DeVRIES and CHITTENDEN: SPAWNING TO MORTALITY OF SILVER SEATROUT 



Maximum Size, Life Span, and Mortality 



Silver seatrout off Texas are small fish whose 

 maximum life span (t L ) is about 1-1.5 yr. The 

 largest of the 17,820 specimens that we captured 

 were only 190 mm SL (230 mm TL). Almost 90% 

 of the C. nothus captured off Port Aransas were 

 <110 mm SL(Fig. 10), 99.1% were<160mm SL, 

 and 99.9% were <180 mm SL. Off Freeport, 85% 

 were <110 mm SL, 99% were <160 mm SL, and 

 99.9% were <180 mm SL (Fig. 11). All C. nothus 

 disappeared off Texas when they were slightly 

 older than age I (Table 3). 



The total annual mortality rate of C. nothus in 

 the gulf off Texas approaches 100% and has a 

 best estimate of 99.83%. Values of total annual 

 mortality (1 — S) in each of the 9 mo from October 

 1977 through July 1978 off Freeport were 100% 

 based on the expression S = N,/N where 5 = rate 

 of survival and No and N, are the number of fish 

 in consecutive year classes and t. Only one year 

 class was present off Freeport in those months so 

 that N, = 0. For the same reason, 1 — S was 100% 

 off Port Aransas in each of the 4 mo from Febru- 

 ary through May 1977, during November and 

 December 1977, and during March 1978. Mor- 

 tality estimates were 98% and 99.9% for Septem- 

 ber and October off Port Aransas, assuming that 

 fish >140 and >150 mm SL were from the older 

 year class. For June, July, and August, 1 — S 

 could not be estimated from the Port Aransas 

 data, because the younger year class had just 

 hatched and was incompletely recruited. How- 

 ever, if the predominant group in August had 

 hatched in spring 1977, then 1 — S would ap- 

 proach 100% in that month also. Following the 

 first procedure of Robson and Chapman (1961), 

 we calculated an average value of 1 — S = 99.83% 

 by pooling the identifiable No and N, values for 

 each month. 



Distribution and Availability 



Larger C. nothus seem more susceptible to 

 trawling during the day. Few fish >100 mm SL 

 were taken in night collections at 11, 18-24, and 

 29-31 m (Fig. 12), but many were taken in day 

 collections at 7, 13-15, and 18-24 m. 



Large silver seatrout almost disappeared dur- 

 ing winter. Fish >120 mm SL from the May77- 

 and August-September 7 6 spawned groups were 

 common during November off Port Aransas 

 (Fig. 3) and during October and November off 

 Freeport (Fig. 4), but very few were captured 



110 130 150 



TOTAL LENGTH (mm) 



Figure 10.— Length frequency (moving averages of three) and 

 cumulative percentage of all C. nothus collected off Port Aran- 

 sas, Tex., February-December 1977. 



STANDARD LENGTH 



Figure 11.— Length frequency (moving averages of three) and 

 cumulative percentage of all C. nothus collected off Freeport, 

 Tex., October 1977-July 1978. 



from December through March. The larger spe- 

 cimens of the August77-spawned group also dis- 

 appeared about December, which may be why 

 the August- and September-spawned groups 

 were not distinct thereafter. Many large fish 

 were again taken in May or June. 



Total Weight- and Girth- 

 Standard Length and 

 Standard Length-Total 

 Length Relationships 



Regression and related analyses for total 

 weight-standard length, girth-standard length, 

 and standard length-total length relationships 

 are presented in Table 4. All regressions were 



495 



