588 



Fishery Bulletin 97(3), 1999 



LoQe N, = 10.786 • 139(age) 



LoQeN, = 8.350-0 134(age) 

 r2 = 0.84 



^ 1995 

 A 1994 



20 30 40 50 



Age (d) 



Figure 6 



Regression plots of Log^, abundance on age 

 of postsettlement red drum collected from 

 the Aransas Estuary in 1994 and 1995. Re- 

 gression equations are given. 



7] 



6 



5- 



4- 



3- 



2 



H 





 20 



7- 



6 



5 



4 



3 



2-1 



1 







Logg N, = 11 609-0 193(age) 

 r2 = 0.87 



cohort B 



30 40 50 



Lege N, = 6.169 -0.106(age) 

 r2 = 0.34 



cohort D 



7- 



6 



5 



4 



3- 



2- 



1- 





 20 



7 



6- 



5 



4 



3 



2 



1 







LoggN, = 8.503-0 127(age) 

 r2 = 0.78 



cohort C 



30 



cohort E 



20 



30 



40 



50 



20 

 Age (d) 



30 



Figure 7 



Regression plots of Log, abundance on age for 10-d cohorts of postsettlement red drum 

 collected from one site (AB2) in the Aransas Estuary in 1995. Regression equations and 

 plots are shown for cohorts B, C, D, and E. 



horts were approximately twofold higher, approach- 

 ing 259(^/d for the late-season cohort. Seasonal fluc- 

 tuations of this magnitude have been reported in 

 several studies on marine fish larvae and attributed 

 to changes in environmental conditions (e.g. tempera- 

 ture, prey availability) which can directly or indi- 

 rectly influence mortality (Rutherford and Houde, 

 1995; Secor and Houde, 1995). 



Temperature is often implicated as a critical fac- 

 tor in recruitment because it has the potential of in- 

 fluencing growth rates and causing episodic mortal- 

 ity during early life (Houde and Zastrow, 1993; 

 Houde, 1996). Temperature has been shown to be a 

 primary determinant of growth variation in labora- 

 tory and wild populations of red drum (Holt et al., 

 1981; Lee et al., 1984; Rooker et al., 1997). More- 

 over, Rocker and Holt (1997) determined that sea- 

 sonal trends in growth variation for red drum were 

 related to temperature. Mean temperatures experi- 

 enced by successive cohorts declined throughout the 

 season; however, growth rates were highest for mid- 

 season cohorts (20 Septem- 

 ber- 10 October) and lowest 

 for early and late cohorts. 

 Conversely, mortality rates 

 were lowest for mid-season 

 cohorts and highest for early 

 and late cohorts. Growth 

 and mortality data appear 

 to be best described by cur- 

 vilinear relationships (qua- 

 dratic function), suggesting 

 that there may be an opti- 

 mal temperature range at 

 which growth and survival 

 of red drum are enhanced 

 and that the midpoint of 

 this range occurs at approxi- 

 mately 26' 'C. 



Variability in other biotic 

 and abiotic factors may also 

 be responsible for observed 

 trends in mortality. Similar 

 to other marine fishes, 

 growth and survival rates of 

 red drum larvae are associ- 

 ated with prey availability 

 (G.J. Holt, unpubl. data). 

 Since spatial and temporal 

 variation in prey density 

 (i.e. meiofauna) are common 

 in estuarine habitats in 

 south Texas (Montagna and 

 Kalke, 1992), seasonal trends 

 in prey abundance may 



40 



50 



LoggN, = 11 918-0.265(age) 

 r^ = 0.91 



40 



50 



