Population structure of blue whales in the North Pacific

Do some blue whales belong to different “countries” that speak different “languages”? This idea of how blue whales are broken down into separate groups is called “population structure.” How would you figure out the population structure? Before going into that, we need to take a step back and look at what information we have on the whales.

Whaling vessels had on-board biologists which kept remarkably detailed and accurate data on thousands of blue whales over the decades of exploitation. Most of what we know about their basic biology comes from the data: how long they are, when they reach sexual maturity, what they eat and when, differences between males and females, etc. (The whaling data offer great value, and I think it would be a shame to not learn as much as possible from it, but that’s a different topic and I digress). Whaling for blue whales ended in the 1970s – what have we learned since then? A lot, it turns out, and without killing any blue whales for research. One population of blue whales feeds within miles of the coast of southern California, and return there summer after summer. This “California” population is a godsend to biologists trying to peer into the world of this mysterious animal, because normally they are too far offshore to easily study. The consistent proximity lets dedicated biologists execute a wide variety of research studies, many of which we will write about later. It may sound a bit strange at first, but it’s hard to find and study the largest animal on the planet.

One reason it’s hard to find them, is because their habitat is the Pacific Ocean—something we often overlook. Don’t believe me? Just look at any standard global map: the continents are square in the middle and the Pacific Ocean is a little strip on the outer edges of the map. This does the ocean a great injustice! It turns out the Pacific Ocean is absolutely enormous – it covers roughly 1/3 of the planet! See for yourself. From this perspective it’s the continents that only make up a sliver on each side. Don’t get me wrong, I understand the reason we terrestrial creatures ignore the Pacific Ocean, but for blue whales the ocean is the center of the map. The point here is that blue whale habitat is enormous and has no physical barriers. It’s no wonder then that blue whales were caught throughout the Pacific Ocean (and all oceans, but today we’re focusing on the Pacific).

But it raises questions: How far do the California blue whales go when they disappear in the winter months? Do all blue whales in the Pacific belong to the population? After all they looked the same to whalers. It’s clear physical obstacles, like a continent, could prevent blue whales in different oceans from interacting and breeding. But there could also be whales that share habitat during part of the year, but don’t breed because they migrate to different breeding locations in the same ocean. Which populations exist and where (the population structure) remains an important open question regarding blue whales.

It’s an important question because each population must be considered and managed separately when tracking their numbers over time (something we call “population dynamics”). For instance, whales caught or accidentally killed in one population will have no impact on the population size of another. Likewise, two populations may have a dramatically different status: one might be critically endangered while another is nearly rebuilt.

It’s also a difficult question to answer. How do we prove distinct populations exist? Up until the 1990s there wasn’t much to go on besides intuition and speculation from looking at whaling records. But in the late 1990s a series of studies lead by Kate Stafford [1-5] came out with a fascinating glimpse into the world of the blue whale in the North Pacific. The scientists used underwater microphones (hydrophones) to record the sounds made by whales over time, across big swaths of the central and eastern North Pacific Ocean. They found something remarkable: there were two distinct blue whale “calls” – it was as if there were two dialects of the blue whale language (we’ll call them the “western” and “eastern” call types). Further, there were clear patterns in when and where these calls occurred. Only the western call occurred in the Aleutian Islands in Alaska, while only the eastern call was only heard off California and near the equator. Perhaps the most interesting finding is that in the Gulf of Alaska, no whales were detected calling in the winter, but both call types were heard there in the summer and fall.

By looking at when the calls occurred and where, it suddenly made sense that there were two populations that migrated to the Gulf of Alaska for the summer, and then migrated back home for the winter. The western whales seemed to disappear off the radar (or in this case hydrophones) toward Russia and Japan, while the eastern whales seemed to go south toward the equator. The idea of distinct populations made sense, and was further confirmed by a study in 2008 [6] that looked at lengths from whaling records and recent aerial photographs. The researchers found that, on average, blue whales from the western half of the North Pacific were longer than those in the eastern half. The picture of population structure was starting to become a little clearer. If there really are two populations which breed on different sides of the largest ocean on the planet, they would face different environmental pressures for survival (the temperature of the water they spend their time in, their food sources, etc.) and their bodies would adapt accordingly with enough time. But we would expect their social behavior (like different language dialects) to continue to change and adapt as well. So it was pretty convincing there were two populations.

But wait, how do we know that all of the eastern whales are all from one population? Is it even possible that whales could go from the equator to Alaska to feed for a few months? There could be multiple eastern populations that were more similar, so that the lengths and calls were the same, but still distinct. This question was pretty well put to rest after extensive satellite tag tracks were published in 2010 [7, 8]. Blue whales tagged off California zipped up to the Gulf of Alaska and down to the equator off Central America, with some also going into the Gulf of California. So it seems likely that there is one population that spreads out to feed throughout the year. But these tags have also shown us (so far) that blue whales do not cross the equator or go past Hawaii all the way west to Japan and Russia. So we learned that the California population in reality was a subset of the whales stopping to feed offshore and the “eastern North Pacific” (ENP) population would be a better name. This population was the focus of my Master’s thesis.

Blue whale tracks

Figure 1a from Bailey et al. (2010). Shows the tracks of blue whales tagged off California. Individual whales move through the entire eastern North Pacific ocean.

So what do we know about the population structure in other areas? In the western North Pacific, we know there is at least one population that spends a significant portion of their time, including the mating season, too far west for our hydrophones to hear them. The truth is no one knows what populations exist (or existed) in the western North Pacific. Thousands of whales were caught off Japan early in the 1900s, but none have been seen since. Was that a population that was killed off forever? Where do the blue whales go during the winter months to breed? These are exciting questions for future researchers to tackle.

 

References:

  1. Stafford, K.M., C.G. Fox, and D.S. Clark, Long-range acoustic detection and localization of blue whale calls in the northeast Pacific Ocean. Journal of the Acoustical Society of America, 1998. 104(6): p. 3616-3625.
  2. Stafford, K.M., S.L. Nieukirk, and C.G. Fox, Low-frequency whale sounds recorded on hydrophones moored in the eastern tropical Pacific. Journal of the Acoustical Society of America, 1999. 106(6): p. 3687-3698.
  3. Stafford, K.M., S.L. Nieukirk, and C.G. Fox, An acoustic link between blue whales in the eastern tropical Pacific and the northeast Pacific. Marine Mammal Science, 1999. 15(4): p. 1258-1268.
  4. Stafford, K.M., S.L. Nieukirk, and C.G. Fox, Geographic and seasonal variation of blue whale calls in the North Pacific. Journal of Cetacean Research and Management, 2001. 3(1): p. 65-76.
  5. Stafford, K.M., Two types of blue whale calls recorded in the Gulf of Alaska. Marine Mammal Science, 2003. 19(4): p. 682-693.
  6. Gilpatrick Jr, J.W. and W.L. Perryman, Geographic variation in external morphology of North Pacific and Southern Hemisphere blue whales (Balaenoptera musculus). Journal of Cetacean Research and Management, 2008. 10(1): p. 9-21.
  7. Calambokidis, J., et al., Insights into the population structure of blue whales in the Eastern North Pacific from recent sightings and photographic identification. Marine Mammal Science, 2009. 25(4): p. 816-832.
  8. Bailey, H., et al., Behavioural estimation of blue whale movements in the Northeast Pacific from state-space model analysis of satellite tracks. Endangered Species Research, 2010. 10(1-3): p. 93-106.

 

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