Wexler et al Temporal variation in larval growth of Thunnus albocares In the Panama Bight 



15°l 



bongo were averaged. Beginning in 1991. a 0.6- 

 m- Tucker trawl equipped with a 335-mm mesh 

 net, flow meter, and temperature-depth logger 

 was used to sample ichthyoplankton at discrete 

 depths at only the Punta Mala shelf break (MSB). 

 These surveys were designed to study the vertical 

 distribution and in situ growth and starvation 

 rates of tuna larvae and the abundance of their 

 zooplankton prey (lATTCi; IATTC-). Two replicate 

 tows of 4 to 5 minutes were made at each of three 

 or four depth strata: 0-5 (stratum 1), 5-20 (stra- 

 tum 2), 20-40 (stratum 3), and 40-60 m (stratum 

 4). Plankton volumes were standardized (Smith 

 and Richardson, 19771 at each depth stratum and 

 were added together for each sampling day to 

 compare the mean plankton volumes collected by 

 the Tucker trawl with those collected by the bongo 

 tows of the previous year. Mean plankton volumes 

 were compared between collection group periods 

 by using a one-way analysis of variance (ANOVA), 

 the Student-Newman-Keuls multiple range com- 

 parison test (SNK test), and a ^test for unequal 

 variance (o=0.05) when appropriate (Zar, 1984). 

 In 1991, all four depth strata were sampled, but 

 in 1992 only the first three strata were sampled. 

 Additionally, a 73-f(m mesh net with a mouth area 

 of 0.014 m- was nested inside the Tucker trawl in 

 1992 to collect microzooplankton simultaneously 

 with all other plankters. The displaced volume of 

 the microzooplankton was included in the total 

 standardized plankton volume for each sampling 

 day. Water temperatures, surface wind speeds 

 (m/s), and salinity values (psu) were measured 

 (described in Lauth and Olson, 1996) during each 

 sampling day. 



Plankton displacement volumes for all years 

 were also standardized as plankton volume per 

 volume of water filtered (mL/m^) to compare 

 mean values between years and with literature 

 values. The mean of each standardized volume for 

 the 1990 oblique tows (0-50 m) and for discrete 

 depths between and 40 m of the 1991 and 1992 

 data were compared between collection group pe- 

 riods by using ANOVA, the SNK test, and a ^test 

 for unequal variance (a=0.05) (Zar, 1984). 



Sea-surface temperatures and wind stress climatology 



The oceanographic surveys provided physical data 

 within a limited portion of the area where Thunnus 

 larvae potentially occurred since hatching. Therefore, 

 area- and time-specific (monthly averages within 1- by 

 1.5-degree areas) SSTs to 5 m depth and wind stress cli- 

 matology data (all data sets based on a hindcast ocean 

 analysis system model described by Ji et al. [1995]) 

 for the estimated area of each collection group period 

 (Table 2, Fig. 2) were accessed from the internet (IRI''). 

 Wind velocities in m/s were calculated from wind stress 

 values based on a constant drag coefficient of 1.3x10"'^ 

 (Sverdrup et al., 1942; Large and Pond, 1981; Ji et al.. 



Suilace winds 



August 



^ PCif ( l(! i:lU::^p/, /, A ,, y -, . 



tiiiil. 



100"W 



95 =W 



85°W 



Surface currents 



80°W 





95 -W 



85'W 



80 W 



Figure 2 



Monthly fields of surface wind velocity (for August) and sur- 

 face current velocity (for September) representative of the 

 seasonal extremes during the reduced upwelling period in 

 the Panama Bight (after Figure 3 of Fiedler, 2002). Shading 

 indicates surface wind divergence (intertropical convergence 

 zone) during August, NEC = North Equatorial Current, SEC = 

 South Equatorial Current, NECC = North Equatorial Counter 

 Current, and CRCC = Costa Rica Coastal Current. The area 

 between the vertical and horizontal lines and the land mass 

 represents the estimated maximum average area (in degrees) 

 (see Table 2) potentially occupied by each larval yellowfin tuna 

 cohort during its life history. The spawning distribution of 

 yellowfin tuna within the area is presented as the proportions 

 (P) of reproductively active females in relation to the total 

 numbers of mature females captured within 1-degree areas 

 during the second and third quarters between 1987 and 1989 

 (from Schaefer. 1998). 



^ International Research Institute for Climate Prediction 

 (IRI). 2006. Website: http://ingrid.ldeo.columbia.edu/ 

 SOURCES/.NOAA/.NCEP/.EMC/.CMB/.Pacific/.monthly/ 

 (accessed on 14 October 2005). 



