Fishery bulletin of the Fish and Wildlife Service - NOAA

STUDIES ON THE STRIPED BASS OF THE ATLANTIC COAST 61
due to the high value estimated for natural mortality each year, for the amount
added in total growth by allowing the fish to live until they are 4 years old does not
compensate for the numbers lost through natural mortality under these conditions.
ТЛВЪЕ 2.—Theoretical treatment of 1,000 striped bass of the 1984 year-class to show the rate of removal
by the fishery and natural mortality, the numbers and poundage caught, and the market value, when
the fish were caught over а 4-У еат period from 1937-40. Note that in this treatment the fish were
caught for the first time when they were S-year-olds
Assuming 1,000 bass were available In 1836, of which 333 would
die In 1936 (natural mortality, 33 percent), leaving
667 bass available In 1937, of which 167 would be caught in 1837
(fishing mortality, 25 percent); 167 would die in 1937 (natural
mortality, 33 percent ot those not caught), leaving
333 bas available in 1938, of which 60 would be caught in 1938
(fishing mortality, 15 percent); 94 would die in 1938 (natural
mortality, 33 percent of those not caught), leaving
189 bass available in 1939, of which 19 would be caught in 1939
(fishing mortality, 10 percent); 57 would die in 1939 (natural
mortality, 33 percent of those not caught), leaving
113 bass available in 1940, of which в would be caught in 1940
(fishing mortality, 5 percent).
Total number of striped bass caught during 1937-40 , 242.
Age
Years ï
3
4
6
G
Тotal
Average
length
il cm. (16.1
inches).
50 cm. (19.7
inches).
58 cm. (22.8
Inches).
66 cm. (26.0
inches).
Average
weight
Pound*
2.0
3.6
6. fi
8.0
Total
weight
Pound»
334.0
175.0
104.5
48.0
661.5
Average
price per
pound
Cents
9.5
10.0
10.0
10.0
Market
value
In tables 1 and 2 it was shown that the total market value of striped bass taken
from the available stock of 1,000 fish of the 1934 year-class from 1936-40 (bass caught
for the first time as 2-year-olds) was $57.45, as compared with $64.48 when this same
stock was utilized by taking its members for the first time when they were 3-year-olds
over the period from 1937-40. It should be pointed out that the gain from allowing
the fish to become 3 years old before being caught has been figured in these examples
as the least that can result. In the first place, the fishing mortality on the members
of the 1934 year-class was estimated from tagging experiments as 40 percent in 1936
and 25 percent in 1937. It has been arbitrarily placed at 15 percent m 1938, 10 percent
in 1939, and 5 percent in 1940, because they are considered the lowest values
possible. Whether or not this annual decline in the percentage taken is as steep as
indicated above and in tables 1 and 2 is extremely questionable. It is obvious that
if this decline is less sharp, the gain from allowing the fish to become 3 years old before
being caught is relatively greater. Further than this, the natural mortality of the
bass of the 1934 year-class is estimated to be 33 percent of the population (neglecting
fishing mortality) in 1936, and it has been arbitrarily placed at 33 percent for the
years from 1937 to 1940. Actually, it is extremely unlikely that it remains as high
as 33 percent over this period, for it is reasonable to assume that as bass become
older than 2 years of age they are less likely to be killed through natural causes. It
is possible that when bass become much older the death rate increases, but in the
examples in tables 1 and 2 that stage is probably not reached. Thus it is likely that
the annual natural mortality of 33 percent from 1937 to 1940 is far too high. If this
be so, the gam from allowing the fish to become 3 years old before being caught is
again relatively greater than is shown by the total market value in the examples given
above. It is evident therefore that the gam from catching striped bass for the first
time as 3-year-olds is far more than is shown in tables 1 and 2. Nor should it be
necessary to point out that the figures used in the examples in tables 1 and 2 represent
only gross values, and that the net values would be far greater.
It is also of importance that if the fishery first starts to operate on the striped bass
population when its members are 3 years old, a greater proportion of the stock is given
a chance to spawn. It has already been shown (see p. 22) that female striped bass first
mature at 4 years of age. If the stocks available at this age are compared in tables 1
and 2, it will be seen that of the 1,000 original fish of the 1934 year-class only 200 were
$31.73
17.50
10.45
4.80
64.48