(courtesy Illinois State Natural History Survey). 



for sampling rocky stream bottoms. A frame marks 

 out 0.1 m^, and a net downstream catches organisms 

 dislodged as the rocks are removed into a pail, for 

 closer e.xamination (Fig. 5-2a). 



Dredges of various shapes and sizes may be pulled 

 along the bottom for measured distances to get or- 

 ganisms in deep-water, but quantitative determina- 

 tions obtained in this way give population estimates 

 that are generally too low. The dredge commonly 

 does not dig sufficiently deep into the bottom ; often 

 it skips and slides along the surface without picking 

 up all the organisms that are present. Much more 

 reliable are the Ekman bottom sampler, on soft bot- 

 toms, and the heavier Petersen sampler, used also on 

 sand and harder bottoms (Fig. 6-9) . For microscopic 

 organisms small core samples are usually collected 

 and brought back to the laboratory for examination. 



The bottom samples obtained in various ways 

 must ordinarily be washed through sieves to remove 

 the debris, and the animals put into vials or jars for 

 identification and counting. The size of mesh to be 

 used in the sieve depends on one's objectives (Reish 

 19.=i9). We find four nesting sieves efficient, with the 

 top sieve having a coarse mesh (2 per inch) and the 



lower ones of increasing fineness (10, 20, 30 meshes 

 per inch) to capture smaller organisms. Suppliers of 

 limnological and oceanographic apparatus and sup- 

 plies have been listed by Ryther et al. (1959). It 

 should also be noted that for on-site studies of ani- 

 mals under water, increasing use is being made of 

 piiotography and even television. The bottom may 

 also be explored at first hand using diving equipment. 



SUMMARY 



Although determination of relative abun- 

 dance is sometimes useful in projects of limited scope, 

 the measurement of absolute abundance is generally 

 to be preferred. Measurement of absolute abundance 

 requires the counting of individual animals or meas- 

 urement of their biomasses on strip censuses or sam- 

 ple plots. The size, number, shape, and distribution 

 of sample plots and methods of measuring population 

 densities present special problems that must be ad- 

 justed for each habitat and group of organisms con- 

 cerned. The development of improved methods of 

 population sampling is one of the major needs of 

 ecology today. 



Measurement of populations 41 



