Tuesday, 9 August 2011

Braking for whales

In keeping with last Thursday's whale theme, today's post also looks at whale collisions with ships. This time around, there is new research that provides some suggestions for how to minimize encounters--especially lethal ones--between whales and ships.

The work was performed in the Stellwagen Bank National Marine Sanctuary, a 2181-square-kilometer marine protected area in the southern Gulf of Maine.

(Map of the study area.)

The Sanctuary is "seasonally dense" with endangered North American right (Eubalaena glacialis), humpback (Megaptera novaeangliae), and fin (Balaenoptera physalus) whales. It is also a main thoroughfare for ships bound for, or returning from, Boston. Furthermore, the Sanctuary's boundaries overlap with areas that have been designated as "Critical Habitat" for right whales, as well as 2 areas that have been designated as right whale Seasonal Management Areas (SMAs) by the National Oceanic and Atmospheric Administration (NOAA). In other words, it's an important place for both whales and shippers, and so it is vital to find management practices that are beneficial to, and sustainable for, both parties.

In the SMAs, there is a speed restriction of 10 knots or less for vessels of a certain size, but this rule expires in 2013. In order to suggest optimal management practices for beyond this expiry date, collaborators from NOAA and the International Fund for Animal Welfare used mathematical modeling techniques to determine which speed limits are likely to most reduce the probability of lethality (for a whale) from collisions with ships. Their hope was that their findings would be useful not only in SMAs, but also throughout the Sanctuary and, potentially, in other marine habitats where whales can be found.

(A cargo vessel, the most common form of traffic in the Sanctuary--and also the fastest.)

To conduct their analyses, the scientists took advantage of ship traffic and speed data from the U.S. Coast Guard's Automatic Identification System (AIS). This allowed the researchers to examine which speeds and ship types were associated with lethal whale hits across the sanctuary. They then applied hypothetical speed limits of 16, 14, 12, and 10 knots and re-calculated the probabilities of lethality according to previously-generated figures describing the circumstances under which whales are likely to die, rather than be injured, during a collision.

Across the Sanctuary, average ship speed was 13.5 knots--3.5 knots more than the limit imposed in the SMAs. On average, whales were likely to be killed by collisions 67% of the time. This was reduced by 56.7%, to only 0.3%, when the researchers applied their hypothetical speed limit of 10 knots. However, it was only reduced by 3.7% under a hypothetical cap of 16 knots. Clearly, the whales would be safer in an environment where ships were moving more slowly.

Since the fastest-moving vessels were cargo/container ships (which, on average, travel at 16.2 knots), the whales would also be better off in habitats where these are kept to a minimum. Unfortunately, these are currently the most common type of vehicle in the Sanctuary, making up 28% of the boat traffic there.

(Tail fin of a North Atlantic right whale, Eubalaena glacialis)

The authors of this study are well aware that all human traffic cannot be diverted around the entire Sanctuary, as this could have severe negative economic consequences. However, they did not feel comfortable recommending mitigation techniques such as sonic deterrents (to the whales), active sonar or human observers for whale detection, or acoustic detection buoys, as each of these also has drawbacks. Thus, it appears that the best choice, at least for now, is to regulate the speed at which vessels can travel through Sanctuary waters--especially during the times of year when endangered whales are most commonly found there. If ship operators balk at the 10-knot limit, perhaps they can be convinced to at least abide by a 12-knot limit, which successfully reduced lethality by 29.4%. That offers only about half the protection of the 10-knot cap, but is still 10 times better than the 16-knot alternative. In other words, it's better than nothing.

Wiley, D.N., Thompson, M., Pace III, R.M., and Levenson, J. 2011. Modeling speed restrictions to mitigate lethal collisions between ships and whales in the Stellwagen Bank National Marine Sanctuary, USA. Biological Conservation 144:2377-2381.

Thanks to the following websites for providing the photos used in this post:

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