REEVE and BAKER: PRODUCTION OF A CHAETOGNATH AND CTENOPHORE 



animals older than those surviving in these 

 experiments. The potential errors in such a 

 procedure are minimal, because the coefficients are 

 tending towards zero and the biomass involved in 

 the two largest size classes is only a small percen- 

 tage of the total. 



The Mnemiopsis growth curves were treated 

 differently because their slopes decreased 

 progressively with age. In order to facilitate com- 

 putation of the required slopes, the curves were 

 divided into segments, the junctions of which were 

 assigned by visual inspection to be at 3- and 50-mg 

 ash-free dry weight. The slopes of the individual 

 segments A, B, and C were individually calculated 

 from the population mean points within them by 

 linear regression analysis. Unlike S. hispida, 

 where growth rate is proportional to temperature 

 between 21° and 31° C, M. mccradyi grows faster 

 at 26°C than at either end of the range. Survival 

 was poor at 31°C, populations dying out by the 

 25th day. Since no points exist from which to 

 compute a slope for segment C at 31°C, it was 

 taken to be the same as that for the 21°C 

 experiments, since segments A and B at the two 

 temperatures are almost identical. 



Sampling dates were divided into three groups 

 on the basis of the proximity of the ambient water 

 temperature to 21°, 26°, and 31°C so that growth 

 coefficients derived for these temperatures could 

 be applied to the standing stock data. Similarly, 

 mean mortality coefficients were derived for the 

 three temperature ranges by averaging the 

 numbers of animals in each size class over the 

 sampling dates in each temperature range. These 

 mean numbers were used to obtain mean ratios of 

 Y/X (as did MuUin and Brooks 1970) where X and 

 Y are the numbers of the earlier and later of two 

 successive size classes. This ratio, and the duration 

 of development in each of the two successive size 

 classes, enables calculation of the exponential 

 coefficient of daily mortality between the two size 

 classes using computer-generated tables. We 

 recognize that this procedure is an approximation 

 which probably oversimplifies actual conditions by 

 making unproven assumptions regarding con- 

 stancy of mortality rate with time and between 

 adjacent size classes, yielding a single value for m 

 rather than a measure of its possible range (see 

 Fager 1973). 



The duration of development in each size class at 

 each temperature range was estimated from the 

 arbitrarily defined limits of each size class and the 

 laboratory growth rate data. 



Net production of a size class on a given 

 sampling date, taking into account animals which 

 die before the end of the day, is the product of the 

 mean biomass and the daily exponential coefficient 

 of growth for that temperature range. The day is 

 assumed to start ^t_the time of sampling, and the 

 mean biomass {WN) of that size class over the 

 subsequent 24 h is obtained by application of the 

 relationship given by Mullin and Brooks (1970) 

 which utilizes the initial biomass, growth, and 

 mortality coefficients. The initial biomass {WN in 

 ash-free dry weight) is the product of sampled 

 numbers (AO and mean ash-free dry weight {W) of 

 an individual organism of that size class. Summing 

 the production values for each size class provides 

 an estimate of the total net production of the 

 population on that day. No attempt was made to 

 estimate egg production in either species. 



Net production was determined for chaetog- 

 naths of the Card Sound population only; values 

 quoted below for the Biscayne Bay population are 

 estimated by applying the mean population 

 production /biomass ratio for Card Sound to es- 

 timated total biomass in Biscayne Bay. An es- 

 timate of annual production is obtained by taking 

 each sampling date as the midpoint of each 

 sampling period, summing the product of daily 

 production and number of days in that sampling 

 period, and summing the total production for each 

 sampling period and adjusting for 365 days. In the 

 ctenophore population, which was sampled for 17 

 mo, and passed through two biomass peaks which 

 Baker (1973) considered to be an annual winter 

 event (Table 2), two values were computed (see 

 Table 3), one for 365 days from the beginning and 

 one for 365 days up to the end of the sampling 

 program. 



Results and Discussion 



Seasonal Changes 



Summaries of the population dynamics and 

 production data are contained in Tables 1 and 2, 

 computed as detailed above from tabulations by 

 sampling date and size class. Figure 1 contains the 

 laboratory growth rate data. The standing stock 

 and production data are summarized in Tables 1 

 and 2, and are derived from the Card Sound 

 population of Sagitta and the Biscayne Bay 

 population of Mnemiopsis, since these populations 

 had been the most effectively sampled. For each 

 size class (Table 1) averaged over the entire 



243 



