Appendices 5-13 - Nautilus Cares - Nautilus Minerals

sources to close range, and that the radius of avoidance of these sources was considerably
less than the radius at which the source was audible. Richardson et. al. (1995) present
summary tables of the broadband levels at which avoidance occurred in various
observations and experimental trials, and present the level at which 10% of migrating gray
whales avoided a semi-submersible drilling rig at 114 dB re 1"Pa, 50% avoidance at 117
dB re 1"Pa and 90% avoidance at > 128 dB re 1"Pa. For spring-migrating bowhead
whales passing drilling operations they estimated strong behavioural changes at levels
near 124 dB re 1"Pa and typical closest approach to levels of 131 dB re 1"Pa. Using the
130 dB re 1"Pa value as an estimate of avoidance ranges for the mining vessel under its
DP operating state gives a 90% avoidance range from the vessel of 15 km.
Perhaps the best long term study of whale response to vessel approaches is that of
Watkins (1986). He reports on more than 25 years of observations made of whales and
vessels in the vicinity of Cape Cod, on the eastern US seaboard. Watkins (1986) reported
that over the years the response of minke and finback whales to nearby vessels changed
from frequent positive responses to uninterested reactions, the response of northern right
whales did not change and humpbacks changed from mixed, often negative encounters, to
generally positive responses. Given the long time frame of the mining operations it is
probable that resident marine fauna will eventually acclimate to its nearby presence.
Watkins (1986) stressed that the most vigorous whale responses came from noise sources
that changed suddenly, rapidly increased (such as an approaching vessel) or were
unexpected. He also noted that whales that were preoccupied with some activity were less
responsive than whales that were inactive. Richardson et. al. (1995) reiterates this in his
summary of baleen whale responses to vessels, stating that when vessels approach whales
slowly and non-aggressively, the whale response is similar, whereas rapidly changing
vessel noise often resulted in strong avoidance responses from nearby whales. McCauley
et. al. (1996) estimated the underwater noise level received at humpback whales involved
in whale watching encounters and simultaneously measured their behavioural reactions.
The most vigorous responses consistently came from vessels which either produced the
most erratic noise levels with many sharp increases in level, or which had deliberately
approached whales too closely (tens of metres).
The noise of the mining vessel in DP mode, while relatively intense, has the redeeming
feature of being produced by a stable platform which either does not move, only moves
25