|Abstract or Summary
- The results of experiments conducted in 1957 and 1958 to
isolate the cause of post-transportation mortalities among hatchery-reared
rainbow trout, Salmo gairdneri Richardson, in Oregon are
reported. Although death rate varies greatly, the average loss
occurring from one to seven days following liberation has been estimated
to be ten percent of the fish transported. Studies conducted
over the past 15 years on problems arising from the live
shipment of fishes are reviewed.
General experimental procedure was to place fish in aquaria
and subject them to certain environmental conditions perhaps associated
with delayed planting mortalities. After a specified interval,
animals were removed from test containers and placed in observation
troughs. Subsequent losses were recorded and compared to the measured experimental conditions in an effort to identify causal
Simulated in-transit motion, density of fish transported, and
pre-hauling starvation period ha. d no apparent influence on the magnitude
of delayed trout deaths. Zinc from zinc chloride and from
galvanized iron plating was toxic to trout, and, at low concentrations,
produced delayed mortalities characteristic of those that
occurred following tank truck transportation in Oregon.
The toxic effect of zinc increased with increasing zinc concentration,
and with increasing length of exposure to the metal.
Of the trout exposed for six hours to an initial zinc concentration of
0.10 mg/1, an average of 8.2 percent suffered a delayed mortality.
When the concentration was increased to 1.00 mg/1 for the same
time period, the delayed loss increased to an average of 63.2 percent.
When length of trout exposure to various zinc concentrations
was increased from two to eight hours, the mean delayed mortality
increased from 39 to 68 percent.
The toxicity of zinc increased with decreasing total water
hardness. At 0.75 mg/1 zinc, mean trout mortalities increased
from five to 28 percent when total water hardness was decreased
from 40 to 20 p. p. m. When hardness was decreased from 20 to
zero p.p.m. , mean mortalities increased from 28 to 88 percent.
On the basis of research previously conducted in Oregon, it was shown that delayed trout losses were inversely correlated with
total alkalinity of the transporting water. Assuming that calcium
was the principal cation involved, the relationship was attributed
to the effects of increased water hardness.
Samples of transportation water from 32 inter-hatchery
transfers of fish were taken at the completion of the station to station
trips and analyzed for zinc. In every instance where delayed
mortalities greater than one percent occurred, appreciable quantities
of zinc were detected. When delayed losses of trout were
much less than one percent, no zinc was detected in the water used
during the transfer.
Based on the results of the field and laboratory tests reported,
it was concluded that zinc from galvanized iron surfaces was responsible
for the post-transportation mortalities of trout in Oregon.
Fish exposed to low zinc concentrations in liberating tanks were
subjected to threshold toxic conditions in which a varying portion
of the animals died a delayed death. Relatively small changes in
the natural water hardness or zinc concentration could either increase
or eliminate subsequent mortalities.