Hence the erection of a series of small troughs in the open air, which
gave such excellent satisfaction that enlargement took the same
direction; and it has thus come about that the rearing operations of
the station down to the present time have been almost exclusively
conducted in open-air troughs. A series of ponds has been constructed,
but with the exception of a few small ones none of them have been as
yet brought into use.

The troughs are for the most part such as are used in the hatchery for
the maturing of spawn, and their form and size have been adapted to the
hatching apparatus which has been in use at the Maine station for many
years. The eggs are developed on wire-cloth trays measuring 12 and one
half inches in width and length, and the troughs are therefore 12 and
three quarter inches wide. Their depth is 9 inches and their length is
10 feet 6 inches. Such short troughs were adopted for two reasons:

(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