Variations in gray whale feeding behavior in the presence of whale-watching vessels in Clayoquot Sound, 1993-1995
Date
2018-02-01
Authors
Bass, Joanna
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Abstract
The growing industry of whale-watching is allowing increasing numbers of people access to whales in their natural environment, and constitutes a non-consumptive use of the whales compared to whaling. At the same time, questions are often raised about the hidden effects of whale-watching on the whales. A population of gray whales (Eschrichtius robustus) which spends the summer feeding in Clayoquot Sound, on the West Coast of Vancouver Island, is regularly observed by whale-watchers from the nearby tourist centre of Tofino. Concern among whale-watching business operators and tourists about the possible effects of whale-watching on the feeding whales was heightened in the years preceding this study by an apparent northward movement of the whales, taking them farther from the Tofino, the point of departure for whale-watching tours. This study attempts to explain this apparent trend by finding out whether the whales' short-term behaviour is affected by the presence of whale-watching vessels, and by examining their short and long-term behaviour in the wider context of some of the features of their environment.
Whales were observed from a small research vessel for three feeding seasons, in five locations within the area known as Clayoquot Sound. The whales' ventilations were recorded continuously and their location and the number of whale-watching vessels present was recorded at regular intervals. The whales' benthic prey was sampled in all three seasons and their planktonic prey in 1995. A series of variables were calculated from the ventilation data and compared to the number of vessels.
The whales' dive behaviour was correlated much more strongly with feeding location than with vessel number. Even with these two factors taken into account, much of the variation in their behaviour remains unaccounted for. Because of this, although the behavioural change in the presence of vessels is statistically significant, there is reason to doubt whether it is biologically significant. The effects of feeding location are probably a composite of the effects of depth, prey type and other factors which are difficult to measure. Of the two main components, prey type appears to have a greater effect than depth on gray whale behaviour.
Interaction exists between the effects of site and those of whale-watch vessels, meaning that the effects of vessels are different at different sites. The general pattern is that the effects of vessel presence are more pronounced in shallow sites than in deep, although there are some exceptions to this trend.
Gray whale prey shows considerable variation in location, density and composition from year to year. The long-term patterns of gray whale habitat use more closely resemble a prey-selection-driven pattern than a pattern of avoidance of whale-watch vessels.
The relatively small influence of vessel numbers on gray whale feeding behaviour suggests that the current guidelines in place for whale-watching vessels are effective in limiting disturbance of the whales. The variable nature of their prey supply suggests that gray whales use all the sites in Clayoquot Sound, and that the availability of a diverse selection of prey is necessary for their success in the tertiary feeding grounds.
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Keywords
Clayoquot Sound, Whale watching, Marine mammals, Gray whale