Lutcavage et al.: Aerial survey of Thunnus thynnus in the Straits of Florida 
309 
the Gulf Stream’s edge intercepts the Bank (or both), 
producing stronger northerly flow. Previous reports 
have also noted the bluefin tuna’s apparent avoid- 
ance of the “dirty water” tidal flow from the Bank, 
which varied a good deal over the survey period. 
However, on at least two occasions we observed blue- 
fin tuna in turbid water. In the present study, the 
period of highest sightings occurred in the six days 
following the new moon. Although aerial sightings 
were not given in relation to lunar phase for the 
1974-76 surveys, this period coincided with the low- 
est catch per boat day for 11 Cat Cay bluefin tuna 
tournaments from 1941 to 1960 (Rivas, 1978). 
It is possible that the apparent relation of strong 
winds with appearance of bluefin tuna on Tuna Al- 
ley may be driven by oceanographic conditions oc- 
curring in adjacent staging areas. In general, flow 
over the Great Bahama Bank in the Bimini area is 
weak and driven by wind and tide (Lee 3 ). Although 
the Bank constitutes a topographic wall, it is not 
associated with strong upwelling. Much stronger flow 
and upwelling occurs where the Loop Current leav- 
ing the Gulf of Mexico impinges on the north coast of 
Cuba. The dynamics of eddy systems near Cay Sal 
Bank and northern Cuba could conceivably influence 
travel routes of bluefin tuna, a concept that is rein- 
forced by the reports of giant bluefin tuna on Cay 
Sal Bank and the Old Bahama Channel by anglers 
and fish spotters (Rivas, 1954; 1978; Mather et al., 
1995), and one that would explain the large variabil- 
ity in numbers of bluefin tuna sighted on Tuna Alley 
from year to year (e.g. an order of magnitude differ- 
ence in sightings between 1974 and 1975 (Rivas, 
1978). 
During the survey, surface sea water temperatures 
in the Straits of Florida and adjacent to the Great 
Bahamas Bank, obtained from advanced high-reso- 
lution radiometer ( AVHRR) satellite imagery, ranged 
from 26° to 30°C, nearly 10°C higher than the mean 
sea surface temperature associated with bluefin tuna 
schools in the New England region (Lutcavage et al. 4 ). 
However, our opportunity to examine additional en- 
vironmental conditions that might have influenced 
bluefin tuna occurrence on the Great Bahama Bank 
region in 1995 was limited. Sea surface temperatures 
across the Straits of Florida are somewhat uniform 
in the late spring and summer, and to our knowl- 
edge there were no current meters or buoy data re- 
3 Lee, T. 1995. Rosenstiel School of Marine and Atmospheric 
Science, Univ. Miami, Miami, FL. Personal commun. 
4 Lutcavage, M., J. Goldstein, and S. Kraus. 1996. Sustaining 
tuna fisheries — issues and answers. Proceedings of the 47th 
tuna conference; Lake Arrowhead, CA, 20-23 May 1996. 
fleeting the precise boundary of the Gulf Stream edge. 
This information, along with tide and wind stress 
records, might have provided a more specific rela- 
tion between environmental conditions and the ap- 
pearance of bluefin tuna on the Great Bahama Bank. 
There are numerous reports of giant bluefin tuna 
in other areas of the Bahamas and Straits of Florida 
beyond Tuna Alley, particularly to the east and north- 
east off Walkers Cay, the Abacos, and also in deep 
water regions west and southwest of the Great 
Bahama Banks and off Cuba (Rivas, 1978; Mather 
et al., 1995; Murray 5 ). Recent longline captures also 
corroborate the presence of bluefin tuna in the east- 
ern areas, well before and concurrent with the as- 
sumed migration period of fish that transit Tuna Al- 
ley (Turner 6 ). In addition, giant bluefin tuna were 
landed in the first week of June in the Gulf of Maine, 
nearly coincident with our first sightings on the Great 
Bahama Bank. At present there is little information 
that would identify whether fish travelling in this 
region are members of the same assemblage, and 
further, whether they had recently exited the Gulf of 
Mexico, or had travelled from areas to the south and 
east, or from the Windward Passage, as suggested 
by Mather et al. (1995). 
Mather et al. (1995) reported that the Great 
Bahama Bank migration area continues along the 
western edge of the Little Bahama Bank, and they 
sighted giant bluefin tuna travelling north between 
the Great and Little Bahama Banks in May-June 
1968. It is clear that without complementary oceano- 
graphic surveys, the sporadic appearance and dif- 
fuse aggregation behavior of giant bluefin tuna on 
the Great Bahama Bank present serious problems 
for direct aerial assessment in this region. Although 
fish can be seen and enumerated under suitable con- 
ditions during daylight hours, there is no way of de- 
termining how many fish transit the Straits of 
Florida in deeper water, or determining their pres- 
ence and abundance in other regions of the Bahama 
islands. Lacking this information, the use of direct 
counts of bluefin tuna in this region as an index of 
spawning biomass seems unwarranted. Alternatively, 
aerial surveys on the Great Bahama Bank, in con- 
junction with direct sampling of landings, may pro- 
vide an index of regional abundance and informa- 
tion on the size classes and reproductive status of blue- 
fin tuna transiting the area. In the future, examina- 
tion of oceanographic conditions occurring in the Loop 
5 Murray, Captain E. 1996. 8101 Nashua Dr., Palm Beach 
Gardens, FL 33418. Personal commun. 
6 Turner, S. 1995. Southeast Fish. Sci. Center, Natl. Mar. Fish. 
Serv, NOAA, Miami, FL. Unpubl. data. 
