Path of the Kuroshio Current affects the presence of several goby species in the brackish water area in northeastern Kyushu Island, Japan: results of a decade-long survey in the Kita River

The Kuroshio Current transports various marine organisms, including tropical and subtropical fishes, from the southern regions to the northern Pacific coast of Japan (Senou et al. 2006; Matsuura 2012). The demersal fishes, including brackish water gobies, are transported during the pelagic larval phase (Senou et al. 2006; Maeda et al. 2007; Matsuura 2012). However, the path of the current shows various changes along the Japanese archipelago at various time periods, from days to decades (Kawabe 1995; Yamashiro and Kawabe 1996; Nagano and Kawabe 2004). Thus, it is possible that the path changes of the Kuroshio Current influence the presence/absence of fishes along the coastal areas of the archipelago. In this study, we examined the relationships between the path of the Kuroshio Current and the presence of several gobies by conducting a decadelong monitoring of the northeastern region of the Kyushu Island (Fig. 1a). Data on the presence/absence of fishes were collected from 1999 to 2011 in the brackish water area of the Kita River, Gokase River system, located at the northeastern region of the Kyushu Island and flowing into the Nobeoka Bay (Fig. 1b). Monitoring was conducted in an artificial backwater zone, ‘‘Wando-pool,’’ located approximately 4 km upstream of the river mouth (Nakajima et al. 2008). Since many gobioid species were shown to inhabit the artificial backwater zone in our previous study (Eguchi et al. 2008), we selected gobies as the target species for analysis. The survey was conducted 2–5 times each year, and annual surveys were conducted during July and November in 2002, 2004, and 2007–2011. Therefore, for this analysis, we used only the data collected from July to November. The surveys in 2000 and 2003 were excluded from this analysis because no data were collected in these months during both years. Most individuals collected during the surveys were estimated to be of the age class 0, because we used a small seine net with 1.5-mm mesh size, which is often used to capture juveniles and immature fishes. Because the sampling site had brackish water during all tides (Nakajima et al. 2008), we analyzed 24 gobioid species (see Table 1) whose adults and juveniles inhabited brackish water areas (Eguchi et al. 2008). Data on the Kuroshio axis were gathered from a public database (Japan Meteorological Agency 2011). Using analysis maps of the current axis on the 20th of every month, we measured the shortest distance from the Nobeoka Bay to the axis (Fig. 1a). Because we did not estimate the length of stay of the gobies since their arrival from the southern regions, we prepared two data sets with different periods: one covering each survey month up to a year ago (12 months) and the other covering each survey month up to half a year ago (6 months). In addition, location of the current axis indicated irregular changes (Fig. 1c). Therefore, we prepared average and minimum distances for each period. A receiver-operating characteristic (ROC) curve (Metz 1978) was generated to illustrate the relationship between the presence/absence of each goby species and the current path of the Kuroshio Current. The presence/absence of N. Onikura (&) S. Oikawa Fishery Research Laboratory, Kyushu University, 4-46-24 Tsuyazaki, Fukutsu, Fukuoka 811-3304, Japan e-mail: onikura@agr.kyushu-u.ac.jp