Batı Karadeniz’de karaya vuran Cetacea türlerinin kalkan balığı avcılığı ile etkileşimi

Mehmet Arda TONAY

Batı Karadeniz’de karaya vuran Cetacea türlerinin kalkan balığı avcılığı ile etkileşimi

Thesis Type: Postgraduate

Institution Of The Thesis: Istanbul University, Faculty of Aquatic Sciences, Department of Marine and Freshwater Resources Management, Turkey

Approval Date: 2010

Thesis Language: Turkish

Student: Mehmet Arda TONAY

Supervisor: Bayram Öztürk

Open Archive Collection: AVESIS Open Access Collection

Abstract:

The Black Sea is almost an enclosed sea, and therefore very sensitive to threats such as pollution and deterioration of habitats of marine organisms, including marine mammals. There are three Cetacean species living in the Black Sea; the harbour porpoise (Phocoena phocoena), bottlenose dolphin (Tursiops truncatus) and common dolphin (Delphinus delphis). Cetaceans in the Black Sea are faced with several threats such as accidental catches in fishing gear (bycatch), habitat degradation causing the reduction of prey resources, marine pollution and epizootics resulting in Cetacean mass mortality events. However, there are not sufficient studies, conducted on a periodical basis, on mortality and life stories of the protected Cetacean species in the region. Therefore it is important to assess the threats and gather basic knowledge about these animals to elaborate and implement effective conservation measures.The aim of this study is to gather information on Cetacean bycaught in bottom gillnets and stranded ashore, to determine the number of Cetaceans that died this way, and to collect biological information on dead Cetaceans in the area, out of the turbot fishing season. The age determination was conducted to understand the age-size composition of the harbour porpoise population living off the Turkish Western Black Sea coast during spring and summer, and effects of turbot fishery on the harbour porpoise population.In this study, the mortality of Cetaceans bycaught in turbot nets and stranded on the Western Black Sea coast was examined. This study consists of three parts. In the first part, the cooperative turbot fishing boat was observed during two fishing seasons in 2007 and 2008, and Cetaceans bycaught in their nets were examined. In the second part, the Turkish Western Black Sea coast was surveyed seasonally between the spring of 2007 and summer of 2009, stranded Cetacean species were determined, and its relation with turbot fishery was studied. In the third part, effects of turbot fishery on the harbour porpoise population, the most common species found dead, was examined through age determination using teeth, and information on the life history of harbour porpoises in the Western Black Sea was obtained.A total of 108 Cetacean specimens were examined in this study during 2007-2009, 25 of which were bycaught and 83 were stranded. In turbot fishery, the CPUE was found to be 0.19 Cetacean individuals per kilometer in both 2007 and 2008. It was observed that 75% of the bycatches occurred during May and June, when turbot fishery was banned. Among three Cetacean species, turbot fishery causes the death of harbour porpoises the most (98% ). It was estimated that the number of harbour porpoises died during turbot fishing was 361(±332)-1829(±675) (SE), in 2007 and 608(±408)-2249(±790) (SE) in 2008. The estimated number of bottlenose dolphins died in the same way was 153±142 (SE) for 2007.Among the stranded Cetaceans, only a few individuals (16% ) showed some evidence of net entanglement because stranded Cetacean specimens were at an advanced stage of decomposition. However, more strandings were observed during late spring and early summer (72% ), which includes the illegal turbot fishing season, compared to autumn and winter (28% ). In the summer of 2009, there was a case of unusual mass stranding (at least 114 individuals of all three species), which mostly affected the common dolphins.Most dead harbour porpoises were individuals between 111-120 cm in length for males and 121-130 cm for females. According to the age determination of 32 harbour porpoises (19 bycaught and 13 stranded individuals), most stranded animals were observed in the age of 1 (31% ), 4 (23% ) and 5 (23% ), and among bycaught animals in the age of 4 (26% ) and 5 (21% ). Turbot fishing by using bottom gillnets, especially in May and June, is a threat to the sustainability of harbour porpoise stocks. Especially the death of pregnant and nursing females as well as immature and maturing (age 3-4) individuals would increase the negative effect. The proper implementation of the ban on turbot fishing will result in a decrease in the mortality of this Black Sea subspecies.When examining the body length and age data of the harbour porpoises bycaught in turbot nets (n=91) in 1999, 2002 and 2003, and of the stranded harbour porpoises (n=232) in 2003-2005 in the same area (as this study was conducted), it was found out that females were approximately 10 cm longer than males, the avarage body length of the females was 121.1±22.37 cm (95% CI), and the average body length of the males was 111.1±28.93 cm (95% CI). The maximum body length was 140 cm for females and 132 cm for males. The smallest newborn was 63 cm. The breeding period was April-June. Asymptotic length of the species was 120.4 cm for females and 116.6 cm for males. The oldest individual found was 8 years old. The asymptotic length of harbour porpoises in this area was found to be about 10 cm shorter than those living the Northern Black Sea and the Azov Sea. According to the results of this study, the harbour porpoises living in the Southeastern Black Sea are the smallest among the other harbour porpoises living in the Black Sea and other seas in the world.It is envisaged that the results and suggestions presented by this study can be used in designing and implementing a better elaborated conservation plan for Cetacean populations in the Western Turkish Black Sea coast.