NOAA Technical Memorandum
NMFS-AFSC-108
Results of a pilot program to document interactions between sperm whales and longline vessels in Alaska waters
Abstract
Interactions between sperm whales (Physeter macrocephalus) and longline fisheries have been well-documented in the Southern Ocean, but in Alaska waters only anecdotal reports of sperm whales interacting with longline operations have been available. In 1996, NMFS received reports from observers on commercial fishing vessels that sperm whales were preying on sablefish (Anoplopoma fimbria) targeted by longline vessels in the Gulf of Alaska. As a result, a pilot project was initiated to characterize the nature and extent of the interactions between sperm whales and the commercial longline fishery in Alaska. Between 17 May and 14 December 1997, fishery observers aboard 16 different vessels monitored 557 longline sets and recorded observations and behavior of sperm whales and any damage to fish brought aboard. Likewise, between 31 March and 14 November 1998, fishery observers aboard 41 different vessels monitored 1,060 longline sets. Sperm whales were not present during any of the 1,075 sets in the Bering Sea. Whereas, sperm whales were present in 28.5% of the 562 sets in the Gulf of Alaska and observers recorded fish damage in 46.2% of the sets in which sperm whales were present. However, few damaged fish (n = 65, x = 3.45, SE = 0.28) were landed during sets in which depredation was reported. There was no evidence that mortality or serious injury to sperm whales was occurring as a result of this interaction. An initial exploration of the catch data suggested that average standardized catch (metric tons/1,000 hooks) depended on the fish species caught, the North Pacific Fishery Management Council (NPFMC) statistical area fished, and the bottom depth. The presence of sperm whales appeared to be related to bottom depth and NPFMC statistical area. To control for these factors, we computed the average standardized catch for trips that had at least three sets when both sperm whales were absent and present and which met the following criteria: 1) predominant fish species was sablefish; 2) sets were conducted within NPFMC statistical areas 630 through 680; and, 3) bottom depth was greater than 200 fm. This data selection provided 12 trips with 285 sets (113 without sperm whales present and 72 with sperm whales present) for the comparison. The Wilcoxson signed rank test of a difference in catch associated with sperm whale presence (V = 62, n = 12) was not significant (P = 0.08). The average difference in catch between sets with sperm whales present and absent was -0.095 t (SE= 0.057). At an average weight of 3.5 kg , 27 fish per set were lost to account for the 0.095 t estimated difference in catch between sets in which sperm whales were present and absent. Our finding of no significant difference does not mean that there was no loss. Sperm whale interactions clearly reduced the catch because some fish were damaged (and were therefore worthless) and other fish were presumably removed from the line. However, at. present, the difference in catch was too small to estimate precisely with the current sample size and high variability in catch per set.
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