Brill et al.: Horizontal and vertical movements of juvenile Thunnus thynnus 



159 



75.0 



74.8 



74.6 



74.4°W 



755 75,4 



75.3 



75,2 75.1 75,0"W 



Figure 3 



Movements offish 1, 2, and 5. Symbols 

 represent hourly intei-vals: open sym- 

 bols indicate daytime and solid sym- 

 bols, nighttime. 



butions. Although they encountered water temperatures 

 of =10°C during their brief descents, fish spent ~90'7f of the 

 time in water greater than 20°C and =50% of the time in 

 water greater than 24°C. 



Occupancy of specific water masses 



Figure 8 shows fish movements and sea surface tempera- 

 tures (SST). Note that specific water masses (except for 

 the Gulf Stream, a portion of which is clearly visible to 

 the southeast) appear ill-defined by SST (Fig. 8). (During 

 the 21-day study period, there was no evidence that an 

 instability in the Gulf Stream resulted in discharge of 

 warm, high-salinity water onto the Virginia continental 

 shelf, as has been occasionally obser\'ed IChurchill et al., 

 19931.) 



Figure 9 presents fish movements and chlorophyll-n 

 concentrations (i.e. phytoplankton abundance) and water 

 clarity measured as the diffuse attenuation coefficient 

 (1/m) at an in vacuo wavelength of 490 nmi (a low diffuse 

 attenuation coefficient indicates high water clarity). Note 

 that, in contrast to sea surface temperature data (Fig. 8), 

 the turbid, plankton-rich water leaving Chesapeake Bay 

 (the "Chesapeake Bay plume") is clearly visible. The Ches- 

 apeake Bay plume flows to the south and remains trapped 



Figure 4 



Movements of fish 3 and 4. Symbols represent hourly intervals: open sym- 

 bols indicate daytime and solid symbols, nighttime. The edge of the continen- 

 tal shelf is shown by the 50, 100-, and 200-m isobath lines. 



inshore because of the Coriolis effect (Reiss and McCo- 

 naugha 1999), which is to the right in the Northern Hemi- 

 sphere. Also visible is the clear oligotrophic mid-Atlantic 

 slope water eastward of the continental shelf- With the 

 exception of fish 4, which made a brief excursion into the 

 mid-Atlantic slope water east of the continental shelf, fish 

 remained in the mid- Atlantic shelf water (i.e. between the 

 extremes of water clarity and phytoplankton abundance 

 immediately to the east and west) (Fig. 9). 



Figure 10 presents the frequency histograms for the 

 specific chlorophyll-a concentrations and diffuse attenua- 

 tion coefficients of water occupied by juvenile bluefin tu- 

 na. These data confirm that juvenile bluefin tuna restrict- 

 ed their movements primarily to waters with a relatively 

 narrow range of clarity and phytoplankton abundance. 



Discussion 



Although juvenile bluefin tuna remained within rela- 

 tively restricted geographical ranges while being tracked 

 (Table 1, Fig. 1), none of these fish showed the repetitive 

 daily behaviors often demonstrated by yellowfin tuna and 



- Wright, W. R. 1976. Physical oceanography. In A summary 

 of environmental information on the continental slope Canadian/ 

 United States border to Cape Hatteras, N.C., p. 4-1-4-109. Pre- 

 pared by TRICOM, The Research Institute of the Gulf of Maine, 

 South Portland, MA, for the Bureau of Land Management, 

 Marine Minerals Division. [Available from National Technical 

 Information Service, U. S. Department of Commerce, Spring- 

 field. VA 22161.] 



