Fishery bulletin of the Fish and Wildlife Service - NOAA

164 FISHEBY BULLETIN OF THE FISH AND WILDLIFE SEBVICE
places of least intense spawning are areas with broken bottom where tidal and general
circulation produce extensive vertical turbulence, drawing cold water from the depths
to the surface, thereby delaying the vernal warming of the upper strata, as a rule,
until the mackerel spawning season is over. As nearly as may be determined from the
information on hydrographie conditions (Bjerkan, 1919, pp. 379-403, Bigelow, 1928,
pp. 550-585) and on spawning times and places (see above), the dividing line between
good and poor spawning areas may be drawn at a vernal temperature of about 8°C.,
(46° F.). The areas that receive little or no spawn are, during the spawning season,
usually colder, and those that receive much spawn are usually warmer than this
temperature.
NUMBER OF EGGS SPAWNED AND SIZE OF SPAWNING STOCK
A rough estimate of the total number of eggs spawned in the region between Cape
Cod and Cape Hatteras can be made from the data of the 1932 survey of spawning.
The average catch during the first seven cruises was about 1,000 eggs per 17 square
meters of sea surface (table 19), or an equivalent of about 200 million eggs per square
nautical mile. Talcing 25,000 square miles as the areas surveyed, this would amount
to a total of 5,000 billion eggs. Since this figure is based on the average concentration
during a 50-day period, and since the period of incubation would average about 7 days
at the prevailing temperature, there must have been about 7 renewals or approximately
35,000 billion e°gs spawned to maintain this average concentration. From a curve of
numbers of eggs taken in successive cruises, it appears that perhaps one-seventh should
be added to allow for the fact that the cruises did not begin early enough or extend
late enough to include all the spawning. This raises the figure to 40,000 billions eggs.
These are in all stages, and it may be computed from mortality rates of eggs (table 7)
that this would be equivalent to 1.6 times as many newly-spawned eggs. Applying
this factor, the final estimate of eggs spawned in this area in 1932 becomes about
64,000 billion.
It is difficult to appraise the reliability of this estimate because of the uncertainty of
its components. Judging these as well as may be, it appears that at best it may be
within 25 percent of the true value and at worst only within the true order of magnitude.
But this is only personal judgment, and since it is impossible to study statistical
probabilities, there is utility in testing the result by deriving a related statistic from
an entirely different source.
During 1932 the catch of mackerel on or near spawning grounds during the
spawning season; that is, in area XXITI (Fiedler; Manning, and Johnson 1934, p. 96),
and in area XXII, west of Nantucket Shoals during April, May, and June, was about
13,000,000 pounds. From unpublished records on size composition of this catch, it
appears that about 10,000,000 pounds of it consisted of fish of spawning size, and that
then- average weight was nearly 1.9 pounds. Thus, a take of about 5,000,000 spawners
is indicated. ' («*а в
"0
To estimate from this the size of the spawning stock it is necessary to know what
percentage this was of the spawning stock in 1932. This may be done only in an
indirect manner. The 1923 class of mackerel, after reaching spawning age, declined
at a rate of 20 percent per year as measured by the catch per purse seine boat during
the four seasons, 1928 to 1931 (Sette, 1933, p. 17). This decline was so steady that it
probably should be ascribed to mortality rather than to other causes, such as changes
in availability. Of course one cannot be sure that the spawning population in 1932
was subject to the same mortality as the 1923 class during the previous years, but