FISHERY BULLETIN: VOL. 75, NO. 2 



TABLE 5. — Mean speed crossing temperature front 1 and mean speed within 5-nmi radius 



before and after. 



'A7"20.5 C. 1.0 nmi. 



crossing the front in all three cases, and 2) the 

 mean speed was slower when crossing the front 

 than after crossing the front in two cases. These 

 data should be viewed with caution, however, be- 

 cause in two instances, daytime and nighttime 

 data were used together and some of the differ- 

 ences in speed may be due to variation associated 

 with time of day. The relationship did hold up well 

 in the single case when daytime data only were 

 used. 



We think the changes in swimming behavior 

 observed at temperature fronts reflected percep- 

 tion and response to the increased temperature 

 gradient per se. In the case of the alteration in the 

 swimming pattern of fish number 5 as it encoun- 

 tered a temperature front at lat. 36°53'N, long. 

 122°27'W (Figure 3), there was no sharp gradient 

 in any of the other environmental parameters we 

 measured. 



That tunas can perceive abrupt temperature 

 changes as small as 0.1°C has been demonstrated 

 by Steffel et al. (1976) for captive kawakawa, 

 Euthynnus affinis. Moreover, a mechanism has 

 recently been suggested (Neill et al. in press) 

 whereby tunas might be able to orient themselves 

 in temperature gradients much gentler than those 

 of our fronts, perhaps even as slight as 

 0.0001°C/m; this speculative mechanism invokes 

 the large thermal inertia of tunas as a device for 

 thermal "memory." 



Movements of Albacore in Relation to 

 Vertical Thermal Structure 



The availability of albacore in offshore waters 

 has been shown to be related to vertical thermal 

 structure (Laurs and Lynn 7 ). However, no obvious 



7 Laurs, R. M., and R. J. Lynn. 1974. The offshore distribution 

 and availability of albacore during early-season and the 

 migration routes followed by albacore into North American 

 waters. SWFC Admin. Rep. LJ-74-47: 19-46. 



354 



relationship was observed in this study between 

 the movements of sonic-tagged albacore in coastal 

 waters and subsurface temperature structure. 

 This may be due to the complicated vertical 

 temperature structure that was observed in the 

 areas where fish were tracked and the lack of data 

 on the depth of the fish. 



SUMMARY 



Six albacore were tagged and tracked with 

 ultrasonic equipment for periods ranging from 2 

 to 50 h and distances ranging from 6.5 to 150.7 km 

 (3.5 to 81.3 nmi). The average swimming speed 

 for these fish tracked between 27.8 and 50.0 h was 

 1.6 knots (82 cm/s) with each fish exhibiting 

 slightly faster swimming speeds during the day 

 than during the night. The mean swimming 

 speeds observed during the tracking experiment 

 are similar to estimates of swimming speed 

 derived from passive tagging results and about 

 twice the calculated minimum swimming speed 

 necessary to maintain hydrostatic equilibrium. 



The tracking experiment indicated that ocean- 

 ographic conditions may play an important role in 

 the local concentrations and movements of alba- 

 core in coastal waters. The movements of fish 

 appeared to be related to the distribution of sea 

 surface temperature, with transmitter-tagged 

 fish spending very little time in water with 

 surface temperatures less than 15.0°C. The results 

 also indicate that upwelling temperature fronts 

 may markedly influence the local concentration of 

 albacore, with albacore tending to concentrate in 

 the vicinity of upwelling fronts, presumably to 

 feed, and moving away from the immediate area 

 when upwelling ceases and the upwelling front is 

 no longer present at the surface. There was also 

 some indication that albacore tended to slow down 

 when crossing sea surface temperature fronts 



