|
Objectives:
The main aim of the study was to
present the first census-based estimate of the population (total
number) of bottlenose dolphins in the Croatian part of the Adriatic
Sea in winter. Additionally, we studied the size and composition of
dolphin schools, the number of cubs, and their spatial distribution in
December. Recordings of any other species of cetaceans and of other
sea mammals were also anticipated.
Study team:
The research team was recruited from the Faculty
of Veterinary Medicine, University of Zagreb, Zagreb, Croatia. Aircraft pilots
were members of the "Krila Kvarnera" Aeroclub, Rijeka, Croatia. In each of the
four aircraft used there was at least one trained member of the research team,
as well as a skilled pilot with experience in observation of the sea surface and
the islands. The research team members were Prof. Dr. Hrvoje Gomercic DVM, Prof.
Dr. Duro Huber DVM, Assistant Professor Dr. Damir Mihelic DVM, Hrvoje Lucic DVM,
Tomislav Gomercic DVM student, and Martina Duras DVM student. The pilots were
Andrej Bagar, Anton Micetic, Zoran Premus, and Josip Zezelic (Figs.
6, 7).
|
|
| Fig. 6. Part of the
research team and pilots at Grobnik Airport on the morning of 27
December 1998 immediately prior to take-off. From right to left:
Tomislav Gomercic, DVM student, Martina Duras, DVM student, Assistant
Professor Dr. Damir Mihelic, pilot Andrej Bagar, pilot Zoran Premus,
Prof. Dr. Duro Huber, and Prof. Dr. Hrvoje Gomercic. |
|
| Fig. 7. Part of the
research team and pilots in the restaurant at Grobnik Airport in the
evening, after landing. From right to left: Prof. Dr. Hrvoje Gomercic,
pilot Anton Micetic, Tomislav Gomercic, DVM student, Hrvoje Lucic, DVM,
Assistant Professor Dr. Damir Mihelic, pilot Zoran Premus; from the
back, pilots Josip Zezelic and Andrej Bagar.
|
|
|
Preliminary study:
A crew-training flight was performed in order to
obtain experience and to standardise methods of work. In the preliminary study
we used two single-engine high-wing monoplanes: a Cessna 172, Registered Number
9A DDH (H. Gomercic, D. Huber, H. Lucic, and pilot A. Micetic), and a Citabria
CH-10, Registered Number 9A CAF (T. Gomercic, and pilot A. Bagar). The aircraft
were in permanent radio contact one with another and with aircraft control
centres in Rijeka and Pula. They were provided with Global Positioning System
(GPS): Garmin GPS 12 XL Personal Navigator for exact positioning of dolphin
schools and positioning of aircraft. A preliminary flight took place on 13
December 1998, between 12:18h and 16:04h in the Northern Adriatic Sea (Gulf of
Rijeka, Kvarneric, and Kvarner). Weather conditions were: wind speed 0-7 km/h;
wave height 0-10 cm; horizontal visibility 20-30 km; 3/4 cloudy sky; seawater
temperature 14-15°C; air temperature 7-11°C (Fig. 8). Flight
speed of the Citabria CH-10 was c. 150 km/h, and of the Cessna 172 c. 200 km/h.
Flight altitude was between 50 and 500 m.
The aircraft made three to six passes over the
dolphin groups found in order to allow the researchers and pilot to count the
number of animals and to assess the composition of the dolphin school.
In this study, Citabria CH-10 found three dolphin
schools (P1, P2, P3). P1 and P2 were in Kvarneric between the islands of Rab and
Cres. School P3 was in Kvarner between the Istrian Peninsula and the island of
Cres (Fig. 9). According to the findings of a team member in
the Citabria CH-10, the P1 school comprised two dolphins, P2 of one dolphin, and
P3 of two dolphins. Between 5 and 13 minutes later the second aircraft (Cessna
172) arrived at the same positions (P1, P2 and P3). The researchers in this
aircraft found two dolphins at site P1, zero dolphins at position P2, and six
dolphins (including one cub) at position P3.
According to the results of the preliminary study
it was verified that the best results of an aerial survey of dolphins (under
these weather conditions in the Adriatic Sea and at an airspeed of 150-200 km/h)
were to be obtained from an altitude of 200 m. This agrees with the findings of
many authors (Ross et al., 1989; Cockroft and Peddemors, 1990; Yoshida et al.,
1997, 1998; Carretta et al., 1998; Forney and Barlow, 1998; Würsig et al.,
1998). Under such weather conditions and at an altitude of 200-m, the estimated
visible search strip (trackline - flight path) was 4 km (2 km on either side of
the aircraft).
- Fig. 8. Fax from the
Naval Meteorological Centre (State Hydrometeorological Department), Split,
Croatia, containing weather conditions on 13 December 1998.
- Fig. 9. Map of the
northern part of the Croatian Adriatic Sea (Gulf of Rijeka, Kvarneric, and
Kvarner) showing courses of aircraft Citabria CH-10 (green line), and Cessna
172 (violet line) during tandem preliminary study on 13 December 1998, with
positions (P1, P2, and P3) of sighted dolphin schools.
The most probable actual number of
dolphins at the three mentioned sites was nine specimens (P1=2 dolphins,
P2=1 dolphin and P3=6 dolphins). However, at these locations the first
aircraft found only five dolphins (56%) while the second aircraft found
eight dolphins (89%) of the nine actually present. We conclude that
under the above described weather conditions, and at an airspeed of
150-200 km/h, we were able to see only about 56-89% of dolphins within
the surveying track. This means that the census of each individual
survey flight must be positively corrected by 12-80% (average 46%). This
is in agreement with the conclusion of Carretta et al. (1998), in that
abundance estimates from tandem survey averaged 53% higher than dolphin
counts obtained from individual survey flights.
Materials and Methods:
Aerial surveys of dolphins in the Croatian part
of the Adriatic Sea were performed on 27 December 1998. Sunrise was at 7:26, and
sunset was at 16:24, according to standard time (Central European Time = 1 hour
plus Greenwich Mean Time). During the several previous days we received
Meteorological Reports, together with a weather prognosis, telefaxed twice-daily
(at 6 and 12 hours) from the Adriatic Sea Naval Meteorological Centre (State
Hydrometeorological Department), Split, Croatia.
We used four single-engine high-wing monoplanes (Figs.
10, 11, 12): Cessna 172, Registered
Number 9A DDH (flight and aircraft A), Citabria CH-10, Registered Number 9A CAF
(flight and aircraft B), Cessna 150, Registered Number 9A DAH (flight and
aircraft C) and Cessna 150, Registered Number 9A DAJ (flight and aircraft D)
simultaneously.
All aircraft were provided with Global
Positioning System (GPS): Garmin GPS 12 XL Personal Navigator with Remote
Antenna (Fig. 13)for exact positioning and timing of dolphin
schools and for aircraft positioning. Binoculars, naval maps and 35-mm still
cameras were also used. For the planning of tracklines (flight paths) for each
of the four aircraft, and for pooling, processing and mapping of data from all
aircraft, we used PC (Pentium) (Figs. 14, 15)
and FUGAWI Moving Map Software for GPS Receivers, Version: 2,18 (1998), and
naval maps 1:100,000 (STATE HYDROGRAPHIC INSTITUTE, 1996). Data for this Report
were calculated by Fugawi Software (1998) and on the basis of the "Autokarta
Hrvatske 1:500,000" map (Birin and Stefanac, 1997). All positions of aircraft
and dolphin schools were recorded by GPS equipment in degrees, minutes, seconds
and tenths of seconds of northern latitude and eastern longitude. However, in
this Report all measured positions are presented to the nearest degree and
minute.
|
|
| Fig. 10. Aircraft used
at Airport Grobnik, in the morning, before take-off. From right to left:
Cessna 172, Cessna 150, Cessna 150 and Citabria CH-10. |
|
| Fig. 11. Citabria CH-10
above the Gulf of Rijeka, with Mount Ucka in the background. |
|
| Fig. 12. Cessna 172
above the Gulf of Rijeka, with the island of Krk and tanker terminal in
the background. |
|
| Fig. 13. Equipment
used for navigation, Garmin GPS 12 XL Personal Navigator with remote
Antenna, PC Interface Cable with Cigarette Lighter Adapter and PC Pentium. |
|
|
| Fig. 14. Garmin GPS 12
XL Personal Navigator and PC Pentium with displayed map of the Gulf of
Rijeka in FUGAWI Moving Map Software. |
|
| Fig. 15. Transferring
data from GPS equipment to PC Pentium at Grobnik Airport after the flight. |
|
|
Fig. 16. Rijeka, Croatia's largest harbour,
near Grobnik Airport. |
|
| Fig. 17. Landing at
Grobnik Airport, near Rijeka. |
|
| Fig. 18. Landing at
Resnik Airport, near Split. |
|
| Fig. 19. Landing at
Cilipi Airport, with Dubrovnik in the background. |
|
|
Aircraft A was provided with a Canon EOS 5000 and an
AF Canon Zoom 80-200 mm; aircraft B was provided with a Nikon N50 and an AF
Zooms 28-80 and 70-310; aircraft C was provided with a Minolta DYNAX 500si and
an AF Sigma Zoom 100-300 mm, and aircraft D was provided with a Minolta X-700
and a Quantary Zoom 70-210 mm. For photodocumentation we used Kodak Select Elite
Chrome EB ISO 100/21°, and 400/27° film for colour slides. Slides were copied on
Kodak Gold GB ISO 200/24° film for prints.
All researchers and pilots counted the dolphins
in every dolphin school, with the highest confirmed number of dolphins in each
school being recorded. Those dolphins that were smaller by about one-third of
their body length were recorded as cubs.
All aircraft initially took off from Grobnik
airport, near Rijeka, ultimately returning to Grobnik but landing for refuelling
at airports at Zadar (D), Split (C) and Dubrovnik (A and B). During flights the
aircraft were in radio contact with aircraft control centres in Rijeka, Pula,
Zadar, Split and Dubrovnik. All aircraft flew pre-programmed courses. All
courses were planned with a minimum overlap of each other. The recorded sortie
lengths (in km) of each aircraft were exclusive of 3-6 passes over every dolphin
school. In the event of the discovery of any sea mammal other than a bottlenose
dolphin, the working protocol was to instruct the nearest aircraft to detour
from its own pre-planned flight path and to assist in consulting and in
confirming the species.
In addition to the pilot (Anton Micetic), a team
of three observers (Hrvoje Gomercic, Damir Mihelic, and Martina Duras) were
present on flight A.
In addition to the pilot (Andrej Bagar), one
observer (Duro Huber) was present on flight B.
In addition to the pilot (Josip Zezelic), one
observer (Tomislav Gomercic) was present on flight C.
In addition to the pilot (Zoran Premus),
one observer (Hrvoje Lucic) was present on flight D.
Working Area - the Croatian part of the
Adriatic Sea:
The Republic of Croatia has a total of surface
area of 88,198 km2, including 31,660 km2 of
sea surface area. The outer limits of Croatian territorial waters are 12
nautical miles from the Croatian coastline, islands or islets. Croatian
territorial waters lie between N4° South and N45°34' North. The longest stretch
of Croatia's international boundary at sea merges with the international waters
of the Adriatic Sea, while northward it measures 16 km adjacent to the Republic
of Slovenia, and 25 km adjacent to the Italian Republic. In the Mali Ston
Channel (Fig. 20) in the neighbourhood of the town of Neum,
there is an international sea boundary extending for 12 km adjacent to
Bosnia-Herzegovina, and in the south a 27-km international sea boundary adjacent
to Montenegro. Total coastline length is 5,790 km, of which 1,185 islands and
islets (total surface area of 3,271 km2) account for 4,012 km
of that figure, 1,778 km being continental coastline. Sixty-six of the islands
are inhabited.
In Croatian internal waters there are three
National Parks: the Brijuni Archipelago - 36 km2 (Fig.
21); the Kornati Archipelago - 224 km2 (Fig.
22), and Mljet Island - 31 km2 (Fig. 23).
There is also one Nature Park: the Bay of Telascica (Fig. 24),
on the island of Dugi Otok. Immediately abutting the sea lies the Mount Velebit
Nature Park and UNESCO Biosphere Reserve: Mount Velebit - 2,274 km2
(Figs. 25, 35, 36), the
Paklenica National Park- 36 km2 (Fig. 25)
and, near the town of Sibenik, the Krka National Park - 142 km2.
All islands and islets have a very similar
geological structure, being composed essentially of limestone rocks, except for
the island of Susak (Fig. 26) which is composed essentially of
sedimentary river sand, and the two volcanic islets of Jabuka (Figs.
27, 28) and Brusnik (Fig. 29).
The depth of the Croatian part of the Adriatic
Sea increases gradually from north to south. The shallowest area (20-30 m) is at
the extreme northern end, along the western coast of the Istrian Peninsula, and
the Gulf of Rijeka (50-60 m). In the south, depths are somewhat greater, but in
general are less than 100 m, except around the island of Jabuka (250 m) and in
the extreme south (up to 400 m). The sea bottom of this area is essentially
sandy and muddy, although in some places it is gravelled and rocky. |
|
|
Fig. 20. The ancient Croatian town of Mali
Ston, near the end of the Mali Ston Channel. |
|
| Fig. 21. Brijuni
Archipelago National Park (Photo: I. Bralic). |
|
| Fig. 22. Kornati
Archipelago National Park (Photo: S. Strikoman). |
|
| Fig. 23. Mljet Island
National Park, with Benedictine monastery dating from the XII century,
located on a small island in Veliko jezero. |
|
| Fig. 24. Telascica Bay
Nature Park, with the saltwater Mir Lake, 1982 (Photo: H. Gomercic). |
|
| Fig. 25. Paklenica
National Park on Mount Velebit, near the Adriatic Sea. |
|
|
Water temperature of the Adriatic Sea is between
12°C in February and 27°C in August. The main sea streams through this area flow
northwards from the Ionian Sea.
Four hundred and seven species of fish have been
recorded as living in the Adriatic Sea (Soljan, 1965; Jardas, 1996) and eight
species of cephalopods (Basioli, 1978; Grubisic, 1990). Many of these are
possible dolphin prey.
For the purposes of this study, Croatian
territorial waters were divided into inshore and offshore waters (Fig.
30). Inshore waters have a surface area of 15,254 km2, and
offshore waters a surface area of 16,406 km2. Inshore waters
include all Croatian islands and islets except the islands of Jabuka - N43°16';
E15°27'13'' (Figs. 27, 28), Svetac -
N43°02'; E15°45' (Fig. 45), Brusnik - N43°00'40''; E15°48' (Fig.
29), Susac - N42°46'; E16°31', Palagruza - N42°23'40''; E16°16' (Fig.
46) and their islets, and the islet of Galijula - N42°22'53''; E16°19'58'' (Fig.
47). Inshore waters correspond to Croatian internal sea waters, although
their boundary is some 5 km offshore (from the boundary of internal sea waters
and of the continental coast, or of the outer coastline of offshore islands and
islets, except for Jabuka, Svetac, Brusnik, Susac, Palagruza and Galijula). |
 |
| Fig. 26. Island of
Susak, composed essentially of sedimentary river sand. |
|
| Fig. 27. The volcanic
islet of Jabuka. |
|
|
Fig. 28. The volcanic islet of Jabuka. |
|
| Fig. 29. The
volcanic island of Brusnik (Photo: H. Gomercic).
|
|
|
Results:
Aerial surveys of dolphins in the Croatian part
of the Adriatic Sea were carried out on 27 December 1998.
Weather conditions at 10 a.m. in the Croatian
part of the Adriatic Sea were, on average, semi-cloud cover; wind speed, 0 km/h
in most places, and in others up to 7 km/h; wave height in the range 0-10
(locally up to 25) cm; horizontal visibility 10-30 km; atmospheric pressure
1,026-1,028 hPa; air temperature 5-10°C; sea temperature 13-14°C (Fig.
31).
Aircraft A took off at 9:09 from Grobnik Airport
and flew a pre-programmed course (violet line on Figs. 32,
33a, b, c, d and
e). At 9:45 and at position N44°38'28.9''; E14°10'22.9'' it
recorded dolphin school A1, and at 10:25 dolphin school A2 (Fig.
37) at position N43°57'48.5''; E14°46'41.1''. School A1 comprised two, and
school A2 five, adult bottlenose dolphins. This aircraft flew a distance of
702.6 km until, at 13:15, it landed at Dubrovnik Airport in Cilipi. It took off
again at 14:48. During the second leg of its flight, at 15:52 and at position
N43°54'29.7''; E15°05'12.0'' it recorded dolphin school A3; at 16:05 and at
position N44°05'56.0''; E14°52'51.4'' school A4; at 16:13 and at position
N44°11'55.0''; E14°43'00.9'', school A5, and at 16:29 and at position
N44°31'07.6''; E14°22'47.1'', school A6. School A3 comprised two adult dolphins,
A4 nine adults, A5 four adults, and A6 three adult dolphins and two cubs (Fig.
30). This aircraft flew a distance of 538.5 km on the second leg of its
flight. At 17:05 it landed at the base airport of Grobnik. Altogether, it flew
for 6:49 h and covered 1,241.1 km, 32.9 km of this overland and 1,208.2 km over
the sea.
Aircraft B took off at 9:17 from Grobnik Airport
and flew a pre-programmed course (green line on Figs. 32,
33a, b, c, d and
e). At 11:06 and at position N43°41'56.5''; E15°45'47.1'' it
recorded dolphin school B1; at 11:17 dolphin school B2 at position
N43°39'29.4''; E15°48'39.8'', and at 11:51 school B3 (Fig. 39)
and at position N43°07'23.6''; E16°30'47.0''. School B1 comprised eleven, and
school B3 six adult dolphins. School B2 comprised four adult dolphins and three
cubs. This aircraft flew 571.4 km on its first leg, landing at 12:53 at
Dubrovnik Airport in Cilipi. It took off again at 13:38. At 13:58 it recorded
school B4 at position N42°45'08.0''; E17°43'54.3''; at 15:18 school B5 at
position N43°40'28.2''; E15°35'25.1''; at 15:20 school B6 at position
N43°41'36.5''; E15°33'55.0''; at 15:51 school B7 at position N44°16'35.3'';
E14°52'06.9'', and at 16:04 school B8 at position N44°30'58.2''; E14°43'50.8''.
School B4 comprised five adult dolphins and one cub; dolphin schools B5
comprised four, B6 six, B7 two, and B8 nine, adult bottlenose dolphins (Fig.
30). This aircraft flew 486.9 km on its return flight and landed at the base
airport of Grobnik at 16:40. Altogether, it flew for 6.38 h and covered 1,058.3
km, 24.6 km of this overland and 1,033.7 km over the sea.
- Fig. 30. Map of the
inshore (15,254 km2) and offshore (16,406 km2) waters of
the Croatian part of the Adriatic Sea (Croatian territorial waters - 31,660 km2),
with sighted bottlenose dolphin schools on 27 December 1998. Small map:
Position of Croatia (black) in Europe.
- Fig. 31. Fax from the
Naval Meteorological Centre (State Hydrometeorological Department), Split,
Croatia, showing weather conditions on 27 December 1998 at 10:00.
- Fig. 32. Map of the
Croatian part of the Adriatic Sea, showing aircraft courses: aircraft A
(violet line), B (green line), C (red line) and D (blue line).
- Fig. 33a. Map of the
northern part of the Croatian part of the Adriatic Sea with shaded search
strip (trackline - flight path) of aircraft A in violet, B in green, C in red,
and D in blue, with position of sighted school of bottlenose dolphins.
- Fig. 33b. Map of part
of the Croatian part of the Adriatic Sea with shaded search strip (trackline -
flight path) of aircraft A in violet, B in green, C in red, and D in blue,
with positions of sighted school of bottlenose dolphins.
- Fig. 33c. Map of part
of the Croatian part of the Adriatic Sea with shaded search strip (trackline -
flight path) of aircraft A in violet, B in green, C in red, and D in blue,
with positions of sighted school of bottlenose dolphins.
- Fig. 33d. Map of part
of the Croatian part of the Adriatic Sea with shaded search strip (trackline -
flight path) of aircraft A in violet, B in green, and C in red, with positions
of sighted school of bottlenose dolphins.
- Fig. 33e. Map of the
southern area of the Croatian part of the Adriatic Sea with shaded search
strip (trackline - flight path) of aircraft A in violet, and aircraft B in
green, with positions of sighted school of bottlenose dolphins.
|
|
| Fig. 34. The bridge to
Krk Island, on 27 December 1998 at 9:33 a.m. |
|
| Fig. 35. The new
Maslenica bridge on 27 December 1998 at 10:56 a.m., with the Mount Velebit
Nature Park and UNESCO Biosphere Reserve in the background. Before the
bridge, the ruin of the former bridge, destroyed by Yugoslav Army units in
21 November 1991. |
|
| Fig. 36. The bridge to
Pag Island on 27 December 1998 at 11:08 a.m., with the Mount Velebit
Nature Park and UNESCO Biosphere Reserve in the background. The environs
of this bridge are the last known habitat of two specimens of
Mediterranean monk seal, before the bridge was bombed and rocketed by
Yugoslav Army units in 1992. |
|
|
Fig. 37. Some dolphins of dolphin school A2. |
|
| Fig. 38. Some dolphins
of dolphin school B1. |
|
| Fig. 39. Some dolphins
of dolphin school B3. |
|
| Fig. 40. Some dolphins
of dolphin school C1 |
|
| Fig. 41. A dolphin of
dolphin school D1. |
|
|
Aircraft C took off at 9:22 from Grobnik Airport and
flew a pre-programmed course (red line on Figs. 32,
33a, b, c
and d). At 11:30 and at position N44°13'10.3'';
E15°06'36.8'' it recorded dolphin school C1 (Fig. 40). This
school comprised six adult dolphins and one cub. This aircraft flew 432.5 km
until 12:28, when it landed at Split Airport Resnik, near Trogir. It took off
again at 13:31 and at 15:48 it recorded dolphin school C2 at position
N44°33'36.4''; E14°46'39.6''. This school comprised four adult dolphins (Fig.
30). The aircraft flew 431.9 km on its return flight and landed at the base
airport of Grobnik At 16:32. Altogether, it flew for 6.07 h and covered 864.4
km, 21.0 km of this overland and 843.4 km over the sea.
Aircraft D took off at 9:18 from Grobnik
Airport and flew a pre-programmed course (blue line on Figs. 32, 33a, b,
c). At 10:30 and at position N45°10'54.9''; E13°33'37.9'' it recorded
dolphin school D1 (Fig. 41). This was a single adult
dolphin. The aircraft flew 455,2 km until 12:42, when it landed at Zadar
Airport in Zemunik. It took off again at 13:07 and at 13:57 it recorded
dolphin school D2 at position N43°39'27.1''; E15°37'06.3''. This school
comprised six adult dolphins and one cub (Fig. 30).
The aircraft flew 382.2 km on its return flight and landed at the base
airport of Grobnik At 16:20. Altogether, it flew for 6.37 h and covered
837.4 km, 32.3 km of this overland and 805.1 km over the sea.
The summarised data of sighted bottlenose dolphin
schools on 27 December 1998 are presented in Table 1 and
in Figs. 30, 32, 33a,
b, c,
d and e.
All four aircraft were in the air for a total of
26 hours and 11 minutes and travelled 4,001.2 km, 110.8 km of this overland and
3,890.4 km over the sea. As the search strip (trackline - flight path) was four
km wide (two kilometres either side of the aircraft), all four aircraft surveyed
a total of 15,561.6 km2 of the Croatian part of the Adriatic
Sea. However, as the search strips intersected at a number of places (violet,
green, red, and blue shaded search strips in Figs. 33a,
b, c, d and e) the
actual total surveyed surface was only 14,507.4 km2,
or 1,054.2 km2
less than the absolute total. Average time of aircraft passes over
dolphin schools (3-6 passing rounds) for each of 18 groups was 3 minutes and 46
seconds (minimum 2 and maximum 14 minutes).
Table 1. Sighted schools of bottlenose
dolphins in the Croatian part of the Adriatic Sea on 27 December 1998, listed in
chronological order (double-bordered schools had travel speeds approaching 3.5
km/h, but their group sizes were more than double, and they were not resighted;
shaded schools were probably resighted) |
|
Identification
of dolphin
schools |
Position of
dolphin schools:
Lat(N) Long (E)
|
Time of sighting
(Central
European Time)
|
Composition of
dolphin school
adult+cub / total
|
Comments; differences in: time / distance; average speed between - -
differences of school size |
| A1 |
44ş 38' 14ş 10'
|
9:45
|
2+0 / 2
|
A6 (6.44
/ 21.3 km)
A1-A6=3.16 km/h - >50% |
| A2 same
as A4 |
43ş 58' 14ş 47'
|
10:25
|
5+0 / 5
|
A4 (5.40
/ 16.5 km)
A2-A4=2.9 km/h - <50% |
| D1 |
45ş 11' 13ş 34'
|
10:30
|
1+0 / 1
|
|
| B1 |
43ş 42' 15ş 46'
|
11:06
|
11+0 / 11
|
|
| B2 |
43ş 39' 15ş 49'
|
11:17
|
4+3 / 7
|
|
| C1 |
44ş 13' 15ş 07'
|
11:30
|
6+1 / 7
|
B7 (4.21 / 20.3 km)
B7-C1=4.7 km/h - >50% |
| B3 |
43ş 07' 16ş 31'
|
11:51
|
6+0 / 6
|
|
| D2 same as B5 |
43ş 39' 15ş 37'
|
13:57
|
6+1 / 7
|
possibly the same as B5
group size difference <50% |
| B4 |
42ş 45' 17ş 44'
|
13:58
|
5+1 / 6
|
|
| B5 same
as D2 |
43ş 40' 15ş 35'
|
15:18
|
4+0 / 4
|
D2
(1.21 / 2.5 km)
B5-D2=1.9 km/h - <50% |
| B6 |
43ş 42' 15ş 34'
|
15:20
|
6+0 / 6
|
|
| C2 |
44ş 34' 14ş 47'
|
15:48
|
4+0 / 4
|
*C2-B8=22.6 km/h - >50% |
| B7 |
44ş 17' 14ş 52'
|
15:51
|
2+0 / 2
|
C1 (4.21 / 20.3 km)
B7-C1=4.7 km/h - >50% |
| A3 |
43ş 54' 15ş 05'
|
15:52
|
2+0 / 2
|
|
| B8 |
44ş 31' 14ş 44'
|
16:04
|
9+0 / 9
|
*B8-C2=22.6 km/h - >50% |
| A4 same as A2 |
44ş 06' 14ş 52'
|
16:05
|
9+0 / 9
|
possibly the same as A2
group size difference <50% |
| A5 |
44ş 12' 14ş 43'
|
16:13
|
4+0 / 4
|
|
| A6 |
44ş 31' 14ş 23'
|
16:29
|
3+2 / 5
|
A1 (6.44 / 21.3 km)
A1-A6=3.16 km/h - >50% |
| * Distance
between C2 and B8 was 6.1 km, and time difference was 16 minutes; swimming speed
should have been 22.6 km/h, and group sizes were more than double. These are the
reasons why we are certain that C2 and B8 were two different dolphin schools.
|
|
|
Discussion::
Although numerous cetacean species - common
dolphin, bottlenose dolphin, striped dolphin, Risso's dolphin, harbour porpoise,
northern bottlenose whale, long-finned pilot whale, false killer whale, sperm
whale, fin whale, minke whale, northern right whale, Cuvier's beaked whale
(Hirtz, 1922, 1937, 1940; Fink, 1937; Riedel, 1963; Nishiwaki, 1972; Garms and
Borm, 1981; Gomercic and Huber, 1989; Notarbartolo di Sciara and Bearzi, 1992;
Gomercic et al., 1994a; Notarbartolo di Sciara et al., 1994; Gomercic et al.,
1998) have been found and described in the Adriatic Sea, on 27 December 1998
between 9 a.m. and 5 p.m. we saw only bottlenose dolphins on the surveyed
surface, i.e., 45.8% of the Croatian part of the Adriatic Sea. During that
period we saw no other species of marine mammal. This tends to confirm that
today, only the bottlenose dolphin is resident in the Adriatic Sea (Gomercic and
Huber, 1989; Bearzi and Notarbartolo di Sciara, 1995; Bearzi et al., 1998;
Gomercic et al., 1998).
In a study of the abundance of dolphins there are
two factors used for determination of group resightings (Carretta et al, 1998).
Dolphin schools are unique (not resighted - or not recaptured, according to
Carretta et al., 1998) if the distance between them would have required the
group to exceed an assumed maximum group travel speed of 3.5 km/h (between 1 and
6 km/h) (Cockcroft et al., 1992; Mate et al., 1995). Average group travel speed
is 3.5 km/h for the coastal bottlenose dolphins of South Africa (Cockcroft et
al., 1992). The second factor in the determination of school resightings is the
observed group size. If the sizes of twice sighted schools (at the same place -
or sufficiently close to the same place) at different times are less than double
it can be considered a resighted group. However, if the sizes are more than
double, the groups are not counted as resighted. According to these criteria
(criteria for discrimination of potentially resighted schools) of our 18 sighted
dolphin schools - schools A2 and B5 - were resighted (Table 1). For this reason,
schools A2 and B5 were not taken into consideration for further calculations.
Hence, on 27 December 1998 in the Croatian part of the Adriatic Sea, we saw 16
different bottlenose dolphin schools comprising a total number of 80 different
adult dolphins and 8 different cubs. Carretta et al. (1998) demonstrated by
using mark-recapture analysis that bottlenose dolphin counts obtained from
individual survey flights must be corrected by plus 53%, which was the result of
the tandem aerial surveys method. There was a close similarity in our study,
where two schools were sighted twice in a tandem survey (term by Carretta et
al., 1998). In school A2, five dolphins were counted during the first aircraft
pass, but 5 hours and 40 minutes later nine dolphins were counted in school A4
(probably the same school as school A2). School B5 comprised four dolphins, but
1 hour and 21 minutes earlier school D2 (probably the same school as school B5)
comprised seven dolphins. This means that in tandem surveys school A2/A4 was 80%
higher than the dolphin count obtained from the individual survey flight, and
tandem surveys of school B5/D2 showed a 75% higher figure than the dolphin count
obtained from individual survey flights. On the basis of our preliminary study
and tandem surveys of schools A2/A4 and B5/D2, it is clear that abundance
estimates from tandem surveys averaged 61.8% higher than dolphin counts obtained
from individual survey flights. According to Carretta et al. (1998) this is, on
average, 53% higher. Due to the fact that our method of aerial survey (flight at
an altitude of almost 200 m and at an averaged airspeed of 158 km/h) of dolphin
number estimation is very similar to the methods of Carretta et al. (1998) -
flying at an altitude of 213 m and at an airspeed of 185 km/h - we accept that
counts obtained from individual survey flights must be increased by only 53% to
achieve an exact number of dolphins on the surveyed sea surface. |
|
| Fig. 42. The islands
of Ilovik (right) and Sv. Petar (left) at the junction of Kvarneric and
Kvarner. In the background (left) is the island of Silba where a young
fin whale (11 m long) was stranded in 1990. |
|
| Fig. 43. Mount
Biokovo, in the southern part of the Croatian Adriatic coast, behind the
Makarska Riviera. The Peljesac Peninsula can be seen in the background. |
|
| Fig. 44. Komiza, a
small town on the west coast of Vis Island, with numerous fishermen and
a developed fishing industry. |
|
| Fig. 45. Svetac
Island and Kamik islet, and the volcanic island of Brusnik in haze.
There are many caves here once used by monk seals. |
|
|
Fig. 46. Palagruza Island, the most distant
offshore Croatian island. In spring and summer 1993 a specimen of Mediterranean
monk seal was seen here. |
|
| Fig. 47. Galijula
islet, the most southerly point of Croatian land, at N42o22'53;
E16o19'58'' and Prof. Dr. Hrvoje Gomercic, the author of this
Report. Palagruza Island can be seen in the background. (Photo: T.
Gomercic). |
|
| Fig. 48. The
Prevlaka Peninsula, with Cape Ostra (N42°23'24'';E18°32'24''),
the southernmost part of continental Croatia. In the background is the
entrance to Boka Kotorska Bay, and the Montenegrin coast, with Mount
Lovcen (1,749 m) under snow. |
|
|
For calculating the total number of dolphins on the
surveyed surface of the Adriatic Sea we totalled all our sighted dolphins,
except for schools A2 and B5 (these were considered as resighted) and schools D2
and A4 (these were tandem aerial surveyed). The sum was 72 dolphins in 16 school
plus 53%. This resulted in 110 dolphins in 24 schools plus D2 and A4 (these were
tandem aerial surveyed), which amount to a total of 126 dolphins (including 12
cubs) in 26 schools. According to this calculation and our data (Table
1) we can conclude that on the surveyed surface of the Adriatic Sea
(14,507.4 km2) there were 126 bottlenose dolphins, and that of this
number 10% were cubs. Also, that they were grouped in 26 dolphin schools.
The total surface area of the Croatian part of
the Adriatic Sea is 31,660 km2. The aerially surveyed surface
area of the Adriatic Sea was 45.8% of the total surface area of the Croatian
Adriatic Sea. If we calculated the number of dolphins for the entire Croatian
part of the Adriatic Sea, we would expect that in the whole Croatian part of the
Adriatic Sea on 27 December 1998 there lived 57 bottlenose dolphin schools with
249 adult animals and 26 cubs (a total of 275 individuals). Average school size
was 4.8.
The surface area of inshore waters is 15,254 km2,
and of offshore waters 16,406 km2. We defined all sea surface area
over 5 km from the outermost major islands as offshore waters. The aerially
surveyed surface area of inshore waters measured 11,752 km2
(77%), while the surveyed surface area of offshore waters measured 2,755 km2
(16.8%). All sighted dolphin schools were in inshore waters, except for school
A2, which was in offshore waters 19.0 km from the nearest coastline of Dugi Otok
island. In A2 school we sighted five adult dolphins, but 5 hours and 40 minutes
later this school was found in inshore waters, recorded as A4 dolphin school (Table
1). The actual number in this school (A2/A4) was nine adult dolphins.
According to the foregoing, we were able to calculate that the total number of
dolphins in surveyed (77%) inshore waters was 114 adult bottlenose dolphins and
12 cubs in 26 dolphin schools, or 148 adults and 16 cubs in 34 bottlenose
dolphin schools in the entire (100%) inshore waters.
The actual total number of dolphins in the
surveyed (16.8%) surface area of offshore waters was nine adult dolphins in one
school, or, by calculation, 54 adult dolphins in 6 schools in the entire (100%)
offshore waters.
When calculating the number of dolphins in
inshore and offshore waters separately, we can expect that in the whole of
Croatian territorial waters on 27 December 1998 there lived 40 bottlenose
dolphin schools, comprising 202 adult animals and 16 cubs (a total of 218
individuals). Average school size was 5.5.
This estimated abundance is in accordance with
our very rough population estimate made in 1993, in which it was speculated that
some 150-300 individual bottlenose dolphins live in the Croatian part of the
Adriatic Sea (Gomercic et al., 1994).
One or more cubs were present in 31.3% of dolphin
schools.
The estimated abundance of bottlenose dolphins in
the Croatian part of the Adriatic Sea is: in inshore waters 10.75/1,000 km2,
and in offshore waters 3.3/1,000 km2.
To conclude this Discussion, we stress that
the size and composition of particular dolphin schools were not able to be
precisely photo-documented. This was because it was not feasible to see all
animals of the school at the same moment on the sea surface and on the same
screen.
Conclusion:
On 27 December 1998 we saw 16 different
bottlenose dolphin ((Tursiops truncatus) schools comprising a total number
of 80 different adult dolphins and 8 cubs in 45.8% of the total surface area of
the Croatian part of the Adriatic Sea.
From this data we calculated that on this surface
area (14,507.4 km2) there were 114 adult bottlenose dolphins
and 12 cubs in 26 dolphin schools. We calculate that in the whole of the
Croatian part of the Adriatic Sea (31,660 km2) there were 40
bottlenose dolphin schools comprising a total of 218 specimens (202 adult
animals and 16 cubs).
In the winter period in question, dolphin cubs
accounted for 10% of the population, while 31.3% of dolphin schools had one or
more cubs that were smaller by about one-third of their body length compared to
adults. Average school size was 5.5.
The estimated abundance of bottlenose dolphins in
the Croatian part of the Adriatic Sea is: in inshore waters 10.75/1,000 km2,
and in offshore waters 3.3/1,000 km2.
Only one sighted bottlenose dolphin school was in
offshore waters, 19.0 km from the nearest coast, but 5 hours and 40 minutes
later this school was sighted in inshore waters. All other sighted bottlenose
dolphin schools were in internal waters, or less than 5 km from the coast.
Not one specimen of any other marine mammal
species was observed.
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Authors:
-
Hrvoje
Gomercic
-
Duro Huber
-
Tomislav Gomercic
-
Hrvoje Lucic
-
Damir Mihelic
-
Martina Duras
Credits::
-
Report conducted for the Regional
Activity Centre for Specially Protected Areas (United Nations Environment
Programme - - Mediterranean Action Plan), Tunis and
-
The Faculty of Veterinary
Medicine, University of Zagreb, Zagreb as a contribution to the Action Plan
for the Conservation of Cetaceans in the Mediterranean Sea by Hrvoje Gomercic,
Department of Anatomy, Histology and Embryology, Faculty of Veterinary
Medicine, University of Zagreb, Zagreb
Financed by RAC/SPA (UNEP/MAP),
Tunis, Tunisia State Directorate of the Environment, Republic of Croatia, Zagreb
Ministry of Development and Reconstruction, Republic of Croatia, Zagreb
With the contribution of Ministry of
Sciences and Technology, Republic of Croatia, Zagreb, Faculty of Veterinary
Medicine, University of Zagreb, Zagreb
Zagreb, December 1998
Source:
See also:
|
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