Do ship strikes threaten the recovery of California blue whales?

Do ship strikes threaten the recovery of California blue whales?

This question motivated my thesis, and in this post we’ll discuss the question, and then answer it. The question was raised because a dedicated group of scientists have been estimating the number of California whales since the early 1990s [1]. These scientists found that from 1993 to 2008 there was no clear sign that the population had increased. We would expect the population size to go down during whaling, and then come back up (recover) once whaling has stopped – similar to how we expect the grass to grow back after we mow the lawn. In fact that is what’s happened to other populations and species: they are recovering to pre-whaling levels. But the California population shows no signs of this, to the distress of scientists and conservationists. So why don’t we see recovery? For many years the main suspect was that we haven’t stopped killing blue whales: we are actually killing them incidentally through fatal ship strikes.

Ships striking and killing whales has been a documented occurrence since the late 1800s, however it was infrequent until about 1950 [2]. Now, with the number of large vessels increasing and moving at faster speeds, it has become a pressing issue. The issue came to the forefront with the California blue whales in 2007 when four whales were mortally struck, something the US government deemed an “Unusual Mortality Event” [3]. For North Atlantic right whales, ship strikes are a serious concern, and much time and energy has gone into research and mitigation techniques for this population. Is it possible ship strikes are preventing the California blue whales from recovering? This ‘ship strike’ hypothesis makes sense intuitively, and has precedent in other populations. If ships are striking and killing whales faster than they can reproduce, then the population won’t be able to recover.

Just how many blue whales are killed by ships anyway? Well, that’s a tricky question. A few things need to align to observe a ship strike: it has to either be noticed at impact or detected later, the carcass needs to be correctly identified, and a necropsy done if there is no obvious evidence for physical trauma. As such, we wouldn’t expect to observe and record all struck whales, especially since blue whales sink initially after dying. In any case, because of a low chance of detecting dead blue whales, observed ship strikes are certainly lower than the actual numbers. We considered 10 per year as a reasonable starting place, but also considered higher numbers in case that was too low (our results didn’t change, so we focus on the case with 10 per year). This ship strike hypothesis has been on the radar of scientists and conservationists for decades, but it is difficult to predict just what kind of impact they are having on the population.

First we need to take a step back and talk about what ‘recovery’ actually means in the scientific world. It is related to an important ecological concept called carrying capacity. Carrying capacity is the amount of animals (blue whales in our case) that the environment can hold or “carry”. It’s not entirely clear what limits blue whales naturally, but clearly their population cannot grow forever or the ocean would be overflowing with whales! Food, space on breeding grounds, disease, etc. are issues that probably limit their growth in some way. For our purposes, a carrying capacity is the number of animals we would expect to see without any human intervention. As such, ‘recovery’ is the process of getting back to the carrying capacity. The US government, in the Marine Mammal Protection Act, defines a population as ‘recovered’ if it is above 60% of the carrying capacity. So for example if a population has a carrying capacity of 10,000 but there are only 5,000 individuals, then they are at 50% and would not be considered recovered. If there were evidence the population was growing, we would say they are recovering. So to know whether the population is recovered, recovering, or failing to recover, we need to know the carrying capacity.

Estimating the carrying capacity was a major goal of my research. We used a model that tracks the number of animals over time and calibrated it with data on past whaling catches, current ship strikes, vessel traffic patterns, and basic blue whale biology.  Once calibrated, the model gave us estimates of carrying capacity and the historical population trend.  We also used the model to project the impact of ship strikes into the future as vessel traffic grew.

Let’s come back to our original question: “Do ship strikes threaten the recovery of California blue whales?” The answer, according to our analysis, is no.  They aren’t recovering because they’ve already recovered! We estimated they are at 97% of their carrying capacity. So it’s no wonder we haven’t seen their abundance grow over time – they’re limited by their natural environment. Ship strikes undoubtedly impact this population, we just found that it did not have much of an impact on their abundance.

So what does that mean for them now? Well that’s a tricky question. There are concerns (rightfully so) that the level of ship strikes are too high, and are almost certainly above legal limits. Our findings aren’t meant to undermine the protection these creates have. In fact, the only reason they’re recovered is because we did enact protections and stopped whaling.

So how about the proposed mitigation strategies, like moving shipping lanes or slowing ships down? Management decisions for mitigation are complex and difficult to make because they balance competing interests. Naturally we don’t want any blue whales killed by ships – any whale saved is a positive. But what if mitigation action against blue whale ship strikes impacts other species or has broader issues like increased carbon emissions? Our paper cannot provide the best solution for how to balance such issues, only the facts to help guide that decision. In this case all we’ve shown is that there is no immediate population level threat to these blue whales. Personally, I believe the monitoring of ship strikes and population abundance is vital for gauging the health of this population moving forward.

References:

  1. Calambokidis, J., et al., Photographic identification of humpback and blue whales off the U.S. West Coast: Results and updated abundance estimates from 2008 field season, S.F.S. Center, Editor 2009.
  2. Laist, D.W., et al., Collisions between ships and whales. Marine Mammal Science, 2001. 17(1): p. 35-75.
  3. Berman-Kowalewski, M., et al., Association between blue whale (Balaenoptera musculus) mortality and ship strikes along the California coast. Aquatic Mammals, 2010. 36(1): p. 59-66.

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