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



153 



Our inferences, from this study, in relation to spotted 

 seatrout peak spawning are based on hatchdate distribu- 

 tions and should be viewed with caution because hatch- 

 dates are based on survivors. Differential survival for early 

 life history stages can bias results. Hatchdate distributions 

 are valuable when compared to egg or recently hatched lar- 

 val densities and might suggest processes responsible for 

 differential cohort survivorship. Because spotted seatrout 

 undergo a protracted spawning period and because there is 

 high variation associated with icthyoplankton samples ( Cyr 

 et al., 1992), intensive and extensive sampling of recently 

 hatched larvae would be required over a long duration to 

 answer these process-oriented mortality questions. 



The daily instantaneous mortality rate of juvenile spot- 

 ted seatrout was higher in Florida Bay than those reported 

 from northwestern Florida systems (Nelson and Leffler, 

 2001). Mortality rates of juvenile spotted seatrout from 

 Florida Bay were 5.7%/d; whereas, for the other systems, 

 rates approximated 3%/d. In general, mortality rates might 

 increase with increasing estuarine temperatures (Houde 

 and Zastrow, 1993). Although we were unable to estimate 

 instantaneous daily mortality rates for larval spotted seat- 

 rout, these data have been estimated for larvae (3.5-6.5 

 mm) in two southwestern Florida estuaries (Peebles and 

 Tolly, 1988). Highly variable rates were reported between 

 the two Florida estuaries (Naples Bay: 0.70 or 50%/d; and 

 Fakahatchee area: 0.37 or 31%/d). Houde ( 1996) reported a 

 generalized instantaneous daily mortality rate for marine 

 fish larvae of 0.239 (21%/d). Estimating mortality rates for 

 larval spotted seatrout in Florida Bay will be critical for 

 calculating G:M ratios in order to evaluate stage-specific 

 survival and to develop credible spatially explicit models. 



Mortality rates of spotted seatrout cohorts could be cal- 

 culated for only three of six cohorts (B, May; D, July; and 

 F, September) because slopes were significantly different 

 from zero for only these cohorts. Furthermore, mortality 

 rates of two of the three cohorts (B and F) were associated 

 with low r 2 values (Table 5); hence the G:M ratios along 

 with the mortality rates for these three cohorts should 

 be considered "rough" estimates. Attaining more accurate 

 mortality estimates for spotted seatrout would be valuable 

 in linking cohort variability with potential recruitment and 

 stage-specific survival. For example, larval cohorts of bay 

 anchovy [Anchoa mitchilli) from Chesapeake Bay, a tem- 

 perate estuary, exhibit growth rates that are temporally 

 variable and mortality rates that are spatially and tem- 

 porally variable (Rilling and Houde, 1999). Temperature, 

 zooplankton prey and gelatinous predators are believed to 

 influence growth and mortality rates of the bay anchovy. 

 For striped bass iMorone saxatilis ) in a subestuary of Ches- 

 apeake Bay, cohorts exhibited highly variable seasonal G:M 

 ratios that were strongly influenced by temperature 

 (Houde, 1997). In a subtropical estuary, cohort-specific 

 mortality rates for juvenile red drum varied temporally; 

 early and late season cohorts exhibited the highest mortal- 

 ity rates, which coincided with highest growth rates and 

 G:M ratios for midseason cohorts (Rooker et al., 1999). We 

 agree with Houde ( 1997) that future research should focus 

 on the variability and causes of variability in growth and 

 mortality, both of which interact to determine stage-spe- 



cific survival. The developmental stage or age where G:M 

 variability is greatest, along with the relationship of this 

 variability to recruitment, need to be determined for spot- 

 ted seatrout in Florida Bay. No doubt a relationship exists 

 between G:M ratios and recruitment. Future research 

 should also determine if cohort G:M ratios and somatic 

 growth rates are seasonally or spatially variable. If they 

 are, then a limited spatial and temporal sampling program 

 could be designed to annually evaluate G:M ratios at highly 

 variable stages or ages as an index of year-class strength 

 of spotted seatrout in Florida Bay. Such an index could be 

 verified by examining year-class catch rates on an annual 

 basis or by virtual population analysis. 



In our study there was little temporal difference in 

 growth of juvenile spotted seatrout cohorts. Larval growth 

 and mortality, which was not treated adequately in our 

 study, could be influenced by copepod prey — an important 

 dietary component of larval spotted seatrout (McMichael 

 and Peters, 1989). The copepod Acartia tonsa is dominant 

 in Florida Bay, but egg production rates for this species are 

 low in the bay compared to those in other systems (Kleppel 

 et al., 1998). We suspect the "bottleneck" to recruitment of 

 spotted seatrout could occur during the larval stage. Hence, 

 future research should examine mortality and growth of 

 larval and recently settled spotted seatrout; in particular 

 the patterns of larval production potential (G:M ratios). 

 Research in these areas should increase our understand- 

 ing of the degree of variability in stage-specific survival 

 and recruitment of spotted seatrout in Florida Bay (Houde, 

 1996). 



For most species, especially those with protracted spawn- 

 ing habits, it is most informative to analyze cohort growth 

 and mortality. For example, striped bass and bay anchovy 

 cohorts in Chesapeake Bay exhibit highly variable growth 

 rates, mortality rates, and stage durations (Rutherford 

 and Houde, 1995; Rilling and Houde, 1999). This variabil- 

 ity could cause differential survival for cohorts and result 

 in frequency distributions of survivor hatchdates that do 

 not resemble recently hatched larvae or egg-production 

 frequency distributions (e.g. Crecco and Savoy, 1985; Rice 

 etal., 1987). 



We are unable to interpret the significance of the abso- 

 lute value of the G:M ratio for juvenile spotted seatrout, 

 because interannual comparisons were not made, but we 

 presented the ratio for future comparisons. Generally, the 

 G:M ratio is <1.0 during the early larval stage, indicating 

 a decline in biomass. However, the G:M ratio of a cohort 

 will eventually exceed 1.0 as a result of a relative decline 

 in mortality as larvae grow (Houde and Zastrow, 1993). 

 Clearly, stage specific analysis of the spotted seatrout from 

 egg through juvenile stage would have been more informa- 

 tive in determining when the maximum G:M ratio occurs 

 (when cohort biomass increases at a maximum rate) and 

 in providing insight into stage-specific dynamics of spotted 

 seatrout (Houde, 1997). A constraint of our study was our 

 inability to estimate larval mortality rates; hence early life 

 history stage dynamics could not be examined. 



Size-selective mortality in the juvenile life history stages 

 can have important consequences for recruitment. Sogard 

 ( 1997 ) argued that "within-cohort size-selective mortality" 



