SECTIONAL TRANSACTIONS.— D. 361 



thought to be dependent on periodic environmental changes and not, as 

 Weissmann suggested, on inherent internal rhythms. 



Both female- and male-producing parthenogenetic eggs undergo only one 

 maturation division resulting in no reduction in the diploid chromosome 

 number (20 in Daphnia magna, 24 in D. pulex). The same diploid number 

 was found in the spermatogonia of D. magna. Spermatogenesis appeared 

 normal. The mechanism of sex determination appears then to be extra- 

 chromosomal. 



Mr. K. R. Allen. — The ecology of young salmon (12.0). 



In the rapid rivers which salmon usually frequent there is no true plank- 

 ton, and therefore fish living in them must feed either on the bottom or upon 

 objects floating on the surface. As surface food is only available in summer, 

 and is not eaten by salmon in their first year, the main food supply of young 

 salmon is derived from the bottom. Although algae may be abundant, the 

 salmon, even when very small, feed only upon the fauna. It has been found 

 that there are consistent differences in the relative extent to which different 

 species of food animals are eaten ; these differences appear to be correlated 

 with differences in the availability of the food animals to the fish. 



Seasonal changes in the type of water inhabited by young salmon have 

 been found to occur. In summer the fish live in comparatively shallow 

 water, but in winter they retire to deep pools and probably lie quietly in 

 sheltered places, only occasionally emerging to feed. There is a check in 

 growth during this time. 



Miss W. E. Frost. — Trout food and river fauna (12.30). 



Work done by him on lakes and rivers in Ireland suggested to the late 

 Mr. R. Southern that they could be divided into two fairly well defined types 

 as regards their brown trout : on the one hand, alkaline waters derived 

 from limestone rocks, where the trout grow rapidly and attain a weight of 

 2, 3 or more pounds ; on the other hand, acid waters derived from non-lime- 

 stone rocks, where the fish grow slowly and seldom exceed \ to g lb. in weight. 



In 1929 a biological survey of the river Liffey was begun in order to obtain 

 quantitative and qualitative data concerning the food of brown trout from 

 acid and alkaline waters, and thus throw some light on this problem of widely 

 differing growth rates. 



The two stations on the Liffey chosen for detailed investigation were 

 Ballysmuttan, where the water is acid (pH 4 • 6 to 6 • 9), and Straffan, where it 

 is alkaline (pH 7-6 to 8 "3). Brown trout were captured during all the 

 months of the year, the stomach contents noted, and simultaneously observa- 

 tions on the river fauna were made. The type of food organism taken by 

 the trout varies somewhat with the time of year at both stations, but not 

 so much as would be expected. This variability seems to depend more on 

 the size of the fish, the height and condition of the water, etc., rather than on 

 the season. 



The abundance of some of the aquatic animals is to a certain extent 

 reflected in the diet of the trout. At Ballysmuttan Ephemerid larvae are 

 scarce and Perlid larvae are common : the reverse applies to Straffan. These 

 larvae are found in corresponding proportions in the stomach contents of 

 the fish from the two places. Certain organisms (fish, crayfish) abundant 

 in the river form but a small part of the trout's food. Trout, both spent and 

 immature fish, with full stomachs, have been taken during the winter. 



The differing growth-rate in alkaline and acid waters does not, generally 

 speaking, appear to be due to a food shortage in the latter waters. 



