46 



No instrument would be so warmly welcomed by investigators handling 

 routine plankton collection as would a mechanical plankton sorter. In our 

 studies of sardine recruitment, for example, we examine several thousands of 

 plankton collections each year for fish eggs and larvae. The separation is now 

 done by a group of laboratory aids, examining each sample under a low-power 

 microscope. 



Considerable experimentation has been done on mechanical separation of 

 fish eggs and larvae from the plankton, but without marked success. One of 

 the instruments developed was called an elutriator. Elutriation, as the name 

 implies, is an attempt to obtain a separation of plankton organisms by washing 

 in water, the separation being effected by difference in specific gravity and 

 buoyancy of the organisms. The device, designed by Isaacs and Folsom, intro- 

 duces plankton organisms slowly into a cross current, allows a sufficient dis- 

 tance of travel to obtain a stratification of the material due to differences in 

 buoyancy, and then effects a recapture in a series of compartments. Although 

 the elutriator was fairly effective in separating out certain plankton groups, such 

 as euphausiids, the separation of fish eggs and larvae was to partial to be of 

 value in our work. This is unfortunate, for the elutriator was a wonderfully 

 ingenious device. 



On the basis of our experience it is clear that considerable preliminary 

 experimentation will be needed to develop methods for mechanically separating 

 plankton. It may be found that one process will be sufficient for accomplishing 

 a separation, but in all probability it will require two or more. Once the tech- 

 nique is established, an instrument may be designed to effect the separation. 



DISCUSSION: Gordon A. Riley 



It seems to me we have made considerable strides, which you (Dr. Ahl- 

 strom)*have summarized very well, in the basic problem of sampling a local 

 water mass and describing the populations and the biological events therein. We 

 have largely failed to make use of the opportunities afforded by present day 

 physical oceanographers for rapid coverage of large areas. The available in- 

 formation on local variability of physical properties, and what little we know 

 about plankton patchiness, make it apparent that routine methods, with stations 

 some miles apart, are largely inadequate for analyzing processes of physical 

 dispersal of organisms and inorganic nutrients. Closely spaced collections for 

 biological and chemical study obviously create large demands for manpower and 

 ship space. Indeed these demands are so large that such work probably never 

 will be of the greatest aid in evaluating the biological role of horizontal water 

 movements. 



Since you have discussed the need for instruments that will demonstrate 

 zooplankton patchiness, I shall say nothing further about that aspect of the prob- 

 lem. The chemists and phytoplankton investigators require an automatic sea 

 sampler of the type now in use but of much larger capacity. I would say that 

 an instrument taking five 500 milliliter samples between the surface and 150 

 meters would be adequate, although near the minimum limits as to both capacity 

 and depth range. 



* -Dr. Riley, who was unable to attend the symposium, made his com- 

 ments on Dr. Ahlstrom's paper in a letter to the author, which is reproduced 

 in part. 



