Abstract-— Trawl collections were 

 made for Atlantic threadfin Polydac- 

 tylus octonemus from 5 to 100 m in 

 the Gulf of Mexico along a cross-shelf 

 transect off Texas during October 

 1977-August 1981. Threadfin gener- 

 ally mature at 165-210 mm TL as 

 they approach 7-9 months of age. 

 Spawning primarily occurs in one 

 period. mid-December-mid-March, 

 and spans 45-120 days overall; 90% 

 of successful spawning may occur in 

 only 59% of that period. Threadfin 

 in the northwestern Gulf range from 

 < 5 to 27 m depths in the demersal 

 stage but are most abundant at <5 

 to 16 m. Young-of-the-year recruit in 

 waters <5-16 m when 2-4 months 

 old. Fish begin to disperse to deeper 

 waters in early summer and form a 

 positive size gradient from the estu- 

 aries seaward. Threadfin in the de- 

 mersal phase are not abundant in the 

 northwestern Gulf after 9-11 months 

 of age and reach only 15 months 

 there. Observed mean and predicted 

 sizes were 135-165 mm TL at 6 

 months, 165-215 mm at 9 months, 

 and 180-205 mm at 12 months. Fitted 

 von Bertalanffy parameters were 

 2.17-2.92 (A', annual). 195-230 {LJ. 

 and -0.03-0.08 years {(„). Maximurn 

 size in the demersal phase is 230 mm 

 TL in the northwestern Gulf, but 

 more typically only 200-205 mm. 

 Typical maximum life span (/; ) is 

 about 1 year but may exceed that if 

 individuals survive in a pelagic stage 

 after spawning. Apparent mean time 

 and cohort-specific total annual mor- 

 tality rates are 97-100% in the north- 

 western Gulf. Population dynamics 

 parameters presented are termed 

 apparent because of the unknown 

 effects of recruitment, movements, 

 random variation, gear selectivity, 

 etc. Spawning grounds seemingly lie 

 along the Outer Continental Shelf, 

 slope, or further offshore, and cur- 

 rents of the cyclonic shelf gyre off 

 Texas and western Louisiana trans- 

 port the young to estuarine and in- 

 shore nurseries. 



Reproduction, Movements, and 

 Apparent Population Dynamics of 

 the Atlantic Threadfin Polydactylus 

 octonemus in the Gulf of Mexico* 



Michael W. Dentzau 



Department of Wildlife and Fisheries Sciences. 

 Texas ASM University, College Station, Texas 77843 

 Present address Florida Department of Environmental Regulation 

 2269 Bay Street, Fort Myers, Florida 33901 



Mark E. Chittenden Jr. 



Department of Wildlife and Fistieries Sciences, 

 Texas A&M University, College Station, Texas 77843 



Present address' College of William and Mary, Virginia Institute of Marine Science 

 Gloucester Point, Virginia 23062 



Manuscript accepted 26 March 1990. 

 Fishery Bulletin, U.S. 88:439-462. 



The Atlantic threadfin Polydactylus 

 octonemus occurs along continental 

 shelves from Massachusetts to Brazil, 

 and in the Gulf of Mexico (Breder 

 1948, Briggs 1958, Dahlberg 1975, 

 Fischer 1978). Although uncommon 

 on the Atlantic coast (Hildebrand and 

 Schroeder 1928, Anderson and Geh- 

 ringer 1965, Anderson 1968, WOk and 

 Silverman 1976), it is abundant in Gulf 

 of Mexico (Gull) coastal, sui-f, and estu- 

 arine waters (Hildebrand 1954, McFar- 

 land 1963, Chittenden and McEachran 

 1976). However, its annual abundance 

 seems to flucttiate greatly (Reid 1956, 

 Gallaway and Strawn 1974, Ogren 

 and Brusher 1977). 



Despite its abundance, the life his- 

 tory of P. octonemus has not been 

 studied in detail, and little supporting 

 data have been published. General 

 notes occur in many faunal studies in- 

 cltiding Gunter (1938a, 1945), Reid 

 (1955), Miller (1965), and Juneau 

 (1975). These notes indicate P. octo- 

 nemus is a small, inshore species that 

 occtirs from spring through fail in the 

 northern Gulf and spawns from fall 

 through spring. 



•Contribution no. 1586 of The College of Wil- 

 liam and Mary, Virginia Institute of Marine 

 Science. 



Our paper describes maturation, 

 spawning periodicity, bathymetric 

 distribution, recruitment, movements, 

 age determination and growth using 

 length frequencies, maximum size, 

 life span, mortality, and relationships 

 of length-weight, length-girth, stan- 

 dard-total length and fork-total length 

 of P. octonemus in the northwestern 

 Gulf. It also discusses spawning areas 

 and larval dispersal in relation to Gtilf 

 hydrography, as well as how recruit- 

 ment, movements, and randomiza- 

 tion affect parameter estimation and 

 interpretation. 



Materials and methods 



Collections for Polydactylus octone- 

 mus were made from nearly 3000 

 trawl tows in 71 monthly or twice- 

 monthly cruises, October 1977-Aug- 

 ust 1981, along a cross-shelf transect 

 in the Gulf off Freeport, Texas (Fig. 

 1) aboard a chartered shrimp trawler. 

 Gear used were twin 10.4-m (34-ft) 

 trawls with a 4.4-cm stretched-mesh 

 cod end and a tickler chain. Initial 

 stations usually were located at 

 depths of 9, 13, 16, 18, 22, 27, 36, and 

 47 m. Sampling was expanded to in- 

 clude stations at 5 and 24 m after 

 November 1978 and at 55, 64, 73, 82, 



439 



