Are tropical seagrass beds and mangrove areas important as nursery sites for juvenile fishes?

2006 Fiscal Year Final Research Report Summary

• Project/Area Number: 16380127
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: General fisheries
• Research Institution: The University of Tokyo
• Principal Investigator: SANO Mitsuhiko The University of Tokyo, Graduate School of Agricultural and Life Sciences, Associate Professor, 大学院農学生命科学研究科, 助教授 (50178810)
• Co-Investigator(Kenkyū-buntansha): KUROKURA Hisashi The University of Tokyo, Graduate School of Agricultural and Life Sciences, Professor, 大学院農学生命科学研究科, 教授 (50134507) ; SHIBUNO Takurou Seikai National Fisheries Research Institute, Ishigaki Tropical Station, Research Scientist, 西海区水産研究所石垣支所, 室長 (10372004) ; HORINOUCHI Masahiro Shimane University, Research Center for Coastal Lagoon Environments, Associate Professor, 汽水域研究センター, 助教授 (30346374)
• Project Period (FY): 2004 – 2006
• Keywords: seagrass bed / mangrove estuary / juvenile fish / nursery / stable isotope ratio / Ishigaki Island

Research Abstract

To clarify whether juveniles of some tropical coastal fishes use seagrass beds or mangrove estuaries as nurseries, visual censuses and stable isotope analysis were conducted at Ishigaki Island, Ryukyu Islands, Japan, from 2004 to 2006.
Five 20 × 1 m belt transects were established in 6 habitats (mangrove estuary, seagrass bed, sand area, coral rubble area, branching coral area on the reef flat, and tabular coral area on the outer reef slope), and all fishes within the transect area were counted. Total length of each fish also was estimated in size class of 0.5 or 1.0 cm. During the study period, a total of 319 fish species were recorded in the 6 habitats. Among these fishes, species of which juveniles (small individuals) were observed only in the seagrass bed and mangrove estuary were 4 (mainly lethrinids) and 3 species (mainly lutjanids), respectively, suggesting that these species utilize the two habitats as a nursery.
In a lutjanid species of which small individuals occur in the mangrove estuary but larger ones inhabit coral-dominated areas, we examined whether this species has an ontogenetic habitat shift using stable isotope analysis. δ^<13>C values of individuals collected in the mangrove estuary had -23 to -17‰, whereas those of individuals in the coral areas shifted from a mangrove signature to a coral reef signature (-16 to -11‰) with growth. These results demonstrated that this lutjanid species has a clear ontogenetic habitat shift from the nursery to adult habitats.