146 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



fully the natural conditions of the lake in all its main features, a knowl- 

 edge of which shows its capacity for fish life, its suitability for the intro- 

 duction of new species, and the measures necessary for the i>rotectiou 

 of its fisheries. Most attention in the line of biology was paid to the 

 smaller forms of life, such as the larval and adult insects, crustaceans, 

 mollusks, and Avorms, which serve as food for fishes, both adults and 

 young. 

 In his conclusions, Prof. Forbes remarks : 



It is evident that even iu a lake of so moderate sizeas this the smaller inliahitants 

 are (Hiito clearly divided into pelagic and littoral gronps, the latter containing the 

 greater number of species, but the former not less numerous iu individuals; and a 

 comparison of the results of dredging shows that this difference applies to the ani- 

 mals of the bottom as Avell as to those swimming freely above it. * * * It is 

 also apparent, from the product of the towing net iu deep water under vaiyiug con- 

 ditions, that the pelagic eutomostraca avoid the surface by day, whether it borough 

 or calm, or the -weather cloudy or clear ; but they do not necessarily withdraw to 

 any great depth, hauls 10 feet below yielding " good" or "largo" collections when 

 the sun was shining. By night, on the other hand, the yield at the surface was 

 large, even iu a high wind. 



The examination of Lake Mendota was made for the purpose of 

 studying a most remarkable mortality among the common x^erch {Pcrca 

 Jiavescens) in August, 1884, and was repeated in August of the follow- 

 ing year. A study of the lake similar to that of Lake Geneva was also 

 instituted, but upon a less comprehensive scale. The fish mortality 

 began early in July of the former year and continued until about the 

 middle of August, the (luantity of fish dying during that period being- 

 estimated at above 300 tons. About 90 per cent of the fish which per- 

 ished were the common perch, after which in numbers came the lake 

 herring, followed by a few other species. Prof. Forbes's studies of the 

 subject were thorough and covered all the questions tliat could be sug- 

 gested, including an examination of all the conditions of the lake, both 

 physical and biological, and of the morphological and histological con- 

 ditions of both the dead and living fishes. It Avas impossible, however, 

 to arrive at satisfa(;tory deductions respecting the cause of the mor- 

 tality, Prof. Forbes not having been a])prised of its occurrence until it 

 was practically over, and no subsequent ox)portunity having arisen to 

 study similar phenomena. However, all the facts have been carefully 

 recorded and discussed, and are available for future consideration. 

 Tlie more significant facts connected Avith the jiuntality are stated in 

 considerable detail, but they are too long to nuiko more than a brief 

 reference to them in this connection. 



A diseased condition of some parts of the viscera was noticed, espe- 

 cially in the herring, but exteriuilly the dying fishes seemed to be in 

 good condition. The herring are subject to some mortality of the same 

 character every year, but to a much less extent than in 1884. It is 

 probable that the causes afiecting the two species were the same, ami 

 it is also possible that the disease is contagious and Avas taken directly 

 from the herring. The majority of the dead perch were full grown, 



