Biological and Existing Data Analysis to Inform Risk of Collision and Entanglement Hypotheses

Report

Title: Biological and Existing Data Analysis to Inform Risk of Collision and Entanglement Hypotheses
Authors: Kropp, R.
Publication Date:
December 01, 2013
Document Number: PNNL-22804
Pages: 42
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Citation

Kropp, R. (2013). Biological and Existing Data Analysis to Inform Risk of Collision and Entanglement Hypotheses. Report by Pacific Northwest National Laboratory (PNNL). pp 42.
Abstract: 

A literature search was conducted to identify articles that would be useful to help assess the likelihood that migrating whales, particularly gray whales, would encounter wave parks that might be proposed for off the U.S. west coast and collide with and/or become entangled in a park’s underwater cables. These concerns have been mentioned in several review articles without any documentation that such encounters would be likely. Therefore, this literature search was focused on trying to obtain the necessary information to support or challenge the hypothesized concern. As an example of the potential interaction, this review focuses on a wave park placed off the coast of Oregon.

 

The review identified a considerable body of literature that documents the severity of entanglements involving marine mammals, particularly large baleen whales. However, these entanglements involved fixed or derelict fishing gear, such as various types of nets and the cables used to attach floats to lobster and crab traps. The review did not identify any cases of whales being entangled in mooring cables such as those planned for use in offshore wave parks.  One of the key properties with fishing lines is that there is usually a considerable amount of slack in the lines. This slack enables the lines to wrap around whale body parts, which ultimately leads to entanglement. The various mooring lines and cables associated with wave parks would be taut under most circumstances and would not have enough slack to allow a whale to become entangled.

 

Three species of baleen whales–gray, humpback, and blue–are the most likely large species that would encounter a wave park placed about 2 to 3 nmi off Oregon. Of the three species, the gray whale is the most likely species to encounter the park because it undergoes an 18,000-km (11,185-mi) migration during which it passes through Oregon nearshore waters twice per year. Most of the more than 18,000 gray whales that pass the Oregon coast would swim within 3.5 to 6.5 km (1.9 to 3.5 nmi) of shore while migrating north to feeding areas in the Bering Sea and within 5.6 to 7.0 km (3.0 to 3.8 nmi) of shore on the return trip south to breeding areas in Mexico. Thus, it is likely that many gray whales would encounter a nearshore wave park. However, the encounter likelihood is not uniform throughout the year. Based on the timing of the migrations, the most likely encounter period would be during the northbound migration from late February through about late May when the adult males, pregnant females, and immature whales follow a migratory path through shallower Oregon waters that has strong overlap with the location of the proposed nearshore wave park. During the southbound migration most whales pass the Oregon coast in January at distances farther offshore than the location of a proposed wave park. The number of whales that might encounter a park is further reduced by the tendency of the whales to generally follow depth contours while swimming past Oregon. Thus, whales swimming at depths shallower than a nearshore wave park would tend to stay at those depths and whales swimming deeper than the park would tend to stay deeper. Humpback whales primarily use the waters off Oregon for feeding and occasionally occur relatively close to shore although most frequent offshore deeper-water banks. Blue whales do not spend much time in Oregon waters, passing through them to more northerly locations. Both species are relatively rare in Oregon waters.

 

Anatomical studies of whale vision and hearing allow some hypotheses about the detectability of a wave park to be made. Whale vision, especially that of gray whales, is fairly poor, and it seems unlikely that whales would be able to visually detect underwater cables and lines at a distance sufficient to allow them to avoid them. One study showed that minke whales could detect and avoid highly contrasting (black and white) lines, but whether other large whales would be able to do so is not known. The literature suggests that baleen whales, unlike many toothed whales, do not use sound to detect underwater objects but to communicate with others of the same species.

 

Despite the lack of information about some aspects of large whale population density and behavior, the described features of the wave park cables and mooring lines, the relative rarity or absence of most baleen whales in nearshore Oregon waters, and the gray whale migration pattern inshore of a nearshore wave park allow the hypothesis that entanglement in wave park cables should not be a significant issue for baleen whales.

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