Fishery bulletin of the Fish and Wildlife Service - NOAA

FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 291
Since a constant proportion of the migrating population cannot be marked day
by day during the migration period, and as the 1932 experiments indicated that early
migrating fish tended to return after a shorter period of ocean life, it is apparent that
critical comparisons of the returns of any two or more years cannot be made. The
longer the period of time spent in the ocean the greater the mortality will be, consequently,
for exact comparisons between marking experiments of 2 or more years, it is
necessary that the fish of one experiment have remained in the ocean the same length
of time as the fish of the other experiments.
In view of the possible errors in the calculated percentage return from the marking
of any one age of seaward migrants, especially in the returns of the 2- and 5-year age
groups, it is believed that the best average value for the ocean survival is the mean of
the several yearly values, i. e., 21.45 percent.
Unfortunately, there is no way of knowing whether or not a differential mortality
exists between marked and unmarked fish, although a differential mortality was found
to exist between fish marked by the amputation of the adipose and one pectoral fin,
and the fish marked by the amputation of the adipose and one of the ventral fins.
This might be caused by any one, or a combination, of the following:
1. Kegeneration of the pectoral fins. The pectoral fins were amputated as close
to the body of the fish as possible, and it does not seem probable that any of the amputated
fins could have regenerated to such an extent as to be unrecognizable. None of
the marked fish recovered showed the slightest sign of regeneration of this fin.
2. Mortality of the fish as a direct result of the operation. Some of the fish were
held in a pen for several days after being marked and then carefully examined. The
wounds had begun to heal and the fish showed no ill effects other than that a few specimens
appeared to have a slight "list." Consequently, the marking probably did
not have a direct influence on the mortality.
3. Mortality caused by the inability of the fish to elude their enemies to as great
an extent as could the fish marked by the amputation of the adipose and one ventral
fin. The pectoral fins are used, almost entirely, for maintaining equilibrium, and it
is possible that fish marked by the amputation of the adipose and one pectoral fin
were handicapped. Such a handicap should not hinder fish feeding on plankton.
However, it might be a serious disadvantage when being pursued by predators. This
is considered the most likely of the several possible explanations for the differential
mortality found between the two groups.
There may have been a differential mortality between the unmarked fish and
those marked by the amputation of the adipose and one, or both, of the ventral fins.
It is not believed that the differential mortality could have been very great in view of
the relatively good returns from all the experiments. If the factor used by Foerster
at Cultus Lake, to correct for differential mortality, were applied to the Karluk data,
the survival of unmarked Karluk fish would be in excess of 56 percent.
MORTALITY IN FRESH WATER
Having ascertained the probable average ocean mortality of Karluk red salmon to
be 78.55 percent, as determined by the marking experiments, it is of interest to calculate
the mortality of this species between the egg stage and the seaward migrant stage. The