2o 4 POPULAR SCIENCE MONTHLY. 



Tomales Bay, caught salmon fry coming back into the stream at flood 

 tide. This indicates that they regularly travel back and forth with the 

 tides. We should expect this, as it is hardly probable that they know 

 any directions, except with or against the current. 



Many experiments were performed at the Hopkins Seaside Labora- 

 tory for the purpose of determining the effect of sea-water upon 

 young salmon of various ages. 25,000 salmon were hatched in the 

 laboratory, and at various ages a few were placed in separate tanks and 

 subjected to various mixtures of fresh sea-water. Without going 

 into details of the experiments, the following are the results obtained : 



The young of any age can bear with impunity a density of 25 per 

 cent, sea- water; that is, 1 part sea-water to 3 parts fresh water. Not 

 until forty days old, at the time of the complete absorption of the 

 yolk-sac, could they withstand a density of 50 per cent, sea-water. 

 At the age of 50 days 75 per cent, sea- water could be endured. Pure 

 sea-water could be endured at the age of 60 days, though there was a 

 slight loss. It is doubtful whether they can enter salt water with com- 

 plete impunity until 3 or 4 months old. The loss was much less 

 when the density alternated from low to high and back again, simulating 

 the change of density in the estuaries with the change of tides. In all 

 cases the young salmon died when changed directly from fresh to salt 

 water, or when the density was rapidly increased until that of sea- 

 water was reached. 



To summarize the notes on migration: The fry begin their down 

 stream migration as soon as they are able to swim. In clear water they 

 travel more at night; in muddy water, as much or more during the 

 day. Much of the time they float down stream tail first. In the larger 

 streams they travel more or less in schools. Their regular migration 

 is not dependent upon the height of the water, but upon age. Their 

 rate of progress is about 10 miles a day, and they are about six weeks in 

 reaching brackish water. They are probably four or five months old 

 when they reach the ocean. 



A Parr, Size upon entering the Ocean. 



Summer Residents. 

 In the upper portion of Sacramento Eiver and its tributaries there 

 remains after the winter and spring migration a large number of young 



