FISHERY BULLETIN: VOL. 73, NO. 3 



Two cultured food organisms, G. splendens and 

 B. plicatilis, were used aboard ship to determine if 

 any particular batch of test larvae vi^as in good 

 condition and would feed. Details on the culture of 

 these organisms are given by Thomas et al. (1973) 

 for G. splendens, and Theilacker and McMaster 

 (1971) for B. plicatilis. Usually a control 5-liter 

 seawater sample was seeded with one or the other 

 cultured organism and a feeding experiment with 

 larvae run concurrently with samples of natural 

 seawater. 



Larvae were permitted to feed for 8 h then 

 siphoned out and sucked down onto a membrane 

 filter (pore size O.Sfim) using a vacuum pump. This 

 rapid removal of larvae from the experimental 

 containers and their fast immobilization on the 

 filters prevented defecation. After air drying, 

 microscopic examination of the transparent larvae 

 permitted counts to be made of larvae which had 

 been feeding and those which had not. The relative 

 proportion of feeding to nonfeeding larvae was 

 subsequently correlated with sizes of the food 

 particles, chlorophyll content, species composition, 

 and the number of particles available to the larvae. 



A 1-liter subsample of each seawater sample 

 was preserved with Formalin (final concentration 

 5%) (Beers and Stewart 1970b). Later the particles 

 in the preserved seawater were settled out and the 

 species enumerated. The method of Utermdhl (as 

 described by Lund et al. 1958) was used to concen- 

 trate organisms from known volumes of preserved 

 seawater, usually 100 ml. At least 100 cells larger 

 than 20 /i.m in diameter were counted from each 

 settled water sample. 



On a cruise of the NOAA RV David Starr Jor- 

 dan, 18-21 March 1974, a 16-channel electronic 

 particle counter with a 280-/u,m pore, Coulter 

 Counter Model Ta, was used to determine the size 

 distribution and numbers of sized particles in 

 seawater samples used for feeding experiments. 

 Only particles 20ju,m and larger were counted. Very 

 good agreement was obtained between the elec- 

 tronic counter particle counts and those obtained 

 with the inverted microscope. A comparison is 

 shown in Figure 1. The speed of counting and siz- 

 ing particles makes the multichannel particle siz- 

 ing instrument desirable for rapid field assess- 

 ment of larval fish food organisms. Because the 

 electronic counter was unavailable for two sub- 

 sequent cruises. 8-12 and 22-23 April 1974, the 

 results for these are given from microscope counts 

 only. 



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NO. OF PARTICLES PER ML FROM INVERTED 

 MICROSCOPE COUNTS ^ 20|jm DIAMETER 



Figure L-Comparison between instantaneous particle counts 

 per milliliter taken with Model Ta Coulter Counter and mean 

 particle count per milliliter observed through an inverted 

 microscope. A 100-ml sample was concentrated and at least 100 

 particles of the dominant organism were counted in any visual 

 field. Only particles larger than 20 fxm were counted. 



To ascertain if anchovy larvae were present at 

 particular depths, plankton tows were taken with 

 a 0.333-mm mesh, 0.5-m mouth diameter net. 

 Because an opening and closing net was unavail- 

 able for this work, an open net was dropped rapidly 

 to the desired depth, towed for 10 min at a main- 

 tained wire angle of 45°, then pulled up rapidly. 

 The total proportion of time the net spent in water 

 other than in the desired stratum never exceeded 

 5% of the total towing time. All larvae captured 

 were measured, sorted to species, and counted. 

 Larval counts were corrected to a comparative 

 volume of 1,000 m^ (Kramer et al. 1972). A flow- 

 meter in the mouth of the net provided a record of 

 the volume of water filtered. 



The shipboard experiments were ordinariy done 

 at temperatures of between 15° and 19°C, whereas 

 concentrations of larval fish food organisms and 

 anchovy larvae were occasionally found between 

 14° and 15°C. It was desirable, therefore, to de- 

 termine the feeding response of first-feeding 

 anchovy larvae at different environmental 

 temperatures. Experiments to determine this 

 were identical to those done at sea except that the 

 concentration of the cultured organism G. splen- 

 dens was varied from 5 to 200 cells/ml, and the 

 water temperature was controlled within ±0.2°C. 



456 



