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Becker, George C. / Fishes of Wisconsin

Silverside family - atherinidae,   pp. 767-773 ff. PDF (2.7 MB)

Page 771

Brook Silverside  771 
an immature, white-yellow generation of eggs was 
0.3-0.5 mm diam. From a nearby area on the same 
date, a 75-mm, 2.08-g female in prespawning condi- 
tion, with ovaries 10% of body weight, held 450 ma- 
turing eggs, 0.5-0.8 mm diam, and a small number 
of immature eggs, 0.2-0.5 mm diam. 
  On 17 July in the Wisconsin River at Boscobel (Grant 
County), an 80-mm 2.46-g fish, with ovaries 14% of 
body weight, held 700 maturing to mature eggs, 0.8- 
1.1 mm diam, and a few immature eggs, 0.4-0.6 mm 
diam. Females examined from Pewaukee Lake (Wau- 
kesha County) on 3 August and from Lake Monona 
(Dane County) on 10 August still had well-devel- 
oped ovaries containing a number of ripe eggs. It is 
likely that spawning may continue over a period of 
days or weeks. In Indiana, Nelson (1968b) noted 
similar ranges in egg sizes and differences in egg 
  The brook silverside's egg is well supplied with a 
number of oil globules and with a flotation-adhesive 
organ in the form of a gelatinous filament, which is 
about 6 times as long as the diameter of the egg. In a 
water current, this filament may act as a flotation body 
transporting the egg for some distance. The sticky 
filament becomes attached to the first thing with 
which it comes into contact, either vegetation or bot- 
tom material such as bulrushes or gravel. The egg it- 
self is not sticky. 
  In Oconomowoc Lake, the eggs hatched out in 8-9 
days at depths of 41-94 cm in 22.8-24°C (73-75°F) 
water. In Indiana (Nelson 1968b), eyed eggs from 
Crooked Lake hatched in the laboratory into young 
4 mm TL. Fish (1932) described and iillustrated the 
27-mm stage. 
  After hatching, young brook silversides wiggle to 
the surface, and assemble in schools of 30-200 indi- 
viduals. They confine themselves to the upper 3 cm 
below the surface, are constantly active, and move as 
a school from the shallow water outward to assume 
a pelagic habitat over deep water. This habitat may 
be over water 3-5 m deep, but it is generally over 
water 10-20 m deep. 
  In their offshore habitat, the young eat largely zoo- 
plankton and grow rapidly. The small size of young 
brook silversides, together with their inconspicuous 
coloring and nearly transparent bodies, gives them 
protection from enemies. Despite their camouflage, 
they have been seen scattering wildly with the ap- 
proach of a tern or a large fish. Hubbs (1921) and 
Cahn (1927) emphasized the constant activity of the 
young. The brook silverside is among the most active 
of our freshwater fishes, and, according to Cahn, 
young fish 22-mm long swam an average distance of 
211 cm in 2 minutes, or an estimated 886 m in the 
course of a day. 
  With the coming of darkness at night, young brook 
silversides become inactive and float motionless just 
under the surface. Members of the school point in all 
directions, and there is no marked orientation. From 
mid-July on the young engage in increasingly fre- 
quent nocturnal migrations to the shallows, from 
which they return to their pelagic habit over deep 
water during the day. Cahn has suggested that this 
movement is triggered by differences in water tem- 
perature: at night the young seek out the warmer 
surface temperature of the shallow water (1 m deep) 
rather than the colder surface temperature of the deep 
water (16 in). This shoreward journey is accom- 
plished rapidly (Cahn 1927:78-79): 
... they swim at top speed straight for the shore .... The 
minnows arrive together, which fact is significant. In from 
ten to twenty-five minutes the entire silversides population 
of the lake is inshore. 
Upon completing their inshore movement, the young 
have joined the adult silversides that typically inhabit 
inshore surfaces through the 24-hour day. Following 
hatching, young brook silversides grow 1 mm per 
day for 2 weeks, after which growth is slower. In 
Oconomowoc Lake, the average growth during the 
first year of life was: 22 June, 11.2 mm; 6 July, 28.6 
mm; 17 August, 46.2 mm; 28 September, 63.9 mm; 
and 26 October, 65.6 mm. Growth ceases in mid-Oc- 
tober and resumes the following May: 18 May, 68.6 
mm; 1 June, 72.2 mm; 22 June, 75.2 mm, and 1 Au- 
gust to I September, 76.2 mm. Most growth (approxi- 
mately 80-90%) occurs during the first year of life. 
The total life-span of the brook silverside is 15-17 
months. The single annulus is deposited in June and 
July. In Indiana, Nelson observed brook silversides 
about 100 mm FL, which must have been between 21 
and 23 months old; none lived into a second breeding 
  The largest known Wisconsin specimens of brook 
silversides were 96 and 98 mm TL; they were cap- 
tured 3 August from Pewaukee Lake (Waukesha 
County). In Indiana (Nelson 1968b), one brook sil- 
verside was 109 mm (4.3 in) FL. 
  In Oconomowoc Lake, young brook silversides 
prior to their inshore migrations at night, ate Cyclops, 
Daphnia, Bosmina, rotifers, diatoms, and other algae. 
While inshore in mid-August they consumed mostly 
insects, including Diptera, Coleoptera, and unspeci- 
fied larvae; they also ate copepods, cladocerans, os- 

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