Fishery bulletin of the Fish and Wildlife Service - NOAA

152 FISHERY BULLETIN JOF THE FISH AND WILDLIFE SERVICE
Encouraged by this success in waters south of the previously known distribution
of larvae, search was in 1927 extended south of Cape Cod. Here eggs were found in
abundance from the offing of Cape Cod nearly to the mouth of Chesapeake Bay. As
in Massachusetts Bay, larvae were present in abundance also.
To determine whether this was the usual condition, the survey was repeated in a
single cruise during May of 1928, when approximately the same conditions were
found.
These three seasons of prospecting for mackerel eggs and larvae completely altered
the previous notion that spawning was more successful in the northwest portions
of the range of the species. Not only were specimens regularly obtained from Massachusetts
Bay to Chesapeake Bay, but the numbers of individuals per tow were greatly
in excess of those taken by similar methods in the Gulf of St. Lawrence during the
Canadian Fisheries expedition of 1914-15. Evidently this southerly region was far
more important than previously supposed, and hence a suitable one in which to study
variations in the survival rate during early stages.
However, it was still necessary to determine the length of the spawning season
and the duration of the period of larval development. For this purpose, successive
cruises were made during the spring and early summer months of 1929. These proved
that in the area between Cape Cod and Cape Hatteras spawning began in early April,
and larval development had nearly run its course by the end of July.
In 1930 and 1931, such successive cruises during the spawning season were repeated
and every opportunity was taken to devise methods of estimating the abundance
of the various young stages.
This development of quantitative technique required the determination of vertical
distribution so that the proper levels would be fished; determination of the incubation
and growth rates so that cruises might be planned at proper intervals to include
all the important events; and finally, it required devising a reliable method of measuring
the amount of water strained by the tow nets so that hauls would be comparable
from time to time and place to place. By 1932 knowledge and techniques
were advanced sufficiently for the survey of that season to provide adequately quantitative
data for the more important sections of this report dealing with growth,
drift, and mortality. Toward the close of this season, the Albatross II was withdrawn
from service as a Government economy measure. This prevented continuing the
research into its next phase, that is, the measurement of mortality and its accompanying
hydrobiological conditions through a series of seasons, to see how mortality is
affected by particular conditions in seasons of good survival contrasted with other
conditions in seasons of poor survival. Since the hoped-for resumption of surveys
has not yet been possible, the present available results arc now reported.
SYNOPSIS OF RESULTS
Most mackerel roach reproductive maturity when 2 years old. Some precocious
individuals, usually males, first spawn a season earlier and others of both sexes a year
later. Tho percentage of the latter is higher among the females than the males.
Mackerel are said to spawn 360,000 to 450,000 eggs in a season, but this is a point
needing further study. Doubtless smaller individuals spawn fewer and larger individuals
more eggs than this. The eggs are ripened in successive batches; it is not
known how many batches or what interval of time intervenes between their discharge.
Spawning takes place over nearly the entire spring and summer range of the species,
from off Chesapeake Bay to Newfoundland. By far the most important ground is