Ecology and conservation of the bottlenose dolphin Tursiops truncatus in Jervis Bay, NSW
thesisposted on 2022-03-28, 17:26 authored by Fiona-Gai Mandelc
Tursiops truncatus were observed from a small boat throughout Jervis Bay (120 sq. km) on the south coast of New South Wales (NSW). This main study within the Bay involved 98 survey days between January 1990 and June 1993. In addition, three Regional Surveys were conducted for three consecutive days each, simultaneously in the Bay and along the coast immediately north and south of the Bay. In total 151 sighting events were recorded in Jervis Bay, on 107 days involving approximately 709 hours and travelling 6,626 km searching for dolphins. Tursiops were sighted on 71% of all survey days in the Bay. These included: 49 on the Search and Encounter Survey; 26 from the transect line on the Transect Survey; 54 in transit between sampled transects on the Transect Survey; and 22 on the Regional Survey. In the Bay two survey methods were used. The first method involved recording opportunistic sightings around the periphery of the Bay, referred to as the Search and Encounter Survey. The second was a line-transect survey designed to quantitatively investigate if dolphins were distributed equally across the major habitats of the Bay. Sightings from the latter survey were divided into three subsets for the purpose of analyses. These were: sightings made from the transect line; initial sightings made at transect sites when these were not being sampled; and all sightings made in transit between sampled transect sites. Habitat was defined for the purpose of this study by depth and substratum, i.e. over seagrass, sand or rocky complex areas. Based on these two different types of surveys the estimated dolphin density in the Bay was 0.12 and 0.13 per sq. km with no clear seasonal peak in density. Both surveys indicated a significant difference in the number of sightings across different habitats when the area of each was considered. Dolphins were sighted more frequently in shallow waters (< 10 m) and over the rarer substrata of seagrass and rocky complex areas. In waters < 10 m in depth animals were more abundant over seagrass than sand but the density of Turslops was greatest in rocky areas. Four ancillary sources recording over 700 sightings of bottlenose dolphins in Jervis Bay were also investigated; and four major faunal studies within the Bay were reviewed in terms of the distribution and abundance of potential prey items of this species. Across my Surveys and all ancillary data sets, where information was available, a number of general trends in the distribution of dolphin sightings were apparent, such as dolphins being present in the Bay throughout the year and their distribution varying significantly and consistently across different areas of the Bay. Seventy-six individual dolphins were identified by photographs of their dorsal fins, 69 from inside the Bay. Because of the opportunistic nature of photographic sampling effort, this is seen as a bare minimum estimate of the number of animals that utilised the Bay during the study period. Of these 69 individuals, 19 (27%) were sighted only once, while 50 (73%) were resighted two or more times. The number of days on which an individual dolphin was sighted varied from 1-15. The possible residency status of individuals was investigated in terms of : the number of days an individual was sighted; the resighting opportunity ratio; the interval between first and last sighting; the average number of days between sightings; the number of years sighted; sightings on consecutive photographic surveys and the average number of days between consecutive survey sightings. These data suggest that the duration of time spent in and out of the Bay is highly variable between identified individuals. In the concluding chapter, the results of this study are considered in terms of two proposed management strategies for the study area. My recommendations relate to increased protection and representation of habitats where high numbers of Tursiops sightings occur. The value of local area population studies and local habitat conservation measures are discussed as a general conservation strategy for coastal bottlenose dolphins, particularly in NSW.