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Short-beaked common dolphins around the island of Ischia, Italy (southern Tyrrhenian Sea)

Authors:
Barbara Mussi at Oceanomare Delphis
Barbara Mussi
  • Oceanomare Delphis
Angelo Miragliuolo
Angelo Miragliuolo
Giovanni Bearzi at Dolphin Biology and Conservation
Giovanni Bearzi
  • Dolphin Biology and Conservation
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Abstract

The short-beaked common dolphin (Delphinus delphis) -once one of the commonest cetacean species in the Mediterranean Sea -has faced a dramatic decrease in the last few decades. Between July-September 1997-2001, relatively large groups (mean group size = 65.48 SD=23.94, N=41, range 35-100) were consistently encountered off the island of Ischia, Italy. Observations, totalling 79 h 45 min, were carried out from a 18 m sailing vessel. Most of the survey effort was concentrated north of the island, within 11 km from the coast, during 256 daily surveys covering nearly 8,500 km. The animals were always sighted over the submarine canyon of Cuma, a highly productive marine area characterised by high pelagic biodiversity and multi-species associations. The area represents an important feeding site for other cetacean species, including Stenella coeruleoalba, Grampus griseus and Balaenoptera physalus. A total of 46 individuals could be opportunistically photo-identified based on natural marks on their dorsal fins. Of these, 19 individuals were re-sighted in different years, suggesting high levels of site fidelity. Breeding activities were often observed, and a high percentage of calves were always present in one or more of the group sub-units. Surface feeding was recorded frequently, and apparently focused on the skipper (Scomberesox saurus), a seasonal fish that is highly valued on local markets. Local fishermen claim that co-operative fishing may occur in the area, with fishermen taking advantage of fish aggregations that are actively schooled by short-beaked common dolphins near the surface. In the past, fish rewards were offered to the dolphins in reciprocation. Based on interviews conducted locally, the skipper fishery fleet has decreased by one order of magnitude due to declined fish stocks. The creation of a marine protected area has been proposed, which might provide research and conservation benefits in the future.
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SHORT-BEAKED COMMON DOLPHINS AROUND THE ISLAND OF ISCHIA, ITALY
(SOUTHERN TYRRHENIAN SEA)
Mussi, B. (1), A. Miragliuolo (1) and G. Bearzi (2)
(1) Studiomare, via P.L. D'Abundo 82, 80075 Foro d'Ischia (NA), Italy
(2) Tethys Research Institute, c/o Acquario Civico, Viale G.B. Gadio 2, 20121 Milano, Italy
Abstract:
The short-beaked common dolphin (Delphinus delphis) - once one of the commonest cetacean species in the
Mediterranean Sea - has faced a dramatic decrease in the last few decades. Between July-September 1997-2001,
relatively large groups (mean group size = 65.48 SD=23.94, N=41, range 35-100) were consistently encountered off the
island of Ischia, Italy. Observations, totalling 79 h 45 min, were carried out from a 18 m sailing vessel. Most of the
survey effort was concentrated north of the island, within 11 km from the coast, during 256 daily surveys covering
nearly 8,500 km. The animals were always sighted over the submarine canyon of Cuma, a highly productive marine
area characterised by high pelagic biodiversity and multi-species associations. The area represents an important feeding
site for other cetacean species, including Stenella coeruleoalba, Grampus griseus and Balaenoptera physalus. A total of
46 individuals could be opportunistically photo-identified based on natural marks on their dorsal fins. Of these, 19
individuals were re-sighted in different years, suggesting high levels of site fidelity. Breeding activities were often
observed, and a high percentage of calves were always present in one or more of the group sub-units. Surface feeding
was recorded frequently, and apparently focused on the skipper (Scomberesox saurus), a seasonal fish that is highly
valued on local markets. Local fishermen claim that co-operative fishing may occur in the area, with fishermen taking
advantage of fish aggregations that are actively schooled by short-beaked common dolphins near the surface. In the
past, fish rewards were offered to the dolphins in reciprocation. Based on interviews conducted locally, the skipper
fishery fleet has decreased by one order of magnitude due to declined fish stocks. The creation of a marine protected
area has been proposed, which might provide research and conservation benefits in the future.
INTRODUCTION: Short-beaked common dolphins in the Mediterranean have undergone a
dramatic decline in abundance during the last few decades, and have almost completely disappeared
from large portions of their former range. This study provides information about a relic population
unit in the central Mediterranean.
METHODS: Observations, totalling 79h 45min, were carried out between July-September from a
18m sailing vessel. Most of the survey effort was concentrated north of the island of Ischia, within
11km from the coast, during 256 daily surveys covering nearly 8,500km.
RESULTS: A total of 51 short-beaked common dolphin sightings were between 1997-2001 (Fig.
1). Group size data presented here are based on a sub-sample of 41 sightings, for which best group
size estimates were available. Delphinus groups were relatively large (mean=65.5 SD=23.94, N=41,
range 35-100 individuals). In 18% (N=41) of the sightings, short-beaked common dolphins were in
mixed groups with striped dolphins (Stenella coeruleoalba).
The animals were mostly sighted over the submarine canyon of Cuma, a highly productive marine
area characterised by high pelagic biodiversity and multi-species associations. The area represents
an important feeding site for other cetacean species, including striped dolphins, Risso’s dolphins
(Grampus griseus), and fin whales (Balaenoptera physalus; Mussi et al. 2001).
A total of 46 individuals could be photo-identified based on long-term natural marks on their dorsal
fins (both nicks on the dorsal fin’s trailing edge and fin pigmentation pattern where considered). Of
these, 19 individuals were re-sighted in different years, suggesting high levels of site fidelity.
Breeding activities were often observed, and calves were always present in one or more of the
group sub-units.
Surface feeding occurred frequently and appeared to focus on skippers, Scomberesox saurus, a
seasonal fish that is highly valued on local markets.
Local fishermen claim that co-operative fishing may occur in the area, with fishermen taking
advantage of fish aggregations that are actively schooled by short-beaked common dolphins near
the surface. In the past, fish rewards were offered to the dolphins in reciprocation. Based on
interviews conducted locally, the skipper fishery fleet has decreased by one order of magnitude in
the last two decades due to declined fish stocks.
DISCUSSION: Observations took place in the busy summer seasons, when pleasure boats and
ferries crowd these waters. Commercial and passenger traffic in the Gulf of Naples and nearby
islands (Ischia, Procida and Vivara) reportedly exceeds 200,000 trips/year, and up to 2,000 pleasure
boats may be moored during the summer in the ports of Ischia.
Ship collisions in the area have been documented for cetacean species including striped dolphins,
common bottlenose dolphins (Tursiops truncatus) and fin whales, while a dramatic harassment
event on Risso's dolphins has been reported by Miragliuolo et al. (2001). Despite the relatively high
abundance of vulnerable cetacean population units, the waters around Ischia are commonly used for
extemporaneous offshore races, implementation of coastal speed limits being virtually null.
Another potential threat to short-beaked common dolphins and other cetaceans in the area is
represented by the illegal driftnet fishery, sadly known for the heavy toll paid by Mediterranean
cetaceans (Di Natale and Notarbartolo di Sciara 1994, IWC 1994, Silvani et al. 1999). This fishery
is reportedly impacting local cetacean communities, with documented bycatch events involving
striped dolphins, common bottlenose dolphins, sperm whales (Physeter macrocephalus) and fin
whales.
The creation of a marine protected area has been proposed, with the intent of supporting
conservation actions and ultimately protect the local cetacean fauna against threats posed by human
activities.
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Pages 1-72 in W. F. Perrin, G. P. Donovan and J. Barlow (eds.) Gillnets and Cetaceans. Rep. Int.
Whal. Commn. Spec. Iss. 15, Cambridge.
Miragliuolo, A., Mussi, B., Bearzi, G., 2001. Risso's dolphin harassment by pleasure boaters off the
island of Ischia, Central Mediterranean Sea. In European Research on Cetaceans - 15. Proc. 15th
Ann. Conf. ECS, Roma, Italy (in press).
Mussi, B., Miragliuolo A., De Pippo, T., Gambi, M.C. and Chiota, D., 2001. The submarine canyon
of Cuma (Southern Tyrrhenian Sea, Italy), a cetacean key area to protect. In European Research on
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Silvani, L., Gazo, J.M., Aguilar, A. 1999. Spanish driftnet fishing and incidental catches in the
western Mediterranean. Biological Conservation 90:79-85.
... In shallower waters, it can be observed in mixed groups with the common bottlenose dolphin Tursiops truncatus [5,6]. In offshore waters over the continental slope it can be observed in mixed groups with the striped dolphin Stenella coeruleoalba [3,[7][8][9][10][11][12][13][14]. A specific case exists in the Gulf of Corinth where D. delphis has usually been recorded in mixed groups with the striped dolphin and the Risso's dolphin Grampus griseus [15][16][17][18][19]. ...
... The small-sized aggregation of common dolphin groups is probably due to a behavioral feeding strategy caused by the distribution of prey (small pelagic fish) over many small patches in the study area [5,28,54], which leads dolphins to split into smaller groups for more efficient predation [55]. On the other hand, the presence of highly productive marine areas and the high seasonal abundance of prey leads to the formation of larger groups of D. delphis as observed in the western Alborán and southern Almeria [4,11], in the Tyrrhenian Sea [8,13,14] and in waters off Malta Island [31]. Indeed, the presence of small pelagic fish such as sardine and anchovy resulted in an important explanatory variable in the modelling of the distribution of the common dolphin as reported by the authors of [28] in the Aegean Sea. ...
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Bearzi G., Notarbartolo di Sciara G., Reeves R.R., Canadas A., Frantzis A. 2004. Conservation Plan for short-beaked common dolphins in the Mediterranean Sea.
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A review of the passive fishing nets and trap fisheries in the Mediterranean Sea and of the cetacean bycatch
Article
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Spanish driftnet fishing and incidental catches in the western Mediterranean
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  • BIOL CONSERV
The Spanish driftnet fishery operating since 1994 on the Mediterranean side of the Gibraltar Straits was an illegal activity whose main target was the swordfish Xiphias gladius but which also caught other species incidentally, particularly sunfish Mola mola. Observations during the 1993 and 1994 seasons showed that the fleet was composed of 27 boats deploying nets 3–4 km long. Swordfish constituted 7% of the catch in 1992, 5% in 1993 and 7% in 1994. Sunfish represented 71% of the catch in 1992, 93% in 1993 and 90% in 1994. Sea turtles, mainly loggerhead turtles Caretta caretta, constituted 0.32% of the total catch in 1993 and 0.92% in 1994. The incidental capture of small cetaceans, composed entirely of common dolphins Delphinus delphis in 1992 and of striped dolphin Stenella coeruleoalba and common dolphins in roughtly equal proportions in 1993 and 1994, constituted 0.9% of the catch in 1992, 0.6% in 1993 and 1% in 1994. The bycatch rate of dolphins was 0.1 individuals per km of net set. The total catch of dolphins can therefore be estimated at 366 (95% confidence interval 268–464) animals for the 1993 fishing season and 289 (CI 238–340) for that of 1994. If these figures are added to the undetermined catches of dolphins by the Italian and Moroccan driftnet fleets also operating in the region, it is possible that these catches are not sustainable. ©
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Report of the workshop on mortality of cetaceans in passive fishing nets and traps. Pages 1-72 in
  • Jan 1994
  • Iwc
IWC. 1994. Report of the workshop on mortality of cetaceans in passive fishing nets and traps. Pages 1-72 in W. F. Perrin, G. P. Donovan and J. Barlow (eds.) Gillnets and Cetaceans. Rep. Int. Whal. Commn. Spec. Iss. 15, Cambridge.