body. Survey methods also use cue counting, line transects, passive acoustic monitoring (PAM), sight-resight method, aerial and others.