(1) It was thought that a greater length might involve the exposure of the eggs near the lower end to the danger of a partial exhaustion of the air from the water by the eggs above them;

(2) these short troughs are very convenient to cleanse and to move about for repairs or other purposes.  They are made of pine boards seven-eighths inch thick.  On the inside they are planed and varnished with asphaltum.  When used for rearing fish each trough is fitted with a pair of thin wooden covers reaching its entire length hinged to the sides and meeting each other, when closed, at a right angle, forming; as it were, a roof over the trough.  When closed they protect from predatory birds and other vermin; when open they are fixed in an upright position, in effect adding to the height of the sides and preventing the fish jumping out.  The time spent in opening and closing the troughs is by this arrangement reduced to a minimum.

Water is fed through wooden tubes, and the volume admitted is regulated by slides The exit of the water is through another tube or hollow plug standing upright near the lower end of the trough, and by its height governing the depth of the water.  The outlet tube is movable and is taken out in cleaning.  A wire-cloth screen just above the outlet tube prevents the fish escaping.

In a trough of standard size 2,000 fry are generally placed, and to accommodate the large numbers of fish reared we bring into use sometimes nearly 200 troughs which are of necessity placed in the open air.  They are arranged in pairs with their heads against the feed troughs, supported by wooden horses at a convenient height from the ground.  They are given an inclination of about 2 inches to facilitate cleaning.

The volume of water fed to each trough has varied from time to time, but is ordinarily about 5 gallons per minute, which renews the water every four minutes.  The ordinary arrangement is to use the water but once in the troughs, letting it waste into some small ponds in which yearling and older fish are kept; but there is one system of 52 troughs arranged in four series, which use in succession the same water.  From these we have learned that young salmon thrive quite as well in the fourth series as in the first.  Indeed, by an actual test, with fish of like origin and character in each series, the fish reared in the fourth series were found to grow faster, to an important degree, than those in the first.  This phenomenon probably resulted from a somewhat higher temperature which the water acquired in passing through the several series.  A like observation has been made on a few salmon maintained for a few weeks, in the warmer water of a neighboring brook.

As already stated, the activity of the station has been mainly occupied with Atlantic salmon, but there have been reared each year a few landlocked salmon and brook trout, and occasional lots of other salmonoids, such as Loch Leven, Von Behr, Swiss-lake, rainbow, and Scotch sea trout.  All these have received the same treatment.  With the exception of the rainbow trout, they are all autumn-spawning fishes, and their eggs hatch early in the spring.