155 



Abstract— Wi' omplin'od ultrasonic trans- 

 mittLT.s lo follow (for up to 48 h) the hor- 

 izontal and vertical movements of five 

 juvenile (6.8-18.7 kg estimated body 

 mass) bluefin tuna [Thunnus thynntis^ 

 in the western North Atlantic (off the 

 eastern shore of Virginia ). Our objective 

 was to document the fishes' behavior 

 and distribution in relation to ocean- 

 ogi'aphic conditions and thus begin to 

 address issues that currently limit pop- 

 ulation assessments based on aerial 

 surveys. Estimation of the trends in 

 adult and juvenile Atlantic bluefin tuna 

 abundance by aerial sun'eys, and other 

 fishery-independent measures, is con- 

 sidered a priority. 



Juvenile bluefin tuna spent the major- 

 ity of their time over the continental 

 shelf in relatively shallow water (gen- 

 erally less then 40 m deep). Fish used 

 the entire water column in spite of rela- 

 tively steep vertical thermal gradients 

 (=24°C at the surface and =12°C at 40 m 

 depth), but spent the majority of their 

 time (=90'"r) above 15 m and in water 

 warmer then 20°C. Mean swimming 

 speeds ranged from 2.8 to 3.3 knots, 

 and total distance covered from 152 to 

 289 km (82-156 nmi). Because fish gen- 

 erally remained within relatively con- 

 fined areas, net displacement was only 

 7.7-52.7 km (4.1-28.4 nmi). Horizontal 

 movements were not correlated with 

 sea surface temperature. We propose 

 that it is unlikely that juvenile bluefin 

 tuna in this area can detect minor 

 horizontal temperature gi-adients (gen- 

 erally less then 0.5°C/km) because of 

 the steep vertical temperature gi-adi- 

 ents (up to =0.6°C/m) they experience 

 during their regular vertical move- 

 ments. In contrast, water clarity did 

 appear to influence behavior because 

 the fish remained in the intermediate 

 water mass between the turbid and 

 phytoplankton-rich plume exiting Ches- 

 apeake Bay ( and similar coastal waters ) 

 and the clear oligotrophic water east of 

 the continental shelf 



Horizontal and vertical movements of 

 juvenile bluefin tuna (Thunnus thynnus), 

 in relation to oceanographic conditions 

 of the western North Atlantic^ determined 

 with ultrasonic telemetry 



Richard Brill 



Pelagic Fisheries Research Program 

 Joint Institute for Marine and 



Atmospheric Research 

 School of Earth and Ocean Science Technology 

 University ol Hawaii at Manoa 

 Honolulu, Hawaii 96822 

 E mail address rbnllia'honlab nmfs hawaii.edu. 



Molly Lutcavage 



Edgeiton Research Laboratory 



New England Aquanum 



Boston, Massachusetts 02110-3399 



Greg Metzger 



Department of Biology 

 Southampton College 

 Long Island University 

 Southampton, New York 11968-4198 



Peter Bushnell 



Department of Biological Science 

 Indiana University South Bend 

 South Bend, Indiana 46634-7111 



Michael Arendt 



Jon Lucy 



Sea Grant Program 

 Virginia Institute ol Marine Science 

 College of William and Mary 

 Gloucester Point Virginia 23062-1346 



Cheryl Watson 



Department of Biological Sciences 

 Central Connecticut State University 

 New Britain, Connecticut 06053-2490 



David Foley 



NOAA CoastWatch Program, 



Hawaii Regional Node 

 Pelagic Fisheries Research Program 

 Joint Institute for Marine and 



Atmospheric Research 

 School of Earth and Ocean Science Technology 

 University of Hawaii at Manoa 

 Honolulu, Hawaii 96822 



Manuscript accepted 6 Julv 2001, 

 Fish. Bull. 100:1,5.5-167 12002). 



Current estimates of spawning biomass 

 for Atlantic bluefin tuna iThiintnis 

 thynnus) remain controversial (Butter- 

 worth and Punt. 1993; Restrepo et al., 

 1994; Restrepo, 1996), although the 

 most conservative predicts that a pop- 

 ulation eight times the current size 

 would be needed to produce maximum 

 sustainable yields (Sissenwine et al,, 

 1998), The current strict catch quotas 

 are based on abundance assessments 

 for both adult and juvenile (i.e, "school- 

 ing") fish (age classes 1-5 years, body 

 mass =6-60 kg). Adult abundance is 

 derived from commercial landings data; 

 juvenile abundance has, since 1985, 

 been based on fishing effort and land- 

 ings data obtained from dockside inter- 

 cepts and telephone polling of the 

 largely recreational fishery for juvenile 



bluefin tuna conducted by the National 

 Marine Fisheries Service's Large Pelag- 

 ics Survey (Turner et al,, 1993, 1997), 

 The usefulness of both data sets can 

 be compromised, however, because the 

 relationship between catch-per-unit-of- 

 effort (CPUE) data and real abundance 

 is not known with certainty (Bakun et 

 al,, 1982; Hilborn and Walters, 1992; 

 Lauck. 1996), This problem is especially 

 critical with highly mobile schooling 

 fishes like tunas because of environ- 

 mental influences on fish distribution 

 and vulnerability to specific fishing 

 gears, as well as the introduction of 

 new fishing techniques (Sharp, 1978; 

 Clark and Mangel, 1979; Brill, 1994; 

 Bertrand and Josse, 2000). 



Juvenile Atlantic bluefin tuna appear 

 in the surface waters off the east coast 



