HIGHER ANIMALS IN ECONOMY OF LAKES 



139 



ouglily familiar with conditions in the field 

 before drawing conclusions from laboratory 

 experiments. It must also be recognized 

 that laboratory experiments give physio- 

 logical answers rather than ecological ones. 

 This is both a pitfall and an advantage. 

 Too often the attention of the experimenter 

 is focussed upon an effort to duplicate in 

 the laboratory physical and chemical con- 

 ditions which may be found in the field, 

 and he does not take thought to consider 

 fully just in what way these conditions are 

 acting upon his experimental animals. 

 There have been experiments by which the 

 relative sensitivity of species towards un- 

 favorable conditions have been measured 



Thus, although it is desirable to get a gen- 

 eral index of resistance, the rate of dying 

 in a lethal concentration of dissolved gases 

 is dependent upon a very complex interac- 

 tion of factors and Wells' scale of sensitiv- 

 ity, while holding true for the conditions 

 of the experiments, cannot be trusted to 

 predict the reactions of those species to sub- 

 lethal concentrations of the gases in ques- 

 tion, nor even to be a measure of the order 

 of concentrations of gases which are the 

 lethal thresholds of each species. 



Other measures of sensitivity towards 

 carbon dioxide have been made which give 

 a scale differing from that of "Wells. If 

 fish are sealed in bottles containing water 



TABLE 1 



Comparison of the Relative Resistaxce of Various Species as Measured by Wells (1918) and the 

 Order of Sensitivity Towards Carbon Dioxide as Measured by Powers et al. (1938) 



AND BY Fry and Black (1938). 



Order of resistance 

 (Wells) 



Order of Sensitivity 

 (Powers et al., Fry and Black) 



Cafosfo m us co m m erson nii 



Semotiliis atromaculaius 



(Chrosomus erythrogaster) 



Micropterus doJom ieit 

 Hyborhynchus notatus 

 Notropis cornutus 



Ambloplites rupestris 



Perca flavescens 

 Ameirus melas 



I Lake Michigan 

 \ X. 111. 



Perca flavescens 



S L. Erie 

 ] Alg. Pk. 



Catostomus commersonil Alg. Pk. 



,,. , , , . ( L. Erie 



Micropterus dolomieu -I ., p, 



( Amhloplites rupestris ( L. Erie 



\ Notropis cornutus Alg. Pk. } Alg. Pk. 



Hyborhynchus notatus Alg. Pk. 



/y-„ \ \ Toronto 



(C hrosomus eos) -j .. p, 



Semotilus atromaculatus Alg. Pk. 

 Ameiurus iiebulosus Alg. Pk. 



by comparing the rates of dying of indi- 

 viduals in conditions which were lethal to 

 the hardiest. The experiments of Wells 

 (1918) from which a scale of hardiness of 

 certain species of fishes was derived, were 

 of this type. Because of the importance of 

 this scale of hardiness, it may be proper to 

 insert here some criticism of the interpreta- 

 tion of the experiments on which it is based. 

 When a fish is placed in a concentration 

 of dissolved gases in which it dies rapidly, 

 the rate of death depends on the ratio of 

 surface to volume, on the rate of circula- 

 tion of the blood, on the rate of gaseous 

 exchange at the gills, on the rate of utiliza- 

 tion of oxj'gen by the tissues, and on the 

 ability of the animal to endure anoxia. 



high in oxygen and charged with various 

 concentrations of carbon dioxide, and al- 

 lowed to die there of asphyxiation, it will 

 be found that the amount of dissolved oxy- 

 gen remaining in the water will vary ac- 

 cording to the species and to the amount of 

 carbon dioxide present. Such experiments 

 have been carried out by Powers et al. 

 (1938) and by Fry and Black (1938). 

 Data from a number of such experiments 

 are plotted in Fig. 3. The order of resis- 

 tance of certain species in Wells' scale of 

 sensitivity are compared with the order of 

 the same species as measured by Powers 

 et al and by Fry and Black in Table 1. This 

 comparison must be considered only as ten- 

 tative, since there is considerable subspe- 



