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Issue Aquat. Living Resour.
Volume 19, Number 1, January-March 2006
Page(s) 59 - 68
DOI 10.1051/alr:2006005
Published online 01 April 2006

Aquat. Living Resour. 19 (2006) 59-68
DOI: 10.1051/alr:2006005

Environmental effect on diet, fecundity and condition of an endangered fish Neosalanx reganius (Osmeriformes) in the Chikugo Estuary, in the upper Ariake Bay, Japan

Md. Shahidul Islam1, Manabu Hibino2, Taro Ohta3, Kouji Nakayama1 and Masaru Tanaka1

1  Laboratory of Estuarine Ecology, Field Science Education and Research Center, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan
2  Aichi Fisheries Promotion Fund Department of Sea-Farming, 1-3 Ichizanmatu Nakayama Atsumi-cho Astumi-gun, Aichi 441-3615, Japan
3  Tottori Prefectural Fisheries Research Center, Ishiwaki, Tomari, Tohaku-gun, Tottori 689-0602, Japan

(Received 29 August 2005; Accepted 21 December 2005 / Published online: 1 April 2006)

Abstract
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 $583 \pm 173$) oocytes individual-1 and relative fecundities ranged 6.8-15.6 (mean $10.1 \pm 2.4$) oocytes mm-1 TL and 0.8-2.5 (mean $1.6 \pm 0.5$) 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 $43.5 \pm 7.9$%) 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.


Résumé
Neosalanx reganius est un poisson Salangidé peu étudié et dont l'aire de répartition est limitée aux fleuves Chikugo et Midori se déversant dans la baie Ariake, Kyushu, Japan. Des échantillons de N. reganius ont été collectés dans les zones à eaux saumâtres du fleuve Chikugo lors de 8 campagnes de 1998 à 2004 afin de caractériser la répartition, l'écologie alimentaire, en relation avec les proies disponibles du milieu, la fécondité et l'indice de condition du poisson. Un total de 244 individus, de 36 à 71 mm (longueur totale) et de 111 à 1301 mg, ont été collectés. Les captures par unité d'effort (CPUE; nombre de poissons collectés par trait de 20 min) sont correlées positivement avec la turbidité et négativement avec la salinité. N. reganius est planctonivore, il se nourrit de copépode calanoïdes Sinocalanus sinensis, proie dominante (97 % des proies) à toutes les stations et durant toutes les campagnes ; les autres proies sont d'autres calanoïdes et cyclopoïdes, Daphnia sp. et mysidacés. S. sinensis est aussi le copépode le plus abondant dans le milieu. Les CPUE montrent un corrélation significative avec la biomasse sèche de copépode qui augmente vers l'amont ( r = 0,90; p < 0,05). La fécondité est de 347 à 995 ovocytes (moy. $583 \pm 173$) par individu et la fécondité relative est de 6,8 à 15,6 (moy. $10{,}1 \pm 2{,}4$) ovocytes mm-1 TL et 0,8-2,5 (moy. $1{,}6 \pm 0{,}5$) ovocytes mg-1 de poids corporel (poids sans les gonades). La fécondité montre une relation significative avec la taille et le poids, de même pour l'indice gonado-somatique (de 29,4 à 58,8 % - moy. $43{,}5 \pm 7{,}9$ %). Les individus prêts à pondre ont un coefficient d'allométrie (b) et un facteur de condition (K) plus élevés que les autres individus. La zone amont oligohaline de l'estuaire du Chikugo présente des aires importantes de ponte et d'alimentation pour ce poisson, avec un maximum de turbidé qui semble avantageux à sa reproduction et à son alimentation. Nous suggérons que de futures recherches mettent en relief la reproduction et le cycle de vie de N. reganius afin de formuler une action de conservation effective.


Key words: Feeding habits / Fecundity / Condition factor / Estuarine turbidity / Salangidae / Neosalanx reganius

Corresponding author: msi@kais.kyoto-u.ac.jp

© EDP Sciences, IFREMER, IRD 2006


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