Title: Impact of whale-watching vessels on humpback whale calling behavior on an Icelandic foraging ground during the Covid-19 pandemic
Authors: Amelie Laute, Thomas J. Grove, Marianne H. Rasmussen, Adam Smith, Olli Loisa, Michelle E. H. Fournet
Journal: MEPS (Marine Ecology Progress Series)
Blog written by Amelie
We have recently published a paper about one of our acoustic studies and would like to share the results with you! Here are the hard facts about the publication:
(Head to the bottom of the page for a visual summary of the paper!)
One of our key projects in Whale Wise is to find out how humpback whales may change their behavior in response to whale-watching vessels, particularly in Skjálfandi Bay. One way they might react is acoustically, changing their calling behavior, since whale-watching vessels make sounds underwater that can be heard by the whales. Discovering if whales do actually change their calls due to whale-watching vessels, is usually difficult to answer, due to a lack of comparable time without whale-watching (in science this comparable time is called a control). You could compare the calling behavior of the whales during whale-watching with nighttime, winter, or times when there is a storm, and no whale-watching is happening. However, we must consider that the whales might just call differently because their behavior could be different at night, or in winter. And storms themselves make a lot of noise, so these occasions are still not truly comparable to periods in which whale-watching would occur.
However, we recently had the opportunity to have a suitable control: during the Covid-19 pandemic the vessel traffic was reduced drastically, and we used this time to find out how the whales react to the vessel noise. We deployed a hydrophone (underwater recording device) during the pandemic in 2020 and used recordings of 2018 as a non-pandemic comparison. Additionally, we knew the number of vessels and in particular whale-watching vessels from their GPS positions (Automatic Identification System, AIS data). We first calculated the number of vessels and found that indeed the number of whale-watching trips decreased by two-thirds (68.6 %) from 42.1 trips per day in 2018, to 13.2 trips per day in 2020. We then manually looked at and listened to our recordings and also found that we heard vessels significantly less often (only 40.5 % of the times in 2020 compared to 60.3 % in 2018). In contrast, we heard a lot more whale calls in 2020 (45.9 % of the time compared to 23.6 % in 2018). We also heard whales more at night when there was more vessel sound during the day.
These results could still potentially be due to a natural change in behavior where the whales call more at night (e.g. due to feeding activities), or the higher number of calls in 2020 could just be due to more whales by chance. To figure out if there was a causal effect of the increase in calling with a decrease in vessel noise, we did some fancy statistical analysis (for experts: models including GLME, LME, and LRM). We found that the probability of detecting a whale call indeed significantly decreased with every additional whale-watching vessel within 5 km distance in both years, independent of the time of day! This made it more likely that the whales change their behavior as a response to the vessel’s presence. However, we had to make sure that the decreasing call rate wasn’t simply because we couldn’t hear the calls due to loud vessel noise.
Therefore, we did another analysis, where we only included times, where we knew there were humpback whales within 3 km of the hydrophone based on land-based surveys. By only including times with whales close by, we knew we wouldn’t miss a call due to loud background noise (we also only included times when the background noise wasn’t exceeding a certain limit). We found that it was more likely to hear a call when there were more whales around (obviously), but also that it was more likely for the whales to call when there was no vessel noise. So, it really seems like the whales call less in the presence of vessel noise.
We must be careful interpreting too much into these results. We don’t know if this reduction in calling effort is meaningful for the whales, or if it bothers them. But we do know that calling is an important way of communicating for the whales in an underwater world, where vision is difficult, while sound travels quickly and far. It would be therefore good if we try to keep the amount of vessel noise that we introduce into the ocean as low as possible to minimize the potential of disturbance.
There is also some good news for the whales: we measured the overall sound level in both years and found that Skjálfandi Bay is relatively quiet (for experts: around 90 dB re 1 µPa, RMS in the 25 – 1300 Hz band). Interestingly, this overall sound levels weren’t much different in 2020 compared to 2018, so apparently the whale-watching vessels don’t make the Bay much louder. This is great for the whales! It is still important to find out in more detail, if the whales change their calling behavior differently with varying vessel types or numbers of vessels present and longtermly, and whether this change in behavior is meaningful for the whales. We will continue with our research and hope to find out more about our impact on the humpbacks of Iceland!
If you want to look at this present study more in detail check out the paper here (https://www.int-res.com/abstracts/meps/v701/p159-173/ , if you by chance have a subscription to MEPS), or email Amelie (email@example.com) and she will send you a copy of the paper.
Impact of whale-watching vessels on humpback whale calling behavior on an Icelandic foraging ground during the Covid-19 pandemic
Acknowledgements: We acknowledge Aðalgeir Bjarnason and Charla Basran for hydrophone deployment, retrieval, and support. Thank you to Paul Wensveen for providing the deck unit and to Aran Mooney for providing the DSG recorder used to collect the 2018 acoustic data. We extend our gratitude to the whale-watching companies, whale- watching guides, the research center staff, and the people of Húsavík for the good collaboration and their support. Special thanks to Benjamin Hildebrand for the intellectual support and critical thinking. AIS data were provided by the Icelandic Coast Guard. Earthquake data were provided by the Icelandic Meteorological Office. Partial funding for this research was provided by an NSF-Fulbright Arctic Research Scholar fellowship awarded to A.S.