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Usinger: Introduction 



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LARVAL DENSITY PER SQUARE FOOT 



Intro, fig. 53. Correlation between forage ratio and density of 

 mosquito larvae (Hess and Tarzwell, 1942). 



1 indicates that an organism is either being selected 

 in preference to other organisms, or that it is more 

 available than others; and a forage ratio of less than 

 1 indicates that an organism is either less preferable 

 or less available. In a test using top minnows (Gam- 

 busia affinis) and anopheline and culicine larvae, 

 Hess and Tarzwell (1942) found that there was a 

 direct correlation between forage ratio and population 

 density (intro. fig. 53). "This would seem to indicate 

 that as Anopheles became more abundant, Gambusia 

 became more accustomed to feeding upon them and 

 began to select them in preference to other food 

 organisms present." 



A knowledge of the biology of aquatic insects 

 should be used in conjunction with forage ratio data 

 in order to interpret the results correctly. For example, 

 during one period of their development different spe- 

 cies of food organisms may be of great value to the 

 fish and at another period be of no value. Hess and 

 Swartz cite emerging caddisfly pupae as a case in 

 point. Trichoptera larvae may be relatively unavail- 

 able to the fish owing to their stone cases; the pupal 







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Intro, fig. 55. Composition of food material from the stomachs 

 of trout feeding in the same area at the same time as Intro, fig. 

 54 (Neill, 1938). 



stage is quiescent and spent under a stone case, but 

 when the pupae emerge and make their way to the 

 surface of the water, they are free of their cases and 

 are readily available to all fish. 



The forage ratio may also be used to make more 

 meaningful the calculation of food grades of streams 

 (Hess and Swartz, 1941). Food grades have commonly 

 been determined without regard to the value of the 

 various organisms as food for the fish. The following 

 method was proposed to correct this: The forage 

 ratio for a given group multiplied by the mean density 

 of the group per square foot will give a measure of 

 the "effective food grade" in terms of that group, in 

 relation to the species of fish for which the forage 

 ratio was determined. Only those organisms which 



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April 



BY WEIGHT 

 BY NUMBER 



June 



I 



July 



Intro, fig. 54. Composition of the bottom fauna in a limited Intro, fig. 56. Seasonal fluctuations in amount of food taken by 



area of the River Don (Scotland) during spring and summer trout (based on the amount of Ephemeroptera, Trichoptera, and 

 (Neill, 1938). Simuliidae found in the stomachs) (Neill, 1938). 



