Rooker et al.: Variability In growth, mortality, and recruitment of Saaenops ocellatus 



589 



C D E 



Cohort (1995) 



1994 1995 

 Year 



Figure 8 



Recruitment potential (G;2 ratios) of 10-d cohorts 

 of postsettlement red drum collected from the 

 Aransas Estuary in 1995. Interannual estimates 

 of G.Z are given. 



have affected early life survival. Conversely, preda- 

 tor fields also vary spatially and temporally and sur- 

 vival may be a function of predator abundance. 

 Postsettlement red drum are exposed to a suite of 

 predators in the seagrass meadows and the abun- 

 dance and distribution of these predators are highly 

 variable (senior author's unpubl. data). Rooker et al. 

 (1998a) examined predation rates on newly settled 

 red drum in experimental mesocosms and demon- 

 strated that predators inhabiting these seagrass 

 meadows, are capable of consuming large numbers 

 of red drum (Z=2-7'^/[h • predator] ). Consequently, 

 fluctuations in prey and predator densities appear 

 critical to the survival of red drum and the addition 

 of this information to future studies should enhance 

 our understanding of early life mortality. 



Growth and mortality estimates were combined to 

 evaluate the relative recruitment potential iG.Z) of 

 red drum cohorts. According to Houde and Zastrow's 

 (1993) review of 188 species, G:Z ratios for marine 

 fish larvae are generally less than 1.0 (mean 

 G.Z=0.89) and larvae inhabiting estuarine ecosys- 

 tems tend to have low G;Z ratios (range: 0.34-0.82). 

 However, recent work in Chesapeake Bay on striped 

 bass {Morone saxatilis) has demonstrated that G:Z 

 ratios can be >1 in estuarine ecosystems (Ruther- 

 ford and Houde. 1995; Secor and Houde, 1995). Simi- 

 larly, G:Z ratios for postsettlement red drum in the 

 Aransas Estuary were >1 in both 1994 and 1995 (1.21 

 and 1.19, respectively) and, as a result, gross growth 

 efficiency was positive and the recruitment poten- 

 tial of 1994 and 1995 year classes appears to have 

 been favorable. 



A high degree of correspondence in G:Z ratios be- 

 tween years was present, suggesting that conditions 

 for growth and survival were similar in 1994 and 



1995. Although the assessment of physical and bio- 

 logical conditions was limited (no data on prey avail- 

 ability or predator fields), parameters measured in 

 this study (e.g. temperature, salinity, settlement size, 

 and density ) were relatively equal between years. In 

 contrast, G:Z ratios of 10-d cohorts and the environ- 

 mental conditions experienced by these individuals 

 were highly variable within a single season. The G:Z 

 ratios were highest (1.3-1.6) for midseason cohorts 

 and settlement densities at these times were at maxi- 

 mum levels. Thus, it appears that physical and bio- 

 logical conditions in the seagrass meadows were op- 

 timal for growth or survival, or for both. The oppo- 

 site trend was observed for individuals arriving early 

 and late in the season. Because G:Z ratios of indi- 

 viduals from these cohorts were low (0.6-0.9), we 

 postulated that nursery conditions experienced by 

 early and late-season cohorts do not favor early life 

 survival (low recruitment potential). 



Acknowledgments 



We thank Kathy Binney, Sharon Herzka, Patti 

 Pickering, Cameron Pratt, and Andy Soto for pro- 

 viding assistance in the laboratory and field. The 

 manuscript benefited from the comments of three 

 anonymous reviewers. This work was supported by 

 grants from the Texas Higher Education Coordinat- 

 ing Board Advanced Research Program (grant 

 003658-392 ), Texas A&M Sea Grant Program (grant 

 NBR;RIF65), Sid W. Richardson Foundation, and 

 fellowships to J.R.R (E.J. Lund, Julian C. Barton). 



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