Bluefin l\ina, Thunnus thynnus. Larvae in the Gulf 



Stream off the Southeastern United States: SatelHte 



and Shipboard Observations of Their Environment 



Michael F. McGowan and William J. Richards 



ABSTRACT: The primary spawning area of the 

 western Atlantic stock of bluefin tuna is presumed 

 to be in the Gulf of Mexico. However, bluefin tuna 

 larvae were collected in April and May 1985 along 

 the shelf edge from Palm Beach, Florida to Cape 

 Fear, North Carolina and offshore as far as 260 km 

 east of Jacksonville, Florida over the Blake 

 Plateau. Satellite and shipboard sea-temperature 

 data indicate that the larvae over the shelf edge 

 were advected there in meanders of the Gulf 

 Stream. Bluefin larvae previously reported in the 

 Straits of Florida and off Cape Hatteras were also 

 in the Gulf Stream according to retrospective anal- 

 yses of temperature and salinity data. Based on 

 age-length relationships and current velocity, one 

 small larva was probably spawned north of Miami, 

 Florida while others could have been advected into 

 the Gulf Stream from the eastern Gulf. Spawning 

 by a few unspent migrating adults could also ac- 

 count for some bluefin larvae in this region. The 

 estimated total larvae off the southeastern United 

 States in 1985 could have been produced by 5% of 

 the spawning stock. Bluefin larvae were found 

 within a narrow range of sea surface temperatures 

 and salinities at offshore stations. Preliminary 

 assessment of larval habitat indicates that waters 

 off the southeastern United States are unfavorable 

 for growth and sur\'ival of bluefin larvae relative to 

 hypothesized larval retention areas in the Gulf of 

 Mexico. 



The western Atlantic stock of bluefin tuna, 

 Thunnus thynnns, spawns from about mid-April 

 to mid-June in the Gulf of Mexico, based on 

 seasonal and areal distribution of their larvae 

 (Richards 1976, 1977). Bluefin tuna larvae have 

 also been collected in the Straits of Florida north 

 of Cuba and east of Miami (Richards and Pott- 

 hoff 1980; Brothers et al. 1983) and off Cape 

 Hatteras, NC (Berrien et al. 1978). 



Michael F. McGowan, Rosenstiel School of Marine and At- 

 mospheric Science, University of Miami, 4600 Rickenbacker 

 Causeway. Miami, FL 33149. 



Willaim J. Richards. Southeast Fisheries Center, National 

 Marine Fisheries Service, NOAA, 75 Virginia Beach Drive, 

 Miami, FL 33149. 



Surveys of the Gulf of Mexico for bluefin tuna 

 larvae are made annually during the spawning 

 season to estimate the total abundance of larvae 

 and to calculate a fishery-independent index of 

 adult spawning stock size (Richards et al. 1981; 

 McGowan and Richards 1986). This index of 

 stock size is used to calibrate vii-tual population 

 analysis (VPA) of the western Atlantic bluefin 

 tuna population to enhance management of the 

 stock (Anonymous 1986). Fishery catch statistics 

 and the index of abundance of larvae show that 

 the bluefin tuna population size is smaller than it 

 was previously, and is below optimum levels 

 (Brown and Parrack 1985; McGowan and Rich- 

 ards 1986). To redress this problem, directed 

 fishing on bluefin in the Gulf of Mexico and in 

 other spawning areas, has been prohibited since 

 1982 (Anonymous 1987). 



Finding bluefin larvae outside the primary 

 spawning areas raises a potential problem be- 

 cause if significant spawning of the remaining 

 stock occurs outside the Gulf of Mexico, then the 

 ichthyoplankton survey in the gulf may not give 

 a rehable index of the stock, and prohibition of 

 fishing in the gulf may not have the desired ef- 

 fect on stock recovery. In 1985 bluefin larvae 

 were widely distributed off the southeastern 

 U.S. coast. The hypothesis that they were 

 spawned in this area needed evaluation. 



In this paper we quantitatively describe the 

 occurrence of bluefin tuna larvae in 1985 near 

 Miami and Cape Hatteras, where they have been 

 reported before, and also their occurrence over 

 the Blake Plateau north of the Bahamas Islands, 

 where they have not been reported previously. 

 We use satellite observations of the position of 

 the Gulf Stream and shipboard hydrographic 

 measurements to describe the habitat where 

 bluefin tuna larvae were collected. We review 

 previous evidence for bluefin tuna spawning in 

 this area to determine if these larvae were in 

 similar, oceanographically defined habitat, and 

 summarize this evidence to assess the waters of 

 the southeastern U.S. coast and the Blake 



Manuscript Accepted March 1989. 

 Fishery Bulletin, U.S. 87:615-631. 



615 



