42 



Usinger: Introduction 



WATER 





- 



SURFACE 





- 





1 











A 



NYMPH 



YOUNG 

 NYMPH 



EGG 



- IMAGO 



-PUPA 



-LARVA 



EGG 



BAETIS 



SIMULIUM 



Intro, fig. 58. Relative accessibility of Baetis and Simulium 

 during the aquatic stages of the life cycle. Degree of shelter — 

 black. Degree of exposure — white (Neill, 1938). 



period of time; 4. Seasonal variations in amount of 

 food (intro. fig. 56) and composition of food (intro. 

 fig. 57) were recorded; 5. The effects of current, 

 vegetation, type of bottom, and chemical character 

 of the water were related to densities of food organ- 

 isms; 6. Fish were taken at a time to correspond to 

 the end of a given day's feeding, thus ensuring ade- 

 quate fresh material; they were captured mostly by 

 angling rather than by seining, (but no significant 

 differences were found in amount and kinds of food 

 in the stomachs of 496 fish whether taken with nets 

 or by angling, Dimick and Mote, 1934); 7. All organ- 

 isms were identified to the species level with 

 notations as to developmental stages; 8. For each 

 group the life cycles of the species present were 

 described, so that the times of greatest abundance 

 could be associated with the presence of various 

 organisms in the food; 9. The relative accessibility 

 (intro. fig. 58) of each of the principal food organisms 



was worked out by grouping the whole fauna into 

 five categories, to each of which a conservatively 

 estimated "coefficient of accessibility" was assigned. 

 All available bionomic data were taken into account 

 in assigning these figures which ranged from 1.0 for 

 freely exposed groups (e.g., Simulium) to 0.0 for com- 

 pletely secure groups (Oligochaeta) (table 5). 



Total numbers in the bottom samples were multiplied 

 by the appropriate coefficient to bring out the acces- 

 sible fauna. In spite of the somewhat arbitrary method 

 of ranking, a high degree of correspondence was found 

 when figures for the "accessible" fauna were com- 

 pared with those for stomach contents. This led 

 Neill to the conclusion that "the predatory relation- 

 ship between the brown (rout and its organic environ- 

 ment therefore is that the trout feeds on the whole 

 range of animals present in whatever type of habitat 

 it finds itself to an extent dependent on the degree 

 of accessibility and the extent of their representation 

 in the fauna. This is sufficient to account for the 

 nature of its stomach contents without invoking dis- 

 crimination on the part of the fish." 



Summary of the methods of assessment of the food 

 of fresh-water fish (after Hynes, 1950). — Most workers 

 studying the food of fresh-water fish have based their 

 conclusions on study of the contents of the stomach 

 or, more rarely, of the entire gut of captured fish. 

 Digestion is less advanced in the stomach, and thus 

 identification of the contents is usually more satis- 

 factory. There has been great variety in the methods 

 of analyzing and presenting the data. Some authors 

 have merely listed the food organisms found in each 

 fish, but this, without analysis, gives no indication 

 of the relative importance of each type of food organ- 

 ism, and most workers have analyzed their data by 

 one or more of the following methods. 



(a) The occurrence method. The number of fish in 

 which each food item occurs is listed as a percentage 

 of the total number of fish examined. Often the number 

 of occurrences of all items is summed and scaled 

 down to a percentage basis to show the percentage 

 composition of the diet. 



(b) The number method. The total numbers of indi- 

 viduals of each food item are given, and are also 

 usually expressed as percentages of the total number 

 of organisms found in all fish examined. 



(c) The dominance method. The number of fish in 

 which each food item occurs as the dominant foodstuff 

 is expressed in one of the two ways used in the 

 occurrence method. 



TABLE 5 (Introduction) 

 'Coefficient of Accessibility" of common groups of Aquatic Organisms 



Freely exposed 



Partly protected or hidden 



Largely secure 



Secure 



1.0 



0.7B 



0.5 



0.25 



0.0 



Simulium 

 (all stages) 



Ephemerid 

 nymphs 



Trichoptera 

 larvae and pupae 



Coleoptera 

 larvae 



Oligochaeta 



Coleoptera 

 (adults) 



Plecoptera 



nymphs 



Hirudinea 

 Nematoda 



Diptera 

 larvae 



Hydracarina 



Diptera pupae 











Crustacea 











Mollusca 











