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Humpback Whale Entanglement in Fishing Gear in Northern Southeastern
Alaska
Janet L. Neilson1,2,4, Christine M. Gabriele1, and Janice M. Straley3
Abstract
The prevalence of non-lethal entanglements of humpback whales in fishing gear in northern southeastern Alaska
(SEAK) was quantified using a scar-based method. The percentage of whales assessed to have been entangled ranged from
52 percent (minimal estimate) to 71percent (conditional estimate) to 78 percent (maximal estimate). The conditional estimate
is recommended because it is based solely on whales with unambiguous scars. Eight percent of the whales in Glacier Bay/Icy
Strait acquired new entanglement scars between years, although the sample size was small. Calves were less likely to have
entanglement scars than older whales and males may be at higher risk than females. The percentage of whales with entanglement
scarring is comparable to the Gulf of Maine where entanglement is a substantial management concern. Consequently, SEAK
humpback whale-fisheries interactions may warrant a similar level of scrutiny.
204 Proceedings of the Fourth Glacier Bay Science Symposium
Introduction
From 1997 through 2004, 52 humpback whales
(Megaptera novaeangliae) were reported entangled in fishing
nets and/or lines in Alaska (or were reported elsewhere
and were confirmed to be entangled in Alaskan fishing
gear.) Seventy-seven percent of the reports involved SEAK
humpback whales (unpublished data, National Marine
Fisheries Service (NMFS) Alaska Regional Office). Wounds
resulting from entanglements can often be seen on the
posterior caudal peduncle (the narrowing of the body at the
insertion point of the flukes). These wounds can remain
visible as unique scarring patterns years after the entanglement
incident.
Robbins and Mattila (2001) examined whales’ caudal
peduncles for entanglement-related scarring and concluded
that 48–65 percent of the humpback whales photographed
annually between 1997 and 2002 in the Gulf of Maine had
been entangled. Until now there have been no systematic
efforts to quantify the prevalence of humpback whale
entanglement in Alaska. Managers in southeastern Alaska
have had to rely on eyewitness reports as the only estimate of
the magnitude of the problem, but not all entangled whales
are found or reported. In 2001, NMFS acknowledged the
pressing need for a detailed assessment of humpback whale
entanglement in Alaska.
The objectives of this study were to (1) estimate the
percentage of humpback whales in northern SEAK that have
been non-lethally entangled based on caudal peduncle scars,
1 Glacier Bay National Park & Preserve, P.O. Box 140, Gustavus, AK
99826
2 University of Alaska Fairbanks, School of Fisheries & Ocean Sciences,
Fairbanks, AK 99775
3 University of Alaska Southeast, 1332 Seward Avenue, Sitka, AK 99835
4 Corresponding author: Janet_Neilson@nps.gov, 907 697-2658
(2) analyze the entanglement scar data in conjunction with
existing long-term humpback whale demographic data to
identify any particularly vulnerable segments of the humpback
whale population and (3) describe the distribution of scarred
humpback whales in relation to the distribution and amount
of commercial fishing in the study area. This paper focuses on
objectives 1 and 2 only.
Methods
We conducted 1,139 hours of vessel-based surveys for
humpback whales in northern SEAK between May 2003 and
November 2004. We approached the whales in outboard-
driven motorboats 4–6.5 m in length and took high resolution
photographs of each whale’s caudal peduncle by operating
the boat parallel and slightly forward of each whale as it
dove. In order to reduce observer bias towards scarred whales,
we collected caudal peduncle photographs from all suitably
positioned whales. Whales were identified based on the
pigmentation and morphology of the ventral surface of their
tail flukes and dorsal fin.
We used a photographic coding technique developed and
ground-truthed in the Gulf of Maine by Robbins and Mattila
(2001) to assess the likelihood that a whale had been entangled
in the past. We divided each whale’s caudal peduncle into
six areas, coded these areas for signs of entanglement-related
scarring (table 1) and assigned an overall entanglement status
code (table 2) to whales with adequate photographic coverage.
Table 1. Summary of scar code descriptions (after Robbins
and Mattila, 2001).
Code Scar Code Description
S0 No visible marks
S1 Non-linear marks or apparently randomly oriented linear
marks
S2 Linear marks or wide areas lacking pigmentation, which
did not appear to wrap around the feature
S3 Linear or wide scars which appeared to wrap around the
feature
S4 At least one visible linear notch or indentation (generally
on the dorsal or ventral peduncle)
S5 Extensive tissue damage and deformation of the feature
SX Feature could not be coded due to lack of photographic
coverage or inadequate photo quality
Table 2. Summary of entanglement status codes (after Rob-
bins and Mattila, 2001).
Code Likelihood of Past
Entanglement Entanglement Status Code
E0 NONE No evidence of entanglement (no
marks present)
E1 LOW
Marks were observed, but did
not suggest a previous en-
tanglement. Scar codes did
not generally exceed S2 in any
documented region
E2 AMBIGUOUS
Entanglement-like elements were
present, but there was no consis-
tent pattern. At least one region
was generally assigned a scar
code of S3 or higher
E3 HIGH
Marks appeared to be entangle-
ment-related and minor tissue
damage was evident. At least
two regions were generally
assigned scar codes of S3 or
higher
E4 HIGH
Marks appeared to be entangle-
ment-related and major tissue
damage was evident. At least
two regions were assigned scar
codes of S3 or higher. At least
one region was coded as S5
Janet L. Neilson and others 205
Three methods were used to estimate the percentage of
whales that had been non-lethally entangled:
Minimal Entanglement Scarring Percentage =
E E
E E E
3 4
0 1 2
∑+∑
+ + + EE E3 4
∑+∑∑∑∑
Conditional Entanglement Scarring Percentage =
E E
E E
3 4
0 1
∑+∑
+ ++ ∑+∑∑∑ E E3 4
Maximal Entanglement Scarring Percentage =
E E E
E E
2 3 4
0 1
∑+ − ∑∑
+ ++ ∑+ + ∑∑∑∑ E E E2 3 4
where:
E0, E1, E2, E3 and E4=the number of whales assigned entan-
glement status codes E0, E1, E2, E3 and E4, respectively.
Two-tailed Fisher’s exact tests of independence (Zar,
1999) were used to test for significant differences between
percentages.
Individual whales with adequate photographs in both
years were used to estimate the annual rate of entanglement
scar acquisition between 2003 and 2004. The whale’s caudal
peduncle photographs from both years were compared and
assessed to estimate the amount of new entanglement-related
scarring occurring between 2003 and 2004. This rate was
calculated by dividing the number of whales in 2004 with
an increase in entanglement scarring by the total number of
individuals with adequate photographic coverage in both
years.
Results
We photographed the caudal peduncle of 303 humpback
whales and assigned entanglement status codes to 180 unique
individuals. The photographic coverage and/or quality of
123 whales was insufficient to assign codes (i.e., photographs
were too distant, blurry and/or were taken at a poor angle).
The percentage of whales assessed to have been
entangled ranged from 52 percent (95 percent CI: 45 percent,
60 percent) (minimal estimate) to 71 percent (95 percent CI:
62 percent, 78 percent) (conditional estimate) to 78 percent
(95 percent CI: 72 percent, 84 percent) (maximal estimate).
The conditional estimate is recommended because it is based
solely on unambiguous scars. Eight percent (95 percent CI:
1 percent, 25 percent) of the whales in Glacier Bay/Icy Strait
acquired new entanglement scars between 2003 and 2004.
The whales with adequate quality photographs consisted
of 62 females, 33 males and 85 whales of unknown sex. The
minimal scarring percentage of males (82 percent) was higher
than that of females (55 percent) and the difference was
significant (P=0.013). However, males and females did not
have significantly different conditional scarring percentages
(males 87 percent, females 72 percent) (P=0.165) or maximal
scarring percentages (males 88 percent, females 79 percent)
(P=0.402).
206 Proceedings of the Fourth Glacier Bay Science Symposium
The whales with adequate quality photographs consisted
of 12 calves (i.e., whales less than one year old) and 168
older whales. The minimal scarring percentage of calves
(17 percent) was lower than that of older whales (55 percent)
and the difference was significant (P=0.015). In addition, the
conditional scarring percentage of calves (29 percent) was
lower than that of older whales (73 percent) and the difference
was significant (P=0.023). However, calves and older
whales did not have significantly different maximal scarring
percentages (calves 58 percent, older whales 80 percent)
(P=0.137).
Discussion and Conclusions
The minimal, conditional and maximal entanglement
scarring percentages indicate that the majority (52–78 percent)
of the humpback whales in northern SEAK have been
entangled at some point in their lives. Most apparently shed
the gear on their own, unless whales are being disentangled
by humans much more often than is reported. The conditional
estimate (71 percent) is recommended because it is based
solely on whales with unambiguous scars. The estimate of
the annual rate of entanglement scar acquisition (8 percent) is
highly uncertain due to the small sample size of whales with
adequate photographs in both years. Similar rates of annual
entanglement scar acquisition were found in the Gulf of Maine
from 1997 through 2002 (8–25 percent) (Robbins and Mattila
2004).
These results indicate that entanglements are much more
common in northern SEAK than previously thought based
on reports of entangled whales. Nevertheless, a scar-based
approach is expected to underestimate the true frequency of
entanglement because it cannot account for (1) whales that
died before they could be detected, (2) entanglements that
did not involve the caudal peduncle and (3) entanglement
injuries that were so old or faint that they had healed beyond
recognition. In addition, whales that were entangled once were
coded the same as whales that were entangled multiple times.
The minimal estimates indicate that male humpback
whales may be more likely to become non-lethally entangled
than female humpback whales. It is unknown why males
would have a higher minimal entanglement percentage
than females. The fact that males’ and females’ maximal
and conditional scarring percentages were not significantly
different indicates that the difference in minimal scarring
percentages is attributable to differences in the number of
whales of each sex with an ambiguous entanglement history.
The minimal and conditional estimates suggest that
calves are less likely to become non-lethally entangled
than older whales. A lower incidence of scarring in calves
is expected because calves had less time to accumulate
entanglement scarring than adults. However, the minimal
scarring percentage of calves in northern SEAK (17 precent)
was higher than in the Gulf of Maine, where only 9 percent
of calves were assessed to have been entangled (Robbins
and Mattila, 2001), but this is not a significant difference.
Continued sampling of calves in SEAK would elucidate if the
scarring percentages found during this study are typical.
Management Implications
From a management perspective, data on the rate of
serious injury and mortality due to entanglements would
be most useful but are difficult, if not impossible, to
obtain. Scarring data cannot be used to estimate the lethal
entanglement rate. Managers also need to know the effects
of non-lethal entanglements on humpback whale fitness. For
instance, female humpback whales in the Gulf of Maine that
survived being entangled were less likely to be lactating than
females that had not been entangled, suggesting that non-lethal
entanglements may have an impact on reproductive success
(Robbins and Mattila, 2001).
While the specific circumstances that led to most past
entanglements will never be known, a description of the
current distribution of commercial, subsistence and sport
fishing gear in SEAK which overlaps with areas of high whale
numbers seasonally would increase managers’ understanding
of sources of current potential threats to this population on
a regional scale and could help inform management actions
aimed at preventing entanglements. This approach would
entail identifying areas where humpback whales regularly
concentrate in SEAK and examining how these areas overlap
with fishing “hotspots” to identify areas that may warrant
monitoring and/or special protection. Prevention is the key
and may mean that some gear modifications are needed.
Disentangling whales from fishing gear is a last resort that
requires proper training and NMFS authorization.
Humpback whale-fisheries interactions in northern SEAK
may warrant a similar level of management scrutiny as the
Gulf of Maine where entanglement has been identified as a
substantial management concern, based on similarities in the
amount of non-lethal entanglement scarring between the two
populations.
Acknowledgments
Funding for this study was provided by the Alaska Sea
Grant College Program, the National Marine Fisheries Service
and Glacier Bay National Park and Preserve. Data collection
was conducted under NMFS Scientific Research Permits
#945-1499-00 and #473-1433. We thank Jooke Robbins
(Center for Coastal Studies), Sue Hills (University of Alaska
Fairbanks), Kaja Brix and Mary Sternfeld (National Marine
Fisheries Service), Jen Cedarleaf (University of Alaska
Southeast) and Betsy Wilson for their help with this study.
References Cited
Robbins, J., and Mattila, D.K., 2001, Monitoring
entanglements of humpback whales (Megaptera
novaeangliae) in the Gulf of Maine on the basis of caudal
peduncle scarring. Unpub. report to the 53rd Scientific
Committee Meeting of the International Whaling
Commission. Hammersmith, London. Document # SC/53/
NAH25. 12 p.
Robbins, J., and Mattila, D., 2004. Estimating humpback
whale (Megaptera novaeangliae) entanglement rates on the
basis of scar evidence: Report to the Northeast Fisheries
Science Center, National Marine Fisheries Service. Order
number 43EANF030121. 21 p.
Zar, J.H., 1999, Biostatistical analysis. 4th ed., Upper Saddle
River, NJ, Prentice Hall.
Suggested Citation
Neilson, J.L., Gabriele, C.M., Straley, J.M., 2007, Humpback
whale entanglement in fishing gear in northern southeastern
Alaska, in Piatt, J.F., and Gende, S.M., eds., Proceedings of
the Fourth Glacier Bay Science Symposium, October 26–
28, 2004: U.S. Geological Survey Scientific Investigations
Report 2007-5047, p. 204-207.
Janet L. Neilson and others 207
