~AQUACULTURE update
Number:R-
April 24. 1989
PARASITIC IIORHS IN TROUT IN LAK£ CAGE CULTURE OPERATIONS
Parasitic
worms
are
k.nown
to
occur among salmon and trout cultured
in lake cages in B.C.. but no
mortalities
have
been
previously
reported.
Recently,
mortalities
caused by the parasitic nematode
Philonema agubernaculum were seen in
yearling steel head trout in a lakebased operation on North Vancouver
Island.
Dead and moribund fish had
severe abdominal distension caused by
massive adhesions and large numbers of
the creamy-white worms loosely coiled
in the body cavity.
Although
P.
agubernaculum
can
trout. the parasite is released in the
stomach of the fish and migrates to the
wall of the swimbladder of the fish and
later to the body cavity where it
increases greatly in size. matures. and
mates. When infected adult fish spawn.
ripe/gravid female worms are discharged
into the water where they burst
releasing a myriad of larvae. which are
taken up by the Cyc lops. undergo
deve 1apment to the stage i nfecti ve for
fish. and the cycle is repeated.
The
parasite is believed to have an annual
cycle. In juvenile fish. gravid worms
may not escape from the body cavity but
possibly die and degenerate there.
reach 25 on in-'ength. in tnis case
the
maximum
length
approximately 15 em.
the adhesions
bound
noted
was
In most flsh.
the
enti re
body
cavity into a soft, sol id mass. The
damage appeared severe enough to have
a serious impact upon the reproduction
and growth of any fi sh that survives
infection.
Fi sh size ranged between 5 and 31
grams.
In some. up to 10% of their
gross weight could be attributed to
the mass of worms in the body cavity.
rhe main disease signs were sluggish
behaviour and abdominal distension.
The life cycle of Philonema
species involves development of the
infective larval stage in a lakedwelling copepod and the adult in a
salmonid fiSh. For P. agubernaculum.
the copepod host is Cyclops sp. Fish
feed on the lake's planktonic fauna
containing
Cyclops
sp.
After
ingestion of infected copepads by
•+.
FIsheries
and Oceans
Canada
P&ches
et Oceans
All farms involved in intensive
lake cage culture must be prepared to
address the possibility of poor fish
health
and
performance caused by
parasitic worms.
No treatments are
known and because of the continual
exposure of caged fish to a parasite's
alternate
or
intermediate
host.
avoidance
is
not
likely
to
be
practical.
Even though the high.
sudden losses often encountered with
viral
or
bacterial
diseases
are
unlikely to occur with diseases caused
by parasitic wonns. infection with such
parasites can have a major impact on a
lake cage operation by causing poor
growth. chronic mortalities. poor flesh
quality, reduced fertility. and an
inability of fish to adapt to sea
water.
For further information contact:
Mr. Gary E. Hoskins (756-7062)
or Dr. Leo Mar90lis (756-7032)
Pacific Biological Station
Nanaimo, B.C .
Terry Tebb· Head, Aquaculture DIvision
Pacific Biological Station
Nanaimo, B.C.
V9R5K6
Telephone 756-7048