BAGLIN. JR.: REPRODUCTIVE BIOLOGY OF WESTERN ATLANTIC BLUEFIN TUNA 



fish was conducted using histological tech- 

 niques. Ovaries were sectioned at 8 yu and stained 

 either with haematoxylin and eosin or trichrome 

 stain. The oocytes were grouped into stages 

 according to the classification system of Kraft 

 and Peters (1963) and Smith (1965). From each 

 prepared slide, a measurement was taken of the 

 largest egg diameter. The egg diameters were 

 measured with an ocular micrometer at 30 X 

 magnification, and the orientation of egg 

 diameters was assumed to be random. Stage of 

 maturity was thus based on the histological 

 examinations. 



A test was made for heterogeneity of egg size 

 within the ovary. Thin cross sections were taken 

 from the anterior, middle, and posterior parts of 

 one ovary of a mature fish and each section was 

 subdivided into three subsamples, representing 

 the center, midregion, and periphery of the 

 ovary (Otsu and Uchida 1959). Egg diameters 

 from each area were then measured and 

 compared statistically. 



Fecundity, defined as the potential number of 

 mature eggs (yolked ova) that could be spawned 

 during one reproductive season, was estimated 

 by using a dry weight method. This consisted of 

 taking samples from the anterior, middle, and 

 posterior parts of each ovary. The eggs from each 

 of these sections and from the remainder of the 

 ovaries were then separated from the ovarian 

 tissue by straining them over a wire screen 

 under running water. The egg samples from 

 each section, in an aqueous solution, were stirred 

 and a subsample from each was pipetted into a 

 beaker. These eggs were stirred and approxi- 

 mately 1-2 g wet weight were taken to be used for 

 the fecundity estimate. The yolked eggs, which 

 were counted and fecundity estimated, were 

 divided into two size categories. Eggs 0.46 mm 

 and larger that were counted were well 

 developed and fully yolked. A second size 

 category for which eggs were counted included 

 smaller eggs (0.32 mm in diameter) that were not 

 in quite as advanced stage of development, but 

 that could possibly undergo further development 

 and be spawned during one reproductive season. 

 The subsample was then weighed to the nearest 

 0. 1 mg, and the weight of the remaining eggs was 

 recorded in grams. Fecundity estimates, 

 rounded to the nearest 0.1 million eggs, were 

 calculated from the relationship C - (AD/B) + A, 

 where A is the number of mature ova in the 

 subsample, B is the weight of the ova in the 

 subsample, C is the number of mature ova, and D 



is the weight of ova from both ovaries minus the 

 weight of the subsample. 



RESULTS AND DISCUSSION 



Sex Composition 



From 1974 through 1977, sex was determined 

 for 283 small and medium bluefin tuna from the 

 Middle Atlantic Bight during June, July, and 

 August (Table 1). No significant difference from 

 an expected 1:1 sex ratio was found. Sampling 

 for the remaining months was inadequate. 



From 1975 through 1978, sex was determined 

 for 3,429 giant bluefin tuna from the Gulf of 

 Mexico, March through June; Bahamas, April 

 through June; and from the northeast coast of the 

 United States, July through October (Table 1). 

 The deviation from an expected 1 : 1 sex ratio was 

 significant for April, May, July, and August. 

 Females were more prevalent than males in 

 spawning aggregations during April and May. 

 Males were more prevalent in feeding schools 

 during July and August. No significant dif- 

 ference from an expected 1:1 sex ratio was 

 found for March, June, September, and October. 

 Sampling during the remaining months was in- 

 adequate. These findings suggest that some 

 giant bluefin tuna segregate into distinct areal 

 groups according to the predominating sex and 

 that sex ratios may change with season. 



Table 1.— Monthly sex ratios for small and medium (1974-77) 

 and giant (1975-78) western Atlantic bluefin tuna. 



'Significant departure from null hypothesis at 0.05 level (chi-square). 



Gonosomatic Index, Gross Morphology, 

 and Size of Ova 



The external appearance alone of tuna ovaries 

 is inadequate for gross classification of maturity 

 stage (Bunag 1956). The GSI (also called gonad 

 index, maturity index, gonadosomic or gonadal- 



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