Figure 13. — Capture of Atlantic menhaden larvae. 



from the buckets in 100-ml. beakers and placed 

 in laboratory aquariums. By these techniques 

 menhaden larvae were kept immersed at all 

 times and could easily be separated from larvae 

 of other species. 



Two porcelain bathtubs served as laboratory 

 aquariums. To minimize the bumping of the 

 newly captive larvae against the tub walls, the 

 interior was painted black and illuminated con- 

 tinuously by pink fluorescent lamps mounted 

 2 m. above the water. Filtered sea water was 

 continuously supplied to each aquarium and 

 the level controlled by a 10-mm. -diameter 

 standpipe to provide a volume of 180 liters for 

 about 300 larvae, 13 to 28 mm. long. During the 

 establishment period, salinity varied between 

 28 and 33 p.p.t. (parts per thousand) and tem- 

 perature, between 16.0° and 19.0° C. 



The larvae were left undisturbed for at least 

 12 hours after transfer to the laboratory 

 aquariums. Dead or dying larvae were removed 

 daily. Total mortality during establishment 

 was less than 5 percent. 



Experimental Procedures 



Experiments were begun 3 to 5 days after 

 larvae were established in the aquariums. In- 

 dividual tests were made in 100-ml. beakers, 

 250- and 500-ml. bowls, or 8-liter battery jars, 

 all painted black, and filled with sea water 

 from the aquarium supply. Larvae were trans- 

 ferred to the test container with a 100-ml. 

 beaker, one or two specimens at a time. From 

 1 to 10 larvae were placed in each beaker or 

 bowl and as many as 20 larvae in each battery 

 jar. 



Procedures differed for specific studies. They 

 are described separately. 



Light and feeding. — Larvae were trans- 

 ferred to 250-ml. bowls, eight larvae to a bowl, 

 and left unfed in total darkness for 24 hours 

 before the experiments were started. Artemia 

 nauplii in suspension were dropped into the 

 bowls {Artemia were used in preference to 

 copepods because their pink color made them 

 readily distinguishable after ingestion by the 

 larvae) . Light intensity of a 500-watt, tung- 

 sten filament lamp suspended 75 cm. above the 

 water was varied by a transformer and meas- 

 ured with a light meter at the surface. We 

 used penlight illumination periodically to de- 

 tect the presence of food in larvae kept in 

 darkness or exposed to low light intensities. 



Food passage and digestion. — -Food inges- 

 tion and digestion times were determined vis- 

 ually and with a radioactive tracer. For the 

 visual observations, several lots of eight larvae 

 each were placed in 500-ml. bowls. They were 

 left unfed for 24 hours before the start of the 

 experiments and then fed Artemia nauplii. 

 When they had stopped feeding, the larvae 

 were transferred to separate 100-ml. beakers 

 for observation. Rate of food passage was esti- 

 mated from the time that feeding stopped until 

 the alimentary tract of each larva was com- 

 pletely empty. 



For the radioactive tracer tests, several lots 

 of eight larvae each were placed in 500-ml. 

 bowls and fed 10 to 50 copepods {Centropages 

 and Acartia) that had been immersed in sea 

 water containing 0.001 yuc./ml. (microcurie per 

 milliliter) of Zn**^ and rinsed in fresh sea water 

 before they were fed to the larvae. When the 



506 



U.S. FISH AND WILDLIFE SERVICE 



