Anyone
who has caught more than few speckled trout has noticed that
some of them emit loud croaks after being caught. All of the
hundreds of members of the drum family found worldwide produce
sound during spawning season. Names like croaker and drum
recognize the sound-making ability of fish in the family.
Sound
is a very effective means of communication in water, much
better than sight. Even in the clearest water, light is lost
rapidly and almost completely gone by 5,000 feet deep. In
the less-than-clear waters such as in coastal Louisiana, light
weakens and vision declines in inches. The opposite is true
of sound, which moves five times faster in water than air.
Because of this, it isn’t surprising that so many marine
creatures––fish, shrimp, barnacles, porpoises
and whales––communicate with sound.
The speckled
trout is not known to produce sound except at the time of
spawning. Only male trout produce sound, and they do so by
vibrating the membrane of the gas bladder with sonic muscles.
The illustration at left shows the gas bladder and sonic muscle.
The gas bladder is silvery-white and tough enough to interfere
with a knife during filleting. The bright red sonic muscles
surrounding the gas bladder are easily noticed if a freshly
captured, recently calling male is cut open. The anterior
(front) horns of the air bladder contact the skull of the
fish.
During
courtship and spawning, specks produce four major sound types:
dual-pulse short grunts, long grunts, longer series of grunts
called multiple pulses, and staccatos, which are long series
of many short pulses, sounding almost like knocking.
When males
gather in spawning schools, the most common call is the dual
pulse (double grunt), with occasional long grunts. The rarest
call, the staccato, is only heard during the period of maximum
sound production, usually an hour or two after sunset.
Mature
spawning speckled trout begin to form groups in late afternoon,
an hour or two before sunset. Calling begins before sunset,
but is usually limited to occasional single dual pulses (double
grunts). Only after sunset do group calls start. Most calls
are made from sunset to three hours after sunset, although
some calls are heard as late as 1:00 or 2:00 a.m. No male
calling sounds are heard in the daytime, except those produced
just before sunset.
Scientists
can only speculate about the advantages and disadvantages
of speckled trout announcing their spawning sites with noise.
Apparently, the attraction of females to spawning sites with
large numbers of males ready to spawn makes egg fertilization
more efficient. The advantages of spawning in noisy groups
must be great enough to offset disadvantages, such as the
attraction of trout predators like bottlenose dolphin and
bull sharks, which hunt by sound.
Speckled
trout calling occurs on all moon phases, but is most common
on the full moon or within three to four days after the moon.
The lowest amount of calling is during the last quarter moon.
First quarter moon periods have slightly more calling than
during the new moon. Calling takes place only from April to
October. In contrast to research in Louisiana, which found
that calling and spawning took place near passes and channels,
research in Indian River Lagoon in Florida showed speckled
trout calling took place almost exclusively over sea grass
beds. There, calling males avoided waters near passes and
freshwater sources.
When
males have spawning on their minds, they don’t feed.
Repeated attempts to catch males on baited hooks from calling/spawning
groups have failed. Females are caught quite often before
and after sunset, but not at spawning.
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