them, in the waters between the south- 

 em Iberian coasts and the Canary Is- 

 lands. This hypothesis was derived froni 

 studies of the tunas and their fisheries 

 off the Atlantic coast of Morocco, and 

 a hydrographic survey of the above 

 waters. In early June 1964, tuna were 

 found between Lanzerote Island (Ca- 

 naries) and the African coast in water 

 with a temperature of 20.06°C and sa- 

 linity of 36.40 o/oo at the surface, and 

 18.37°C and 36.56 o/oo at a depth of 

 50 m. Aloncle believed that these tuna 

 were pushed northward with the sea- 

 sonal warming of the water, and kept 

 away from the Moroccan coast by iso- 

 therms of temperatures greater than 

 2I°C. In the course of this movement, 

 he believed that they spawned in the 

 area between Lanzerote, Conception 

 Bank and the Moroccan coast. 



e. Other Eastern Atlantic 

 Areas 



The only other parts of the eastern 

 Atlantic where bluefin tuna are believed 

 to spawn are in the vicinit>' of the Azores 

 (Ferreira 1932) and in equatorial wa- 

 ters south of Sierra Leone (Richards 

 1969, Richards and Simmons 1971, 

 unpublished data reports for Geronimo 

 cruises 3, 4 and 5). 



In May and June, the months in 

 which bluefin reportedly spawn around 

 the Azores, the surface temperature in 

 the area varies between about 1 7.0°C 

 and about I9.7°C, while the salinity 

 remains at about 36. 1 o/oo. 



In the area bounded by latitudes 

 7°N and 8°S, and longitudes 1 3°W and 

 15°W, three larval T. ihynnus thynmis 

 were collected in March 1963, in wa- 

 ters with surface temperatures exceed- 

 ing 27°C and surface salinities of from 

 36.4 o/oo to 38.8 o/oo (Richards 1 969). 

 These temperatures are much higher 

 than those in other reported spawning 

 areas of the species in the eastern At- 

 lantic and the Mediterranean. 



4. Western Atlantic 



a. Introduction 



In comparison with what is avail- 

 able for the eastern Atlantic and the 

 Mediterranean, ver>' little has been pub- 

 lished on the relationships between the 

 spawning of bluefin tuna in the western 

 Atlantic and environmental conditions. 

 Therefore our discussion of this corre- 

 lation depends almost entirely on ob- 



servations which weie made concur- 

 rently with investigations, but have not 

 been synthesized or analyzed, and data 

 in atlases or survey reports. 



b. Gulf of Mexico 



The best documented spawning of 

 bluefin in the western Atlantic takes 

 place in the Gulf of Mexico in from 

 late April to early July (Section VD3). 

 Tlie heaviest concentrations of larvae 

 evidently occur in the deep (more than 

 200 m) area behveen latitudes 23°N 

 and 30°N and longitudes 84°W and 

 94°W (Figure 68) (Juarez 1974b, 

 Montolio and Juarez 1977). During 

 April, May and June the surface tem- 

 perature of these waters increases from 

 between 22°C and 24°C to a little over 

 2TC (Galtsoff 1954). The temperature 

 in the upper layer is fairly constant 

 down to 30 m. TTie salinity in the upper 

 50 m layer is typically 36.00 o/oo. In 

 contrast, the salinity of this layer over 

 the Campeche and Florida Banks, 

 where few larvae have been collected, 

 is 36.25 o/oo, possibly because of up- 

 welling. 



c. Straits of Florida 



The Straits of Florida at the Mi- 

 ami-Bimini line present an unusual situ- 

 ation in that adult bluefin are found in 

 numbers, and almost exclusively, on 

 the eastern (Bahamas) side (Section 

 IVC2), but the abundance of larvae is 

 minimal on the eastern side and maxi- 

 mum near the Florida side (Section 

 VD3). Most of the fish examined at 

 Bimini were spent, with a very few ripe 

 ones among them (Rivas 1954). Evi- 

 dently most of the spawning occurs 

 south of this line. 



The surface water temperature 

 across the Straits of Florida between 

 Miami and the Florida Keys and the 

 Bahamas (25°N to 26'N and 79°W to 

 80°W) is fairly unifomi in the deeper 

 central area. On the edges of the Strait 

 where the bottom rises sharply tem- 

 peratures are cooler (Sverdrup et al. 

 1942). The surface temperature in the 

 deep water is about 26°C to 26.5°C in 

 April, increasing to about 27°C in May 

 and about 28°C in June (Pyle 1962). ' 



Nearly all of the catches of bluefin 

 tuna in the Straits of Florida have been 

 taken by rod and reel from schools 

 traveling close to the surface (Rivas 

 1978). The fish are seen near the sur- 

 face in this area only under certain con- 



ditions of wind and cuiTL-nt, however, 

 and it is presumed that the\, must spend 

 much of the migrating period at deeper 

 levels. A male with milt was taken by 

 longline at an estimated deptli of 55 m 

 on the eastern side of the Straits just 

 north of Bimini (Mather and Bartlett 

 1962). The water temperature at the 

 location was 26.7°C at the surface and 

 25.7°C at 55 m. The salinity in the 

 upper 50 m layer in this region ranges 

 from 35.5 o/oo to 37 o/oo, with an 

 average of about 36.2 o/oo, from April 

 through June 1977. The average phos- 

 phate and oxygen levels in this period 

 were about 0. 1 and 4.65 ppm, respec- 

 tively. 



d. East and North of the 

 Bahamas 



In the waters east of the Bahamas 

 the research vessel Crawford caught 

 bluefin with maturity indices from 28 

 to 126 (mostly 60-90) at several loca- 

 tions in May 1961 (Figure 70, Table 

 25). Fhe surface water temperatures 

 ranged from 23.6°C to 26.5°C, but most 

 of the fish were taken in waters of 

 25°C. At 55 m the temperature range 

 was 22.4°C to 25.3°C, but most of the 

 fish were taken where the temperature 

 was about 24°C. 



A little north of the Bahamas the 

 Crawford caught eight fish with matu- 

 rity indices of 21 to 98 (Table 25) in 

 water with surface temperature beUveen 

 24.2°C and 25.6°C and temperature at 

 55 m between 20.8^C and 24.1 T. 

 Zahrov (1965) reported that spawning 

 fish caught north of the Bahamas (Fig- 

 ure 70) at the end of May and the 

 beginning of June were in water with 

 surface temperature of 25°C to 26.4°C. 



e. Northeastern United States 



The bluefin showing signs of 

 spawning taken by the M/V Delaware 

 about 500 km off southern New Jersey 

 and the Delmarva Peninsula during 

 cruise 57-5 (Section VD3) were gener- 

 ally in cooler waters than were any of 

 the previous fish. These catches were 

 made in waters of surface temperatures 

 between 1 8.3X and 25.5°C. The most 

 successful catches were in water w ith a 

 temperature around 20' Cor 2 rC. Fish- 

 ing depths were estimated to be 52 m to 

 57 m and most of the fish were taken 

 where the water temperature at this 

 depth ranged from 1 5 .5 "C to 2 1 . 1 °C. 



91 



