Powell et al.: Growth, mortality, and hatchdate distributions for Cynoscion nebulosus 



143 



25°20' 



25° 10' 



2.V00' 



— 24"50' 



81 "00' 



80"45' 



so Mr 



Figure 1 



Location of sampling sites for spotted seatrout (Cynoscion nebulosus) in Florida Bay, 

 Everglades National Park, Florida, including Florida Bay Subdivisions. 



of the Everglades, including a return of historic freshwater 

 flows into Florida Bay. 



Two conceptual frameworks have been advanced to couple 

 the role of growth and mortality in influencing cohort dy- 

 namics. Anderson ( 1988), in a review of hypotheses relating 

 survival of prerecruits to recruitment, advocated a growth- 

 mortality hypothesis as a rational framework for early life 

 history studies that address recruitment variability. This 

 concept predicts that survival of a cohort is directly related 

 to growth rates during the early life stages. The growth- 

 mortality framework, which includes several important in- 

 tegrated components and is based on bioenergetic principles 

 of growth and ecological theory that predict growth rate, is 

 directly related to survival. If it can be demonstrated that 

 survival is a function of growth during the early life stage, 

 then a valuable tool becomes available for examining mecha- 

 nisms influencing recruitment of marine fishes. 



Another framework suggests that the mortality rate does 

 not operate alone in determining stage-specific survival, 

 but it is the mortality:growth (M:G) ratio (mortality per 

 unit of growth) that determines stage-specific survival (see 

 citations in Houde, 1997 ). Houde ( 1997 ) advanced the idea 

 of using the M:G ratio as an estimator of production and 

 potential survivorship especially in early life stages when 

 both mortality and growth are high and variable. This con- 

 cept was partly based on the strong coupling of growth and 

 mortality demonstrated by Ware ( 1975 ) who argued that 

 when growth rate is poorer than average, larvae would be 

 exposed to sources of mortality over a longer period and 

 hence their mortality rate would increase. Growth and 



mortality values for successive cohorts would tend to form 

 a cluster of points around a regression of mortality on 

 growth based on average values for a particular species. 



Our intent is not to test the growth-mortality hypothesis 

 (sensu Hare and Cowen, 1997) as outlined by Anderson 

 (1988), nor fully to develop the M:G ratio concept (Houde, 

 1997), but rather to use these concepts as a framework 

 for our study. The major goal is to provide information on 

 growth and survival of larval and, mainly, juvenile spotted 

 seatrout that can ultimately be used to develop a spatially 

 explicit model that can be linked to Everglades restoration 

 activities. Therefore, the major objectives of this paper are 

 1 ) to determine overall growth rates of larval and juvenile 

 spotted seatrout in Florida Bay; 2) to determine and com- 

 pare juvenile growth rates geographically; 3) to estimate 

 natural mortality rates of juveniles; 4) to estimate hatch- 

 date distributions; 5 ) to compare cohort growth and mortal- 

 ity rates and G:M ratios for juveniles; and 6) to evaluate 

 the effects of salinity and temperature on otolith growth — a 

 surrogate for somatic growth. 



Methods and materials 



Field collections 



Larval fish used for otolith microstructure analysis were 

 collected from September 1994 through July 1997, mainly 

 in the Gulf transition, western, and central subdivisions 

 (Table 1, Fig. 1). These subdivisions designated by the 



