Davis and Stanley Movements of Thunnus maccoyii in the Great Australian Bight 



463 



to be well above ambient water temperature. Stomach 

 temperature increased after feeding, whicli was at Iribuled 

 to the hydrolytic processes of digestion and an increase 

 in metabolic rate. Possibly raised stomach temperature 

 speeds digestion and enables a higher feeding frequency 

 when food is abundant. The capacity to maintain higher 

 stomach temperature is enhanced by thermal isolation 

 through heat exchangers, the gas bladder, and the fatty 

 body wall (Carey et al., 1984). Therefore, stomach tem- 

 perature would reflect activities associated with feeding 

 and digestion, rather than temperature changes occurring 

 in the body. SBT, while very much smaller than the giant 

 bluefin tuna tracked by Carey, also showed a marked dif- 

 ferential between stomach and ambient temperatures. 

 The largest differential of 9°C (track 11) was found in the 

 largest SBT tracked, which may be relevant. Because tem- 

 perature increased steadily during the track, it was prob- 

 ably due to digestion of a meal. All pole-and-line-caught 

 SBT tracked during these experiments were initially 

 chummed with pilchards and could have eaten pilchards 

 just before tracking. In some instances, tunas regurgitated 

 pilchards while the tag was placed in their stomachs. A 

 caecal temperature differential reaching 10°C has also 

 been observed in a 103-cm SBT after being fed in cage 

 experiments (Gunn et al.^). The smaller temperature dif- 

 ferential observed in the other two tracked tuna (tuna 10 

 and 13) may have been a consequence of their having less 

 food in their stomach to digest, or their smaller size. 



Tuna 11 showed the characteristic changes in stomach 

 temperature associated with the swallowing of prey or 

 water or both. Two swallowing events occurred, a minute 

 apart, resulting in a drop in stomach temperature to ambi- 

 ent temperature. Stomach temperature recovered rapidly, 

 but not quite to the level before swallowing. Carey et al. 

 (1984) managed to have Atlantic bluefin tuna retain tags 

 in their stomachs for up to 13 days by feeding them to 

 ensure that their stomachs never remained empty. Sev- 

 eral hours before regurgitation of the tag, a series of cold 

 pulses were observed in stomach temperature, presum- 

 ably from swallowing water which lowered the stomach 

 temperature briefly. If these cold pulses persisted, the tag 

 would be spat out a few hours later. If the tuna was fed, 

 the tag was usually retained. If regurgitation to purge 

 indigestible items from the stomach normally occurs after 

 digestion is complete and the stomach is relatively empty, 

 as suggested by Carey et al. (1984), then it is likely that 

 the swallowing events observed in tuna 11 involved little 

 or no food. Because a very marked drop in stomach tem- 

 perature was observed in tuna 11, a large amount of water 

 must have been swallowed. 



=* Gunn, J., T. Polacheck, T. Davis. M. Sherlock, and A. Betle- 

 hem. 1994. The development and use of archival tags for 

 studying the migration, behaviour and physiology of southern 

 bluefin tuna, with an assessment of the potential for transfer of 

 the technology to groundfish research. Proc. ICES mini-sym- 

 posium on migration, St. Johns, Newfoundland. ICES CM. 

 Mini:2.1, 23 p. International Council for the Exploration of the 

 Sea. Palaegade 2-4, DK-1261 Copenhagen K, Denmark. 



The effects of chumming on tuna behavior 



Obsci-vations on tracked tuna being caught up in chum- 

 ming by commercial pole-and-line fishing have provided 

 us with insights into these operations. Tunas 5, 6, 8, 

 and 15 were associated with chum lines for part of their 

 tracks. They all followed the chum line at a depth of about 

 20 m for 15-30 minutes, and some came to the surface for 

 brief periods. The tuna schools extended to at least 200 m 

 behind the vessel that was chumming. Pole-and-line opera- 

 tors usually steam forward at about 2 m/s, chumming with 

 live and, sometimes, frozen pilchards. They progress from 

 tuna school to tuna school hoping to combine schools. Most 

 tuna appear to be below the surface and well behind the 

 boat. Fish progress to the front of the school and come to 

 the surface to take chum, replacing tuna that were there 

 before them. Based on our tracking, it appears that tuna 

 after spending a short time at the surface, break away 

 from the chum line in groups or in schools. 



Effects of tagging and tracking on tuna behavior 



There is a concern that capture, tagging, and tracking 

 procedures change the behavior of the fish being observed. 

 A number of criteria are used to support the view that the 

 fish behave normally: they remain within a school (Yuen, 

 1970; Cayre, 1991; Cayre and Marsac, 1993); show simi- 

 larity in vertical and horizontal movements across tracks 

 (Holland et al., 1990a; Cayre, 1991) or through experimen- 

 tal procedures such as evaluating swimming performance 

 when tags are attached externally (Arnold and Holford, 

 1979; Blaylock, 1990) or observing food consumption when 

 tags are placed in the stomach (Lucas and Johnstone, 

 1990). 



There is however, evidence that SBT do react adversely 

 to being tagged. Hampton (1986) found that lower than 

 expected numbers of SBT tagged with dart tags were re- 

 captured in the first 5 days after release. Also, recaptured 

 fish were in significantly poorer condition than untagged 

 fish — the effect being greatest in those at liberty for 5-20 

 days (Hampton, 1986) and 13-24 days (Hearn, 1986). 

 Presumably, the adverse effects were related to reduced 

 feeding. 



Fish generally undergo a phase of recovery from the 

 trauma of capture and tagging. The resumption of "typi- 

 cal" behavior is often taken as a sign that the fish is be- 

 having normally (Holts and Bedford, 1990). In our study, 

 five tuna initially dived to the bottom, five dived below the 

 depth distribution of the tuna school, and six returned 

 immediately to the school. All returned to schools, which 

 were usually near the surface, within 20 minutes. Most 

 significantly, four tracked tuna actually joined the chum 

 lines of commercial pole-and-line vessels shortly after tag- 

 ging. However, it could not be determined whether these 

 four tuna were feeding because they did not have tempera- 

 ture-pressure tags in their stomach. 



Stress associated with capture and tagging is more 

 obvious in larger fish: blue and striped marlin remain at 

 depth for many hours before returning to normal depths 



