Science Advisory Report 2017/038

Assessing the risk of ship strikes to humpback (Megaptera novaeangliae) and fin (Balaenoptera physalus) whales off the west coast of Vancouver Island, Canada

Summary

• Baleen whales are at greater risk of being struck by large ships than other marine mammals because of their large body size, limited ability to manoeuvre away from oncoming vessels and because of behavioural characteristics that likely contribute further to their vulnerability.
• Southwest Vancouver Island, Canada is the approach corridor to major ports. The route into and out of Juan de Fuca Strait is regularly transited by large, deep sea vessels. Studies have demonstrated that strikes from ships travelling at speeds exceeding 12 knots are likely to cause mortality, whereas strikes from ships travelling at speeds ≥18 knots are almost certain to be fatal.
• The spatial distribution of whales and ship traffic was compared to identify areas where strike risk is greatest. Risk was expressed as the relative probability of a ship and a whale occurring in the same “space”.  Ship speed was used to calculate the probability that a collision would be fatal to a whale.
• During 2012 through 2015, 34 systematic aerial surveys were conducted in fall and winter to predict relative densities of humpback and fin whales over a region overlapping the shipping corridor off the west coast of Vancouver Island.
• Humpback whales were primarily observed on the continental shelf, with highest occurrence along the shelf edge (~200 m depth contour), fin whales were observed to occur off the shelf in deeper water (>400 m depth).
• Blue whales are also vulnerable to ship strike, but their distribution could not be quantified because of very low numbers of sightings.
• Ship traffic density and speed in the study area was available in a spatially compiled format for an entire year (2013).
• For humpback whales, the highest risk of whale-ship encounters (strikes) was predicted to occur along the continental shelf break at the 200 m depth contour, and inside western Juan de Fuca Strait. For fin whales, the regions of highest predicted risk were in a corridor offshore of the shelf break and west of the entrance to Juan de Fuca Strait, as well as inside western Juan de Fuca Strait.
• The mean probability of a fatal collision was predicted to be slightly higher for fin whales than humpback whales even though fin whales occurred at much lower densities than humpback whales.  This is likely related to the primarily offshore distribution of fin whales, which exposes them to marine traffic travelling at higher mean speeds.
• Ship strike mortalities in British Columbia may have a greater impact on fin whales because the population is smaller than the humpback whale population and because they are present year-round.
• Even in the areas of highest predicted risk for whale-ship strikes, the estimated probability was less than a 1% chance for either species. However, the impact of this risk on the populations cannot be determined until the abundance of these species in the study area is known.
• The ship strike models likely underestimate true collision risk because they did not take into account possible species-specific differences in vulnerability.
• It is reasonable to expect that the future risk of ship strikes will increase in the study area as a result of greater traffic, larger ship sizes, and growing whale populations. However, it is not possible to estimate future risks without having more specific information on the factors influencing risk.

This Science Advisory Report is from the February 23-26, 2016 National Marine Mammal Peer Review Committee (NMMPRC): Part II. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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