Fishery bulletin of the Fish and Wildlife Service - NOAA

BIOLOGY OF THE ATLANTIC MACKEREL 173
It is improbable that the daytime descent was beyond the 20 meter level at this
station or was ever beyond the thermocline. During 1930, 1931, and 1932, when
the nets were hauled obliquely below as well аз above the thermocline, the lower tows
seldom caught larvae that could not be accounted for as contaminants resulting from
passage through the upper layers.
From the length-distribution of the larvae it appears (table 4) that the larger
individuals (6 to 9 mm.) were more stongly inclined to migrate, reaching the surface
at night, while the smaller ones (4 to 5 mm.) tended to stay in the intermediate
5- to 10-meter levels.
Though these observations do not provide a precise description of vertical distribution
and migration, they do demonstrate the necessity of sampling all levels
down to the thermocline to get the representative statistics needed for the studies
on growth and mortality to follow.
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20 LARVAE
EVENING MIDNIGHT DAWN NOON в в 10 12 14 (б 18
ТЕКРЁНД-ЧЯЕ. *С.
FIGUBE £.—Vertical distribution of mackerel larvae at several points of time in the dirunal cycle In relation to temperature. Tlie
solid lines connect observational points. Tbo broken lines Indicate the probable vertical position of the bulk of the population
of larvae.
GROWTH
Very little has been published on the growth of marine fishes during that early
period of the life history spent in the plankton community, and nothing on the
growth of the mackerel during this stage. Of the data collected during the present
investigation, only those of 1932 were collected m a manner sufficiently quantitative
and at short enough intervals of time to be used in deducing growth rate.
The method of analysis consists, essentially, in following the advance in position
of the mode of homologous groups of larvae by comparing sizes collected in successive
cruises. But this cannot be done in a simple and direct manner. Mackerel eggs
are spawned over a period of several months. The larvae are subject to high mortality.
As a result, almost always there are vastly more small larvae than large ones,
and the predominance of small larvae is so great during most of the season that the
groups of larger ones do not form distinct moçles. Instead, in ordinary arithmetic
frequency distributions they are apparent principally as a lengthening of the "tail"
of the distribution at its right-hand side (table 5).