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Review of Possible Ways that Increasing Ocean Noise May Affect Marine Mammals

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Peter L. Tyack. Implications for marine mammals of large-scale changes in the marine acoustic environment. Journal of Mammalogy, 89(3): 549-558, 2008. [DOWNLOAD PAPER(pdf)]
In this wide-ranging literature review, Peter Tyack of Woods Hole Oceanographic Institute sketches the history of research into the effects of noise on marine life, with some references as well to effects noted on terrestrial creatures. He begins by noting that while acute disturbance of individuals attracts the most attention, the likely more profound effects of chronic disturbance on population vitality (success in foraging and mating) are much harder to discern. Several examples are presented of studies that documented both temporary and long-term abandonment of key habitat when loud noise was present (including grey whales abandoning a birthing lagoon for several years, then returning when the salt production facility was abandoned, and dolphins moving away from foraging habitat when shipping traffic is heavy).

Next, Tyack turns to a detailed examination of the question of whether global shipping may be dramatically decreasing the area within which whales can hear each other’s calls, beginning with the thought that the unintentional consequences of increased shipping noise may be creating unexpected problems analogous to those created by the introduction of industrial-waste gasses into the atmosphere, which went unnoticed for decades. Following on models created in the 1970s, updated to take into account the hundred-fold increase in shipping noise since then, he notes that “the increase in ambient noise from shipping seems to have reduced the detectable range of low frequency whale calls from many hundreds of kilometers in the prepropeller ocean down to tens of kilometers in many settings today.” (For example, the finback whale range shrank from at least 400km in the pre-engine ocean to 90km in the 1960s, down to 32km today.) He notes that, as populations of great whales fall, the separation between them may increase, with these increases in shipping noise compounding the challenge of finding mates or sharing information about active feeding grounds. However, he then goes on to point out that there is, so far, no clear evidence that the great whales do indeed communicate over long distances; clear responses to the calls of other whales have been seen only at ranges of 10km or less; the fact that a human acoustic sensor can detect a signal at 400km does not necessarily mean that the whales themselves rely on hearing signals at such distances. He suggests that acoustic tags may help to clarify whether distant, faint signals from conspecifics (whales of the same species) do in fact trigger any discernable reaction (calling in response, or moving toward the distant whale).

While noting that it may be impossible to design scientifically valid studies to uncover the possible cost of “lost opportunities” when communication is drowned out by shipping noise, an indirect way to get at this question is within reach of researchers: if animals alter their calls in noisy conditions, we can infer that the noise is disrupting their normal communication channels. And indeed, Tyack notes a long list of studies that show such changes, such as beluga whales and manatees increasing the volume of their calls in noisy conditions, and an apparently dramatic increase in the frequency (pitch) of right whale calls in sections of the ocean where low-frequency shipping noise is more intense. While noting that these and other studies “suggest that vessel noise clearly does interfere with communication in marine mammals,” Tyack also notes that we do not know how costly these adaptations are, or what noise level would preclude such compensation. Also, he asks, “When does noise so degrade the usefulness of a habitat that animals leave? Can this level be predicted by the compensation behavior?” As of yet, these are unanswered, and difficult to answer, questions.

Finally, Tyack turns to research that show clear disturbance reactions to ocean noise, including killer whales staying 4km away from acoustic harassment signals near fish farms, dolphin numbers dropping to 8% of normal within 3.5 km of similar noise-makers on other fish farms (with those small numbers implying that the avoidance distance was far greater). He notes that the degree of displacement or behavioral response is not necessarily a direct indicator of the severity of impact, suggesting that “if an animal is in bad enough condition that the risk of altering behavior is high, then it may be less likely to show a disturbance response.” For example, hungry animals will linger in a feeding area the longest. He also notes that some responses to noise may be caused by noise sources that resemble a predator’s call (as in recent modeling Tyack has done that suggests beaked whale decompression sickness may result from a long series of near-surface dives as the whales flee sonar signals that they mistake for orca calls). He cites some compelling studies on terrestrial animals showing that repeated disturbance exacts high costs in reproductive success and overall health (including a study of geese that showed that when undisturbed, geese increased their body mass and had a 46% breeding success, whereas in nearby areas where farmers scared them off their fields, they did not gain mass and had a breeding success of only 17%).

To conclude, Tyack suggests that there are several lines of research that have so far received little attention, which could help to move key understanding of noise impacts forward, including: focusing on the most vulnerable animals as subject of study into the effects of disturbance, further study of the possibility that predator responses underlay key behavioral impacts (including fleeing, increased vigilance, and avoiding habitats), and following up on the recent theory of allostasis (behavioral changes that allow an animal to maintain equilibrium in the face of external environmental changes or stressors) as a way of understanding the costs and benefits of changing behavior in the face of noise.

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