Fishes of the Western North Atlantic 



449 



Pelvic fin: length 2.05-2.4 in hi. 

 Pectoral fin: length 1.2— 1.35 in hi. 

 Distances: predorsal 1.9—2.25; prepelvic 



1.9—2.25; anal origin to caudal 



base 2.6—3.1; all in SL. 



Scales: lateral series 41—48 ; between dor- 

 sal and anal fins 14—17; around 

 body 28—36; around caudal pe- 

 duncle 16—20 (all counts based on 

 25 specimens covering the size 

 range of this species). 



Scutes: prepelvic 15—18, av. 16.79 {^7^)'i 



postpelvic 8-12, av. 9.95 (178); 

 total ventral 23—29, av. 26.75 

 (176). 



Gill rakers: ca. 300 at 66 mm SL, 330 

 at 117 mm, 350 at 131mm, and 

 about 440 at 180 mm; fine and 

 numerous (rakers counted in situ). 



Fin rays: dorsal 11— 14, av. 12.13 (i7^)> 

 caudal 19; anal 17—27, av. 22.38 

 (179); pelvic 8, rarely 7; pectoral 

 12-17, av. 14.39 (354, 177 fish). 



Vertebrae: 40—45, av. 42.23, including 

 urostyle (30). 



As in D. cepedianum., there appear to be no reliable external features that will 

 distinguish the sexes. 



Color. In life, body bright silvery, especially on sides, opercles, and underparts, 

 but back and upper sides bluish-black or dark olivaceous, washed with gold and over- 

 lain with horizontal rows of dark stripes. A conspicuous, jet-black round or oval spot 

 behind upper part of opercle. Caudal with middle portion of each fin lobe bright golden 

 yellow, fading basally and distally, the posterior border dusky; dorsal fin dusky, washed 

 with yellowish olive; anal fin intensely yellow; paired fins yellow basally. A broad golden 

 stripe (often represented only by gilt reflections) in the light band between back and 

 sides, but almost no gilt on sides of head. 



Size and Age. The Threadfin Shad is a smaller species than the gizzard shad. 

 In the northern part of its range, the adult generally does not exceed 5 or 6 inches 

 TL; in southern states it may grow to 7 inches, and in Guatemala to nearly 8.75 

 inches or 180 mm SL (specimen from Laguna de Yalac; UMMZ 143386). The 

 species has a short life-span, for few individuals appear to exceed two years of age. 



Variations in Size with Season. In bays along the Texas coast, adults were most 

 abundant in August and October while the young were entirely absent; between August 

 and November the size increased from a mode between 103 and 108 mm TL to 

 1 18 mm. In January, only fish with a modal length of 63 mm came into the catch, and 

 in February the same group was present along with a few fish 1 14 and 1 17 mm long. 

 Possibly early growth takes place only in fresh water. Water temperatures in the bays 

 of Texas varied from about 48 to 86°F over the year. 



Spawning. Threadfin Shad may spawn when they are less than one year old; there 

 appear to be two spawning peaks — spring and fall. Spawning occurs at about 70° F in 

 open waters, near or over plants and other objects. The eggs are slightly adhesive. 



Habits. Threadfin Shad are generally pelagic; they often feed and migrate in 

 schools numbering thousands of individuals. The different age-groups usually remain 

 in separate schools. They are known to concentrate at the downstream faces of dams 



