SPAWNING, AGE DETERMINATION, LONGEVITY, AND 

 MORTALITY OF THE SILVER SEATROUT, CYNOSCION NOTHUS, IN 



THE GULF OF MEXICO 



Douglas A. DeVries 3 and Mark E. Chittenden, Jr. 4 



ABSTRACT 



Cynosciou nothvs females from the Gulf of Mexico off Texas matured at 140-170 mm SL as they 

 approached age I. Spawning occurred from early May through late October but primarily in two 



periods, May and August-September. Greatest spawning occurred in the August-September period 

 when two distinct spawned groups (intrayear class cohorts) were produced. The multiple-spawned 

 group structure within a year class may be important to the population dynamics and stability of C. 

 nothus. This species reached 130-190 mm SLatage I. Only one year class occurred or dominated in 

 any one month, and only two year classes were ever present at once. The largest specimen captured 

 was 190 mm SL, and 99% were<160 mm. The maximum life span (f ; ,) was only 1-1. Syearsoff Texas 

 but might be 2 years in the northcentral gulf. The total annual mortality rate was best estimated at 

 99.83% and probably is no lower than 90% if the life span is as long as 2 years. Larger C. notkm almost 

 disappeared during winter suggesting an offshore movement for overwintering. 



The silver seatrout, Cynoscion nothus, smallest of 

 four congeners found along the U.S. Atlantic 

 coast, ranges from Chesapeake Bay to the Bay of 

 Campeche (Hildebrand and Schroeder 1928; 

 Hildebrand 1955). Cynoscion nothus is one of the 

 more abundant fishes of the nearshore waters of 

 the northern Gulf of Mexico (Hildebrand 1954; 

 Moore et al. 1970; Gutherz et al. 1975). It is not 

 considered an estuarine species (Ginsburg 1931; 

 Hildebrand and Cable 1934), but small numbers 

 of C. nothus have been taken in bays throughout 

 the year (e.g., Gunter 1938, 1945; Swingle 1971; 

 Dahlberg 1972). 



Despite its abundance, no published study has 

 been directed at C. nothus in the gulf. Little is 

 known about its life history, although Mahood 

 (1974) described spawning and monthly size 

 composition in Georgia waters. General notes on 

 C. nothus appear in many faunal studies, includ- 

 ing Hildebrand and Cable (1934), Gunter (1938, 

 1945), Chittenden and McEachran (1976), and in 

 a literature review by Guest and Gunter (1958). 



■Based on a thesis submitted by the senior author in partial 

 fulfillment of the requirements for the M.S. degree, Texas 

 A&M University. 



2 Technical article TA 15557 from the Texas Agricultural 

 Experiment Station. 



department of Wildlife and Fisheries Sciences, Texas 

 A&M University, College Station. Texas; present address: 

 North Carolina Division of Marine Fisheries, Box 769, More- 

 head City, NC 28557. 



department of Wildlife and Fisheries Sciences, Texas 

 A&M University, College Station, TX 77843. 



This paper describes age determination for 

 silver seatrout, spawning seasonality and peri- 

 odicity, growth, mortality, diel changes in avail- 

 ability, and total weight-standard length, girtn- 

 standard length, and standard length-total 

 length relationships. 



METHODS 



Collections were made monthly from Febru- 

 ary through December 1977 and in March 1978 

 in the Gulf off Port Aransas, Tex., aboard the 

 Texas Parks and Wildlife Department's RV 

 Western Gulf, using a 13.7 m otter trawl with 5.1 

 cm stretched mesh in the cod end. Stations were 

 usually occupied at 11 m depth at night and at 7, 

 15, and 18-24 m during the day. Additional night 

 collections were made from May through Octo- 

 ber 1977 at 20-22, 29-31, and 38 m. 



Cynoscion spp. were fixed in 10% Formalin' 

 and transferred to 40% isopropanol before proc- 

 essing. Cynoscion nothus were separated from 

 C. arenarius primarily by comparing the anal 

 fin base to the eye width, a procedure based on 

 the comparatively low anal fin ray counts and 

 larger eye size that Ginsburg (1929) reported in 

 C. nothus. The anal fin width equals or is only 

 slightly greater than eye width in C. nothus, but 



Reference to trade names does not imply endorsement by 

 the National Marine Fisheries Service, NOAA. 



Manuscript accepted January 1982. 

 FISHERY BULLETIN: VOL. 80. NO. 3. 1982. 



487 



