>Are the fish [shad] allowed to spawn naturally or are they killed for their
eggs
>and sperm? Does anyone know how many times an individual shad will spawn in
>the wild (Males? Females?)?
Depending on their geographical location, shads may spawn once and then die
(called semelparity), or they may survive to make several spawning runs per
lifetime (called iteroparity).
As with most migratory fishes, shads will not naturally spawn in aquaria since
the environmental cues that induce spawning (as far as they are known) are too
complex to be simulated. But shads are artificially spawned at hatcheries in one
of two methods. Most "low-tech" hatcheries are located directly on the rivers
where the young are to be released. Hatchery workers collect ripe males and
females as they return to spawn, and squeeze their eggs into a bowl or pail.
They fertilize the eggs by gently stirring in a drop or two of sperm. The eggs
are then placed into hatching tanks that are continuously refreshed with water
pumped from the river. As the newly hatched fry mature, they make their way
through a pipeline that whisks them into their permanent home.
A more "hi-tech" hatchery approach allows shad to spawn on their own with the
help of hormone injections. Again, returning shad are collected from the wild.
They're dipped into an anesthetic to keep them calm, sexed, and tested to see if
their eggs and sperms are sufficiently developed for the hormone to work. A
glass pipette is inserted through the vent and into the testes and ovaries and
small samples of eggs and sperm are removed, which are then examined under the
microscope. If a shad is ready, it's given the hormone injection -- a grain of
rice-sized pellet with the consistency of an aspirin. Since mortaility is high
from the rough landling, hatchery workers are also mixing non-injected shad in
with the injected ones. Hormones released during the induced spawning process
cause the non-injected shad to spawn as well. The resulting orgy-like spawning
act was vividly described in a hatchery report from the Maine Department of
Marine Resources. Here's a summary:
In June and July 1997, around 30 shad were injected and released into a
3,030-gallon, naturally lit spawning tank. During the day, the shad swam
parallel to each other, usually against the current. But as the hormones took
effect and the afternoon waned, the shad got more excited and unpredictable in
their movements. First, occassional individuals "changed lanes." Then they began
doing U-turns and 360 degree loops around the tank, now usually with the
current. Then this looping turned into sudden high-speed bursts, with
individuals racing around the tank one to 1-1/2 times. Now the shad also begen
to get more agressive. A male approached a female from the rear and attempted to
touch its nose to just in front of the female's tail. The female fish didn't
seem to like this, and so the two shad, as a pair, chased around for one or two
circuits of the tank, often splashing and slapping on the surface.
However, the fish have not yet spawned. Instead, the high-speed, pair-swimming
behavior apparently serves as a way to get the other shad more excited.
As darkness fell, more and more shad seemingly lost their inhibitions and began
falling out of their parallel swimming patterns and making random lane changes.
The number and frequency of aggressive encounters, along with the U-turns and
360 degree loops, continued to increase as the evening wore on. Eventually, the
aggressive pairings became mutually agreeable and spawning occurred. Each pair
of shad sawm in a tight 360 degree loop, during which they vibrated or jerked
spasmodically, with the inside shad pushing against the outside shad's body.
During these encounters eggs and sperm were squeezed out. Apparently, the sight
(or scent?) of these looping, vibrating pairs incited unpaired shads to swim in
vibrating circles by themselves. Since they do not have a mate to help them
release their eggs and sperm, they availed themselves of whatever was in the
water, pushing, bumping, or vibrating against filter pipes and the tank walls.
One specific shad was observed several nights in a row slowly bumping its side
against the tank in what seemed like a choreographed manner. It would visit the
same several spots on the tank wall, bump it gently several times, then swim to
another spot and repeat the ritual. After several stops the shad would reverse
direction, swim directly back to the original spot, and begin the pattern again.
Hatchery workers who placed their fingers against the tank glass could detect
some of these vibrations, which were described as "much like [that of] a low
frequency vibrator pad." In this particular tank, the shad produced 0.2-2.3
liters of eggs nightly, at an average of 82,700 eggs per female.
Pretty cool, huh?
Chris Scharpf
Baltimore
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