Article: Jacobson, E.K. et al. “Acoustic evidence that harbor porpoises (Phocoena phocoena) avoid bottlenose dolphins (Tursiops truncatus).” Marine Mammal Science: 31(1): 386-397. January 2015.
Harbor porpoises (Phocoena phocoena) (Fig 1) have long been at home off the coast of central California. They have thrived in this highly productive ecosystem and for a long time only had to worry about killer whales and the occasional shark. However, things started to shift in the early 1980s. An El Nino event in 1982 brought warmer waters to the central coast of California, and along with it, bottlenose dolphins (Tursiops truncatus) (Fig 2). Typically, porpoises had inhabited this region as well as colder waters on more northern coasts, and dolphins inhabited warmer waters farther south, so there wasn’t any range overlap for these two species, that is until warmer waters started creeping up the coast. This overlapping home range resulted in the first reported stranding of a harbor porpoise that was killed by a dolphin in Monterey Bay, CA, occurring in 2005. In 2007, harbor porpoise strandings in central California doubled, and researchers found that the most common cause of death was “blunt force trauma” (Fig 3). Most of these attacks were traced to 21 male dolphins in the area and most of the attacks occurred during the dolphin breeding season (June – November).
Porpoises aren’t prey for dolphins, so why are these attacks happening? There are two potential explanations. First, it is thought that there might be competition for prey between the two species or they may interfere with the feeding of one another. However, a few studies have honed in on the feeding habits and preferences of these two species, finding that there really isn’t much overlap in preferred prey, and these two species occupy and feed at different depths in the water column. Second, male dolphins are known to have infanticidal tendencies. For male dolphins, killing young dolphins frees up female dolphins for reproduction. Infanticidal behavior in dolphins has been observed in other parts of the world. Many porpoises are about the same size as young dolphins, and aggressive male dolphins may “practice” their infanticidal behavior on porpoises. Regardless, there is a well-documented history of interspecies aggression, making this explanation more likely.
This interspecies aggression in central California is a relatively new trend, so researchers decided to investigate whether or not this aggression is altering the behavior of porpoises in this ecosystem.
In Monterey Bay, CA, researchers set up click detectors in the water column to quantify the presence and activity of bottlenose dolphins and harbor porpoises in the area (Fig 4). Remember, both dolphins and porpoises use audible clicking for communication and echolocation. These click detectors are able to pick up and record any clicks made by these species. These two species use different frequency clicks, porpoises have a narrow-band, high frequency click (120-140 kHz); while dolphins use mid-frequency clicks (30-60 kHz). This difference allows for the distinction of the two species.
Using a combination of recorded clicks and aerial observations, researchers were able to piece together the patterns of porpoise and dolphin abundance and activity in Monterey Bay. They honed in on points in time in which both species were present in the same habitat, as indicated by the click recordings. Porpoise clicks before, during, and after the presence of dolphins were noted. Clicks were assessed in relation to time, which was a way for researchers to standardize the data. Over the course of one hour, each minute in which there was an audible train of clicks was counted as a “positive minute,” and the total proportion of positive minutes was calculated. It was found that in the absence of dolphins, the proportion of positive minutes for porpoises was 0.18, while in the presence of dolphins, that number dropped to 0.06. That means porpoises either left the area or were keeping quiet. This pattern holds up when looking at the hours before, during, and after dolphin encounters (Fig 5).
The click frequency evolved by harbor porpoises is advantageous for their ecosystem. The high frequency click is out of the audible range of their natural predator, the killer whale. But the newcomers to the ecosystem, the bottlenose dolphins, have the ability to detect the porpoise clicks up to 2km away. Due to the interspecies aggression by the dolphins, porpoises seem to be on high alert and respond to dolphin encounters by either leaving the area or reducing their vocalization to avoid detection. With ocean temperatures continuing to increase, it is likely that warming water will bring larger numbers of bottlenose dolphins to harbor porpoise habitats. Already threatened by fishing nets, boats, and natural predators, porpoises are now facing a new threat in the form of aggressive dolphins. Even if dolphins don’t attack, porpoises are still negatively affected: avoiding dolphins forces them out of their preferred habitat, cutting them off from necessary resources.
Check out this video for a relatively non-graphic glimpse at dolphin aggression towards porpoises…