Studies on the ecology and conservation status of short-beaked common dolphins (Delphinus delphis) and common bottlenose dolphins (Tursiops truncatus) in the Mediterranean Sea
General introduction
1. Context
2. Study areas and rationale for choosing them
4. Overview of total field research effort
5. List of articles and interpretation of results
6. Conclusions
1. Context
Dolphins have been reported to swim in Mediterranean waters since the times of Aristotle, when they were certainly more abundant components of the marine fauna than today. Aristotle himself – incidentally - provided the first cetological description ever, the first scientific account for Greek coastal waters (1), and the first report of long-term individual identification based on fin markings:
"The dolphin is provided with a blowhole and lungs... and has been seen asleep with his beak above the water, and when asleep he snores. No one is ever been to be supplied with eggs, but directly with an embryo, just as in the case of mankind and viviparous tetrapods. Its period of gestation is ten months, and it brings forth its young in the summer. The dolphin is provided with milk and suckles its young... which accompany it for a considerable period. In fact, the creature is remarkable for the strength of its parental affection. The young grow rapidly, being full grown at ten years of age. It lives for many years; some are known to have lived for more than 25, and some for 30 years; the fact is fishermen nick their tails sometimes and set them adrift again, and by this expedient their ages are ascertain." (Historia Animalium, 350 B.C.)
Regrettably, 2,353 years after this first account information on the ecology, status and conservation of Mediterranean dolphins is still surprisingly poor. As stigmatised by Notarbartolo di Sciara & Gordon (1997)(2):
"One of the first difficulties encountered in the attempts to solve Mediterranean whale and dolphin conservation problems is a lack of adequate knowledge of population distribution, size, discreteness, trends, and dynamics for any of the cetacean species."
I will try to fill in part this gap by presenting data resulting from research conducted on coastal dolphins in two Mediterranean areas. Since 1987 (initially in the context of my “Laurea” degree at the University of Padua, Italy), I have been focusing on the ecology and conservation status of two threatened cetacean species: the short-beaked common dolphin (3) (Delphinus delphis) and the common bottlenose dolphin (4) (Tursiops truncatus). Groups living in two Mediterranean areas - the northern Adriatic Sea and the eastern Ionian Sea - became the focus of my investigations. Extensive field research conducted in those areas over the past decade enabled to describe aspects of the natural history of both dolphin species.
In this dissertation I aim at providing an overview on the status and problems of D. delphis and T. truncatus by means of a selection of articles. The outlined research evaluates aspects of the ecology, behaviour and social organisation of two dolphin species living in Mediterranean coastal waters, and sheds light on the possible reasons that determined the decline of short-beaked common dolphins in portions of their former range.
The cumulative results presented here contribute to the understanding of cetacean ecology and conservation status in Mediterranean waters, and provide some of the needed baseline data from which effective management and conservation strategies can be designed.
Footnotes
(1) Frantzis,
A., Alexiadou, P., Paximadis, G., Politi, E., Gannier, A. & Corsini-Foka,
M. In press. Current knowledge on the cetacean fauna of the Greek Sea.
Journal of Cetacean Research and Management.
(2) Notarbartolo
di Sciara, G. & Gordon, J. 1997. Bioacoustics: A tool
for the conservation of cetaceans in the Mediterranean Sea. Marine Freshwater
Behaviour
and Physiology 30:125-146.
(3) Hereafter “common
dolphin”.
(4) Hereafter “bottlenose dolphin”.
2. Study areas and rationale for choosing them
Field research was conducted in three central Mediterranean study areas. Two of these are situated in the northern Adriatic Sea, and one in the eastern Ionian Sea (Fig. 1). In addition, I have included articles that focus on the whole Mediterranean Sea - to review common dolphin ecology, status and conservation - and on the northern Adriatic Sea - to review the past and present status of cetaceans in this region.
Fig. 1. Location of the three study areas where field research was conducted between 1987-2003. 1) Gulf of Venice, northern Adriatic Sea (1988-2002); 2) Kvarneric, northern Adriatic Sea (1987-1994); 3) Island of Kalamos, eastern Ionian Sea (1993-2003).
Northern Adriatic Sea
The shallow northern Adriatic Sea was chosen for its long history of human impact and habitat degradation, and because preliminary surveys conducted by the candidate and by the Tethys Research Institute (5) indicated that the waters of Kvarneric, Croatia, could represent an ideal place to start a longitudinal study focusing on the local bottlenose dolphin groups. In addition, our observations together with limited literature data (6) suggested that while bottlenose dolphins could be found predictably, common dolphins had disappeared from northern Adriatic waters in recent times. This made it an interesting area to investigate the factors that can promote the decline of coastal species. Preliminary research in the Kvarneric started in 1987 and is still ongoing (7), making the local bottlenose dolphin community the most intensively studied and best known in the Mediterranean Sea. Here, I have included published information obtained between 1987-1994.
Between 1988-2002, observations conducted in the Kvarneric were supplemented by the opportunistic collection of sighting records in northern Adriatic open waters, and by dedicated cetacean surveys conducted off Venice and the Po river in 2001 and 2002. These surveys were aimed at a preliminary assessment of cetacean density, to identify the most appropriate research approach and promote dolphin monitoring in the region.
Eastern Ionian Sea
When I first joined the project that my friend and colleague Elena Politi had started a few years earlier around the island of Kalamos, Greece, I was stunned by the abundance of common dolphins, and impressed by the presence of a few sympatric bottlenose dolphins. That looked to me like a truly pristine situation that was ideal to allow comparisons with the northern Adriatic, where D. delphis had gone and only T. truncatus remained. At Kalamos - I thought - common dolphins were the most successful top predators, those who “won” the local evolutionary battle. The possibility of studying common dolphins in a coastal environment represented a valuable opportunity considering the general lack of information on the few D. delphis groups living in the central Mediterranean. I was hoping to bring new evidence on the critical habitat needs by common dolphins, as well as on the reasons that prompted their decline in other areas. Unfortunately, a decade-long study in eastern Ionian Greece showed that my first years at Kalamos may have been the last opportunity to see those animals thriving. Since 1997, spotting common dolphins has become increasingly difficult even in this area.
Footnotes
(5) Bearzi, G. 1989. Contributo alle conoscenze sulla biologia di
Tursiops truncatus (Montagu, 1821) nel mare Adriatico settentrionale.
Degree
in Biological Sciences unpublished thesis. University of Padova, Italy.
172 pp.
Notarbartolo di Sciara, G., Venturino, M.C., Zanardelli,
M., Bearzi, G., Borsani, J. F. & Cavalloni, B. 1993. Cetaceans
in the central Mediterranean Sea: Distribution and sighting frequencies.
Bollettino
di Zoologia 60:131-138.
(6) e.g. Pilleri, G. & Gihr, M. 1977. Some records of cetaceans
in the Northern Adriatic Sea. Pp. 85-88 in: G. Pilleri (Ed.), Investigations
on Cetacea, Vol.
8.
(7) Since 2000, the Adriatic Dolphin Project is being carried
out by the Croatian organisation Blue World.
3. Methodological approach
Since the beginning of my studies, I have preferred to work on small boats. These offer a number of benefits for coastal dolphin studies. Small boats allow to minimise disturbance while observing dolphins for protracted periods of time, and they facilitate the kind of close approaches that are necessary to take good photos of individual dorsal fins, suitable for photo-identification purposes. Small inflatable craft with fiberglass keels such as those used to record most of the data presented here offer additional advantages. When drifting or moving at slow speeds they do not roll as much as boats with V-shaped keels. They offer comfortable seating and comparatively smoother navigation with wavy sea. They are lighter than rigid boats, therefore reducing gas consumption and allowing the use of less powerful engines... I have always liked to work on small inflatables and I feel “at home” there. Conversely, I’m not an offshore person and I can get badly seasick on a rolling ship.
Inflatables with fiberglass keels between 4.7-5.8 m were used to run the longitudinal studies in Croatia and Greece. In the Gulf of Venice, the open sea environment made us choose a sturdy 7 m speedboat or, alternatively, large research vessels.
Individual photo-identification based on long-term natural marks is a popular method to study cetaceans, worldwide. It is amazingly rewarding to shoot the right photo at the right time, not only from a scientific standpoint. For this study, a total of 36,629 selected slides were included in photo-identification catalogues. Such large catalogues allowed to perform a number of studies, but indeed much remains to be done. Only a portion of the information that may be derived from our catalogues has been presented here. I hope that future research will benefit from the large body of information that has been collected over several years of extensive photo-identification effort - ranging from social organisation to habitat use by individual dolphins.
Recording dolphin behaviour at sea is a demanding task that has been the subject of criticism owing either to methods that are often inappropriate (8), or to a generally skeptical attitude towards its potential benefits. My colleagues and I share the view that behavioural studies represent an important complement to other research approaches. However, we must acknowledge that observing, recording and analysing the behaviour of animals that spend the majority of their time below the surface is a very demanding task, both in the field and at the lab. Between 1991-1994, we recorded bottlenose dolphin behaviour in the Kvarneric for a total of 592 h, based on a 3-min behavioural sampling routine. Some results from this study have been included in one of the published contributions included in this thesis. In addition, between 1997-2002 we recorded the surface behaviour and respiration pattern of common and bottlenose dolphins at Kalamos over more than 1,000 h of behavioural sampling. This second dataset is so large and complex that the analyses will keep us busy for the next several years. Although I could not manage to have it included in this thesis, this dataset served to supplement some of the analyses presented here.
Other methods used across this study have included the collection of drifting fish scales during surface feeding by common dolphins and the analysis of individual bottlenose dolphin photos aimed at the assessment of their nutritional condition. Throughout our work in the northern Adriatic and eastern Ionian Seas, we have always tried to adopt an approach that was as multidisciplinary as possible, engaging in the collection of very diverse data. Today, the challenge facing us is analysing and publishing all this information before it ages.
During most of the time our research approach has been deliberately non-intrusive, meaning that we did no harm to the animals apart from bothering them a little with our boat during photo-identification approaches. However, remote biopsy sampling was occasionally adopted to collect skin and blubber samples suitable for genetic and toxicological analyses, performed by the University of Durham, U.K., and by the University of Siena, Italy, respectively. The rationale and results of Tethys’ biopsy sampling campaigns have been partly reviewed by Bearzi (2000) (9), who stressed the need of adopting a precautionary approach in light of a dramatic event that involuntarily caused the death of a common dolphin. While analyses based on biopsy samples provided information of great importance for conservation and management purposes, which complemented some of the articles presented here, nothing can bring that dolphin back to life.
Footnotes
(8) e.g. Mann, J. 1999. Behavioral sampling methods
for cetaceans: a review and critique. Marine Mammal Science 15(1):102-122.
(9) Bearzi, G. 2000. First report of a common dolphin (Delphinus
delphis) death following penetration of a biopsy dart. Journal of Cetacean Research
and Management 2(3):217-221.
4. Overview of total field research effort
The work presented in this thesis covers a time span of 16 years. However, the research effort was quite uneven among study areas. Fig. 2 outlines the distribution of the survey effort between 1987-2002.
Fig. 2. Survey effort in years 1987-2003.
In the Kvarneric, northern Adriatic Sea, and around the island of Kalamos, eastern Ionian Sea, the photo-identification effort has been intensive. The cumulative number of selected photographs included in the catalogues considered for the work presented here is shown in Figg. 3 and 4. A total of 13,806 bottlenose dolphin photos were catalogued for the Kvarneric study.
Fig. 3. Cumulative number of photographs included in the bottlenose dolphin photo-identification catalogue in the Kvarneric, northern Adriatic Sea, between 1987-1994.
At Kalamos, the catalogues included 16,693 photos of common dolphins and 5,764 photos of bottlenose dolphins.
Fig. 4. Cumulative number of photographs included in photo-identification catalogues at Kalamos, eastern Ionian Sea, between 1991-2002. Tt = Tursiops truncatus, Dd = Delphinus delphis.
These graphs are intended to give an idea of the research effort. However, as methods varied among areas it is difficult - and probably futile - to provide further cumulative details. Detailed descriptions of the effort and methods for each study can be found in the articles included in the thesis.
5. List of articles and interpretation of results
Click here to see separate section
6. Conclusions
While working on some of the articles included here, I have learned to appreciate the power of extensive literature reviews combined with longitudinal field studies. While it is in the field that one can get a grip on the present status of cetacean populations, historical literature is extremely important to understand how the situation was likely to be in the past, before the impact of human activities became such a tremendous threat. Past records often look disappointing as many of them are non-quantitative, of uncertain reliability, or lacking key elements. Still, a critical evaluation of the available historical literature allows to place our baselines in the right place, and glance at what might have been the animals' “original state”.
Shifting baselines (10) can affect our capability to mitigate threats by preventing to put cause-effect relationships into the right historical context. Documenting and acknowledging the long-term impact of mistakes done in the past is very important. As far as Mediterranean cetaceans are concerned, placing our baselines only 30-40 years ago, i.e. when most cetacean studies begun and quantitative evidence started to be produced, may be misleading. Some evidence presented here actually suggests that events having occurred prior to the 1970s - a veritable turning point in the status of marine resources - may have had a significant impact.
When I think about how the Mediterranean looked a few decades ago, when I was a kid in love with the sea, I cannot hold a shiver of sadness. So many of those beautiful animals have become a rare sight.
Still, a few dolphins are left, not far from here. I hope that the future human generations will have opportunities to see and enjoy them the way I have.
Venice,
Italy
June 2003
Footnotes
(10)
Olson, R. 2002. Slow-motion disaster below the waves. L.A. Times, Sunday
Opinion
Section, November 17, 2002.
Pauly, D. 1995. Anecdotes and the shifting baseline syndrome of fisheries.
Trends in Ecology and Evolution 10:430.
Also see: http://www.shiftingbaselines.org/slideshow/index.html