Schaefer Spawning time, frequency, and batch fecundity of Thunnus albacares 



99 



fecundity can be estimated from batch fecundity (the 

 number of oocytes released per spawning I and spawn- 

 ing frequency. However, only at the final stages of 

 oocyte maturation, beginning with the migratory- 

 nucleus phase and followed by hydration, is there a 

 distinct hiatus in the distribution of oocytes from 

 which the batch fecundity can be determined. Previ- 

 ous batch fecundity estimates for yellowfin tuna in 

 the eastern Pacific (Joseph, 1963) were based upon 

 counts of yolked oocytes or upon more advanced 

 stages from ovarian tissue samples that had been 

 placed in Gilson's fluid. This methodology, however, 

 creates a potential bias in batch fecundity estimates 

 because Gilson's fluid causes significant shrinkage 

 of oocytes and compaction of the oocyte size distri- 

 bution, which then complicates partitioning of the 

 most advanced group of oocytes from the adjacent 

 group of smaller oocytes. 



The objectives of this study are 1) to determine 

 the time of day that yellowfin spawn and to develop 

 histological criteria for estimating spawning fre- 

 quency; 2) to estimate batch fecundity from migra- 

 tory-nucleus or hydrated-stage oocytes in order to 

 provide an estimate of total fecundity; and 3) to esti- 

 mate energetic investment in spawning. 



Materials and methods 



Sampling and processing 



Yellowfin tuna were caught by rod and reel aboard 

 the MV Royal Polaris, a San Diego-based long-range 

 sportfishing boat, around Clipper-ton Atoll in the east- 

 ern Pacific Ocean (lat. 10°118'N, long. 109°13'W> 

 during the first two weeks of May during both 1986 

 and 1987 throughout the diel period (Fig. 1). 

 Clipperton Atoll is located in an area in which large 

 catches of yellowfin tuna are taken by purse seiners 

 (Bayliff, 1991). Sea-surface temperatures during 

 these periods were 83° to 84°F (28° to 29°C). 



Specimens close to or greater than 900 mm fork 

 length (FL) were measured to the nearest millime- 

 ter and their gonads removed immediately after cap- 

 ture (Fig. 2). The decision to sample only yellowfin 

 greater than about 900 mm FL was based on earlier 

 research by Orange ( 1961), who found that about 7Q ( /c 

 offish greater than 900 mm FL were reproductively 

 active. A 2-cm cross section from the central region 

 of the ovaries or testes of each specimen was fixed in 

 10% neutral-buffered formalin (Hunter, 1985). The 

 ovaries or testes from each specimen, minus the sec- 

 tion, were frozen at sea, thawed in the laboratory, 

 blotted dry, and weighed to the nearest centigram 

 on a Mettler electronic balance. A portion of each 



0200 0600 1000 1400 1800 2200 



Time of day (h) 



Figure 1 



Number of yellowfin tuna, Thunnus albacares, 

 sampled for reproductive data during May of 1986 

 and 1987 from around Clipperton Atoll in the east- 

 ern Pacific Ocean by times of day 



sample of preserved gonadal tissue was embedded 

 in Paraplast, sectioned at approximately 6 /jm, and 

 stained with Harris hematoxylin followed by eosin 

 counterstain. The slides were viewed by light micro- 

 scopy with a Nikon optiphot microscope fitted with a 

 microflex HFX-II photographic attachment and 

 Kodak T-max 100 film. 



Histological classification 



The classification of yellowfin tuna ovaries was based 

 on a modification of the system of Hunter and 

 Macewicz ( 1985 ). For each ovary the most-developed 

 oocytes were classified as follows: 1 = unyolked, 2 = 

 partially yolked, 3 = fully yolked, 4 = migratory- 

 nucleus stage, and 5 = hydrated. Identification of 

 ovarian atresia (resorption of oocytes) was restricted 

 to advanced yolked oocytes (stage 3). Ovaries were 

 classified into one of three categories of alpha (a) 



