FISHERY BULLETIN: VOL. 80, NO. 1 



and reached a peak during April and May as 

 indicated by the highest GSI. Sampling was 

 inadequate for November through February. 

 Well-developed ovaries were present mainly 

 during May and June in eastern Atlantic giant 

 bluefin tuna from the traps at Barbate 

 (Rodriguez-Roda 1967). In May, the mean GSI 

 for the western Atlantic was greater than that 

 for the eastern Atlantic. No great difference was 

 found between the mean GSI for the western and 

 eastern Atlantic giant bluefin tuna for June, 

 July, and August for which data were available 

 for comparison. The plots of the GSI indicate that 

 giant bluefin tuna spawning occurs earlier in the 

 western Atlantic (the drop in GSI occurring in 

 June) than in the eastern Atlantic (the drop in 

 GSI occurring in July). 



Heterogeneity of Egg Diameters 



A significant difference in egg diameters was 

 found for the center, midregion, and periphery of 

 the anterior section of an ovary from a mature 

 fish (F=6.1;df = 2, 631; P<0.005). No significant 

 difference in egg diameters was found for the 

 center, midregion, and periphery of the middle 

 or posterior sections of the ovary. A significant 

 difference in egg diameters was also found 

 among the anterior, middle, and posterior sec- 

 tions (F= 11.6; df = 2, 1,843; P<0.001). Because 

 some heterogeneity occurred, estimates of 

 fecundity were based on eggs from each section 

 of both ovaries. Heterogeneity of egg size within 

 an ovary has also been shown for albacore, 

 Thunnus alalunga, (Otsu and Uchida 1959); 

 swordfish, Xiphias gladius, (Uchiyama and 

 Shomura 1974); and white marlin, Tetrapturus 

 albidus, (Baglin 1979). 



Histology of the Ovaries 



Microscopic examinations were made of 

 ovarian tissues from 119 small and medium 

 bluefin tuna and 173 giant bluefin tuna. 

 Diameters measured from these prepared slides 

 were considerably smaller than those measured 

 from whole eggs fixed in 10% Formalin (see 

 footnote 5). 



The oocytes were grouped into the following 

 stages of oogenesis using the system of Kraft and 

 Peters (1963), Smith (1965), and Moe (1969). 



Stage 1 — Thin layer of cytoplasm surrounding a 



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relatively large nucleus with a single nucle- 

 olus; oocytes are <0.03 mm in diameter. 



Stage 2 — Dark cytoplasm and many peripheral 

 nucleoli are in the nucleus oocytes are 0.03 

 mm through 0.13 mm in diameter (resting 

 stage). 



Stage 3— Yolk vesicles appear in cytoplasm; the 

 membrane called the zona radiata, also 

 referred to as zona pellucida (Hoar 1969) and 

 vitelline membrane (Bodola 1966), appears at 

 the end of this stage; oocytes are 0.17 mm 

 through 0.30 mm in diameter (early vitello- 

 genic stage). 



Stage 4 — Thick zona radiata, yolk vesicles, and 

 yolk globules are present; oocytes are 0.33 mm 

 through 0.63 mm in diameter (late vitellogenic 

 stage). 



Stage 5— This final stage was seldom observed 

 during histological analysis. It evidently takes 

 place during a short period of time immediate- 

 ly before ovulation. Eggs in this stage have a 

 lightly staining granular yolk mass with few 

 yolk vesicles and yolk globules, a thin zona 

 radiata, and an irregular shape caused by 

 sectioning. 



Histological examination of female bluefin 

 ovary sections from the Middle Atlantic Bight 

 revealed the following: 



Age 1 — Very little sexual differentiation was 

 present in age 1 bluefin tuna (N= 17) collected 

 during May, June, July, and August. Some 

 oogonia were observed within the lamellae. 



Age 2 — The first appearance of oocyte develop- 

 ment occurred in age 2 bluefin tuna collected 

 during July (N - 4). Both stage 1 and stage 2 

 oocytes were present, although stage 2 oocytes 

 were most numerous. 



Age 3— Many stage 2 oocytes and a few stage 1 

 oocytes were found in age 3 bluefin tuna 

 collected during January and June (N - 13). 

 Only stage 2 oocytes were found in age 3 

 bluefin tuna collected during July and August 

 (N= 10). 



Age 4 — Stage 2 oocytes were present in all age 4 

 females collected during June (Af = 36) (Fig. 3). 

 Also in 11% of these fish, some vitellogenic 

 stage 3 oocytes undergoing absorption were 

 present. Only stage 2 oocytes were present in 

 age 4 bluefin tuna collected during July and 

 August (N = 10). 



Age 5— Mostly stage 2 oocytes were present in 

 age 5 fish collected during June (N = 16), 



