Reproduction, movements, and apparent population dynamics of the Atlantic Threadfin Polydactylus octonemus in the Gulf of Mexico.
Chittenden, M.E., Jr.
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Trawl collections were made for Atlantic Threadfin Polydactylus octonemus from 5 to 100 m in the Gulf of Mexico along a cross- shelf transect of Texas during October 1977- August 1981. Threadfin generally 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 water in early summer and form a positive size gradient from the estuaries seaward. Threadfin in the demersal 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 12 months. Fitted von Bertalanffy parameters were 2.17-2.92 (K, annual), 195-230 (L) and -0.03-0.08 years (t0). Maximum size in the demersal phase is 230 mm TL in the northwestern Gulf, but more typically only 200-205 mm. Typical life span (tL) 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 mortality rates are 97-100% in the northwestern 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 currents of the cyclonic shelf gyre off Texas and western Louisiana transport the young to estuarine and inshore nurseries.