Abundance Estimations for Marine Mammals: nailing the numbers

For marine mammals, ideally their numbers would be the opposite!

For marine mammals, ideally the numbers would be the opposite!

After reading about abundance estimation, I have a new found respect for the population estimates of marine mammals… the all important numbers that tell us how certain species are doing and whether they are recovering or declining. These seemingly plain numbers fail to convey the detailed back story involved for each total.

Counting animals who spend most of their time in the water is challenging. For example, a pod of dolphins is spotted in the water and someone on a boat manages to count ten dolphins, but those dolphins dive and another set of dolphins surfaces but then they dive down before the count is complete. Now how many dolphins were there? See the challenges here people?

Thankfully, scientists spend tons of time taking into consideration every possible scenario that could alter a count so we don’t have to worry about it. In the Encyclopedia of Marine Mammals’ article, authors Stephen Buckland and Anne York go into great detail explaining how the scenario I describe above is accounted for during line transect sampling by identifying a “cluster” and “after detection, search effort ceases, and the vessel closes with the detected cluster, to allow more accurate estimation of cluster size.”

What does this really mean? Line transect sampling is fancy scientific lingo for a survey designed with lines. Basically, researchers decide how to best survey an area for marine mammals by mapping out lines to follow in the water in order to cover an area thoroughly yet without overlap. Once a cluster of marine mammals is spotted while traversing one of these predetermined lines, the researchers temporarily abandon the line and get closer to the animals to get a better look and count.

But this is just one of many scenarios described and one of many abundance estimation methods used. Other methods include mark-recapture, photo identification, migration counts and colony counts, but each of these has specific pros and cons that are taken into consideration with great care. Not to mention all the assumptions explained, such as “if one animal in a cluster is detected, it is assumed that the whole cluster is detected.”

The point is that scientists take great pains to make sure that the population numbers we see are accurate. They make sure no stone is left unturned, no line is left untraveled and no marine mammal is left uncounted.


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