Abstract. — Female cobia, Ra- 

 chycentron canadum, were sampled 

 on their spawning grounds in the 

 northern Gulf of Mexico to study 

 changes in proximate analysis (pro- 

 tein, lipid, carbohydrate, and ash) 

 of the ovaries during gonadal matu- 

 ration. Four major stages of oocyte 

 development were studied: stage 1, 

 previtellogenesis; stage 2, vitello- 

 genesis; stage 3, final maturation; 

 and stage 4, postovulation. Cobia 

 are multiple spawning fish; there- 

 fore, ovaries engaged in a sequen- 

 tial round of oogenesis were distin- 

 guished as stages 1' and 2'. Protein 

 was the major constituent of cobia 

 ovaries and its contribution re- 

 mained fairly constant (49-55% of 

 the dry weight) throughout all 

 stages of development. Lipid was 

 the second most abundant compo- 

 nent but the levels, ranging from 

 21 to 41%, changed depending on 

 the stage of ovarian development. 

 Lipid concentration increased from 

 stage 1 through 3 and decreased 

 slightly in stage 4; it was lower in 

 stage- 1 than in stage- 1' ovaries but 

 was the same in stages 2 and 2'. 

 Carbohydrate was the least abun- 

 dant component (3-4%) whereas 

 ash ranked third (6-20%). Most 

 cobia were in prespawning condi- 

 tion ( stages 1-3 ) when they arrived 

 in the northern Gulf of Mexico in 

 April and May; some prespawning 

 fish (stages 1 and 2) were also ob- 

 served in August and September 

 about a month or two before migra- 

 tion to the overwintering grounds 

 normally occurs. Cobia undergoing 

 sequential spawning episodes 

 (stages 1" and 2) were captured 

 from April through August. 

 Gonosomatic indices (GSI) were 

 calculated both for ovarian devel- 

 opmental stage and for month of 

 capture. Mean GSI increased as 

 ovarian development proceeded 

 and decreased during postovula- 

 tion; GSI for month of capture was 

 highest during April and May when 

 the prespawning fish first ap- 

 peared in northern Gulf of Mexico 

 waters. 



Biochemical and histological 

 changes during ovarian 

 development of cobia, 

 Rachycentron canadum, from 

 the northern Gulf of Mexico 



Patricia M. Biesiot 

 Robert E. Cay lor 



Department of Biological Sciences, University of Southern Mississippi 

 Hattiesburg, Mississippi 39406-5018 



James S. Franks 



Gulf Coast Research Laboratory 

 Ocean Springs. Mississippi 39566-7000 



Manuscript accepted 9 May 1994. 

 Fishery Bulletin 92:686-696 ( 1994). 



686 



Cobia, Rachycentron canadum, are 

 large migratory fish with a world- 

 wide distribution in tropical and 

 subtropical seas, except for the Pa- 

 cific coast of North America (Mig- 

 dalski and Fichter, 1983). In the 

 western Atlantic, cobia are found 

 from Massachusetts and Bermuda 

 to Argentina (Briggs, 1958) but are 

 most common in the Gulf of Mexico 

 (Migdalski and Fichter, 1983), rang- 

 ing from Key West along the entire 

 coast to Campeche, Mexico (Daw- 

 son, 1971). Cobia support a popu- 

 lar sport fishery wherever they are 

 present. Total mortality rates for co- 

 bia, including sport and commercial 

 catches plus natural mortality, may 

 be high (Richards, 1967) and it has 

 been questioned whether cobia in the 

 Gulf of Mexico are being exploited at 

 rates beyond which maximum sus- 

 tainable yields can be maintained. 



Cobia undergo extensive seasonal 

 migrations (Fig. 1), moving from 

 overwintering grounds to distant 

 spawning/feeding grounds during 

 the spring and summer (Briggs, 

 1958). They are usually absent from 

 the U.S. fishery in more northerly 

 latitudes during fall and winter 

 months (Dawson, 1971 ) and are be- 

 lieved to spend their winters near the 



Florida Keys. 1 During the spring, co- 

 bia move northwest into Gulf waters' 

 or north along the eastern seaboard 

 of the United States (Richards, 1977). 

 Cobia usually enter north-central 

 Gulf waters (Alabama and Missis- 

 sippi) in March or April and begin the 

 return to their wintering grounds in 

 late September. 1 



Female cobia with ripe ovaries 

 have been collected in the northern 

 Gulf of Mexico from April to May 

 through October. 2 Spawning takes 

 place throughout the summer with 



1 Franks, J. S., J. T. McBee, and M. T. Allen. 

 1992. Studies on the seasonal movements 

 and migratory patterns of the cobia, Rach- 

 ycentron canadum, in Mississippi marine 

 waters and adjacent Gulf waters. Gulf 

 Coast Res. Lab., Ocean Springs, MS 39566- 

 7000. Interim Contract Rep. to Miss. Dept. 

 Wildl., Fish, and Parks/Bur. Mar. Res. and 

 U.S. Fish Wildl. Serv., Atlanta, GA 30303, 

 Project No. F-91, 62 p. 



2 Lotz, J. M., R. M. Overstreet, and J. S. 

 Franks. 1991. Reproduction of cobia, 

 Rachycentron canadum, from the north- 

 eastern Gulf of Mexico. In J. S. Franks, T. 

 D. Mcllwain, R. M. Overstreet, J. T. McBee, 

 J. M. Lotz, and G. Meyer, Investigations 

 of the cobia (Rachycentron canadum) in 

 Mississippi marine waters and adjacent 

 Gulf waters. Gulf Coast Res. Lab., Ocean 

 Springs, MS 39564-7000. Final Rep. to 

 Miss. Dept. Wildl., Fish, and Parks/Bur. 

 Mar Res. and U.S. Fish Wildl. Serv, Atlanta, 

 GA 30303, Project No. F-91, p. 2-1 to 2-42. 



