- This study consisted of determining some of the conditions for
culture of young English sole (Parophrys vetulus Girard) in the laboratory.
Rearing of the larvae to the juvenile stage was attempted
unsuccessfully; techniques used and problems encountered during culture
are discussed. Growth of juveniles was studied at combinations of
temperatures and daily rations of the diet Oregon Moist Pellet ranging
from 9.5 to 21° C and from zero to 16% of dry body weight. The effects
of diet (artificial versus natural), population density, and dominance
behavior on growth were also studied. Acutely tested tolerance (96 hour
bioassay) to salinities in the range of 0 to 34%₀ was determined.
In the temperature-ration experiment, fastest growth of the juveniles
(0.95%/day) occurred at the lowest temperature and highest ration.
Growth declined with decreasing ration and increasing temperature. At
21° C, juveniles lost weight regardless of ration. For fish fed Oregon Moist Pellet the maintenance ration was determined to be 3.1% at 9.5° C,
increasing to 4.7% at 18° C. Highest food conversion occurred at the
lowest temperature and the 8% ration. Food conversion declined with
increasing temperature and at rations greater than 8%. At an 8% ration
and the temperatures 12, 15, and 18° C, growth rate of fish about 16 to
19 months old was approximately the same as for fish about 4 to 7 months
old. However, the approximate maximum daily food consumption rate decreased
from about 20% in 5 gram fish to 16% in 9 gram fish to 10% in
33 gram fish.
At the same restricted ration (8%), juveniles grew much faster on
a clam-shrimp diet (1.16%/day), which was thought to be a more natural
diet, than the Oregon Moist Pellet diet (0.42%/day). However, a greater
total ration of the Oregon Moist Pellet (16%) could be consumed than the
clam-shrimp diet (10%). Despite the faster growth on the natural diet,
mortality during the experiment was higher in groups fed the clam-shrimp
diet (32%) than in groups fed the Oregon Moist Pellet diet (3%). The
reason for this differential mortality was not determined.
Over the range of initial population densities of 0.5 to 5.3 kg/m³
no differences in growth rate were observed. In later experiments
juveniles were reared at densities up to 15.1 kg/m³ without mortalities
nor slowing of growth.
Aggressive behavior was observed between English sole throughout
the experiments. Active fish nipped at the tails of, and grew faster
(p< 0.01) than, less active fish. When active and less active individuals
were separated, both groups grew at the same rate statistically when fed the same ration, although the active fish continued to grow
faster. Active fish (22.7 grams) were significantly smaller (p<0.05)
as a group at the beginning of the experiment than less active, and
slower growing, fish (24.1 grams).
Salinities down to 3.3%o were tolerated by laboratory adapted juveniles
for 96 hours without dying. At 1.3%₀ salinity the time to 50%
mortality at 9.5° C (46 hours) was twice that at 16.5° C (23 hours) for
0-group sole. At 1.6%₀ salinity and 16.5° C, the time to 50% mortality
was 1.5 times greater with 0-group fish (24 hours) compared with I-group
fish (16 hours).
Diseases encountered during the experiments included gyrodactyliasis,
caused by the monogenetic trematode Gyrodactylus, and three
presumed bacterial infections: a fin rot, a coldwater lesion-producing
disease, and a systemic bacterial infection, probably vibriosis. Treatments
and preventive measures used are given. In addition, the occurrence
and consequences of the parasites Philometra americana (Nematoda)
and Glugea (Microsporida) and skin tumors are discussed.