822 



Fishery Bulletin 88(4), 1990 



Laboratory studies of growth are important in light of 

 evidence in other fishes that variability in growth rates 

 in the wild can affect early-life-stage duration, poten- 

 tially resulting in large changes in larval survival and 

 subsequent recruitment (Houde 1987). 



The Achotlnes Laboratory 

 and study area 



The lATTC's Achotines Laboratory is located on a 

 broad south-facing headland on the southern tip of the 

 Azuero Peninsula in the Los Santos Province of the 

 Republic of Panama (Fig. 1 ). This site is located at the 

 northwestern part of the Panama Bight, which extends 

 from Panama to Ecuador (Forsbergh 1969). The Pana- 

 ma Bight is a region with large seasonal variations in 

 atmospheric and oceanic characteristics, in part influ- 

 enced by the seasonal position of the intertropical con- 

 vergence zone (ITCZ). Sea-surface temperature data 

 (Fig. 2) clearly show an upwelling cycle driven by the 

 Caribbean trade winds when the ITCZ is displaced to 

 the south, t.\"[Dically beginning in December and lasting 

 at least through April. The continental shelf is narrow, 

 2-30 km in width, off the Azuei'o Peninsula, l)ut widens 

 abruptly on either side of the Peninsula. Thus, oceanic 

 habitat, thought to be a reijuirement for tuna spawn- 

 ing, occurs close to shore. Scombrid larvae and early 

 juveniles are routinely captured as close as 9 km from 

 Achotines Bay. The following late-larval and early- 

 juvenile scoml>rids have been captured and held in the 

 laboratory with varying degrees of success: black skip- 

 jack tuna, yellowfin and/or bigeye tuna {Thioinus nlbd- 

 cnres and/or T. ohofusy, frigate and/or luillet tuna 

 (Auxis thnzard and/or A. rochei)-. sierra Scotnhero- 

 morus sierra, chub mackerel Scomber japonicus, and 

 Indo-Pacific bonito Sorda orientalis. 



Materials and methods 



Larval and early-juvenile fishes were attracted to a 

 bright underwater light at night (nightlighting) in the 

 vicinity of the 100- and 20')-m isobaths (Fig. 1) during 



'Early -juvenile T. nlhacarex and T. ohesiia cannot he distinguished 

 on the basis of meristic. morphological, pigmentation (Matsumoto 

 et al. 1972). or osteological characters (Potthuff 1974). However, an 

 electrophoretic distinction between yellowfin and bigeye adults pro- 

 vides a means of separating the larvae and earlv juveniles (Graves 

 et al. 1988). 



-Larval and early-juvenile Auxit: thnzard and A. rochei have been 

 distinguished by minor differences in pigmentation and body depth, 

 but identifications are ambiguous (Uchida 1981). dill raker counts 

 can be used for identification of juveniles >2."> mmSL, Imt gill rakers 

 are too tiny and difficult to count in smaller specimens (Uchida 1981). 



Figure 2 



Sea-surface temperatures taken while nightlighting in the sampling 

 areas (Fig. 1) off the Achotines Laboratory. 



October 1986-April 1988. A 24-volt DC 300-watt light 

 was lowered from a drifting boat to a depth of about 

 14 m, left for about 5 minutes, and slowly raised to a 

 depth of 1-2 m. Late-larval and early-juvenile'' black 

 skipjack tuna approaching the light were collected by 

 dipnet and quickly placed into 61 x 76 cm polyethylene 

 bags with rounded corners containing aerated sea- 

 water. A water conditioner, Fritz-guard, was added to 

 the water to minimize damage to the fish from mucous 

 loss caused by abrasion. Time from capture to arrival 

 at the laboratory ranged between 1 and 3 hours. Sea- 

 surface temperature, weather, and sea conditions were 

 recorded. The fish which did not survive nightlighting 

 and transfer procedures were measured (standard 

 length, SL, to nearest 0.1 mm) and weighed (round wet 

 weight to nearest 0.001 g) soon after capture. 



At the laboratory, the captive fish were placed in 

 1.2-m diameter circular fiberglass tanks containing 0.3 

 m" of aerated seawater with a weak current. Water 



'Late larvae are defined as the "postflexion larvae" and "transfor- 

 mation larvae." and early juveniles as the "pelagic or special 

 juveniles" of Kendall et al. (1984). 



Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service. N().'\;\. 



