Accurate descriptions of reproductive
profiles are important for effective conservation
and stock assessment efforts. We examined the
reproductive traits of an Okinawan population of
western Pacific gizzard shad (Nematalosa come)
using histological analyses. The observed lengths
at 50 % maturity were approximately 180 mm
standard length (SL) for females and 130 mm SL
for males, which corresponded to 4-year-old fish
for females and at least 2-year-old fish for males.
The spawning season was estimated to occur from
January to August, peaking from March to June,
with active spawning occurring during an increase
in seawater temperature. The lipidosomatic index
of both sexes decreased until June, presumably as
the fish expended energy to spawn. The fecundity
(F) of females, tested using 62 ovaries, was related
to SL: F=3.471×10−2
×SL3.122. The Okinawan
population of N. come had a longer spawning
season and a higher fecundity than other populations
for which these data are available. Such
favorable traits may contribute to the persistence
of this population at the species’ distributional
margin. Our findings indicate that the spawning seasons
of Okinawan populations of N. come and
Nematalosa japonica overlapped, implying that hybrid
production occurs under natural conditions. Around
Okinawa Island, however, the extent of shallow waters
is rapidly decreasing because of recent land reclamation.
We suggest the diminution of suitable sites as a possible
factor for narrowing spawning opportunities and promoting
the recent appearance of natural hybrids. Shallow
habitats should be conserved to preserve healthy
populations of the species.