identifying individual bowhead whales through aerial photography

Identifying Individual Identifying Individual Bowhead Whales Bowhead Whales Through Aerial Through Aerial

PhotographyPhotography

Supporting organizations• National Marine Mammal Laboratory,

Alaska Fisheries Science Center, NOAA Fisheries Service,Seattle, Washington

• LGL, Ltd. Environmental Research Assoc., King City, Ontario, Canada

• North Slope Borough, Dept. of Wildlife Management, Barrow, Alaska

• Minerals Management Service, Environmental Studies Program, Alaska Outer Continental Shelf Region, Anchorage, Alaska

• Cascadia Research Collective, Olympia, Washington

Available images of bowheads

• There are currently over 17,000 images of bowhead whales in collections archived at LGL and NMML.

• Over the past 30 years, aerial surveys have been conducted mostly near Point Barrow during the spring migration and in the Beaufort Sea in summer.

Whale images are measured for length and corrected relative to camera lens, aircraft altitude (calibrated), and any known photogrammetric biases.

15.5 m

Whale lengths

Growth of individual whales

• Calves are 4.75 - 5.0 m long in May,

and 6.25 - 6.5 m long by October.

• Yearlings in May are 6.5 - 8.0 m long.

• Small whales (~9.5 m) grow 22 - 44 cm/yr.

• Medium size (~12 m) grow 12 - 15 cm/yr.

• Large whales (~15 m) grow 0 - 5 cm/yr.

Components of categorizing bowhead images

ZONESZONES IDENTIFIABILITYIDENTIFIABILITY QUALITYQUALITY

Rostrum H+ 1+

Mid-back H- 1-

Lower back M+ 2+

Fluke M- 2-

U+ 3

U-

X

Four zones used to categorize areas on a bowhead image

Rostrum zone(1)

Mid-back zone(2)

Lower-backzone(3)

Fluke zone(4)

Sizes of marks used in categorizing identifiability

Identifiability by zonesRostrum zone

unmarked(U-)

Mid-back zone(U+)

Lower-back zone (H+) Fluke zone(H+)

Large, fairly

distinctive mark

Large, non- distinctive

markOne medium not unique

mark

Image qualityRostrum zone is quality 1+

(= clearly visible)Mid-back zone is

quality 2+

Lower-back zone is quality 2+ (= adequately visible)

Fluke zone is quality 1+

(=clearly visible)

Some of this zone is obscured

by splash

Computer-assisted matching program

• Because the number of photos in the catalog has been growing, it is becoming necessary to increase matching efficiency through a computer program that prioritizes most likely matches.

Applications of results from bowhead photo-identification

• Abundance estimates

• Growth rates of individual whales

• Calving intervals

• Population dynamics

• Stock structure

Abundance Estimates

n

Photo ID abundance

Ice-based abundance

Spring 1985 641

6,700(3,195-12,561)

6,039(3,300-11,100)Summer 1985 1,069

Spring 1986 401 7,734

(5,400-11,100)Summer 1986 441

Resightings of adult whales with calves indicates calving intervals of 3-4 years

Survival probability is remarkably high for bowheads (0.95 to almost 1.00);

some bowheads live for over a century

Seasonal distribution of bowhead sightings through the spring migration near Barrow

Date bins Census Photos Difference<24 Apr 5.6% 7.8% 2.0%

24-30 Apr 16.5% 6.5% -10.0%1-7 May 24.9% 16.8% -8.0%

8-14 May 30.1% 26.7% -3.0%15-21 May 12.3% 18.8% 7.0%22-28 May 6.6% 13.1% 6.0%

>28 May 4.0% 10.4% 6.0%

Spring migration of bowheads at Barrow(gray = aerial photo data;

burgundy = ice-based census data)

0%5%

10%15%20%25%30%

4/5 to4/21

4/22 to4/28

4/29 to5/5

5/6 to5/12

5/13 to5/19

5/20 to5/26

5/7 to6/10

Stock-structure issues

Resighting dates for bowheads seen on more than one year

during the spring migration

Differences in dates (delta T) between initial sightings and

resightings of individual bowheads migrating past Barrow

0

1

2

3

4

5

6

1 6 11 16 21 26 31

Delta T

Freq

uen

cy

Differences in sighting dates of individual bowhead whales as a

function of whale size

5

10

15

20

0 10 20 30 40

Delta T (days)

Wha

le le

ngth

(m

)

Cumulative frequency of dates of all photographed bowheads

0%

20%

40%

60%

80%

100%

1976-94(not 1985)

1985