When making behavioral observations of your study animal, it’s important to minimize the impact your presence has on the actions of your subject. As a doctoral student at the University of Hawaii at Manoa studying the habitat-use patterns of spinner dolphins in the Main Hawaiian Islands, it is important to remain as inconspicuous as possible, so I can collect accurate data of how dolphins move between their environments. However, as an observer it’s difficult to hide on the open ocean, especially when you make a bunch of engine noise and bubbles as you approach an acoustically sensitive animal. Dolphins are curious creatures who will often change what they are doing to investigate the arrival of a boat. In this case, passive monitoring—one in which only the instrument and not the observer is present—is ideal. I reduce my impact on spinner dolphin behavior, by rarely being there at all.

Hawaiian spinner dolphins (Stenella longirostris) are a small species of dolphin found throughout the Hawaiian island chain (Norris et al., 1994). At night these social animals forage on fish, shrimp, and squid in the waters along the sloping volcanic coastlines (Benoit-Bird et al., 2001; 2003). During the day spinner dolphins return to nearshore waters, often moving into shallow and calm bays, to rest (Norris et al., 1994). Dolphin “sleep” looks very different from our sleep; resting dolphins form small, tightly-spaced groups that swim slowly and in synchrony. They don’t snuggle up to slumber on the ocean floor because they have to (1) go to the surface to breath, and (2) keep an eye out for hungry tiger sharks. 

The downside of this daily routine is that it places dolphins in close proximity to human activity that can interrupt their rest. Cumulative effects of vessel traffic and swim-with-dolphin tours can reduce their foraging success at night (Courbis & Timmel, 2009; Heenehan et al., 2016a, 2016b; Danil et al., 2005). My research aims to identify areas throughout the Main Hawaiian Islands that are important resting areas for spinner dolphins. This knowledge can inform management of areas where human interactions with dolphins should be monitored to ensure harassment doesn’t occur during the day, so that dolphins can get enough rest to hunt successfully at night.

In order to determine when and where spinner dolphins rest, I use a combination of vessel surveys and passive acoustic monitoring. As part of my master’s thesis I spent two weeks during the summers of 2016 and 2017 on Maui conducting vessel surveys with my advisor, Dr. Marc Lammers, the research coordinator for the Hawaiian Islands Humpback Whale National Marine Sanctuary. We started out of Lahaina Harbor in West Maui around 9:00 am and zig-zagged between Maui and Lanai scanning the waves for shiny dorsal fins breaking the surface. When we found dolphins, we monitored the pod (group of dolphins) from a distance for as long as possible, noting their group size, location, behavior (milling, traveling quickly, or jumping out of the water), and sea-state which indicates how rough the water is and therefore, how well we could see dolphins. Estimating group size is a skill that develops with experience; rough approximations are made by assuming that the number of animals seen at the surface at any given time represent about one quarter of the total pod. We’d often stay with the dolphins until around 5:00 pm as they rounded the bottom of Lanai and headed for deeper waters. 

Vessel surveys provide excellent confirmation that a particular species is in a particular location, but there are limitations. If the sea-state is too rough, it can be difficult to spot the low profile of spinner dolphins. Weather can also prevent us from taking the boat out at all. We’re also limited to day-time hours and only see dolphins during the rather fleeting moment when they break the surface to breathe between 1-5-minute dives. Passive acoustic monitoring is a lovely complement to visual data, as it takes advantage of the vocal nature of spinner dolphins.

Spinner dolphins make clicks, whistles, and burst pulse buzzes (similar to blowing raspberries) to navigate and communicate. Therefore, acoustic recordings containing dolphin signals (sounds) indicate that dolphins were present at that location at a particular time of day. For my master’s thesis and my current PhD project, I use Ecological Acoustic Recorders, which conveniently forms the acronym, EARs. EARs are autonomous acoustic recorders that contain a hydrophone (an underwater microphone), solid state drive ( storage for the recordings), and large battery pack protected in a water-tight PVC pipe. Each deployment begins by hooking up the EAR to a laptop and programming when the device should start recording, at what frequency, and how long each recording should be.

I program my EARs to make 30-second recordings every 5 minutes for 6 months or until the batteries die. By making shorter recordings every 5 minutes, I am able to regularly sample the soundscape through different seasons, while balancing the battery life of the EAR.  We then seal the device with o-rings and a couple of bolts and head out on a boat to the dive site. The number of dive sites depends on how much detail, or resolution, I want within an area as well as how many instruments I have available. For my PhD project, I selected two well-known resting bays off Oahu, two in Maui Nui, and two off of Hawaii to get a general idea of how the dolphin acoustic patterns in these areas may vary.  At the dive site, a concrete block molded from a plastic tote to act as a mooring anchor is lowered over the side of the boat 60-100 feet to the sandy bottom. SCUBA divers take the EAR down to the mooring and secure it with hose clamps so that even with ocean currents the EAR would stay put. Once deployed, I cross my fingers and hope that when I return in 6 months the EAR is still there, still sealed, and full of recordings.

EARs record anything that makes a sound whether it’s dolphins, snapping shrimp, damselfish (they sound like purring cats), whales, or anthropogenic sounds like vessel engines, scuba divers, construction, or conversations if the instrument is still running when it’s picked up after the 6 months. These data provide a record of the ocean soundscape at any given time and have a multitude of applications beyond my own project. If we can identify the source of a sound—usually because we have past recordings made with visual confirmation of the source—then we can use passive acoustic monitoring to determine how often that sound occurs. When I’m not conducting vessel surveys, I spend much of the other 50 weeks of the year behind a computer quantifying the dolphin acoustic activity in each recording. From that, I can look at daily, seasonal, spatial, and historical trends in dolphin signals such as: during what time of day are dolphin acoustic signals detected in a bay, if those times change throughout the year, and if the amount of signals has increased or decreased over the years suggesting an increase or decrease in the number of dolphins present (or at least making noise) in an area. I hope that by eavesdropping on dolphins and underwater activity, I can better inform management of which areas are important to protect from human impacts.

Photo credits indicated in the bottom left of the photo

Megan McElligott recently completed her master’s degree in the Marine Biology Graduate Program at the University of Hawaii at Manoa, and has continued within the program as a 1^st year PhD student with Drs. Adam Pack and Marc Lammers. She also collaborates with the Maui-based non-profit, Oceanwide Science Institute. Megan’s research uses passive acoustic monitoring to determine acoustic activity and habitat-use patterns of spinner dolphins in Oahu, Maui Nui, and Hawaii.

References

Benoit-Bird, K. J., Au, W. W. L., Brainard, R. E., & Lammers, M. O. (2001). Diel horizontal migration of the Hawaiian mesopelagic boundary community observed acoustically. Marine Ecology Progress Series, 217(1991), 1-14. https://doi.org/10.3354/meps217001

Benoit-Bird, K. J., & Au, W. W. L. (2003). Prey dynamics affect foraging by a pelagic predator (Stella longirostris) over a range of spatial and temporal scales. Behavioral Ecology and Sociobiology, 53, 364-373. https://doi.org/10.1007/s00265-003-0585-4

Courbis, S., & Timmel, G. (2009). Effects of vessels and swimmers on behavior of Hawaiian spinner dolphins (Stenella longirostris) in Kealakeʽakua, Honaunau, and Kauhako bays, Hawaiʽi. Marine Mammal Science, 25(2), 430-440. https://doi.org/10.1111/j.1748-7692.2008.00254.x

Danil, K., Maldini, D., & Marten, K. (2005). Patterns of Use of Mäkua Beach, O‘ahu, Hawai‘i,

by Spinner Dolphins (Stenella longirostris) and Potential Effects of Swimmers on Their Behavior. Aquatic Mammals, 31(4), 403-412. https://doi.org/10.1578/AM.31.4.2005.403

Heenehan, H. L., Johnston, D. W., Van Parijs, S. M., Bejder, L., & Tyne, J. A. (2016a). Acoustic response of Hawaiian spinner dolphins to human disturbance. Proceedings of Meetings on Acoustics, 27, 10001. https://doi.org/10.1121/2.0000232

Heenehan, H. L., Van Parijs, S. M., Bejder, L., Tyne, J. A., & Johnston, D. W. (2016b). Differential effects of human activity on Hawaiian spinner dolphins in their resting bays. Global Ecology and Conservation, 10, 60-69. https://doi.org/10.1016/j.gecco.2017.02.003

Norris, K. S., Wursig, B., Wells, R. S., Wursig, M., Brownlee, S. M., Johnson, C., & Solow, J. (1994). The Hawaiian Spinner Dolphin. Berkeley, CA: University of California Press, Berkeley.