Neosalanx reganius is a poorly studied salangid fish restricted
to the upper reaches of the Chikugo and the Midori River flowing into the Ariake Bay,
Kyushu, Japan. Samples of N. reganius were collected from brackish water areas
of the Chikugo River by eight cruises in 1998-2004 to characterize the distribution
pattern, feeding ecology in relation to ambient prey concentrations, fecundity and
condition of the fish. A total of 244 specimens were collected, 36 to 71 mm total
length, and 111 to 1301 mg body weight. The catch per unit of effort (CPUE; number of fish
collected by towing a larva net for 20 min) correlated positively with turbidity and
negatively with salinity. N. reganius is a planktivorous fish, fed on a single
calanoid copepod species Sinocalanus sinensis, which was the single most dominant
prey item in all stations during all cruises, contributing as high as 97.0% of the total
diet of the fish; the other prey items (other calanoids and cyclopoids, Daphnia
sp. and decapod mysid) together contributed only 3%. S. sinensis also dominated
in the environmental copepod composition. The CPUE showed significant correlation with copepod
dry biomass which increased upstream (r = 0.90; p < 0.05). Fecundity ranged 347-995
(mean 583 173) oocytes individual−1 and relative fecundities ranged 6.8-15.6
(mean 10.1 ± 2.4) oocytes mm−1 TL and 0.8-2.5 (mean 1.6 ± 0.5)
oocytes mg−1 of net body weight (weight taken after gonad extraction). Fecundity showed significant positive relationship with fish length and body weight. GSI ranged 29.4-58.8%
(mean 43.5 ± 7.9%) and had significant relationship with fish length and
body weight. Spawning individuals had higher allometry coefficient (b) and
condition factor (K) than the non-spawning individuals. The oligohaline upper
Chikugo estuary provides important feeding and spawning grounds for the fish
with sufficient prey abundance and turbidity maximum that seemed
advantageous for feeding and spawning of N. reganius in the Chikugo estuary. We suggest
that future research should emphasize on the spawning and early life ecology
of the fish in order to formulate effective conservation action.
