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Tesi etd-06032015-175346

Thesis type
Tesi di dottorato di ricerca
A demographic approach to cetacean conservation based on photo-identification and stranding data
Settore scientifico disciplinare
Corso di studi
tutor Prof. Santangelo, Giovanni
commissario Prof. Luschi, Paolo
commissario Dott. Mazzariol, Sandro
commissario Dott. Haslberger, Alexander
commissario Prof. Barale, Roberto
commissario Prof. Inga, Alberto
commissario Prof. Manfredi, Pietro
Parole chiave
  • Demography
  • conservation
  • long-lived species
  • fin whale
  • Mediterranean Sea
  • life-history table
  • bottlenose dolphin
Data inizio appello
Data di rilascio
Riassunto analitico
The preservation of the biodiversity is nowadays one of the most difficult challenges for conservation biologists, as an increased number of species has been threatened or overexploited in the last years. Demography, “the science of the populations”, can furnish useful tools to improve natural population management and conservation. The knowledge of life-history traits and demographic parameters is needed to develop demographic models; these models, based on life-history tables and transition matrices, are essential to foster population conservation, as they allow to assess population performance, project population trends overtime and then suggest specific measures for their protection. This is mostly meaningful with long-lived and slow reproducing species, as cetaceans, threatened by many sources of mortality and environment stress. Nevertheless, few studies deal with the demography of these marine mammals because of the difficulty in collecting this kind of data, thus the main cetacean life-history traits are still largely unknown, especially in the Mediterranean Sea.
The aim of my PhD thesis is to improve the demographic knowledge on Mediterranean fin whale (Balaenoptera physalus Linneaus 1758) and bottlenose dolphin (Tursiops truncatus Montagu, 1821), analyzing stranding and photo-identification data.
The most exhaustive long-term photo-identification dataset available for the Mediterranean fin whale, recorded by Tethys Research Institute in the Pelagos Sanctuary over 1990-2007, was analyzed in order to obtain demographic information on the only mysticete regularly occurring in our basin. The analysis of the annual encounter rate revealed an uneven occurrence of the fin whale across the years and months: an anomalous reduction between 2001 and 2004 and a higher abundance of the target species in the Corso-Ligurian Basin in late-spring and summer were observed. Using the photo-identification technique, 431 different fin whales were recognized, 318 of known size: 6 calves (≤10 meters), 33 immatures (10-15 m), 261 adolescents-adults (>15 <20 m) and 18 olds (≥20 m). The size of the sub-population of individuals > 15 m over 1990-1999 was 539 (95% CI = 345-732), using the capture-mark-recapture POPAN model. After correcting the number of calves by a mathematical approach, a static-life history table was set out for the period 1990-2007. The highest survival was between calf and immature (61.1%), while the minimum between adolescent-adult and old (2.5%) vital stages. The overall life expectancy was 6.3 years and 14.3 years at entry in the adolescent-adult stage.
Stranding data are a valuable and quite inexpensive source of information on demography of marine mammals. The analysis of 1105 bottlenose dolphins stranded along Italian and French Mediterranean coasts between 1986-2011 highlighted a not homogeneous spatio-temporal distribution. The highest number of specimens/coastal km were recorded along Italian Adriatic Sea and a significantly higher number and homogeneous distribution of strandings were found over 1999-2011. The finding of newborns throughout the year revealed a continuos reproduction of the target species in the Mediterranean. The sex ratio of strandings was significantly biased toward males (1.38:1). A mortality table was set out sharing female of known size (n=308) into 4 vital stages: cub (≥90 <180 cm, 0-1 yr), calf (≥180 <220 cm, ≥1 <3 yr), juvenile (≥220 <270 cm, ≥3 <7 yr), adult (≥270 cm, ≥7 <57 yr). The population structure is broadly consistent with a steady-state population, cub vital stage shows the highest mortality rate per year and the lower life expectancy. Population replacement is ensured if the fraction of individuals still alive at the maximal age (57 yr) ranges between 1% and 4%, depending on the values of the fertility rate (the average number of cubs delivered yearly by each female).
Photo-identification long-term studies have greatly enhanced the possibilities to collect demographic information on wild living cetacean populations. Female bottlenose dolphin photo-identification data recorded in the eastern Ligurian Sea between 2006 and 2014 were analyzed to find out female reproductive parameters. The individual life-histories of 41 reproductive females and their 70 cubs were analyzed. The average fertility rate was 0.4 dolphins/yr, the first-year cub mortality ranged between 0.17 and 0.47 dolphins/year and the calving interval was 3.15 years. These values are essential to assess population viability and formulate management and conservation plans.
Estimation of demographic parameters, as actual abundance and population growth rate, and elasticity analysis on free-ranging natural populations can aid in decision regarding the efficacy to protect different vital stages. Thus, bottlenose dolphin photo-identification data recorded in the eastern Ligurian Sea between 2006 and 2012 were analyzed to find out such demographic information. Overall, 311 different bottlenose dolphins were photo-identified and about 250 individuals were yearly estimated in the study area by mark-recapture technique. To analyze the population structure dolphins were shared into 4 vital stages as reported above. The distribution of dolphins, normalized by life stage duration, decreases with age and the highest mortality rate was found in the cub vital stage, in agreement with the previous stranding data analysis. Using a stage-structured matrix population model the geometric population growth rate (λ=1.03) was calculated. According to elasticity analysis the adult survival is likely the most important factor for population conservation: a 1% increase of this value will cause a 0.763% increase in population growth rate (λ).