NOTE Olson and Scholey Growth of late-larval and early-juvenile Euthynnus /meatus 



823 



from Achotines Bay was pumped through a sand filter 

 containing #20 silica sand and added to the tanks at a 

 rate sufficient to exchange the water several times per 

 day. Wild zooplankton, predominantly copepods, caught 

 with a 505-Mm mesh net were provided as food in den- 

 sities of approximately 50-150/L two or three times 

 per day until the black skipjack grew to about 13-17 

 mmSL. Then, whole Poec(7/o latipixHd and Poeciliopsis 

 turrubarensis fry or chopped fish of several species 

 were provided four to six times per day. The black skip- 

 jack were fed until satiated. Maximum rations were 

 provided throughout the growth experiments. When 

 the fish reached about 35-45 mmSL, they were trans- 

 ferred into 3.0-, 4.6-, or 6.4-m diameter plastic-lined 

 pools containing 2.3, 11.4, or 24.6 nr' of water. 



Water temperature in the laboratory aquaria was 

 recorded to the nearest 0.1 °C several times per day 

 with a mercury thermometer. Salinity was measured 

 to the nearest l"/"u (readability) several times per 

 month with an optical salinometer. Daylight illumina- 

 tion was supplemented by fluorescent lights over the 

 containers. At night, low levels of indirect fluorescent 

 lighting were maintained to prevent the fish from col- 

 liding with the container walls. 



Because black skipjack are delicate, initial lengths of 

 those held for growth experiments were estimated by 

 visually comparing them with a ruler held near the tank 

 and adjusting for the magnifying effect of the water. 

 Lengths of fish that had recently died or were near 

 death in the tanks soon after capture were estimated 

 in the same way, and subsecjuently measured (SL to 

 nearest mm) after retrieval from the tanks to improve 

 estimating skills and to provide a measure of estima- 

 tion error. The maximum error measured was used to 

 calculate the largest potential errors in growth rates 

 that could have resulted. 



Weights were calculated from lengths based on a 

 weight-length relationship derived from 184 black skip- 

 jack measured (SL to nearest 0.1 mm) and weighed 

 (round wet weight to nearest 0.001 g) soon after cap- 

 ture. Length and weight measurements were con- 

 verted to natural logarithms (In), and regression 

 parameters Ina and h were estimated by the method 

 of least scjuares. 



\nW = Ina + bh^SL, 



(1) 



where W = weight in g, and SL = standard length in 

 mm. Validity of the assumptions of linear regression 

 were tested using residual analyses (Draper and Smith 

 1981). 



Standard lengths and weights at the end of the 

 growth experiments were measured to the nearest 0.1 

 mm and 0.1 g, respectively. The experiments were 



I 

 m 



L. 



O 

 to 

 a. 



n = 103 



"1 1 



10 15 



STANDARD LENGTH AT CAPTURE (MM) 



Figure 3 



Standard leiigth.s of late-larval and early-juvenile black skipjack tuna 

 measured soon after capture by nightlighting in the waters south 

 of the Achotines Laboratory, October 1987-April 1988. 



terminated when the fish died from some accidental 

 or unknown cause or were near death. 



The average daily growth rate of each fish over the 

 duration of captivity was expressed as growth in SL 

 (mm) during captivity divided by days in captivity. The 

 average daily weight increase of each fish was calcu- 

 lated using final weights of the fish and the estimated 

 SL at capture converted to weight. 



Results 



Late-larval and early-juvenile scombrids have been 

 captured in all months of the year, but not in every 

 month since the inception of routine sampling in 1984. 

 During the period of October 1986-April 1988, 212 

 late-larval or early-juvenile black skipjack tuna were 

 captured and transferred to the laboratory. Speci- 

 mens which did not survive live transfer procedures 

 ranged between 7.1 and 18.4 mmSL soon after capture 

 (Fig. 3). A total of 79 (37%) survived handling and liv- 

 ed longer than 48 hours. Sampling dates, numbers 

 caught, and other information are given in Table 1. 

 Young black skipjack were caught during 16 of the 

 18 months in which sampling took place during this 

 period. No nightlighting took place during January 

 1987 due to poor weather conditions. The greatest 

 catches were made during November and December 

 1986, December 1987, and January 1988. Sampling fre- 

 quency was governed by weather conditions, boat 

 availability, and other factors. At times, sampling was 

 terminated prematurely due to poor weather, which 

 resulted in no catch. For this and other reasons, the 



