164 



Fishery Bulletin 100(2) 



«D N "^ 1 "^ C> ^ 



Si- C) O O C) O C) 





S 60 1 



Log (chlorophyll-a) (mg/m) 



Diffuse attenuation coefficient (490 nmi/m) 



Figure 10 



Frequency histogram (mean ±SEM) of chlorophyll-o 

 concentrations ( mg/ni'^ i and water clarity measured as 

 the diffuse attenuation coefficient ( 1/m, at an in vacuo 

 wavelength of 490 nmi ) in waters along the track lines 

 of five juvenile bluefin tuna (Brill and Lutcavage, 2001). 

 The span of the horizontal axes show the approximate 

 range of these variables present off the eastern shore of 

 Virginia. 



ity of fish to sense shallow horizontal temperature gra- 

 (Jients in the face of the steep vertical temperature gra- 

 dients they routinely experience may explain, therefore, 

 why Power and May (1991) and Podesta et al. (1993) could 

 find no correlation between SST "fronts" and the appar- 

 ent abundance of yellowfin tuna in the Gulf of Mexico and 

 swordfish in the western north Atlantic. 



In contrast to SST, water clarity and phytoplankton 

 abundance appear to have a strong influence on the hor- 

 izontal movements of juvenile bluefin tuna (Figs. 9 and 

 10). Tunas are sight hunters, and possess the highest vi- 

 sual acuity of any teleost (Nakamura, 1968). We suspect 

 that juvenile bluefin tuna remain in water masses with a 

 standing phytoplankton biomass sufficient to support con- 

 centrations of prey, but where turbidity is low enough that 

 visual prey detection and prey capture abilities are not 

 impeded. Our conclusion is further supported by the lo- 

 cations of juvenile bluefin tuna schools detected in aerial 



50 

 40 

 30 

 20 



10 ■! 



adult Atlantic bluefin tuna 



JjjWL..^ 



-? 60 -I 



o 



I 50 



I 40 



Cl 



I 30  



t 20- 



E 



P 10 







60 

 50 

 40 

 30 

 20 

 10 



T> f>' ■?> ^*• <^' N*' ■i^' -y v>' <y <:■' -^ ^ 



adult Atlantic bluefin tuna 



^IVi-'"p<"iWi-r- i , , I 



B 



^^-Sl- 



juvenile Atlantic bluefin tuna 



Ik^ 



DAY 

 NIGHT 





CV > H' :S > 53 fe ^ ,* ?l kS> N^ 0- v> n"* 

 <0^ C^ <^ <$- <^ C^ <^ '^ <^^^^^ ^ 



Temperature interval ( C) 



Figure 11 



Frequency histograms (mean +SEM) showing time spent 

 at specific temperatures by adult bluefin tuna tracked 

 in the Gulf of Maine (western North Atlantic) with tem- 

 peratures expressed as water temperature (A), and with 

 temperatures expressed in relation to surface layer tem- 

 perature (B), data taken from Lutcavage et al., 2000). 

 Equivalent data for juvenile bluefin tuna are presented 

 in panel C. Shaded bars indicate nighttime and open 

 bars indicate daytime. 



surveys conducted in 1997.^ Although satellite data show- 

 ing diffuse attenuation coefficients and chlorophyll-a con- 

 centrations are not available for 1997, bluefin tuna schools 

 were located in the areas where the fish carrying ultra- 

 sonic transmitters remained (Fig. 9). Olson and Podesta 

 ( 1987), Olson et al. ( 1994), and Humston et al. (2000) have 

 also concluded that aggregations of highly mobile species 



Lutcavage, M. 1998. Aerial survey of school bluefin tuna off 

 the Virginia Coast, July 1997. Report to the National Marine 

 Fisheries Service (cooperative agreement NA77FM0533). (Avail- 

 able from the author, Edgerton Research Laboratory, New Eng- 

 land Aquarium, Central Wharf Boston. MA 02110,1 



