Research Document - 2004/121

Review of the Potential Hydrophysical-related Issues in Canada, Risks to Marine Mammals, and Monitoring and Mitigation Strategies for Seismic Activities.

By Lawson, J., McQuinn, I.

Abstract

To outline the elements of the physical environment that will effect seismic sound propagation, we subdivided Canadian waters into five hydrophysical areas based on a number of broadly-defined physical features such as depth, bottom type, weather patterns, and surrounding land masses: Enclosed Continental Shelf, Narrow and Broad Continental Shelves, Arctic Shelf, and Coastal Canyons. These characteristics overlap among the areas, and the characteristics effect underwater acoustic environments. The prediction of underwater acoustic propagation is a critical parameter in determining the detection range of acoustic signals by marine mammal receivers.

There are few studies of (1) marine mammal hearing sensitivity (especially for baleen whales), (2) physical structure and underwater sound propagation characteristics of many marine areas in Canada, and (3) the effects of seismic sounds on marine mammal hearing sensitivity or behaviour, both on an individual and population level. How reliably these effects occur, the magnitude of these effects, the range of “recovery times” after effects are detected, and the factors which seem to influence probability, magnitude, and time course of effects are all types of data that remain limited for almost all marine mammals.

Marine mammal reactions to seismic sounds are variable, and fixed exposure criteria for behaviour may be impractical given these variable reactions and the variable nature of sound propagation.

Study of issues related to potential impacts of seismic sound on marine mammals are needed:

1. What are the best sound propagation models for areas likely to host seismic exploration?
2. There is a need for better and more accurate information on naturally-occurring and man-made noise in the ocean.
3. There is a need for significantly more information regarding the reactions of marine mammals (and their prey) to underwater sound from seismic arrays.
4. Is ramp-up an effective mitigation method?
5. Is passive and/or active acoustic monitoring of marine mammals from the source vessel an effective monitoring and/or mitigation strategy?
6. The spatio-temporal distribution, and physiological needs of marine mammal populations.

We have underlined the complexities involved in developing protocols and standards for seismic exploration among the diverse physical environments making up the Canadian marine environment. However, ultimately the Department wants to understand the effects of seismic exploration on individuals and populations of living organisms that exploit these hydrophysical regions so as to find ways to minimise the impacts of this sound source. Due to the complex patterns of sound propagation in these diverse regions, some marine mammals may not necessarily encounter the average sound exposure conditions predicted for a seismic survey. Therefore we must determine and be sensitive to the worst-case conditions that can be encountered to ensure that we do not underestimate the impact upon a particular segment of a marine mammal population.

Especially when dealing with SARA-listed species, detrimental effects suffered by one individual can translate into detrimental effects on the population; in critical situations (e.g., the northern right whale and blue whale), the reduced fitness or loss of a single individual becomes a concern for the health and productivity of the population. The onus falls on DFO to provide the necessary precautionary regulations and mitigation measures to ensure that no additional pressure is exerted on populations already at risk. This may entail extraordinary measures when endangered species are involved in critical behaviours (e.g., calving, feeding, and migration), which might include areas and seasons closed to seismic, or operational shut downs when detection probabilities fall below certain standards due to sub-optimal observation conditions.

The validity of any assessment regarding potentially harmful impacts of seismic sound on marine mammals will depend crucially on the accuracy and applicability of acoustic propagation models and the data used in this process. DFO will have to regulate within a precautionary framework as it is unlikely there will ever be direct hearing sensitivity measures for the large whales, and marine mammals’ behavioural reactions to seismic sounds will always be variable.

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