This study, led by Erin Ashe as part of her PhD project at the University of St Andrews, assesses the health of the population of Pacific white-sided dolphins found in the Broughton Archipelago, BC and nearby waters. This is a demographic study, which means that it uses statistical methods to study a population. The statistics are important, because only the most catastrophic problems are apparent to casual observation, and the goal of conservation biology is to identify whether there are human-caused problems, and if so, mitigate them well before they reach catastrophic levels.
Come to BC and spend some time here. If you’re lucky, you will see groups of hundreds of dolphins. When people see large groups of dolphins, the obvious, superficial interpretation is that the population(s) must be thriving. But that’s not always the case. Remember when we first started hearing about “dolphin-safe tuna”? Those measures were implemented because managers were concerned about levels of bycatch of other dolphin species in tuna fisheries. Spinner dolphins are designated as depleted under the US Marine Mammal Protection Act, even though there are about 613,000 spinner dolphins. That’s a lot of dolphins, but it is less than half of what there were before huge bycatch rates in tuna fisheries. For BC’s dolphins, yes, we see hundreds of dolphins in a group. And don’t get me wrong. Abundance is a hugely important parameter to measure, and a lot of our time is spent producing good estimates of wildlife abundance: it is an essential number to have if you want to know how much fish the dolphins need to thrive, or to estimate the probability that the population will go extinct. But it is tough to detect anything but the most catastrophic population crash from the rough abundance estimates we are likely to produce for these dolphins. The population could still be declining, and no one could “eyeball” a decline from 900 to 850 to 800 dolphins, or even measure it will all the uncertainty and variability in our estimates. Fortunately, there are other reliable ways to measure population health.
The method we’re using is to estimate survivorship, which is the probability of an individual surviving from one year to the next. If this number proves to be lower than what we expect from other, healthy dolphin populations, then we need to look at factors affecting survivorship. If the number comes out to be typical of dolphin survivorship, then management actions are not necessary.
We’re off to a tremendous start. Our good friend and neighbour, Alexandra Morton, has been collecting photographs of the dolphins’ unique markings and dorsal fin shapes since the dolphins returned to the area in the late 1980’s. Alexandra’s exhaustive field efforts and meticulous work has produced records that include thousands of dolphins. With this extensive catalogue and my photographic effort over the last 5 years, we now have the opportunity to learn essential information about Pacific white-sided dolphins and their populations.
Using photographs of unique natural markings on the dorsal fin and sometimes body, we create a record of encounters, an ‘encounter history’ that provides information about each time you saw that dolphin. Using mark-recapture statistics, you can estimate how many there are, the likelihood that an adult dolphin will survive from year to year and lots of other neat things. This helps make decisions about whether we need to take action to confer additional protections from human threats.
Imagine it as a trip to the doctor’s office. The doctor weighs you, takes your blood pressure, and maybe takes some samples. Then she compares your “status” to what she’d like to see in a healthy patient. In much the same way, we’re identifying whether dolphins in BC are surviving as long as we’d like to see. If not, then we’ll do some follow-up work to identify why, and whether we can do anything about it. Sure, dolphins are cool. We like them a lot. But we also like the scientific challenge of studying this species. We are only seeing a small fraction of individuals for brief periods at the surface. It’s forcing us to develop new mathematical tools, and spend lots of time in the field, and everything we learn feels new and exciting.