Ryutei Inui

Evaluating the potential for invasion by alien freshwater fishes in northern Kyushu Island, Japan, using the Fish Invasiveness Scoring Kit

The potential invasiveness of 28 freshwater fishes in northern Kyushu Island, Japan, was evaluated using the Fish Invasiveness Scoring Kit (FISK). The five co-authors scored the level of invasiveness for each species and calculated the total FISK scores; the maximum and minimum scores were then eliminated, and the mean of the remaining three scores was used as the final score for each species. The mean scores ranged from 11.0 (Hypomesus nipponensis) to 31.0 (Cyprinus carpio). The receiver operating characteristic curve indicated that the threshold value between fishes that present a high risk of invasion and the other species were 19.8.

Selection of spawning habitat by several gobiid fishes in the subtidal zone of a small temperate estuary

We examined the spawning sites of several gobiid fishes in the subtidal zone of a small temperate estuary and determined the relationship of the nest abundance of each species with several environmental conditions, including water temperature, salinity, median particle size, proportion of silt and clay, current width, maximum water depth, distance from each site to the river mouth, and density of objects (boulders, cobbles, empty oyster shells, and empty clam shells) in the water. During this study, we found 115 nests belonging to the following six species: Favonigobius gymnauchen, Tridentiger obscurus, Leucopsarion petersii, Acanthogobius lactipes, Rhinogobius giurinus, and Tridentiger trigonocephalus. Stepwise multiple regression analysis, which was used to determine the best model for the nest abundance of all species, revealed that the nest abundance of each species clearly increased with an increase in the density of spawning substrates, such as cobbles and bivalve shells. These results show that the density of spawning substrates is a crucial factor affecting the selection of spawning habitats by each species.

Habitat suitability of eight threatened gobies inhabiting tidal flats in temperate estuaries: model developments in the estuary of the Kuma River in Kyushu Island, Japan

We developed habitat suitability models for eight threatened goby species inhabiting tidal flats in the estuary of Kuma River in Kyushu Island, Japan. The best models selected several environmental factors, which were supported by area under the curve (AUC) values ranging from 0.793 to 0.976. The habitat suitability curves for each variable demonstrated various slopes, suggesting interspecific differences in habitat suitability for each environmental factor. The results suggested that the range of environmental gradients in tidal flats influences the richness of the threatened goby species.

The first record of the spawning nest of Gymnogobius cylindricus and Gymnogobius macrognathos

Gymnogobius cylindricus and Gymnogobius macrognathos inhabit estuarine waters (Onikura et al. 2014; Ministry of the Environment 2015). Gymnogobius cylindricus is distributed from Aichi Prefecture to Kumamoto Prefecture in western Japan, and G. macrognathos is distributed from Miyagi Prefecture to Miyazaki Prefecture (Ministry of the Environment 2015). The genus Gymnogobius includes 13–16 species found in shallow marine, estuarine, and freshwaters throughout Japan, the Russian Far East, Kuril Islands, Korean Peninsula, and Yellow Sea (Stevenson 2002; Akihito et al. 2013). Therefore, clarification of habitat segregation among species of Gymnogobius is important for revealing the mechanisms of evolution and adaptive radiation in these fishes. In addition, G. cylindricus and G. macrognathos are listed in the Red Data Book edited by the Ministry of the Environment (2015) as ‘‘Endangered’’ and ‘‘Vulnerable’’, respectively. Therefore, the ecological information, such as life history, is necessary to effectively conserve these threatened gobies (Inui and Koyama 2014). In particular, spawning ecology of these species must be elucidated, because the knowledge of reproduction is especially important for conservation of fishes (e.g., Geist and Dauble 1998; Rowe and Hutchings 2003; Inui et al. 2010). Previous studies have reported that several species of Gymnogobius, including G. breunigii, G. scrobiculatus, and G. uchidai, spawned in the inner surface of burrows of upogebiid mud shrimps Upogebia major and callianassid ghost shrimp Nihonotrypaea japonica (Dôtu 1954; 1957; 1961; Inui et al. 2011). It is suggested that G. cylindricus may also spawn in the burrow of U. major similar to other species of Gymnogobius (Onikura et al. 2014), but the spawning nest of this goby has not been found. According to Onikura et al. (2014) and the Ministry of the Environment (2015), G. macrognathos mainly spawns in the burrow of N. japonica, but there are no details, for example, the physical environment of the spawning site and the conditions for the egg clutch of this goby. We found the egg clutch of G. cylindricus and G. macrognathos, and collected samples of these eggs at the mouth of the Kuma River (32 280N, 130 330E) in Kumamoto Prefecture. Spawning habitat and description of eggs of Gymnogobius cylindricus. The egg clutch of G. cylindricus was found on March 8, 2016, by shoveling bottom sediment burrowed by several invertebrates (Fig. 1a, b). This egg clutch was attached to an inner surface of a burrow, and half of the wall was covered with its eggs (Fig. 1b). The eggs were laid in a single layer in the burrow, which was in accord with other species of Gymnogobius (Dôtu 1954; 1957; 1961). Although the male G. cylindricus guarding the egg clutch could not be visually observed, a male [52.84 mm in standard length (SL)], which might have been the parent was collected from under the egg clutch (Fig. 1a). In addition, Upogebia major, Upogebia yokoyai, and Nihonotrypaea japonica were found around the spawning site and are the candidates for the host. The host shrimps could be easily distinguished between upogebiid mud shrimps and callianassid ghost shrimps by the burrow structure and the inner mud lining A. Koyama and R. Inui contributed equally to this work.

Symbiotic partner specificity and dependency of two gobies (Apocryptodon punctatus and Acentrogobius sp. A) and four alpheid shrimps inhabiting the temperate estuary of southern Japan

We collected two gobies, Apocryptodon punctatus and Acentrogobius sp. A, and four alpheid shrimps from an estuary in southern Japan, to identify the symbiotic shrimps with gobies and to understand their partner specificity and dependency. Analysis based on generalized linear models identified two specific partners for each goby. A. punctatus frequently coexisted with partners and showed similar habitats, suggesting that this goby might be an obligate mutualist. Acentrogobius sp. A had lower rates of coexistence and appeared in habitats different from one of its partners, suggesting that this goby might be a facultative mutualist.

Spawning sites of Eleotris acanthopoma (Perciformes: Eleotridae) in the Kitagawa and Sumiegawa Rivers, Kyushu Island, Japan

Eleotris acanthopoma belongs to the Eleotridae family (Suborder: Gobioidei), and is distributed in regions of southern Japan, such as along the Pacific side of Honshu Island from Chiba Prefecture to the west and in the Shikoku, Kyushu and Okinawa islands (Akihito et al. 2002; Senou et al. 2004; Iwata 2005). Three ecological studies of this species have been published from the researches conducted in Okinawa Island, such as the instream distribution, feeding habits, morphology of the larvae and juveniles (Maeda and Tachihara 2004, 2005), and the recruitment of pelagic larvae into streams (Maeda et al. 2007). In the Shimanto River, located to the south of Shikoku Island, this species is known as an abortive migration fish (Iwata and Hosoya 2005). There are no studies on the reproduction of this species on Kyushu Island. We investigated the spawning sites of E. acanthopoma in the brackish waters of the Kitagawa and Sumiegawa Rivers in the eastern region of Kyushu Island, Japan (Fig. 1). The Kitagawa River is approximately 51 km in length and flows into Nobeoka Bay. The surveys were conducted at low tide of spring tide, in a brackish water inlet from the mouth of the river to approximately 6 km upstream (32 370N, 131 410E), every month from February 2006 to January 2007 and once in July 2007. Stones used for spawning were observed by snorkeling, and they were counted, photographed and measured (major and minor axes) at each station. In each stone used for spawning, the egg clutch (major and minor axes) and live male (standard length, SL) were counted, photographed and measured. On 26 August 2006, salinities and water temperatures at the bottom of the river, water depths and the materials on the riverbed were investigated at low tide. The Sumiegawa River is approximately 3.5 km in length and flows into Sunue Bay. The surveys were carried out using the same methods and conditions as that in Kitagawa River, in the brackish area from the mouth of the river up to approximately 0.8 km upstream (32 390N, 131 460E), twice in July and August 2007. On 25 July 2007, salinities and water temperatures at the bottom of the river, water depths and the materials on the riverbed were investigated at low tide. One egg clutch was reared in a bucket immediately after the collection at 10:00 a.m. on 25 July 2007, and a part of the eggs and newly hatched larvae were photographed under a microscope and fixed in 5% formalin solution at 7:00 p.m. (FRLK070561, obtained from the collections in the Fishery Research Laboratory, Kyushu University). Spawning sites and periods of E. acanthopoma. In the Kitagawa River, the spawning sites of E. acanthopoma were found on 26 August 2006, and 26 July 2007. On 26 August 2006, the water depth at the site was 40 cm at low tide, the salinity of the riverbed at low tide was 0.8 psu, and several cobbles (10–22 cm) were present on the sand and gravel bed (Fig. 1a). In 2006, the surveys were conducted every month for a year from February to January of the subsequent year; however, the spawning site was only found in July 2006. This species spawns from May to December on Okinawa Island (Maeda et al. 2007), and the spawning periods of this species was shorter on Kyushu Island than on Okinawa Island. In the Sumiegawa River, the spawning site of E. acanthopoma was located on 25 July 2007. On this day, the water depth at the spawning site was 40 cm at low tide, the salinity of the riverbed at low tide was 24.1 psu, and many N. Onikura (&) J. Nakajima T. Nishida R. Inui K. Eguchi M. Nakatani S. Oikawa Fishery Research Laboratory, Kyushu University, 2506 Tsuyazaki, Fukutsu, Fukuoka 811-3304, Japan e-mail: onikura@agr.kyushu-u.ac.jp

Field survey and resin casting of Gymnogobius macrognathos spawning nests in the Tatara River, Fukuoka Prefecture, Japan

We investigated the spawning nests of Gymnogobius macrognathos on a tidal flat in the Tatara River, Fukuoka Prefecture, Japan. Digging uncovered 19 spawning nests. The number of eggs and the standard length of the guarding male were positively correlated. Nine spawning nests were examined using in situ resin casting. All casts had structures characteristic of callianassid shrimp burrows and were most likely those of Nihonotrypaea japonica. Spawning nests had significantly greater average diameters than shrimp burrow openings and may have been widened by G. macrognathos.

Abiotic and biotic factors influence the habitat use of four species of Gymnogobius (Gobiidae) in riverine estuaries in the Seto Inland Sea

Gobies that are phylogenetically related or coexist in the same marine and estuarine systems often exhibit abiotic and/or biotic habitat segregation. Thus, it is possible that species of Gymnogobius inhabiting the same riverine estuaries also exhibit abiotic and/or biotic habitat segregation. The goal of this study was to determine the differences in abiotic and biotic habitat use between these species by sampling goby and host shrimps, and by examining the physical environments of the rivers where these species are found. The surveys of goby and host shrimps were conducted in the estuaries of the Saba and Ibo rivers, which drain into the Seto Inland Sea, a body of water that separates three of the four main islands of Japan. We used hand nets and shovels to collect goby and host shrimps, and measured median sediment particle size, elevation, and salinity at each site. Generalized linear models (GLMs) were used to assess the preferences in abiotic and biotic habitat use by the goby species. Median particle size, salinity, and elevation were used as the abiotic environmental predictors, whereas the presence/absence of host shrimps were re-organized into four categories consisting of “Upogebia major” only, “Nihonotrypaea japonica” only, “Upogebia major & Nihonotrypaea japonica,” and “Upogebia yokoyai,” which were used as the biotic environmental predictors. The GLMs demonstrated that median particle size had the largest influence of the abiotic variables, with goby species segregating according to differences in sediments; moreover, there was some evidence suggesting that the host and symbiont do not always correlate at the species level. Our results indicated that although there is some overlap in abiotic and biotic habitat use among the four species of Gymnogobius, the differences were broad enough to provide an explanatory mechanism as to how these species can coexist in the same river systems.

Removal of water-surface reflection effects with a temporal minimum filter for UAV-based shallow-water photogrammetry: Removal of Water-Surface Reflection Effects for UAV-Photogrammetry

The recent development of structure-from-motion (SfM) and multi-view stereo (MVS) photogrammetry techniques has enabled semi-automatic high-resolution bathymetry using aerial images taken by consumer-grade digital cameras mounted on unmanned aerial vehicles (UAVs). However, the applicability of these techniques is sometimes limited by sun and sky reflections at the water surface, which render the point-cloud density and accuracy insufficient. In this research, we present a new imaging technique to suppress the effect of these water-surface reflections. In this technique, we order a drone to take a short video instead of a still picture at each waypoint. We then apply a temporal minimum filter to the video. This filter extracts the smallest RGB values in all the video frames for each pixel, and composes an image with greatly reduced reflection effects. To assess the performance of this technique, we applied it at three small shallow-water sites. Specifically, we evaluated the effect of the technique on the point cloud density and the accuracy and precision of the photogrammetry. The results showed that the proposed technique achieved a far denser point cloud than the case in which a randomly chosen frame was used for each waypoint, and also showed better overall accuracy and precision in estimating water-bottom elevation. The effectiveness of this new technique should depend on the surface wave state and sky radiance distribution, and this dependence, as well as the applicability to large areas, should be investigated in future research. Copyright