SHIP STRIKE RISK TO WHALES AROUND SOUTH GEORGIA/ISLAS GEORGIAS DEL SUR
Russell Leaper, Manuela Bassoi, Danielle Buss, Susannah Calderan, Emma Carroll, Martin Collins, Paul Ensor, Amy Kennedy, Paula Olson, Jennifer Jackson
Sightings of large whales at South Georgia/Islas Georgias del Sur (SG/GS) have been increasing in recent years as they recover from severe depletion by 20th century industrial whaling. As cruise ship traffic around SG/GS has also increased, there is a need to address the risk of collisions between ships and whales. Here, we use sightings data collected during a research survey around SG/GS in summer 2020 to calculate whale density, together with AIS data on shipping traffic to estimate shipping density (in terms of km travelled per km2), in order to examine ship strike risk and possible mitigation options. During 2430 km of visual transects, there were 337 sightings of humpback whale groups totalling 661 individual whales. The resulting average estimated density was 0.089 individuals.km-2. Densities were particularly high across the shelf to the north of the island, which corresponds to the area most used by cruise ships. There were also 38 sightings of 58 individual blue whales with a resulting density estimate of 0.009 individuals.km-2.
Shipping is dominated by fishing vessels and cruise ships which are separated seasonally. Fishing is concentrated in the winter months while cruise ships visit in summer and follow predictable routes along the north coast of the island, visiting designated landing sites. A simple encounter model was used to estimate a risk index based on whale and cruise ship densities in the summer 2020. This index was the estimated annual number of lethal ship strikes from cruise ships assuming no avoidance by the vessel or whale, and was estimated at 28 humpback whales and 1.5 blue whales. These should not be considered as estimates of likely mortality but do show that in a global context, SG/GS could be considered a high-risk area based on the whale densities observed in 2020.
As it is unlikely that there would be any particularly important areas for humpback whales that could be avoided by simple routeing measures, we suggest consideration of speed restrictions for vessels in SG/GS waters. At present about 35% of the distance travelled by cruise ships on the SG/GS shelf is at speeds less than 10 knots which suggests that it may be possible to limit all speeds to less than 10 knots without huge inconvenience to schedules through careful voyage planning. IAATO cruise ships in areas around the Antarctic Peninsula adhere to a 10 knot speed restriction to mitigate ship strike risks, primarily because of concern about humpback whales. The SG/GS and South Sandwich Islands Marine Protected Area Management Plan includes objectives to manage human activities including shipping to minimise impacts on the marine environment. Slower ship speeds can also have other environmental benefits of lower emissions and reduced underwater noise.