HELGOI~NDER MEERESUNTERSUCHUNGEN Helgol/inder Meeresunters. 47,335-346 (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 areas of the w e s t e r n Baltic a n d the 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. Wulf 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 harbour porpoises, Phocoena phocoena, was conducted in JuneJuly 1992 in the western Baltic (Bay of Kiel, the Great Belt, Little Belt, an 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 and survey results from the two years were compared. The highest density of porpoises was found in the North Sea area, whereas the Bay of Kiel a n d the Little Belt h a d low densities. The Great Belt and the area north of Fyn had intermediate densities and the area north of Fyn h a d a density similar to that found in the same area in 1991. The distribution of the perpendicular sighting distance from the surveys in 1991 and 1992 was identical in locality and dispersal, but gave rise to different estimates of effective search widths. Pod sizes for the 1992 survey w e r e smaller than pod sizes estimated from boat surveys, but were within the r a n g e of those for the 1991 aerial survey. INTRODUCTION In 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 as 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, as u s e f u l for monitoring population trends. We concluded that line transect methods applied with 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 also 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 size 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 sizes 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 pod sizes from incidental sightings from ships may be biased. 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 for 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 . T h e p r e s e n t s u r v e y w a s c o n d u c t e d to i m p r o v e o u r e s t i m a t i o n s of t h e e f f e c t i v e s e a r c h w i d t h , to p r o v i d e a m o r e a c c u r a t e e s t i m a t e of t h e r e l a t i v e a b u n d a n c e of h a r b o u r p o r p o i s e s i n t h e w e s t e r n Baltic, to i n c r e a s e t h e a r e a c o v e r e d b y s u r v e y s , a n d to t e s t t h e f e a s i b i l i t y of a e r i a l s u r v e y s i n t h e m u d d y , t u r b i d w a t e r s of t h e W a d d e n a n d N o r t h Seas. 9 Biologische Anstalt Helgoland, H a m b u r g

336

M. P. Heide-J~rgensen, J. T e i l m a n n , H. Benke & J. Wulf

MATERIALS AND METHODS For the 1992 survey, five areas were selected, two of which were also covered by the 1991 survey. The Great Belt a n d Little Belt were 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 in the western Baltic, from the Bay of Kiel to the area north of F y n (Fig. 1). I n addition, a n area around the island of Sylt i n the North Sea, k n o w n for its conspicuous n u m b e r s of h a r b o u r porpoises, was chosen for testing the feasibility of aerial surveys i n the relatively turbid nearshore waters of the North Sea (Fig. 2). The survey in 1992 closely followed the design 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. The s a m e aircraft was u s e d as in 1991, a n d the average g r o u n d speed on 133 transects was 163 k m / h (95 % CI: 160-166). Observations were only m a d e from the rear seats through the b u b b l e windows; the right front seat was used only for training observers. The surveys w e r e flown on the 17th, 19th, 20th a n d 24th J u n e in 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, the 27th-30th J u n e in the Bay of Kiel, a n d the 3rd July in the North Sea. A total of 62 hours flying-time was allocated for the survey. All flights were conducted 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 the sea state increased to more than 2, or if the observers considered the conditions u n s u i t a b l e . There was generally less than 4/8 of cloud coverage, a n d for most of the s u r v e y the sky was 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 generally followed the procedures outlined by B u r n h a m et al. (1980) with modifications as g i v e n in H e i d e - J o r g e n s e n et al. (1992). All sightings from the rear seats in 1991 a n d 1992 were 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. The r e m a i n i n g sightings were truncated at a p e r p e n d i c u l a r distance of 500 m, partitioned in 25 m intervals a n d fitted to the h a z a r d rate model (Buckland, 1985) using the computer p a c k a g e ,,Distance" developed b y Laake et al. (1991). Confidence intervals (95 %) were calculated using the method d e v e l o p e d b y Burnham et al. (1987). Revised estimates (i.~. excluding right-front observers) for 1991 for the Bay of Kiel a n d the area north of Fyn were 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 around the island of Sylt. Two m e t h o d s of calculating densities were applied: Method 1: sightings from all strata, a n d both years, were pooled to derive a c o m m o n ESW. Method 2: sightings were pooled b y stratum, b u t separated by year. Pod sizes were estimated directly from the surveys in each stratum a n d year, or from collections of incidental sightings from sailors and ferry employees in Little Belt, Great 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 the W a d d e n Sea. For this last area, the data on pod sizes were 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 incidental s i g h t i n g s m a d e in G e r m a n waters of the North Sea. Glare was quantified as the a n g l e from the h e a d i n g of the aircraft at w h i c h observation b e c a m e obstructed by reflections of the s u n on the sea's surface. A o n e - t a i l e d paired

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

Fig. 1. Map of the area in the western Baltic Sea with transects and sightings of harbour porpoises indicated. 9 1 Harbour porpoise; 9 2.8 Harbour porpoises. Aerial survey from 17-30 June 1992

M. P. H e i d e - J ~ r g e n s e n , J. Teilmann, H. B e n k e & J. Wulf

338

8o00 , 55030 '

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Aerial Survey - 3 July 1992 9 l Harbourporpoise 9 2-8

I

55~

Harbour porpoises

I

I 25 km

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o

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~

I

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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 was u s e d to c o m p a r e the n u m b e r of porpoises sighted per h n e a r k i l o m e t e r on the side with glare, to the n u m b e r sighted on the side without glare, For e a c h sighting, an effort w a s m a d e in both 1991 and 1992 to i n c l u d e information about the sighting c u e (e.g. if the porpoise was at or b e l o w the surface w h e n first observed). Also, the reaction by porpoises to the p a s s a g e of the aircraft, the p r e s e n c e of a calf within a body lengtt~ of an adult a n d the p r e s e n c e of gulls n e a r porpoises. Such anciUary data from both years are p r e s e n t e d here.

RESULTS T h e survey m 1992 c o v e r e d 5722 l i n e a r kilometers dispersed over the Bay of Kiel, the Great Belt, the Little Belt. the area north of Pyn an d a selected area in t h e N o r t h S ea around t h e island of Sylt (Figs 1, 2). This effort resulted in a total of 214 e n c o u n t e r s with harbour porpoises, i n c lu d i n g sightings by the pilot and the right-front observer, an d those m a d e by rear observers 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 pods was in the North Sea, w h e r e the sighting rate was almost 10 times g r e a t e r than the lowest density found in the Bay of Kiel (Tables 1 2). Th e h i g h e s t f r e q u e n c y of mother-calf pairs was also found in the N o r t h S ea area w h e r e 28 % of the sightings w e r e of mother-calf pairs. In the Great BeIt an d the a r e a

Abundance

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

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339

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340

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

Table 2. Summary statistics for the line transect estimation of harbour porpoise densities for two of the strata s h o w n in Fig. 1. The effective search half-width (ESW) was estimated by fitting a hazard rate model to the f r e q u e n d e s of sighting distances for the 1991 a n d the 1992 survey separately (Method 2). Sighttngs within 50 m from the trackline were eliminated, and all sightings were grouped at 25 m intervals and truncated at 500 m from the track,line. Confidence intervals are given as 95 % intervals. See Table 1 for comparison, and for additional information on effort and sightings Area Year ESW (l/f[0], m) Coefficient of variation Density of pods Coefficient of variation Density (D, porpoises/km 2) A b u n d a n c e estimate Confidence interval Coefficient of variation

North of Fyn 1991 1992 165.8 0.095 0.087 0.248 0.095 397 339-465 0.262

223.2 0.070 0.055 0.147 0.067 280 205-380 0.157

Bay of Kiel 1991 1992 165.8 0.095 0.031 0.250 0.039 162 99-268 0.262

223.2 0.070 0.009 0.334 0.010 41 2 I-81 0.352

n o r t h of Fyn, 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 . Of 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 Little 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 22 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 = 0.0027). 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 ~. Effect of sea state. 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 with sea state 2 could not be completely avoided. Such conditions sometimes occurred 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 efforts as w e l l as 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 effort 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 , df = 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 for 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 , less 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 . O b s e r v e r e f f i c i e n c y . D u r i n g b o t h t h e 1991 a n d 1992 s u r v e y s , a c o m b i n a t i o n of e x p e r i e n c e d a n d i n e x p e r i e n c e d o b s e r v e r s w a s u s e d , b u t o n l y o n e of t h e o b s e r v e r s h a d a s u f f i c i e n t n u m b e r of s i g h t i n g s to a l l o w a c o m p a r i s o n of e f f i c i e n c y b e t w e e n y e a r s . T h i s o b s e r v e r i n c r e a s e d t h e a r e a of h i s s e a r c h s w a t h c o n s i d e r a b l y i n 1992, w h e r e a s t h e s e a r c h s w a t h for all o t h e r o b s e r v e r s c o m b i n e d o n l y i n c r e a s e d s l i g h t l y ( T a b l e 4). B e c a u s e t h e

A b u n d a n c e and distribution of harbour 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 in parentheses. See Fig. 3 for distribution of the sightings of perpendicular distances Percentage of sightings 1991 1992

Percentage of effort 1991 1992

Median sighting distance (m)

ESW

209 (0.08) 204 (o.og) 79 (0.84)

Sea state 0

59

34

46

20

138

Sea state 1

39

54

42

60

133

Sea state 2

2

12

12

20

91

(m)

o b s e r v e r w h o i n c r e a s e d his search swath in 1991 contributed substantially to the total n u m b e r of sightings in both 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 due mainly to this observer's in c r e a s e d efficiency {Table 2, Fig. 4). Comparison of densities i n 1 9 9 1 a n d 1 9 9 2 . Twomethods ofcalculating densities w e r e applied: M e t h o d 1: both the 1991 and 1992 surveys used the s a m e aircraft (with observations from the rear seats}, the same altitude of 183 m and at least some of the s a m e observers. T h e two surveys can therefore be c o n s i d e r e d comparable, with r esp ect to the p e r p e n dicular distance distribution of the sightings. In both years, there w a s a d e a r t h of sightings within the first 50 m on either side of the trackline, h e n c e this a r e a w as not i n c l u d e d in the calculation of area searched. Sightings more than 500 m from the trackhne w e r e considered to be outliers a n d w e r e therefore omitted. Th 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 sightings in 1991 (n = 65) an 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 an d dispersion, and w e r e not found to differ significantly (Kolmogorov-Smirnoff two sample 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 intervals of 25 m. T h e p o o l e d sighting distances from 1991-92 w e r e fitted to the h a z a r d - r a t e model. Th e estimate of the ESW for the pooled data from 1991 and 1992 (ESW = 198.3, cv 0.062) was i n t e r m e d i a t e b e t w e e n the search w id th for 1991 (ESW = 165.8 cv 0.09) an d 1992 (ESW -- 223.2, cv 0.07, Fig. 4}. T h e c o m m o n ESW for both 1991 and 1992 w as ap p l i ed to the d e n s i t y estimation in all areas for both years (Table 1). M e t h o d 2: alternatively, the two surveys could be t r eat ed 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 observer efficiency and sighting conditions: T h e r e are obvious differences in o b s e r v e r efficiency and sighting conditions b e t w e e n the two years, so separate ESW's could be ap p l i ed for each y e a r (Table 2). T h e two m e t h o d s of deriving the ESW g a v e m a r k e d l y different a b u n d a n c e estimates in the two strata w h e r e the surveys can b e c o m p a r e d (the area north of Fyn an d the Bay of Kiel). M e t h o d 1 provides identical estimates for the area north of Fyn in both years,

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

342 1.4

Sea state 0, n=73, ESW=209 m 1.2 ~

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Sea state 1, n=88, ESW=204 m

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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 the fitted curve shows the expected number of sightings

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

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 between 1991 and 1992 with all the other observers. Confidence intervals (95 %) given in parentheses

Propo~ion of allsighfings ESW(m) Median (m)

One observer 1991 1992

All other observers 1991 1992

33%

40%

67%

60%

161 (85-304) 114

268 (219-327) 159

152 (126-184) 133

189 (156-230) 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 from 1991 to 1992. Both 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 significantly f e w e r p o r p o i s e s in t h e Bay of Kiel in 1992, c o m p a r e d w i t h 1991. Observational cues. For a b o u t half t h e sightings, 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 the 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 surface, 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 aircraft b y diving. T h e o n l y clearly d i s c e r n i b l e r e a c t i o n to t h e aircraft w a s diving; o t h e r t y p e s of r e a c t i o n w e r e not 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 plane, 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 least s o m e of the dives. R e a c t i o n s after t h e p a s s a g e of t h e aircraft c o u l d not b e o b s e r v e d , b e c a u s e our 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 side of t h e aircraft. T h e distribution 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 (Table 5). D u r i n g t h e p a s s a g e of the p l a n e , at least half 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 the surface. T h e p o r p o i s e s w e r e often a c c o m p a n i e d b y gulls (Larus fuscus a n d Larus ridibundus) that w e r e e i t h e r on t h e w a t e r s u r f a c e or flying. T h e s e birds often a t t r a c t e d t h e a t t e n t i o n of the o b s e r v e r to t h e spot of the 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 . Gulls w e r e in the close 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 , associations 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 could also h a v e b e e n m i s s e d by t h e o b s e r v e r s .

Table 5. Three cues noted for the harbour porpoise sightings 1991 (n -- 55) At surface Below surface Dive reaction to passage of aircraft * two of them were swimming belly-up ** five of them were at the surface when diving

62% 29%* 9%

1992 (n = 113) 57 % 32% 11%**

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

344 1.4

1991, E S W = 1 6 6 m e-

1.2 l.0

0

0.8

0.6 0.4 0.2 0.01992, E S W = 2 2 3 m

1.4 1.2

I W

1.0 ~

0.8

'~,

o.6

9~ 0.4 0.2 0.0 ~ 1991-1992, E S W = 1 9 8 m

1.4 9~ r

1.2

,~ 1.0

~

0.8

"~,

0.6 0.4 0.2 0.0

0

50

100

150

Perpendicular

200

250

300

350

400

450

500

distance from trackline (meters)

Pig. 4. Distribution of sightings of harbour 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 (Buckland, 1985) and the fitted curve shows the expected number of sightings

A b u n d a n c e a n d distribution of harbour porpoises

345

DISCUSSION Results of aerial surveys of h a r b o u r porpoises are evidently subject 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 observer efficiency. Previous studies h a v e also d e m o n s t r a t e d the i m p o r t a n c e of sighting conditions. For example, Barlow et al. {1988} n o t e d that estimates of h a r b o u r porpoise density w e r e n e g a t i v e l y b i a s e d by i n c r e a s i n g cloud c o v e r a g e a n d sea states. The effect of variable observer 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, due to the low n u m b e r of sightings p e r observer. Nevertheless, our results indicate that, despite a shght i n c r e a s e in sea state in 1992, the observer w h o contributed the most sightings in both y e a r s w i d e n e d his s e a r c h swath 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, that the first 50 m from the trackline w o u l d be omitted from the analyses. In the context of a t t e m p t i n g to monitor trends in h a r b o u r porpoise densities, two m u t u a l l y contradictory a r g u m e n t s for deriving ESW's s e e m equally a p p r o p r i a t e : (1) Ideally, sightings for e a c h y e a r a n d e a c h stratum should b e t r e a t e d separately. This p r o c e d u r e t a k e s account of the a n n u a l variability in observer efficiency a n d sea states w h e n deriving the ESW's. (2) The pooling of sightings 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 variance. This p r o c e d u r e recognises that confident estimation of variability 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 state a n d observer efficiency. Also, the a n n u a l contribution to the variation should b e c o m e progressively smaller. Both a p p r o a c h e s require that the surveys b e c o n d u c t e d u n d e r similar conditions, e.g. identical survey platform, altitude, speed, s e a states of 0 a n d 1, no cloud coverage. G i v e n these restrictions, it is usually impossible to obtain a d e q u a t e s a m p l e s w h e n calculating ESWs for e a c h year, stratum, a n d p e r h a p s e v e n day. Integration of distance d a t a over several y e a r s will, u n d e r the s a m e restrictions, allow a reliable a n d confident 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 . The d i s a d v a n t a g e of such integration is that it requires the u p d a t i n g of ah p a s t surveys w h e n a n e w survey is a d d e d . H o w e v e r , as the n u m b e r of surveys increases, the 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. Regardless of the m e t h o d e m p l o y e d , densities of harbour porpoises in the Bay of Kiel are low c o m p a r e d with all other a r e a s surveyed. The later timing of the survey in the Bay of Kiel in 1991 (July} m a y explain the h i g h e r densities found in 1991 than 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 cannot exclude the possibility that the porpoises found 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 south into the Bay of Kiel in July. However, although the porpoises o b s e r v e d in the G r e a t Belt w e r e close to the northern b o r d e r of the stratum in the Bay of Kiel, nothing indicates that t h e y m o v e d into this stratum during the survey period. The Bay of Kiel was s u r v e y e d as l o n g as ten days after the G r e a t Belt, b u t densities in t h e Bay r e m a i n e d low. A s h i p - b a s e d survey in S e p t e m b e r 1991 (covering both the Bay of Kiel a n d the southern part of the G r e a t Belt) confirmed that the G r e a t Belt has a much h i g h e r d e n s i t y of h a r b o u r porpoises t h a n t h e B a y of Kiel at that time of y e a r (Benke, unpubl, data). Like the Bay of Kiel, the Little Belt has a density of porpoises that is close to the detection limits, e v e n t h o u g h it was formerly an a r e a w h e r e l a r g e n u m b e r s of p o r p o i s e s could b e c a u g h t in a drive fishery (Andersen, 1982).

346

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 coast, 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. Also, 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 Sylt ( 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 Sea, 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 , as 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, or 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 will 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 . To 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. Paludan Moller, I. Petersen, J. Refn, R. Sonntag. K. Hfillert prepared the maps; L. Petersen (Danish Air Survey ApS.) was the pilot on both surveys; P. Agger, S. Buckland and R, Reeves kindly provided criticism of a n earlier draft of this manuscript. The Danish part of this study was financed by the National Forest and Nature Agency (Danish Ministry of Environment) and the G e r m a n part by G r e e n p e a c e Germany (1991) a n d the Ministry of Environment, Nature Conservation a n d Nuclear Safety.

LITERATURE CITED Andersen, S. H., 1982. C h a n g e in the occurrence of the harbour 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. Perpendicular 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 transect 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 methods for fish survival experiments b a s e d on release-recapture. - Am. Fish. Soc., Bethesda, Maryland, 437 pp. Heide-Jorgensen, M.-P., Mosbech, A., Teilmann, J., Benke, H. & Schult z, W., 1992. Harbour porpoise (Phocoena phocoena) densities obtained from aerial surveys north of Fyn a n d in the Bay of Kiel. - Ophelia 35, 133-146. Laake, J. L., Buckland, S. T., Anderson, D. R. & Burnham, K. P., 1991. Distance sampling: A b u n d a n c e estimation of biological populations. Colorado Coop. Fish and Wildlife Res. Unit, Colorado State Univ., Fort Collins, Colorado, 53 pp.