H E L G O I ~ N D E R MEERESUNTERSUCHUNGEN Helgol/inder Meeresunters. 4 7 , 3 3 5 - 3 4 6 (1993)
A b u n d a n c e a n d d i s t r i b u t i o n of h a r b o u r p o r p o i s e s
Phocoena phocoena
in s e l e c t e d a r e a s of t h e w e s t e r n
Baltic a n d t h e N o r t h S e a
M. P. H e i d e - J o r g e n s e n 1, J. T e i l m a n n 1, H. B e n k e 2 & J. W u l f 2
i Danbiu ApS., Biological Consultants; Tornagervej 2, DK-2920 Charlottenlund,
Denmark
2 Forschungs- und Technologiezentrum Westkfiste, University of Kiel; Werftstrafle i0,
D-25761 Bfisum, Federal Republic of Germany
ABSTRACT: An aerial survey of h a r b o u r porpoises,
Phocoena phocoena,
was c o n d u c t e d in J u n e - July 1992 in the w e s t e r n Baltic (Bay of Kiel, the G r e a t Belt, Little Belt, a n area north of Fyn) a n d a coastal area in the North Sea off the island of Sylt, Survey procedures a n d statistical analysis were identical to those of a similar survey in 1991 a n d survey results from the two years were compared. The h i g h e s t density of porpoises was found in the North Sea area, w h e r e a s the Bay of Kiel a n d the Little Belt h a d low densities. The G r e a t Belt a n d the area north of Fyn h a d i n t e r m e d i a t e densities a n d the area north of Fyn h a d a density similar to that found in the same area in 1991. The distribution of the p e r p e n d i c u l a r sighting distance from the surveys in 1991 a n d 1992 was identical in locality a n d dispersal, but gave rise to different estimates of effective search widths. Pod sizes for the 1992 survey w e r e smaller t h a n pod sizes estimated from boat surveys, b u t were within the r a n g e of those for the 1991 aerial survey.I N T R O D U C T I O N
I n a n e a r l i e r p a p e r ( H e i d e - J o r g e n s e n e t aL, 1992), w e e v a l u a t e d a e r i a l s u r v e y a s a m e a n s of e s t i m a t i n g d e n s i t i e s of h a r b o u r p o r p o i s e s ,
Phocoena phocoena,
a s u s e f u l for m o n i t o r i n g p o p u l a t i o n t r e n d s . W e c o n c l u d e d t h a t l i n e t r a n s e c t m e t h o d s a p p l i e d w i t h a t a r g e t a l t i t u d e of 183 m p r o v i d e a r e l i a b l e a n d e f f e c t i v e w a y of e s t i m a t i n g h a r b o u r p o r p o i s e d e n s i t i e s . H o w e v e r , w e a l s o c o n c l u d e d t h a t s u c h s u r v e y s a r e s e n s i t i v e to t h e t y p e of a i r c r a f t u s e d , e x p e r i e n c e of t h e o b s e r v e r s a n d , m o s t i m p o r t a n t l y , t h e s i g h t i n g c o n d i t i o n s u n d e r w h i c h t h e s u r v e y s a r e c o n d u c t e d . Also, t h e e s t i m a t e s of p o d s i z e a r e s u s p e c t , b e c a u s e i n a e r i a l s u r v e y s a c t u a l p o d s i z e s m a y b e u n d e r e s t i m a t e d . E s t i m a t e s of p o d s i z e s f r o m i n c i d e n t a l s i g h t i n g s f r o m s h i p s m a y b e b i a s e d .S u r v e y s of h a r b o u r p o r p o i s e s i n D a n i s h a n d G e r m a n w a t e r s , c o n d u c t e d a t r e g u l a r i n t e r v a l s a n d u s i n g s i m i l a r m e t h o d s , a r e a p o s s i b l e w a y of e v a l u a t i n g t r e n d s i n h a r b o u r p o r p o i s e p o p u l a t i o n s . M o n i t o r i n g of c o a s t a l h a r b o u r p o r p o i s e p o p u l a t i o n s c a n b e u s e f u l f o r a s s e s s i n g , a l b e i t i n d i r e c t l y , t h e i m p a c t of p o r p o i s e b y - c a t c h e s i n c o m m e r c i a l f i s h i n g o p e r a t i o n s , a n d for e v a l u a t i n g t h e s t a t e of t h e m a r i n e e n v i r o n m e n t .
3 3 6 M. P. H e i d e - J ~ r g e n s e n , J. T e i l m a n n , H. B e n k e & J. Wulf
MATERIALS AND METHODS
For the 1992 survey, five areas were selected, two of which w e r e also c o v e r e d by the 1991 survey. T h e Great Belt a n d Little Belt w e r e i n c l u d e d to e n s u r e c o n t i n u o u s s a m p l i n g i n the w e s t e r n Baltic, from the Bay of Kiel to the area north of F y n (Fig. 1). I n addition, a n area a r o u n d the i s l a n d of Sylt i n the North Sea, k n o w n for its c o n s p i c u o u s n u m b e r s of h a r b o u r porpoises, was c h o s e n for testing the feasibility of aerial surveys i n the relatively turbid n e a r s h o r e waters of t h e North Sea (Fig. 2).
The s u r v e y i n 1992 closely followed the d e s i g n described in H e i d e - J o r g e n s e n et al. (1992), except for all transects b e i n g flown at a n altitude of 183 m. T h e s a m e aircraft w a s u s e d as in 1991, a n d the a v e r a g e g r o u n d s p e e d on 133 transects was 163 k m / h (95 % CI: 160-166). O b s e r v a t i o n s were only m a d e from the rear seats t h r o u g h the b u b b l e w i n d o w s ; t h e right front seat was u s e d only for t r a i n i n g observers. T h e surveys w e r e flown on the 17th, 19th, 20th a n d 24th J u n e i n the area North of Fyn, the 19th a n d 30th J u n e a n d 3rd July in the Little Belt, the 17th, 20th, 24th a n d 30th J u n e in the Great Belt, t h e 2 7 t h - 3 0 t h J u n e in the Bay of Kiel, a n d the 3rd July i n the North Sea. A total of 62 h o u r s flying-time w a s allocated for the survey.
All flights were c o n d u c t e d in w e a t h e r conditions that the observers j u d g e d excellent. Generally, this m e a n t sea states less t h a n 2. Flying was s u s p e n d e d if t h e sea state i n c r e a s e d to more t h a n 2, or if the observers c o n s i d e r e d the conditions u n s u i t a b l e . T h e r e was g e n e r a l l y less t h a n 4/8 of cloud coverage, a n d for most of the s u r v e y the sky w a s clear. The w e a t h e r conditions r e s e m b l e d those of the 1991 survey ( H e i d e - J o r g e n s e n et al., 1992).
Statistical analysis g e n e r a l l y followed the procedures o u t l i n e d b y B u r n h a m et al. (1980) with modifications as g i v e n i n H e i d e - J o r g e n s e n et al. (1992). All s i g h t i n g s from the rear seats i n 1991 a n d 1992 w e r e pooled across strata, to derive a c o m m o n effective search half-width (ESW) for each year, a n d for both years. Sightings w i t h i n 50 m of the trackline were discarded. T h e r e m a i n i n g sightings were t r u n c a t e d at a p e r p e n d i c u l a r distance of 500 m, p a r t i t i o n e d in 25 m intervals a n d fitted to the h a z a r d rate m o d e l (Buckland, 1985) u s i n g the c o m p u t e r p a c k a g e ,,Distance" d e v e l o p e d b y L a a k e et al. (1991). C o n f i d e n c e intervals (95 %) w e r e c a l c u l a t e d u s i n g the m e t h o d d e v e l o p e d b y B u r n h a m et al. (1987).
Revised estimates (i.~. e x c l u d i n g right-front observers) for 1991 for t h e Bay of Kiel a n d the area north of F y n w e r e calculated, as welI as n e w estimates for b o t h areas, the Great Belt, the Little Belt a n d the North Sea area a r o u n d the island of Sylt. T w o m e t h o d s of calculating densities were applied: M e t h o d 1: sightings from all strata, a n d b o t h years, were pooled to derive a c o m m o n ESW. M e t h o d 2: sightings were pooled b y stratum, b u t s e p a r a t e d b y year.
Pod sizes were e s t i m a t e d directly from the surveys in each stratum a n d year, or from collections of i n c i d e n t a l s i g h t i n g s from sailors a n d ferry e m p l o y e e s in Little Belt, G r e a t Belt, the area north of Fyn, the Bay of Kiel (cf. H e i d e - J o r g e n s e n et al., 1992) a n d t h e W a d d e n Sea. For this last area, the data o n p o d sizes w e r e o b t a i n e d d u r i n g 1989-1992, w h e n the University of Kiel h a d a p r o g r a m m e to collect i n c i d e n t a l s i g h t i n g s m a d e i n G e r m a n waters of the North Sea.
A b u n d a n c e a n d d i s t r i b u t i o n of h a r b o u r p o r p o i s e s 337
338 M. P. H e i d e - J ~ r g e n s e n , J. T e i l m a n n , H. B e n k e & J. Wulf
5 5 0 3 0 '
5 5 ~ '
5 4 o 4 0 '
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8 o 0 0 , 8 o 3 0 '
I =
Aerial Survey - 3 July 1992
9 l Harbourporpoise
9 2 - 8 Harbour porpoises
I I
2 5 k m
o ~
I
8 o 0 0 ' 8 o 3 f f
5 4 ~ '
Fig. 2. Map of the area in the North Sea with transects and sightings of harbour porpoises indicated. 9 1 Harbour porpoise; 9 2-8 Harbour porpoises. Aerial survey on 3rd July 1992
t-test w a s u s e d to c o m p a r e t h e n u m b e r of porpoises sighted p e r h n e a r k i l o m e t e r on the side w i t h glare, to t h e n u m b e r s i g h t e d o n the side w i t h o u t glare,
For e a c h sighting, an effort w a s m a d e in b o t h 1991 a n d 1992 to i n c l u d e i n f o r m a t i o n a b o u t t h e s i g h t i n g c u e (e.g. if the p o r p o i s e w a s at or b e l o w the s u r f a c e w h e n first observed). Also, the r e a c t i o n b y p o r p o i s e s to the p a s s a g e of the aircraft, t h e p r e s e n c e of a calf w i t h i n a b o d y lengtt~ of an adult a n d the p r e s e n c e of gulls n e a r porpoises. S u c h anciUary data from b o t h y e a r s are p r e s e n t e d here.
RESULTS
T h e s u r v e y m 1992 c o v e r e d 5722 l i n e a r k i l o m e t e r s d i s p e r s e d o v e r the B a y of Kiel, the G r e a t Belt, the Little Belt. the a r e a north of Pyn a n d a s e l e c t e d a r e a in t h e N o r t h S e a a r o u n d t h e island of Sylt (Figs 1, 2). This effort r e s u l t e d in a total of 214 e n c o u n t e r s w i t h h a r b o u r porpoises, i n c l u d i n g s i g h t i n g s b y t h e pilot a n d the right-front o b s e r v e r , a n d t h o s e m a d e by r e a r o b s e r v e r s that w e r e m o r e than 500 m from the trackline. T h e h i g h e s t density of p o d s w a s in t h e N o r t h Sea, w h e r e the sighting rate w a s almost 10 t i m e s g r e a t e r than the l o w e s t density f o u n d in the Bay of Kiel (Tables 1 2).
A b u n d a n c e a n d d i s t r i b u t i o n of h a r b o u r p o r p o i s e s 339
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3 4 0 M . P. H e i d e - J o r g e n s e n , J. T e i l m a n n , H. B e n k e & J. W u l f
T a b l e 2. S u m m a r y statistics for the line transect estimation of h a r b o u r porpoise densities for two of the strata s h o w n in Fig. 1. The effective search half-width (ESW) was e s t i m a t e d b y fitting a h a z a r d rate m o d e l to the f r e q u e n d e s of sighting distances for the 1991 a n d the 1992 s u r v e y separately (Method 2). Sighttngs within 50 m from the trackline w e r e eliminated, a n d all sightings w e r e g r o u p e d at 25 m intervals a n d t r u n c a t e d at 500 m from the track, line. C o n f i d e n c e intervals are given as 95 % intervals. See Table 1 for comparison, a n d for additional information on effort a n d sightings
Area North of Fyn Bay of Kiel
Year 1991 1992 1991 1992
ESW (l/f[0], m) 165.8 223.2 165.8 223.2
Coefficient of variation 0.095 0.070 0.095 0.070
Density of pods 0.087 0.055 0.031 0.009
Coefficient of variation 0.248 0.147 0.250 0.334
Density (D, p o r p o i s e s / k m 2) 0.095 0.067 0.039 0.010
A b u n d a n c e estimate 397 280 162 41
Confidence interval 339-465 205-380 99-268 2 I-81
Coefficient of variation 0.262 0.157 0.262 0.352
n o r t h of F y n , l e s s t h a n 2 % of t h e s i g h t i n g s w e r e of m o t h e r - c a l f p a i r s . O f t h e r e l a t i v e l y f e w s i g h t i n g s f r o m t h e B a y of Kiel a n d t h e L i t t l e B e l t i n 1992, n o n e w e r e m o t h e r - c a l f p a i r s . I n 1991, 18 % of 2 2 s i g h t i n g s f r o m t h e B a y of Kiel, a n d 4 % of 85 s i g h t i n g s f r o m n o r t h of F y n + w e r e of m o t h e r - c a l f p a i r s .
G 1 a r e . T h e r e w a s n o s i g n i f i c a n t d i f f e r e n c e i n t h e n u m b e r of p o r p o i s e s s i g h t e d p e r h n e a r k i l o m e t e r , for t r a n s e c t s w i t h g l a r e a n g l e s of l e s s t h a n 55 ~ (p > 0.08). H o w e v e r , for t h o s e w i t h a n g l e s of g l a r e b e t w e e e n 55 ~ a n d 110 ~ t h e n u m b e r of p o r p o i s e s s i g h t e d p e r k i l o m e t e r w a s s i g n i f i c a n t l y l o w e r o n t h e s i d e w i t h g l a r e (p = 0 . 0 0 2 7 ) . W e t h e r e f o r e o m i t t e d t h e s i d e w i t h g l a r e o n all t r a n s e c t s h a v i n g a n a n g l e of g l a r e g r e a t e r t h a n 55 ~ .
E f f e c t o f s e a s t a t e . S e a s t a t e h a s a n e g a t i v e e f f e c t o n t h e p r o b a b i l i t y of o b s e r v i n g h a r b o u r p o r p o i s e s . W e p u r p o s e l y a v o i d e d s e a s t a t e s a b o v e 1, b u t s h o r t p e r i o d s w i t h s e a s t a t e 2 c o u l d n o t b e c o m p l e t e l y a v o i d e d . S u c h c o n d i t i o n s s o m e t i m e s o c c u r r e d a l o n g c o a s t s w i t h t u r b u l e n c e , or i n e x p o s e d o f f s h o r e a r e a s . T h e t o t a l e f f o r t s a s w e l l a s t h e n u m b e r of s i g h t i n g s i n s e a s t a t e 2 w e r e l o w ( < 2 1 % ) d u r i n g b o t h t h e 1991 a n d t h e 1992 s u r v e y ( T a b l e 3). H o w e v e r , t h e d i s t r i b u t i o n of t h e e f f o r t a n d s i g h t i n g s i n 1991 o n s e a s t a t e w a s s i g n i f i c a n t l y d i f f e r e n t f r o m t h e 1992 d i s t r i b u t i o n ( c h i - s q u a r e , d f = 2, p = 0.0001).
N o d i f f e r e n c e i n E S W a n d m e d i a n v a l u e s c o u l d b e d e t e c t e d f o r s i g h t i n g s i n s e a s t a t e 0 a n d 1 ( T a b l e 3, Fig. 3). T h e E S W i n s e a s t a t e 2 w a s s i g n i f i c a n t l y n a r r o w e r t h a n i n s e a s t a t e s 0 a n d I ( T a b l e 3, Fig: 3). T h i s c l e a r l y d e m o n s t r a t e s t h a t t h e r e is a r i s k a s s o c i a t e d w i t h u s i n g s u r v e y d a t a c o l l e c t e d i n s e a s t a t e 2. H o w e v e r , l e s s t h a n 13 % of t h e s i g h t i n g s i n o u r s u r v e y s w e r e m a d e i n s e a s t a t e 2 c o n d i t i o n s , a n d t h e e f f e c t o n t h e E S W is t h e r e f o r e c o n s i d e r e d to b e n e g h g i b l e .
A b u n d a n c e a n d distribution of h a r b o u r porpoises 341
Table 3. Distribution of sightings and effort on sea states, median of the frequency of perpendicular distances and effective search half-width (ESW) calculated for the three sea state conditions,
following
the procedures employed for the pooled distance frequency (i.e. hazard rate, left and right truncation at 50 and 500 m, respectively, and 25 m intervals). Coefficients of variation are given inparentheses. See Fig. 3 for distribution of the sightings of perpendicular distances
Percentage Percentage Median ESW
of sightings of effort sighting distance
1991 1992 1991 1992 (m) (m)
Sea state 0 59 34 46 20 138 209
(0.08)
Sea state 1 39 54 42 60 133 204
(o.og)
Sea state 2 2 12 12 20 91 79
(0.84)
o b s e r v e r w h o i n c r e a s e d his search s w a t h in 1991 c o n t r i b u t e d substantially to t h e total n u m b e r of sightings in b o t h years, the m u c h w i d e r ESW a c h i e v e d in 1992 is p r o b a b l y d u e m a i n l y to this o b s e r v e r ' s i n c r e a s e d efficiency {Table 2, Fig. 4).
C o m p a r i s o n o f d e n s i t i e s i n 1 9 9 1 a n d 1 9 9 2 . T w o m e t h o d s o f c a l c u l a t - ing densities w e r e applied:
M e t h o d 1: b o t h the 1991 a n d 1992 surveys u s e d the s a m e aircraft (with o b s e r v a t i o n s from the r e a r seats}, the s a m e altitude of 183 m a n d at l e a s t s o m e of t h e s a m e observers. T h e two s u r v e y s c a n t h e r e f o r e b e c o n s i d e r e d c o m p a r a b l e , w i t h r e s p e c t to t h e p e r p e n - dicular d i s t a n c e distribution of t h e sightings. In b o t h years, t h e r e w a s a d e a r t h of sightings within t h e first 50 m on e i t h e r side of t h e trackline, h e n c e this a r e a w a s not i n c l u d e d in the calculation of a r e a s e a r c h e d . Sightings m o r e t h a n 500 m from the t r a c k h n e w e r e c o n s i d e r e d to b e outliers a n d w e r e t h e r e f o r e omitted.
T h e f r e q u e n c y distributions of the p e r p e n d i c u l a r distance of the s i g h t i n g s in 1991 (n = 65) a n d 1992 (n = I40) w e r e c o m p a r e d with r e g a r d to locality a n d dispersion, a n d w e r e not f o u n d to differ significantly (Kolmogorov-Smirnoff two s a m p l e test, p = 0.13)
w h e n c o m p a r i n g sightings from 50 to 500 m from the trackline in i n t e r v a l s of 25 m. T h e p o o l e d s i g h t i n g distances from 1991-92 w e r e fitted to the h a z a r d - r a t e model. T h e estimate of t h e E S W for the p o o l e d d a t a from 1991 a n d 1992 (ESW = 198.3, cv 0.062) w a s i n t e r m e d i a t e b e t w e e n the s e a r c h w i d t h for 1991 (ESW = 165.8 cv 0.09) a n d 1992 (ESW -- 223.2, cv 0.07, Fig. 4}. T h e c o m m o n ESW for both 1991 a n d 1992 w a s a p p l i e d to the d e n s i t y e s t i m a t i o n in all areas for b o t h y e a r s (Table 1).
M e t h o d 2: alternatively, the two s u r v e y s could be t r e a t e d as two i n d e p e n d e n t samples, with e a c h i n t e g r a t i n g the variations of o b s e r v e r efficiency a n d s i g h t i n g condi- tions: T h e r e are o b v i o u s differences in o b s e r v e r efficiency a n d s i g h t i n g conditions b e t w e e n the two years, so s e p a r a t e E S W ' s could b e a p p l i e d for e a c h y e a r (Table 2).
342
1.4
M. P. H e i d e - J ~ r g e n s e n , J. Teilrnann, H. B e n k e & J. Wulf
1.2
~ LO
~
0.80
0.6
o 0,4
t ~
0.2
0.0,
1.4
"i 1.2 1.0
0.8
0.6
~
.4~" 0,2
0.0,
1.0
0.8
0 . 6
~
~
0.40.2
0,0
0
Sea
state 0, n=73, ESW=209 mm
-
~ Sea state 1, n=88, ESW=204 m
1HMII
Sea
state 2, n=19, E S W = 7 9 mm
50 100 150 200 250 300 350 400 450 500
Perpendicular
distance from trackline (meters)Fig. 3. Distribution of sightings of harbour porpoises at various distances from the trackline for sea state 0 to 2 (1991 and 1992 pooled). Data were fitted to the hazard rate model (Buckland, 1985) and
A b u n d a n c e a n d d i s t r i b u t i o n of h a r b o u r p o r p o i s e s 343 Table 4. Comparison of proportion of sightings, effective search half-width (ESW) and median of the frequency of perpendicular distances, for one observer that changed search swath b e t w e e n 1991
and 1992 with all the other observers. Confidence intervals (95 %) given in parentheses
One observer All other observers
1991 1992 1991 1992
Propo~ion of allsighfings 3 3 % 4 0 % 6 7 % 6 0 %
ESW(m) 161 268 152 189
(85-304) (219-327) (126-184) (156-230)
Median (m) 114 159 133 128
w h e r e a s m e t h o d 2 i n d i c a t e s a 3 0 % d e c l i n e in a b u n d a n c e f r o m 1991 to 1992. B o t h m e t h o d s s u g g e s t t h a t t h e r e w e r e s i g n i f i c a n t l y f e w e r p o r p o i s e s in t h e B a y of K i e l in 1992, c o m p a r e d w i t h 1991.
O b s e r v a t i o n a l c u e s . F o r a b o u t h a l f t h e s i g h t i n g s , n o t e w a s t a k e n as to w h e t h e r t h e p o r p o i s e w a s at t h e s u r f a c e ( b r e a k i n g t h e surface) or b e l o w t h e s u r f a c e , a n d w h e t h e r it r e a c t e d to t h e p a s s a g e of t h e a i r c r a f t b y d i v i n g . T h e o n l y c l e a r l y d i s c e r n i b l e r e a c t i o n to t h e a i r c r a f t w a s d i v i n g ; o t h e r t y p e s of r e a c t i o n w e r e n o t o b s e r v e d a n d r e c o g n i s e d . All d i v e s m a d e d u r i n g t h e p a s s a g e of t h e aircraft w e r e c o n s i d e r e d to b e a r e a c t i o n to t h e n o i s e or s h a d o w of t h e p l a n e , e v e n t h o u g h o t h e r factors m a y h a v e t r i g g e r e d at l e a s t s o m e of t h e d i v e s . R e a c t i o n s a f t e r t h e p a s s a g e of t h e a i r c r a f t c o u l d n o t b e o b s e r v e d , b e c a u s e o u r v i s u a l s e a r c h w a s l i m i t e d to t h e a r e a f r o m 0 ~ to 90 ~ o n e i t h e r s i d e of t h e aircraft. T h e d i s t r i b u t i o n of c u e s w a s v e r y similar b e t w e e n 1991 a n d 1992 ( T a b l e 5). D u r i n g t h e p a s s a g e of t h e p l a n e , at l e a s t h a l f t h e p o r p o i s e s w e r e e i t h e r r e s t i n g at t h e s u r f a c e or s w i m m i n g w h i l e b r e a k i n g t h e surface.
T h e p o r p o i s e s w e r e o f t e n a c c o m p a n i e d b y gulls
(Larus fuscus
a n dLarus ridibundus)
t h a t w e r e e i t h e r o n t h e w a t e r s u r f a c e or flying. T h e s e b i r d s o f t e n a t t r a c t e d t h e a t t e n t i o n of t h e o b s e r v e r to t h e s p o t of t h e w a t e r s u r f a c e w h e r e t h e p o r p o i s e w a s s w i m m i n g . G u l l s w e r e in t h e c l o s e v i c i n i t y of t h e p o r p o i s e s in a p p r o x i m a t e l y 1 3 % a n d 1 6 % of t h e s i g h t i n g s in 1991 a n d 1992, r e s p e c t i v e l y . H o w e v e r , a s s o c i a t i o n s w i t h gulls w e r e n o t c o n s i s t e n t l y r e c o r d e d a n d c o u l d also h a v e b e e n m i s s e d b y t h e o b s e r v e r s .
Table 5. Three cues noted for the harbour porpoise sightings
1991 1992
(n -- 55) (n = 113)
At surface Below surface
Dive reaction to passage of aircraft * two of them were swimming belly-up * * five of them were at the surface w h e n diving
6 2 % 57 %
2 9 % * 3 2 %
1991, E S W = 1 6 6 m
I W
M. P. H e i d e - J o r g e n s e n , J. T e i l m a n n , H. B e n k e & J. W u l f
1.2 e-
l . 0
0.8
0
0.6
0 . 4
0 . 2
0.0-
1.4
1.2
1.0
~ 0.8
'~, o.6
9 ~ 0.4
0.2
0.0 ~
1.4
9 ~ 1.2 r
,~ 1.0
~
0.8"~, 0.6
0.4
0.2
0.0
344
1.4
0
1992, E S W = 2 2 3 m
1991-1992, E S W = 1 9 8 m
50 100 150 200 250 300 350 400 450 500
P e r p e n d i c u l a r d i s t a n c e f r o m t r a c k l i n e ( m e t e r s )
Pig. 4. Distribution of sightings of h a r b o u r porpoises at various distances from the traCkline for both the 1991 and 1992 surveys (rear observers only), Data were fitted to the hazard rate model
A b u n d a n c e a n d distribution of h a r b o u r p o r p o i s e s 345
D I S C U S S I O N
Results of a e r i a l surveys of h a r b o u r p o r p o i s e s are e v i d e n t l y s u b j e c t to c o n s i d e r a b l e variability, d e p e n d i n g on sighting conditions a n d o b s e r v e r efficiency. Previous studies h a v e also d e m o n s t r a t e d t h e i m p o r t a n c e of s i g h t i n g conditions. For e x a m p l e , B a r l o w et al. {1988} n o t e d that e s t i m a t e s of h a r b o u r p o r p o i s e d e n s i t y w e r e n e g a t i v e l y b i a s e d b y i n c r e a s i n g cloud c o v e r a g e a n d s e a states. T h e effect of v a r i a b l e o b s e r v e r efficiency could only b e a d d r e s s e d in a h m i t e d w a y in the p r e s e n t study, d u e to the low n u m b e r of s i g h t i n g s p e r observer. N e v e r t h e l e s s , our results i n d i c a t e that, d e s p i t e a s h g h t i n c r e a s e in s e a state in 1992, the o b s e r v e r w h o c o n t r i b u t e d t h e most s i g h t i n g s in b o t h y e a r s w i d e n e d his s e a r c h s w a t h significantly. Part of this a p p a r e n t i m p r o v e m e n t m a y b e d u e to the decision, t a k e n after the 1991 survey, t h a t the first 50 m from the t r a c k l i n e w o u l d b e o m i t t e d from the analyses.
In the context of a t t e m p t i n g to m o n i t o r t r e n d s in h a r b o u r p o r p o i s e densities, two m u t u a l l y c o n t r a d i c t o r y a r g u m e n t s for d e r i v i n g ESW's s e e m e q u a l l y a p p r o p r i a t e :
(1) Ideally, s i g h t i n g s for e a c h y e a r a n d e a c h s t r a t u m s h o u l d b e t r e a t e d s e p a r a t e l y . This p r o c e d u r e t a k e s a c c o u n t of the a n n u a l v a r i a b i l i t y in o b s e r v e r efficiency a n d s e a states w h e n d e r i v i n g the ESW's.
(2) T h e p o o l i n g of s i g h t i n g s across y e a r s a n d strata s t a n d a r d i s e s a n d r e d u c e s the v a r i a n c e . This p r o c e d u r e r e c o g n i s e s t h a t c o n f i d e n t estimation of v a r i a b i l i t y is difficult w h e n d e a l i n g with small s a m p l e sizes a n d flexible p a r a m e t e r s such as s e a s t a t e a n d o b s e r v e r efficiency. Also, the a n n u a l c o n t r i b u t i o n to the v a r i a t i o n should b e c o m e p r o g - r e s s i v e l y smaller.
Both a p p r o a c h e s r e q u i r e t h a t the surveys b e c o n d u c t e d u n d e r similar conditions, e.g. i d e n t i c a l s u r v e y platform, altitude, s p e e d , s e a states of 0 a n d 1, no cloud c o v e r a g e . G i v e n t h e s e restrictions, it is u s u a l l y i m p o s s i b l e to o b t a i n a d e q u a t e s a m p l e s w h e n c a l c u l a t i n g ESWs for e a c h year, stratum, a n d p e r h a p s e v e n day. I n t e g r a t i o n of d i s t a n c e d a t a over s e v e r a l y e a r s will, u n d e r the s a m e restrictions, a l l o w a r e l i a b l e a n d c o n f i d e n t g e n e r a l i s a - tion of the s e a r c h profile to b e m a d e . T h e d i s a d v a n t a g e of such i n t e g r a t i o n is t h a t it r e q u i r e s the u p d a t i n g of ah p a s t surveys w h e n a n e w s u r v e y is a d d e d . H o w e v e r , as the n u m b e r of s u r v e y s increases, t h e a d j u s t m e n t s of p a s t surveys e v e n t u a l l y b e c o m e s trivial. R e g a r d l e s s of the m e t h o d e m p l o y e d , d e n s i t i e s of h a r b o u r p o r p o i s e s in the B a y of Kiel are low c o m p a r e d w i t h all other a r e a s s u r v e y e d . T h e l a t e r t i m i n g of the s u r v e y in t h e Bay of Kiel in 1991 (July} m a y e x p l a i n the h i g h e r d e n s i t i e s found in 1991 t h a n in 1992. No surveys w e r e c o n d u c t e d in the G r e a t Belt in 1991 a n d w e c a n n o t e x c l u d e the p o s s i b i l i t y t h a t the p o r p o i s e s f o u n d in the G r e a t Belt in J u n e 1992 w o u l d h a v e m o v e d s o u t h into the Bay of Kiel in July. H o w e v e r , a l t h o u g h the p o r p o i s e s o b s e r v e d in the G r e a t Belt w e r e close to the n o r t h e r n b o r d e r of the s t r a t u m in the Bay of Kiel, n o t h i n g i n d i c a t e s t h a t t h e y m o v e d into this s t r a t u m d u r i n g the s u r v e y period. The Bay of Kiel w a s s u r v e y e d as l o n g as t e n d a y s after t h e G r e a t Belt, b u t d e n s i t i e s in t h e Bay r e m a i n e d low.
A s h i p - b a s e d s u r v e y in S e p t e m b e r 1991 (covering b o t h the Bay of Kiel a n d the southern p a r t of the G r e a t Belt) c o n f i r m e d that the G r e a t Belt has a m u c h h i g h e r d e n s i t y of h a r b o u r p o r p o i s e s t h a n t h e B a y of Kiel at t h a t time of y e a r (Benke, u n p u b l , data).
3 4 6 M. P. H e i d e - J o r g e n s e n , J. T e i l m a n n , H. B e n k e & J. W u l f
T h e 1992 s t u d y d e m o n s t r a t e d t h a t a e r i a l s u r v e y is a f e a s i b l e m e t h o d for c e n s u s i n g h a r b o u r p o r p o i s e s i n t h e N o r t h S e a c l o s e to t h e c o a s t , a n d t h a t h i g h d e n s i t i e s of h a r b o u r p o r p o i s e s a r e p r e s e n t a r o u n d t h e i s l a n d of Sylt. A l s o , a r e m a r k a b l y l a r g e p r o p o r t i o n of t h e m o t h e r - c a l f p a i r s t h a t w e r e o b s e r v e d d u r i n g t h e s u r v e y s w e r e a r o u n d t h e i s l a n d of Sylt. T h e a r e a w a s c h o s e n b e c a u s e of t h e m a n y o b s e r v a t i o n s of h a r b o u r p o r p o i s e s m a d e a l o n g t h e c o a s t of S y l t ( B e n k e , u n p u b l , d a t a ) . It is i m p o r t a n t to s t r e s s t h a t t h e d e n s i t i e s o b t a i n e d f r o m t h i s s u r v e y c a n n o t b e a p p h e d to t h e e n t i r e N o r t h S e a , b e c a u s e t h e a r e a s e l e c t e d is c o n s i d e r e d to h a v e e s p e c i a l l y h i g h d e n s i t i e s .
I n d e p e n d e n t e s t i m a t i o n of p o d s i z e s for e a c h s t r a t u m a n d y e a r r e m a i n s a n i n t r i g u i n g p r o b l e m , a s p o d - s i z e e s t i m a t e s p o o l e d o v e r s e v e r a l y e a r s a n d a r e a s m a y m a s k p o s s i b l e t r e n d s b e t w e e n y e a r s . F o r t r e n d a n a l y s i s , it is t h e r e f o r e p r e f e r a b l e to h a v e i n d e p e n d e n t e s t i m a t e s of p o d sizes, o r to m a k e u n c o r r e c t e d c o m p a r i s o n s of d e n s i t i e s o f p o d s .
F u t u r e s u r v e y s w i l l n e c e s s a r i l y h a v e s o m e v a r i a t i o n c a u s e d b y s u b o p t i m a l s i g h t i n g c o n d i t i o n s (i.e. s e a s t a t e 2), a n d of p a r t i c i p a t i n g o b s e r v e r s w i t h v a r i o u s l e v e l s of e x p e r i - e n c e . T o e n h a n c e c o m p a r a b i l i t y , a n a l y s e s of f u t u r e s u r v e y s s h o u l d n o t i n c l u d e m o r e t h a n 10 % of s i g h t i n g s f r o m s e a s t a t e 2 a n d s h o u l d i n c l u d e n o n e f r o m h i g h e r s e a s t a t e s .
Acknowledgements. Besides the authors, the following participated as observers: R. Dietz, P. P a l u d a n Moller, I. Petersen, J. Refn, R. Sonntag. K. Hfillert p r e p a r e d the maps; L. P e t e r s e n (Danish Air Survey ApS.) was the pilot on b o t h surveys; P. Agger, S. Buckland a n d R, Reeves kindly provided criticism of a n earlier draft of this manuscript. The Danish part of this study was f i n a n c e d by the National Forest a n d N a t u r e A g e n c y (Danish Ministry of Environment) a n d the G e r m a n part b y G r e e n p e a c e G e r m a n y (1991) a n d the Ministry of Environment, N a t u r e C o n s e r v a t i o n a n d Nuclear Safety.
L I T E R A T U R E C I T E D
Andersen, S. H., 1982. C h a n g e in the occurrence of the h a r b o u r porpoise, Phocoena phocoena, in Danish waters as illustrated by catch statistics from 1834 to 1970. - F. A. O. Fish. Ser. 5 (4), 131-133.
Barlow, J., Oliver, C. W , Jackson, T. D. & Taylor, B. L., 1988. Harbor porpoise, Phocoena phocoena,
a b u n d a n c e estimation for California, Oregon, a n d Washington: II Aerial surveys. - Fish. Bull. U.S. 85, 433-444.
Buckland, S. T., 1985. P e r p e n d i c u l a r distance models for line transect sampling. - Biometrics 41,
177-195.
Burnham, K. P., Anderson, D. R. & Laake, J. L., 1980. Estimation of density from line t r a n s e c t sampling of biological populations. - Wild. Monogr., Chestertown, 72, 1-203.
Burnham, K. P., Anderson, D. R., White, G. C., Brownie, C. & Pollock, K. H., 1987. Design a n d analysis m e t h o d s for fish survival e x p e r i m e n t s b a s e d on release-recapture. - Am. Fish. Soc., Bethesda, Maryland, 437 pp.
H e i d e - J o r g e n s e n , M.-P., Mosbech, A., Teilmann, J., Benke, H. & Schult z, W., 1992. H a r b o u r porpoise (Phocoena phocoena) densities o b t a i n e d from aerial surveys north of Fyn a n d in the Bay of Kiel. - Ophelia 35, 133-146.
