Davis and Slanley: Movements of Thunmi-i maccoyii in Ihe Great Australian Bight 



461 



the shelf break, which it reached some 18 hours later It 

 remained in that area until tracking ended. Those lump, 

 reef, island and shelf-break associations matched the dis- 

 tribution of sightings by aerial surs'ey (Cowling et al."). 



Surfacing behavior 



Most SBT spend a large part of the day in the upper 10 m 

 (Fig. 10) in the Bight in summer unless they travel to other 

 areas. At night, they tend to move to deeper water, but 

 some tracked tuna remained in the upper 10 m through- 

 out most of the night. The relatively long time spent at the 

 surface during the day in the Bight (nearly 30%) would 

 facilitate sightings by aerial survey, and surfacing behav- 

 ior would significantly influence aerial detection. 



The surface behavior of southern bluefin tuna in the 

 Bight contrasts with that of most other tunas studied. Yel- 

 lowfin tuna near Hawaii have an average daytime depth of 

 71m during tracks offshore and away from fish aggregat- 

 ing devices (FADs), with modes at 0-10 m and 50-60 m. 

 When near FADs, they have an average daytime depth of 

 59 m. At night, yellowfin have modes at 0-10, 30-40, and 

 180-190 m (Holland et al, 1990a). Similar depth distribu- 

 tions for yellowfin tuna were observed in the western Pa- 

 cific (Yonemori, 1982). In the eastern Pacific Ocean, at the 

 northern extent of their range, yellowfin tuna remained 

 at somewhat shallower depths during the day than in 

 previous studies (Block et al., 1997), presumably because 

 of the shallowness of the mixed layer which limited their 

 depth distribution. In the western Indian Ocean, yellowfin 

 tuna tracked both on and off FADs spent most of the day 

 between 60 and 110 m (Cayre and Chabanne, 1986; Cayre, 

 1991;Marsacet al.-'). 



Tracked skipjack tuna spent little time at the surface 

 during the day in waters off Hawaii (Yuen, 1970; Dizon et 

 al., 1978), and Tahiti (Cayre and Chabanne, 1986), although 

 they were surface oriented at night. In the western Indian 

 Ocean, two skipjack tuna tracks were found at somewhat 

 shallower depths but the modal depths were still between 

 10 and 20 meters (Cayre, 1991). Bigeye tuna tracked off 

 Hawaii had a modal depth of 220 m during the day and 

 80 m at night (Holland et al., 1990a). Atlantic bluefin tuna 

 appear to spend some time near the surface in inshore wa- 

 ters off the New England area (Carey and Lawson, 1973), 

 although the modal depth of 0-15 m during the day (Lut- 

 cavage et al., 2000) is somewhat deeper than that for SBT 

 in the Great Australian Bight. This nearsurface orienta- 

 tion of Atlantic bluefin tuna has prompted development of 

 aerial surveys to assess the abundance and distribution of 

 these tuna in these waters (Lutcavage and Kraus, 1997). 

 The extensive surfacing behavior of SBT in the Bight does 

 not appear to occur in the shelf waters of the Indian Ocean 



' Cowling, A., T. Polacheck, and C. Millar. 1996. Data analysis 

 of the aerial surveys (1991-1996) for juvenile southern bluefin 

 tuna in the Great Australian Bight. 1996 Southern bluefin tuna 

 recruitment monitoring workshop report. Rep. RlVrWS/96/4, 87 

 p. CSIRO Marine Laboratories, Castray Esplanade, Hobart, 

 Tasmania, Australia 7000. 



off Western Australia. Of the six SBT tracked in the west, 

 only one spent significant time at the surface during the 

 day (Fishery Agency of Japan''-''). However, this fish oscil- 

 lated rapidly between and 40 m rather than remaining 

 near the surface. The different surfacing behavior of these 

 SBT in the west may have been due to their much smaller 

 size (35-48 cm compared to 67-114 cm), and that they 

 were in a migratory mode moving southwards, assisted by 

 the Leeuwin Current (Shingu, 1967; Maxwell and Cress- 

 well, 1981), rather than in summer residency. 



The conditions in which SBT come to the surface in the 

 Great Australian Bight are quite varied. These tuna re- 

 main within the surface 5 m for extended periods during 

 the day under a range of weather and sea conditions, al- 

 though surfacing is more pronounced on warm days with 

 little wind. A clear example of this type of movement was 

 evident with tuna 9, which remained below the surface 

 in a school of tuna for the first hour of the track, during 

 which winds were documented at over 35 km/h. After the 

 winds dropped, the tracked tuna surfaced and remained 

 there, apart from occasional dives, for the rest of the track. 

 Surface schools of tuna were observed as far as the eye 

 could see during these calm conditions. In contrast, exten- 

 sive surfacing can occur in adverse weather, as observed 

 in fish 7 between 07:00 and 09:00 h during rough seas and 

 winds >35 km/h. However, in these conditions, no surface 

 fish could be seen from the vessel. Protocols for aerial sur- 

 veys of SBT in the Bight restrict surveys to wind speeds 

 of <18 km/h because of a marked decrease in school sight- 

 ings at higher wind speeds (Cowling et al.'). The highest 

 number of sightings per unit of effort occurred when aerial 

 surveys were flown in conditions of low wind speeds and 

 little cloud cover The ultrasonic telemetry suggests that 

 wind affects detection of schools from the air rather than 

 their surfacing behavior. 



One of the objectives of the aerial survey was to reduce 

 uncertainty in aerial survey estimates of surface abun- 

 dance by incorporating environmental and behavioral 

 data into the aerial survey analyses. Although the results 

 from ultrasonic tracking provided preliminary informa- 

 tion on vertical distribution that might affect sighting 

 by aerial survey, the intent of our study was to use the 

 more extensive data that would be obtained from archival 

 tags to model responses in surfacing behavior to environ- 

 mental conditions through space and time. Aerial survey 

 estimates of abundance could then be adjusted by incor- 

 porating the proportion of time that SBT would be visible 

 during aerial surveys based on the environmental condi- 

 tions that occurred at the time of survey flights. 



A distinct surfacing behavior regarded as a characteris- 

 tic of SBT in the Bight during summer is called "rippling" 

 (Hynd and Robins, 1967). This behavior occurs under 

 very calm, sunny conditions. Tuna laze at the surface for 

 extended periods, often breaking this inactive phase by 

 rolling from side to side. Unfortunately we did not have 

 suitable weather conditions to observe this behavior How- 

 ever, data from archival tags suggest that these fish derive 

 significant heating benefits through both insolation and 

 the transfer of heat from the warm surface waters (Gunn 

 etal.2). 



