Ladd Irvine of Oand a team of researchers wanted to take a closer look at blue whale movements off California, an area which also contains a great deal of shipping activity. When looking at movement of any population, it’s really important to get as much historical data as possible because animals can vary their movements year on year. The team used 15 years’ worth of data collected from 171 tagged whales. These satellite monitored radio tags are pretty nifty pieces of kit, collecting the time and location of the whales at the surface and transmitting to satellites. With this wealth of data, the team set to work figuring out which areas of the Californian coast are particularly important to blue whales. Sure enough, some rather interesting patterns emerged.
Blue whales are attracted to the west coast of the US during the summer months to feast on the large numbers of krill found in the California Current System. The krill are super-abundant during this time as a result of upwellings that increase productivity in the region, and by currents and the bathymetry (equivalent to topography – how the sea floor looks) of the area that concentrate the krill into certain areas. As you might expect, the blue whales concentrated in areas where krill was most abundant. Early in the summer the whales tended to stay in the more southerly sections, where upwelling intensity (and thus productivity) is fairly moderate. During October and November the northern section of California experiences much stronger upwellings, and sure enough that’s exactly where the whales tended to hang out. The whales don’t just head up north around the same time every year though. Because the tracking data spanned 15 years, the researchers were able to compare movements over multiple years. The timing of the more intense northerly upwelling varies year on year, and it was quite apparent that the whales tailored their northerly migration in line with this variation. With variation in krill availability, you might expect that the whales would stay in California waters as long as krill abundance was high enough to give a decent meal. Interestingly, the whales didn’t really alter when they continued on their migration out of California in line with productivity timings. There was only one notable time when they did leave significantly later – in 2004. What’s so special about 2004? Well not much. There was a weak El Niño, but the researchers note that it didn’t really have an impact in the California Current System. 1999 was a super-productive year in the California Current System and no doubt prey was super-abundant. Did the whales hang out there for longer than usual? Nope!
So far so good, but the tracking data also revealed some potential issues. The blues tended to be most concentrated alongside areas where human population is high, and where there are busy ports. Busy ports means busy shipping lanes, which run straight through the places where the whales can be found. Now remember that ship strikes are thought to be relatively infrequent, but the question is how do we know if there has been a ship strike? Well first of all we have to know we have hit a whale. Sounds daft, but if you’re in a huge freight vessel you might not notice. You might also hit a whale and injure it (with unknown survival), or you might fatally hit a whale, and just not report it. We do have evidence that on the whole ship strikes are on the increase, and this paper makes a seemingly simple suggestion to reduce the likelihood of a strike on a blue whale: move the shipping lane during the months when the whales are migrating through. Of course such a move is an economic, political, and social minefield, but it’s not impossible – and there is a prescient. Twelve years ago shipping lanes in the Bay of Fundy, Canada were moved just 6 km away from a key North Atlantic right whale feeding ground. Since then, the right whale population has shown a small increase. Of course correlation doesn’t imply causation, but it is food for thought.
This paper is published in the open access journal PLoS ONE. Have a read of it here http://dx.doi.org/10.1371/journal.pone.0102959
Image: Blue whales aren’t the only whale species to be susceptible to ship strikes. This image shows an North Atlantic right whale (Eubalaena glacialis) who died after colliding with a vessel and suffering significant propeller cuts. Credit: NOAA National Marine Sanctuaries/Center for Coastal Studies (Public Domain Licence)