must lie moililicd tor ilifl'ercnt soil tyiK-s ;iiul tax- 

 onoiuic grou]JS to produce tlu- best results (Mac- 

 tadyen 1953). 



W'itii tlie ftotiition method, the litter or soil is 

 placed in a large pan, and warm — not hot — water is 

 l)oured in to thoroughly soak and cover the soil to a 

 depth of 1-2 cm. The warm water stimulates the 

 animals to activity, and they come to the surface 

 where they may be collected with forceps or suction 

 bottle. For greater efficiency, air may be bubbled 

 through the material to break up clumped masses ; 

 chemicals, such as magnesium sulphate, may be added 

 to increase the specific gravity of the water (Kevan 

 1955). 



The microjaiina consists mostly of microscopic 

 forms such as protozoans, rotifers, nematodes, tardi- 

 grades, and turbellarians. To quickly obtain a non- 

 quantitative sample, a few grams of soil or litter may 

 be placed on a screen in a glass funnel, and water, 

 warmed to about 40°C, poured over it. The fluid 

 collected from the funnel may then be centrifuged. 

 A slower but more effective method for collecting 

 nematodes is to wrap the sample in cheese-cloth or 

 muslin and immerse it in water in a funnel that has 

 a clamped rubber tube fastened to the stem. Let it 

 stand for a few hours, and the nematodes will collect 

 in the stem of this Baerman funnel. They may then 

 be released into a petri dish for e.xamination ( Kevan 

 1955). 



To obtain a good idea of the protozoans present, 

 culturing is usually necessary. Edible bacteria are 

 added to a non-nutrient agar or silica jelly in a petri 

 dish, small amounts of properly prepared soil dilu- 

 tions inserted at various points, and the culture in- 

 cubated for two weeks. Final examination of tlie 

 culture is made under the microscope, and the pres- 

 ence or absence of protozoans at the various points 

 determined. Cultures may also be prepared using soil 

 extract or hay infusion (Kevan 1955). 



Fish 



In small streams, a representative section of 

 known length may be blocked ofT at the upper end by 

 stretching a net from one bank to the other. Seining 

 proceeds from the lower end up to the stretched net. 

 There are limits, of course, to the size and depth of 

 streams that can be examined in this manner, and 

 unless care is taken, fish will escape around the ends 

 or underneath the net. Seines can be used efficiently 

 in this manner only when the bottom is free of large 

 stones or other obstacles. 



It is sometimes possible to draw long nets over 

 measured areas of ponds and the shallow waters of 

 lakes, but the data obtained usually give a relative 

 abundance only. In deeper water, trammel, gill, or 



hyke nets may be set, but each type of gear has limi- 

 tations with reference to species, locality, and time 

 of day. Seines, trammel and Fyke nets that catch 

 fish alive are commonly u.sed, however, in applying 

 the Petersen method for obtaining absolute abun- 

 dance. 



When fish are removed from a body of water by 

 gear of some sort or by s|X)rt fishing, the catch per 

 unit-effort may be a basis for estimating total popu- 

 lation. Creel censuses of the catch of fishermen are 

 commonly taken to measure the yield of fish over 

 periods of time. 



In modern practice, artificial ponds are usually 

 built in such a way that they may be drained and unde- 

 sired species or surplus populations removed. Fish 

 may be counted and measured in these operations, 

 and only those species which are desired for re- 

 l)lenishing the population restored to the pond. 



Small bodies of water, or representative areas of 

 larger bodies, can be blocked of? with nets and cen- 

 sused by the use of a poison, such as rotenone (pow- 

 dered derris root). Fish are killed and float to the 

 surface of the water where they may be collected and 

 counted. This method cannot, of course, be used 

 where the population is to be left undisturbed. The 

 ])oison also kills most zooplankton and some, but not 

 all, other kinds of invertebrates (Brown and Ball 

 1942). 



A less drastic method, that of shocking, is most 

 effective in small streams. Two electrodes are in- 

 serted into the water and the electric charge tempo- 

 rarily stuns the fish so that they float to the surface 

 where they can be captured. After the desired data 

 are obtained, the fish are returned to the water and 

 recover rapidly (Lagler 1952). 



Plankton 



Plankton nets are commonly made of silk bolt- 

 ing cloth ; number 20 or 25 is ordinarily the finest 

 mesh used. Tow nets are made with a conical bag 

 attached to a wire frame, to which the tow string is 

 attached by means of cords. Collections may be re- 

 moved by turning the net inside out in a jar of water, 

 or the organisms may be concentrated in a vial 

 screwed in at the tip of the cone. 



For surface plankton, it suffices to use the plank- 

 ton net as a sieve and pour through it a known quan- 

 tity of water. The tow net may be dragged behind a 

 boat either at the surface or submerged to any depth 

 by means of weights attached to the tow line. 



Since the depth at which plankton occur varies 

 with the time of day, vertical hauls sampling all 

 depths are preferred. Comparison of plankton popu- 

 lations at different times or in different areas had 

 best be made in terms of unit surface area. The Wis- 



Measurement of populations 39 



