REEVE and BAKER: PRODUCTION OF A CHAETOGNATH AND CTENOPHORE 



Details of methods for the calculation of 

 production for populations with continuous breed- 

 ing occur in Winberg (1971) and Crisp (1971). They 

 are essentially similar to the method used here and 

 by Mullin and Brooks (1970) and Hirota (1974) ex- 

 cept that no adjustment is made to the sampled 

 biomass (PFAO to compute the mean biomass {WN) 

 during the 24 h immediately following the taking 

 of the sample. This additional step, which we also 

 performed, requires considerable extra effort 

 (depending on the number of size classes and 

 sampling dates involved) as well as access to com- 

 puter services. In these warm waters, however, 

 where growth and mortality rates may be less 

 variable than in regions of more pronounced 

 seasonality, the increase in W tends to cancel out 

 the decrease in A^, the difference between WN and 

 WN for Sagitta and Mnemiopsis being less than 

 10% (93 and 108%, respectively). 



Mortality coefficients tended to increase 

 progressively with age in Sagitta and with 

 increasing temperature. These environmental ob- 

 servations correspond to the conditions of labora- 

 tory cultures with respect to temperature, but in 

 cultures young animals tend to die off more 

 rapidly than juveniles and immature animals 

 (Reeve and Walter 1972). A variety of interacting 

 factors, including differences in predation pres- 

 sure and food adequacy, may be responsible. In the 

 ctenophore population the pattern of mortality is 

 uniformally low except in size class B which 

 corresponds to the time of change from ten- 

 taculate larva to lobate adult. The unmeasurable 

 mortality of size group A can be partly attributed 

 to sampling inefficiency, though this was shown 

 not to be the case for Sagitta (see above). 



Problems of Measuring Growth Rate 



In animals such as copepods, with life history 

 stages marked by recognizable and abrupt 

 changes (i.e., molts), division of the cycle into parts 



may be accomplished on the basis of some 

 biologically meaningful criteria. Both chaetog- 

 naths and ctenophores exhibit more gradual 

 transformation from newly hatched larva to ma- 

 ture adult, and size class separation is based on 

 arbitrary limitations such as preserved length or 

 sieve size. The only real validity of the particular 

 size classes used here is that they represent a 

 progression from the youngest to the oldest 

 animals. Factors such as variability of size of 

 animals of the same age at different temperatures 

 and imprecision of raw measurements (larger 

 ctenophores may pass through a mesh slightly 

 smaller than their diameter by their own weight 

 deforming their shape) tend to blur the sharpness 

 of the line separating one size class from the next. 

 The arbitrary choice of size classes resulted in 

 large variations in the durations of development 

 of each size class. In Sagitta the mean duration 

 (i.e., averaged over the three experimental 

 temperatures) of the initial size class was 12 days, 

 shortening to 2 days as length increased rapidly, 

 and increasing to 9 in the last size class as a final 

 length was approached in the adult. In Mnemiop- 

 sis size class durations proved to be even more 

 erratic (see Table 1). 



On the basis of the definitions used by Reeve 

 (1970) for S. hispida, the larval, juvenile, imma- 

 ture, and mature stages correspond approximately 

 to size classes A and B, C and D, E and F, and G 

 and H, respectively. For M. mccradyi the ten- 

 taculate larva extends to size class C and the first 

 eggs are also produced by size class C animals (29 

 mm and larger). 



The most satisfactory way to determine growth 

 and mortality in a population is to follow the 

 increase in size and decrease in numbers of a 

 cohort of the population over successive sampling 

 dates by inspection of size-frequency histograms 

 (Winberg 1971; Crisp 1971). In warmer waters, 

 although biomass may fluctuate widely, breeding 



245 



