McGOWAN and RICHARDS: BLUEFIN TUNA LARVAE 



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SURFACE ISOTHERMS (tC 

 CRUISE D-66-3 

 APR.6-22, 1966 



STATION 



20 



40 - 



a. 



UJ 60 



o 



80 - 



100 



TRANSECT N 

 CRUISE D-66-3 

 APR. 20 , 1966 



SURFACE ISOTHERMS 

 CRUISE D-66-7 

 JUN 17-29, 1966 



20 - 



60 



80 



100 



STATION 



TRANSECT M 

 CRUISE D-66-7 

 JUN 23, 1966 



Figure 9. — Plots of surface isotherms and temperature sections of the stations where bluefin larvae were collected near Cape 

 Hatteras by Berrien et al. (1978). The plot at upper left shows the position of the temperature fi-ont at the sea surface between 

 stations 3 and 4 on transect N. The section at upper right shows that the front e.xtends to the bottom: station 5, where a bluefin 

 larva was collected, is indicated by an arrow. Plot at lower left shows the surface front inshore of station 4 on transect M. Section 

 at lower right shows temperatures; the two stations where bluefin larvae were caught are indicated by arrows. (Figures redrawn 

 fi-om Berrien et al. 1978.) 



1983:50), the day when the Gulf Stream was at 

 its closest inshore position during the sampling 

 days. Thus nearshore catches were highest when 

 the rapidly flowing core of the Gulf Stream 

 meandered toward shore, carrying bluefin tuna 

 larvae with it. 



DISCUSSION 



Limitations of the Data 



Bluefin tuna larvae are rare on the average in 

 their oceanic habitat. Standard ichthyoplankton 

 tows, which are made to 200 m in order to quan- 

 titatively sample all species, undersample the 

 surface layers where tuna larvae are more abun- 



dant. In addition, we have evidence, to be pub- 

 lished elsewhere, that bluefin tuna larvae are 

 most abundant near specific oceanogi-aphic fea- 

 tures; so they may be undersampled by nonstra- 

 tified survey designs such as the uniform grid 

 often used for logistical reasons. The estimates 

 of abundance will be valid but may have wider 

 confidence intervals than estimates made with a 

 more efficient stratified design. Furthermore, 

 bluefin tuna larvae gi'ow and swim rapidly, so 

 they avoid plankton nets better than larvae of 

 most other species, again contributing to low 

 absolute catches. We acknowledge that low 

 catches limit the precision of results; therefore, 

 we tried to rein in unwarranted speculation. The 

 calculations of adult biomass from larval abun- 



625 



