FISHERY BULLETIN: VOL. 73, NO. 2 



tends to extend over most or all of the year and 

 distinct cohorts can rarely be identified. 



Grov^^th rate, therefore, was measured in the 

 laboratory, and in as large a volume as practical 

 (30-70 liters). No attempt was made to simulate 

 natural food levels. There were various reasons for 

 this. Mean annual zooplankton concentrations of 

 the 200-yu,m mesh, which is the food source of older 

 Sagitta and Mnemiopsis (Reeve and Walter 1972; 

 Baker 1973), were of the order of magnitude of 1 

 organism/liter, an impractically low concentra- 

 tion to work with in these volumes. It is certain 

 that any environmental concentration estimated 

 from a net tow is an average of several small-scale 

 patches of higher and lower density. We have some 

 information from direct observation by scuba 

 (unpubl. data) that patch densities at least an 

 order of magnitude greater occur, as well as in- 

 formation (also unpubl. data) that both Sagitta 

 and Mnemiopsis can ingest food several times 

 faster following a period of starvation than they 

 do under conditions of a constant supply of food. 

 Sagitta is capable, under certain conditions, of in- 

 gesting within 1 to 2 min all the food it consumes 

 in 24 h under conditions of continuous abundant 

 food supply. 



Despite the fact that feeding habits and en- 

 vironmental food concentrations are poorly un- 

 derstood at present, it is clear that for carnivorous 

 zooplankton, at least, maintaining a continuous 

 supply of food at mean environmental concentra- 

 tions in small-scale experimental conditions, 

 would be as artificial as maintaining a continuous 

 abundant supply, even though there must ob- 

 viously be a relationship between total food supply 

 and production in the environment. The latter 

 method does provide a standard (i.e., maximum) 

 growth rate. When better data become available 

 on the interrelationships of feeding, food supply, 

 and growth rate, the production estimates com- 

 puted on that basis can be revised downward. At 

 present, there is little information available to 

 even guess to what extent these growth rates and 

 hence production estimates are overestimations. 

 Hirota (1974) reported surprisingly little 

 difference in growth rates of Pleurobrachia in 

 experiments at food concentrations ranging 

 between 1 and 350 )u.gC /liter, but pointed out that 

 in the 70-m' tank in which the low food concentra- 

 tion occurred, food organisms were not uniformly 

 distributed because some species were concen- 

 trated at the surface during the day. In Card 

 Sound and Biscayne Bay the mean annual con- 



centration of food from the 200-/xm net (the size 

 range fed to adult Sagitta and postlarval 

 Mnemiopsis in our experiments) was 0.8 and 8.1 

 /AgC /liter. Taking into account all organisms down 

 to a 20-)u,m retaining mesh those figures would be 

 increased by a factor of 5 (Reeve and Cosper 1973). 



Production Comparisons 



Sameoto (1971) obtained a value for the net 

 production of S. elegans in Nova Scotia waters 

 (ranging in temperature approximately from 0.5° 

 to 14°C) of 200 mgC/m^ per yr in a 50-m water 

 column, and McLaren (1969) reported a similar 

 range of values for this species from Ogac Lake on 

 Baffin Island (49-196 and 318). Those authors es- 

 timated production/biomass ratios between 1.0 

 and 2.1 on an annual basis. These figures compare 

 with annual net production of S. hispida in Card 

 Sound and estimated in Biscayne Bay of 730 and 

 1,750 mgC/m" per yr and production/biomass ratio 

 of 109 on an annual basis. With a mean annual 

 biomass two orders of magnitude lower, therefore, 

 S. hispida in Card Sound exceeds the net produc- 

 tion of 5. elegans in St. Margaret's Bay, Nova 

 Scotia by virtue of its rapid growth rate and short 

 generation time. The disparity would be even 

 greater on a cubic meter basis because Card Sound 

 is comparatively shallow. 



Hirota (1974) quoted a value for net annual 

 production of the ctenophore Pleurobrachia bachei 

 in waters off California (ranging in temperature 

 approximately from 12.5° to 20°C) of 5,415 mg 

 ash-free dry weight/m- per yr, and a daily 

 production/biomass ratio of 0.02. These figures 

 compare with an annual net production of M. 

 mccradyi in Biscayne Bay of 2,086 to 4,227 mg 

 ash-free dry weight/m- per yr and a production/ 

 biomass ratio of 0.12. As in the previous com- 

 parison, annual production of different species in 

 different regions is surprisingly similar on a 

 water column (square meter) basis. The growth 

 rate of M. mccradyi, however, is some 5 times 

 faster, and its production is supported by a water 

 column depth of 3 m rather than in excess of 40 m 

 in the case of Pleurobrachia bachei. 



The 10-fold difference in the mean annual 

 standing stock of 200-ju,m mesh zooplankton 

 between Card Sound and Biscayne Bay (and in 

 phytoplankton pigment) is probably a reflection of 

 the poor water exchange and limited land 

 drainage into Card Sound as compared with Bis- J 

 cayne Bay (Reeve and Cosper 1973). These 



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