Hiroyoshi Kohno

Food habits of fishes in the mangrove estuary of Urauchi River, Iriomote Island, southern Japan

The food habits of 67 fish species collected from the mangrove estuary of the Urauchi River, Iriomote Island, southern Japan were investigated using gut content analysis. Ontogenetic changes in food preference were recognized in nine species, including mugilids, gerreids, mullids, gobiids and tetraodontids. In most cases, juveniles of these species fed mostly on small crustaceans (e.g. calanoid and cyclopoid copepods and gammaridean amphipods) or detritus. With their subsequent growth, larger prey items (e.g. crabs and polychaetes) became dominant. A cluster analysis based on dietary overlaps showed that the mangrove fish assemblage comprised eight trophic groups (zooplankton, small benthic crustacean, large benthic crustacean, polychaete, fish, detritus, plant and insect feeders). Of these, large and small benthic crustacean feeders, which consumed mainly crabs and gammaridean amphipods, respectively, were the most abundantly represented in terms of species, whereas polychaete and insect feeders were each represented by only two species.

Social Interactions of Juvenile Brown Boobies at Sea as Observed with Animal-Borne Video Cameras

While social interactions play a crucial role on the development of young individuals, those of highly mobile juvenile birds in inaccessible environments are difficult to observe. In this study, we deployed miniaturised video recorders on juvenile brown boobies Sula leucogaster, which had been hand-fed beginning a few days after hatching, to examine how social interactions between tagged juveniles and other birds affected their flight and foraging behaviour. Juveniles flew longer with congeners, especially with adult birds, than solitarily. In addition, approximately 40% of foraging occurred close to aggregations of congeners and other species. Young seabirds voluntarily followed other birds, which may directly enhance their foraging success and improve foraging and flying skills during their developmental stage, or both.

Plunging Behaviour in Chick-rearing Brown Boobies

ABSTRACT Brown Boobies (Sula leucogaster) obtain prey by plunge dives. In order to investigate the foraging behaviour of Brown Boobies underwater, we attached acceleration data loggers to chick-rearing Brown Boobies at Nakanokamishima Island in Japan. We documented, for the first time, Brown Boobies performing many rapid and shallow V-shaped dives and some W-shaped dives during the daylight period. The average and maximum dive depth and duration were 1.03 m and 3.81 m and 1.83 s and 21 s, respectively. Furthermore, Brown Boobies used buoyancy to ascend to the water surface. Our data suggest that Brown Boobies mainly depend on shallow-plunging, contrary to the pursuit divers such as gannets, due to their ecological and/or physiological constraints.

Relationship between coral distributions and physical variables in Amitori Bay, Iriomote Island, Japan

The relationship between coral distributions and physical variables was investigated in Amitori Bay, Iriomote Island, Japan. Field observations were conducted to obtain data on coral distributions, sea temperature, sea salinity, wind speed, and river flow rate. The observed data were then used in ocean and wave model numerical simulations and soil particle tracking analysis to obtain the spatial and temporal distributions of wave height and the numbers of soil particles. The main results of this study indicate that wave height and the number of soil particles have a significant correlation with coral distribution. Higher wave heights result in greater coverage of tabular coral and lower coverage of branching coral. A greater number of soil particles relates to lesser coverage of tabular coral. On the contrary, the number of soil particles is not correlated with branching coral coverage. The potential habitats for tabular corals largely depend on the species. Acropora hyacinthus is distributed in conditions with low numbers of soil particles, A. digitifera is distributed in conditions with low numbers of soil particles and high wave heights, and A. millepora is distributed in conditions with high numbers of soil particles and low wave heights. Averages of diversity index of the coral types at the mouth and inner parts of the bay are lower than average of the whole region, but average of diversity index at the intermediate part of the bay with the intermediate physical disturbances is higher than it, which seems to support the intermediate disturbance hypothesis.

Effect of Wind on the Flight of Brown Booby Fledglings

Abstract There is increasing evidence showing that wind velocity affects the flight and foraging behavior of seabirds; however, few studies have examined these effects on seabirds inhabiting tropical oceans where lighter wind conditions usually prevail. The Brown Booby Sula leucogaster is an example of a tropical seabird with relatively low wing loading; strong wind conditions may be expected to impede the stability of their flight. We examined how different wind conditions affected the duration and flying behavior of Brown Booby fledglings during foraging trips by means of direct observation of nest attendance and by attaching video loggers to birds. The duration of foraging trips by fledglings decreased with increasing wind speed, and during flight, the body rotation of fledglings became greater with increasing wind speed. As expected, fledglings were buffeted by strong winds due to their relative inexperience in flight combined with their low wing loading. Fledglings were probably forced to flap against strong winds in order to adjust the stability of their bodies, offsetting the efficient use of wind for gliding. Furthermore, the height at which fledglings flew fluctuated more at higher wind velocities, which may have constrained their detection and capture of prey. In conclusion, our results indicate that the aerodynamic performance of Brown Booby fledglings is impaired by strong wind conditions, leading to poor flight stability and potentially reduced prey detection.

Breeding Phenology and Chick Growth in the Brown Booby Sula leucogaster (Sulidae) on Nakanokamishima, Japan

Abstract We studied the breeding phenology of the Brown Booby Sula leucogaster on Nakanokamishima, southern Ryukyu Islands, Japan. During the breeding season (April–November) of 1989 we observed the presence and absence of 18 chicks and measured their growth on a total of 17 days. Egg hatching was observed during April and May. The estimated minimum pre-fledging period ranged from 60 to 100 days and the maximum pre-fledging period ranged from 86 to 122 days, whereas the minimum post-fledging dependence period ranged from 24 to 105 days and the maximum post-fledging dependence lasted from 74 to 124 days. All of the monitored fledglings left the colony by early-November, and the total period over which parents cared for their chicks was 119–197 days (minimum) and 146–208 days (maximum). Body weight and culmen length attained 95% of asymptotic values in 65 and 69 days of age and wing length in 91 days. Six out of 18 fledglings were seen again in the colony, three to seven years after initial independence.

Ontogenetic and seasonal changes in the diet of the halfbeak Zenarchopterus dunckeri at Iriomote Island, southern Japan

Ontogenetic and seasonal changes in the diet of the halfbeak Zenarchopterus dunckeri were examined in the Urauchi River estuary, Iriomote Island, southern Japan, using gut content analysis. The diet of this species changed dramatically with growth. Smaller juveniles fed mostly on small zooplankton and small gastropods, with a shift to the consumption of terrestrial insects, such as ants and flies, by larger individuals. Seasonal dietary differences were also apparent in adults. Ants were the most dominant prey item in the spring to fall months, whereas flies were abundant in the diet in the winter and early summer months.

Microhabitat differences in crab assemblage structures in a subtropical mangrove estuary on Iriomote Island, southern Japan

Crab assemblage structures in sandy flat, muddy flat and mangrove forest microhabitats in a mangrove estuary of the Urauchi River, Iriomote Island, southern Japan, differed clearly among the three microhabitats, species and individual numbers being greater inside the mangrove forest than on the flats. A similarity index showed distinct differences in species composition among the microhabitat assemblages, primarily due to the differential distributions of dominant species, such as the soldier crab Mictyris guinotae, sentinel crab Macrophthalmus convexus and sesarmid crab Perisesarma bidens. A canonical correspondence analysis indicated that the differences likely arose from among-microhabitat differences in the physical environment, the mangrove forest being characterized mainly by structurally complex mangrove roots, lower soil porewater temperatures and higher relative substrate elevation, the sandy flat by lower organic content of the sediment and higher porewater oxygen amount, and the muddy flat by higher levels of sediment water and organic material. The findings suggested that microhabitat-related physical differences are important factors determining crab distribution patterns in mangrove estuaries.

Thermodynamics of Coral Diversity — Diversity Index of CoralDistributions in Amitori Bay, Iriomote Island, Japan and Intermediate Disturbance Hypothesis

The relationship between coral distributions and physical variables was investigated in Amitori Bay, Iriomote Island, Japan. Amitori Bay is located in the northeast region of Irio‐ mote Island, Japan. Broad areas of coral have developed in the bay, and their life forms, coverages, sizes, and species vary depending on their locations. In addition, Amitori Bay has no access roads, and the bay perimeter is uninhabited. Thus, this small bay, with its variety of environments and lack of human impact, is considered to be one of the most suitable areas for studying the relationship between coral distribution and physical varia‐ bles. Field observations were conducted to obtain data on coral distributions, sea temperature, sea salinity, wind speed, and river flow rate. The observed data were then used in ocean and wave model numerical simulations and soil particle tracking analysis to obtain the spatial and temporal distributions of wave height and the numbers of soil particles. Our results showed that the life forms, sizes, and species of corals significantly varied de‐ pending on their locations in the bay, because the physical variables differed significantly among these locations. From the results of the above observations and simulations, we calculated diversity index of coral distributions and its relation to physical variables. The diversity index, DI is de‐ fined as DI=-Σ ci log2 ci, where ci is the ratio of i-th group coverage to total coverage. DI is a quantitative measure for the degree in which a dataset includes different types and is related closely to entropy concept in Thermodynamics. The value of DI increases when both the number of types and the evenness increase. For a given number of types, the val‐ ue of DI is maximized when all types are equally abundant. The results show that Aver‐ ages of diversity index of the coral types at the mouth and inner parts of the bay are lower than average of the whole region, but average of diversity index at the intermedi‐

Influence of Wave and Sediment Transport on Coral Distribution in Amitori Bay, Iriomote Isrand

気象および河川の状況を実測し,これをもとに懸濁物の 挙動を再現した結果も,サンゴの分布やサイズ組成と比 較した.これらの結果より,従来から言われているよう に高波浪かつ懸濁物が少ない場所は様々なサイズのサン ゴが高被度で分布していることを確認した. 本研究はこれらの研究に引き続き図-1に示す沖縄県西 表島網取湾においてサンゴ分布の調査および各種物理条 件の調査並びに再現計算を行った.特に土粒子の河川から の湾内への流入パターンを数値計算により再現し,波高 分布とあわせて,サンゴ群集の分布を比較検討した. なお,網取湾には東海大学沖縄地域研究センター網取 施設があり,風向・風速や水温の観測が行われている。 サンゴは同じような形態を持っていても種によって環 境に対する耐性が異なると考えられる.さらに同じように 見える群集形態でも異なるいくつかの種が混在している 場合がある.そこで本研究では,形態による類型のみな らず,種同定が可能であった卓状サンゴ群集については, 種を識別して,種別の分布条件を評価した. 西表島網取湾のサンゴ種別分布に及ぼす波浪および土砂輸送の影響 Influence of Wave and Sediment Transport on Coral Distribution in Amitori Bay , Iriomote Isrand