FISHERY BULLETIN: VOL. 87. NO. 3, 1989 



nearby. Phytoplankton patches can seed other 

 downstream patches in eddies (Heywood and 

 Priddle 1987). This would be favorable for larval 

 tunas offshore of the Gulf Stream front if these 

 patches produced a food chain containing their 

 prey. Much of the eddy-induced production may 

 be flushed offshore rather than contribute to the 

 shelf food chain (Walsh 1986). However, a pulse 

 of nutrients from shelf-edge upwelling would be 

 diluted rapidly by mixing with the Gulf Stream. 

 We need more quantitative knowledge of the 

 trophic results of these linkages between the 

 shelf break and the Gulf Stream and more infor- 

 mation about the food requirements of larval 

 bluefin before the potential benefits of shelf- 

 break upweUing to epipelagic ichthyoplankton 

 can be assessed. 



Dynamic Larval Retention Areas 



Bluefin tuna larvae have been collected over 

 wide areas of the Gulf of Mexico. They appear 

 to occur primarily where curi'ents or eddies en- 

 counter the shelf between the 100 and 1,000 m 

 isobaths (Sherman et al. 1983). They may be 

 most abundant at the cold edge of the Loop 

 Current surface fronts in the eastern Gulf of 

 Mexico (Richards et al. in press). Variability in 

 the Loop Current (Sturges and Evans 1983) will 

 produce variability in the seasonal occurrence 

 and amount of such habitat for the bluefin tuna 

 larvae in the Gulf. The amount of habitat may 

 limit the number of bluefin tuna recruits to the 

 adult stock as has been hypothesized for At- 

 lantic herring stocks (lies and Sinclair 1982). 

 Because it is outside the rapidly flowing Loop 

 Current which feeds into the Gulf Stream, this 

 habitat may be a larval retention area of bluefin 

 tuna. In the larval retention hypothesis devel- 

 oped for Atlantic herring (Sinclair and lies 

 1985), the larvae do not undergo development 

 while drifting passively (e.g., Harden-Jones 

 1968). Instead, they develop into juveniles 

 within a retention region and then migrate 

 actively to juvenile nursery areas. 



Bluefin tuna larvae seem to fit into this life 

 history model. Larvae spawned in the Loop Cur- 

 rent can be advected to the Gulf Stream off the 

 southeastern U.S. where habitat is relatively 

 unfavorable. Larvae just outside the Loop Cur- 

 rent in the Gulf of Mexico retention areas could 

 develop until they are mature enough to begin 

 their migration to feeding areas along the middle 

 and northern U.S. east coast. 



The limited data on distribution of bluefin tuna 



young of-the-year support this hypothesis. 

 Juvenile bluefin tuna appeared in diets of terns 

 at the Dry Tortugas (24°30'N, 82°50'W) from 

 early June to early July (Potthoff and Richards 

 1970). These juveniles ranged in length from 25 

 to 115 mm with all sizes present early in the 

 season but only longer ones present later. No 

 juvenile bluefin tuna were noted in April or May 

 during eight years of observations although 

 juveniles of other species of tuna were being 

 eaten by terns during May. The juvenile bluefin 

 tuna were apparently unavailable within the 24 

 km feeding range of the terns (Robertson 1964) 

 until they began migi-ating through the Straits of 

 Florida in June. The timing of the migration 

 suggests that there is a distinct and discrete 

 time for the young-of-the-year juveniles to mi- 

 gi'ate from their larval retention area to their 

 juvenile habitat. Perhaps the larvae must de- 

 velop enough to begin schooling just as herring 

 do before they begin their migration. Migi-ation 

 by schools of newly transformed juveniles is con- 

 sistent with the cohesive migi'ation of other age 

 classes of bluefin tuna (Brunenmeister 1980; 

 Mather 1980). Larvae that were swept out of the 

 Gulf in April and May would not be in synchrony 

 with the migi-ation of their year class in June and 

 July unless they reached suitable retention areas 

 off the southeastern U.S. There is no evidence 

 for such areas. The larval retention areas thus 

 play a dual role for bluefin tuna by supplying 

 habitat for larval development and by affecting 

 the timing of migrations of different life stages of 

 the species. The larval retention areas we pro- 

 pose for bluefin tuna and other oceanic pelagic 

 fishes differ from those of fixed size proposed for 

 herring stocks because they are dynamic, vary- 

 ing in date of occurrence, geographical location, 

 and area. This variability in the quantity of 

 larval habitat is a density-independent environ- 

 mental factor which may explain a significant 

 amount of variability in recruitment of pelagic 

 fishes. Variations in the quality of larval habitat 

 (coincident prey and predators) would cause den- 

 sity-dependent effects within the constraints of 

 the total larval retention habitat. 



A quantitative knowledge of bluefin tuna 

 larval retention areas will have two practical 

 applications. The precision of larval surveys, 

 which are the only current independent esti- 

 mates of the spawning stock, may be improved 

 by a stratified sampling design once the strata of 

 low and high abundance can be defined. In addi- 

 tion, hypotheses about the importance of appro- 

 priate habitat for bluefin tuna and the effects on 



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