Fishery bulletin of the Fish and Wildlife Service - NOAA

STUDIES ON THE STB1PBD BASS OF THE ATLANTIC COAST 59
different conditions of the fishery and thus determine which is the most advantageous,
not only from the point of view of profit to the fisherman, but also in the light of what
is known about the life history of this species. In other words, it is desirable to discover
at what age (or length) it is most advantageous to start the fishery for striped
bass; i.e., whether the fishery gets the most profit out of taking the fish for the first
time when they are 2-year-olds (averaging roughly three-quarters of a pound and 12
inches in length) as it does at present, or whether it would benefit by allowing the fish
one or two more growing seasons before catching them.
In order to find the answers to these questions it is essential that the fishing
mortality at different ages—the percentage of fish of each age taken by the fishery—
and the natural mortality, be known. This can only be done accurately by careful
studies and the collection of detailed statistics on the annual catches of striped bass
over long-term periods, although the present work has given some information along
these Unes. Considering the dominant 1934 year-class, it has been assumed from
the percentage of tag returns (see p. 57) that approximately 40 percent of its members
were taken by the fishery as 2-year-olds in 1936 and 1937, and that about 25 percent
of the 3-year-olds of 1937 and 1938 were also taken by the fishery. It is known
from vanous catch records from Virginia to Rhode Island that only about onequarter
as many 3-year-old striped bass were caught in 1937 as the 2-year-olds that
were taken in 1936. This is demonstrated in figure 4, where the catches of a poundnet
fisherman at Fort Pond Bay, Long Island, N. Y., were approximately four times
as great by number in 1936 as they were in 1937, and where the catch was over 90
percent 2-year-olds in 1936 and 3-year-olds in 1937. Given this information it is
possible to estimate the natural mortality in 1936 by the following equation:
NM=S 1 -(FM 1 +S 2 ),
wherein NM is the natural mortality in 1936, Si the stock available in 1936, FMi
the fishing mortality in 1936, and S a the stock available in 1937. Si can be given
any arbitrary value, for example, 1,000. If FMi is assumed to be 40 percent of Si
(see above), FMi is 400. S 2 is equal to approximately 4Х^М 2, where FM t is the
fishing mortality in 1937, for tagging experiments indicate that roughly 25 percent of
the 3-year-olds were taken in 1937. FM 2 is known to be % FM b as only one-quarter
as many 3-year-olds were taken in 1937 as there were 2-year-olds taken in 1936.
Under these conditions FM 2 therefore becomes 100, and in the equation above, where S l
was assumed to be 1,000, S 2 becomes 400. Substituting these values in the equation,
the natural mortality in 1936 attains a value of 200. Thus of the original 1,000 fish
in 1936, 400 were caught as 2-year-olds, and of the remaining 600 fish, 200 were lost
through natural mortality. It is therefore apparent that if the estimates on which
the figures making up this equation are based are correct, natural mortality accounted
for about one-third of the 2-year-olds hi 1936 which were not taken by the fishery.
It should be pointed out, however, that slight variations in the percentages assigned
to FMi and FMz, which are only rough approximations, can materially change the
value obtained for NM. ,
Taking the figures in the equation above, since they seem to be the best available,
it is possible to get some estimate of the yield from the stock under the existing conditions
of the fishery. Table 1 is a theoretical treatment of 1,000 striped bass of the
1934 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 of this age group
were caught over a 5-year period from 1936-40 (as 2-, 3-, 4-, 5-, and 6-year-olds).
This treatment, in other words, considers the value when the fishery starts catching
striped bass for the first time as 2-year-olds, which is exactly what occurred in 1936
along the Atlantic coast. The natural mortality is figured at one-third of the population,
excluding those taken by the fishery. The fishing mortality was estimated to
be 40 percent in 1936, 25 percent in 1937, 15 percent in 1938 (when the members
of the 1934 year-class were 4-year-olds), 10 percent in 1939 (5-year-olds), and 5 percent
in 1940 (6-year-olds)—a declining fishing mortality that undoubtedly represents
as sharp a decrease in the percentage of fish of any year-class caught each year as
could possibly exist, and probably over-estimates the decline in the percentage taken
by the fishery as the members of a year-class become older. It will also be noted in
table 1 that the price per pound varies with the different size categories under con-