One fish, two fish, red fish, blue fish

November 28, 2010

When groups are small and individuals are distinctive, you may be able to count every individual in the group. When populations are big, or individuals are not distinctive, you need to estimate abundance from samples.

Some of the most pressing questions in conservation biology concern the health of populations.  “Population” refers to all the animals of a given species that live in the same geographical area and that form an interbreeding community.  Should a species be listed under some environmental legislation?  Should we allow hunts of healthy populations?  Is a species likely to go extinct?  These fundamental questions that inspire us rely on having good information about how many animals are in an area, and whether that number is going up or down.  We specialise in low-cost and creative methods to provide that information.

But why should anyone care that there are 9,120 harbour porpoise in BC waters?

Well, this number is a crucial piece of information to have if you want to assess whether the population can withstand the level of porpoise bycatch in fishing nets.  In some countries, this number takes on legal ramifications — if fishing practices are causing more bycatch than the population can withstand, it will lead to management actions to change fishing practices, reduce bycatch, or reduce fishing pressure altogether in extreme cases.

Still, we know that these numbers seem cold.  Jane Goodall’s chimps were named for a reason.  Actually, for lots of reasons.  We don’t want to know about thousands of whales.  People connect to individuals.  Moby Dick.  Flipper.  Shamu.  Willy (Keiko).  We get it, and we like studying individual whales and dolphins.  But a population is just a collection of individuals.  We study individuals to understand how populations are doing; and we have done coarse studies that estimate abundance overall, to guide future studies on the health of individuals.  The point is:  we use a lot of math in our research, but these animals are not just numbers to us.

We have expertise in the two most widely used methods to estimate abundance of marine wildlife:  mark-recapture statistics; and line transect surveys.   Mark-recapture studies take samples of individually distinctive animals, revisit an area and sample again, and use these samples to make inference about how many animals there must be in the population for us to see the individuals we do.  If you can track individuals through time, you can estimate other population parameters than just abundance:  you can also estimate survivorship, trends in abundance, reproductive rate, social structure and movement patterns.  We have conducted mark-recapture studies on humpback and killer whales, but the focus of our mark-recapture work is on Pacific white-sided dolphins.  Read more about the dolphins here:

Line transect surveys don’t require us to know anything about individual identity.  Instead, we estimate density of whales (or sharks: check this out).  The math is a bit nerdy, but we like it a lot.  We’ve contributed to the methods by testing new methods for distance estimation, survey design for geographically complex regions, abundance estimation from cheap platforms of opportunity, and have field-tested methods to estimate abundance of rare species.  We’re not mathematicians, but we like working with them on our most challenging studies, like Amazon river dolphins, Antarctic minke whales in the sea ice, or marine mammals in the convoluted fjords of BC’s Great Bear Rainforest.  Read more about the field on our colleagues’ page.

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