Biogeography 17. 13–20. Sep. 20, 2015 Horizontal and vertical distribution of Cottus reinii (Pisces: Cottidae) in Lake Biwa, central Japan Yasuhiro Fujioka1,2*, Akihisa Sakai1 and Atsuhiko Ide1 1 Shiga Prefecture Fisheries Experiment Station, Hassaka 2138-3, Hikone, Shiga, Japan 2 Lake Biwa Museum, Oroshimo 1091, Kusatsu, Shiga, 525-0001 Japan Abstract. The distribution of Utsusemi-kazika Cottus reinii in Lake Biwa was investigated by means of small trawl-net, commercial trawl-nets, and commercial set-nets. C. reinii were abundant in the coastal bot- tom waters deeper than 20 m between June and August and deeper than 30 m in October. These were mostly small fish of less than 4 cm SL, showing a mode in the 1-1.99 cm class in June. Fish of more than 4 cm SL also were caught in coastal waters (10-20 m in depth) and offshore deep waters (deeper than 40 m) throughout the year. These results suggest that this species leaves the plankton to begin its benthic life at a size of about 1 cm SL at depths of 20-30 m. After growing to 3-4 cm SL by October at depths of 30-40 m, they expand their range to bottom waters deeper than 40 m. The bottom sediment of the lake habitat is mainly fine mud, very different from the reported of gravel-bottom habitat of this fish in rivers. Key words: Cottus reinii, Utsusemi-kazika, Distribution, Lake Biwa Introduction is chiefly distributed in shallow coastal waters with stony bottoms and occasionally in muddy deep wa- The genus Cottus has a circumpolar distribution ters, and it spawns in rivers (Nakamura, 1975; Goto, in the northern hemisphere and includes more than 1995). Recently, this fish has been shown to inhabit 60 species of sculpin confined to freshwater (Berra, in the lower reaches of rivers flowing into the North 2001; Nelson, 2006; Yokoyama & Goto, 2011). Its Basin of Lake Biwa throughout the year (Fujioka et life cycle is commonly fluvial, but some species al., 2014). The distribution of C. pollux has never are amphidromous (Goto, 1994). In the freshwater been overlap with that of C. reinii. Its occurrence in drainages of the Japanese Archipelago, six species Lake Chuzenji in Tochigi Prefecture is due to unin- of Cottus are found (Nakabo, 2013). Among these, tentional introduction from Lake Biwa (Yokoyama C. pollux mainly inhabits upper reaches of rivers and & Yamamoto, 2012). C. reinii has an amphidromous lays comparatively few large eggs, while C. reinii life style, and its larvae are known to descend to the lives in the middle and lower reaches of rivers and lake and become bottom dwellers (Goto, 1995), but spawns many small eggs. The latter species compris- there has been no detailed study of its distribution es two types (small- and middle-egg types) (Nakabo, in the lake. Such knowledge is important in order to 2013). The small-egg type of C. reinii also lives in elucidate the life-history segregation of this species Lake Biwa and is known there as Utsusemi-kazika; and the evolution of all the freshwater Cottus of its spawned eggs are smallest among Japanese fresh- Japan. The present study was an effort to clarify the water Cottus (Kurawaka, 1976, 1992; Goto, 2001). It distribution of C. reinii in Lake Biwa throughout the ——————————————————————— year. *Corresponding author: [email protected] − 13 − Distribution of Cottus reinii in Lake Biwa Materials and Methods Chinai from February, 1997, to November, 1998, once per month (Fig. 1, Table 2). The nets were set In Lake Biwa, Cottus reinii is already known at 10-20 m depth perpendicular to the shore and the to inhabit deep offshore bottoms during winter captured fish were collected in the morning once (Mizutani et al., 1993). The first part of the present each day. One day’s take thus comprised the monthly study was therefore conducted from early summer to sample. autumn, in daytime. The fishing gear employed was To better comprehend the offshore distribution, a small-sized bottom-trawl net (height 1.4 m, width C. reinii were collected with a commercial bottom 1.3 m, depth 3.8 m with 2 mm mesh openings in the trawl-net (so-called “chyubiki ami”, mesh opening bottom net) with a pair of two-ropes 100 m or 300 m 2 mm) or cage-traps (so-called “ebi-tatsube”, mesh long (for details see Sakai et al., 2002). This net was opening of the bottom net 2 mm) from October, towed on the bottom at the several depths between 1996, to January, 1999, in one day per month except 2 and 40 m in shore and offshore of Hikone (Fig. 1, for July, August, September, and November, 1997, Table 1) for about 250 m during on the daytime of and May, and June, 1998. The chyubiki-ami was 12 June, 1 July, 1 August, and 24 and 25 October, used in the morning. 1996. At the same time, water temperature at several The fish were identified as C. reinii by their ex- depths was measured. ternal appearance and number of pectoral fin rays, In order to investigate the lake shore more fully, following Nakabo (1993). The specimens were fish were caught by a commercial set-net (so-called preserved in 10% formalin, and their standard “Eri”, mesh opening of the bottom net 4.3 mm ) length measured (except for some taken small-size at Imazu from March, 1996, to June, 1997, and at bottom-trawl net surveys). The fish collected by commercial set-net and trawl-net were sexed by ex- amination of gonads. The spawning season of C. reinii in Lake Biwa has been reported as March to April (Nakamura 1963) or from late February to early May (Goto 1995), so we defined “the spawning season” as February to April, “the post-spawning season” as May to July, “the non-spawning season” as August to October, and “the pre-spawning season” as November to January, and Table 1. Seasonal differences in numbers of Cottus reinii caught by small trawl-net at different depths in Lake Biwa Depth (m) Date 2 5 10 20 30 40 12 June, 1996 0 0 1 94 25 - 1 July, 1996 0 0 2 600 197 394 Fig. 1. Map showing sampling sites. 1 and 2 are the set-nets 1 August, 1996 0 - 0 123 60 71* in Chinai and Imazu, respectively. 3-8 indicate the approximate area of commercial bottom trawlng (the 24 and 25 October, 1996 - - 0 0 14 24 numbers correspond to “Location No.” in Table 3). 9 is the * This sample was taken on 25 July, 1996. site of use of the small bottom trawl-net. − 14 − Yasuhiro Fujioka, Akihisa Sakai and Atsuhiko Ide Table 2. Seasonal differences in numbers and standard length of Cottus reinii caught by set-nets in Lake Biwa. Number of fish taken at Number of fish taken at Year Date 1 1 Size range (cm) set net 1* set net 2* 3 Mar. - 9 4.92-7.22 12 Apr. - 11 4.64-9.13 1996 3 May - 6 4.98-6.75 11 Jun. - 7 5.08-7.78 2 Jul. - 5 5.18-7.14 24 Feb. 25 3 3.61-8.96 (3.88-7.16)*3 4 Mar. 13 4 4.23-12.48 (5.22-11.85)*3 1 Apr. 14 24 3.84-8.87 (4.38-9.20)*3 1997 10 May (5 May)*2 20 3 4.63-9.05 (7.19-10.25)*3 16 Jun. - 11 5.91-9.52 27 Nov. 88 - 3.42-9.89 15 Dec. 66 - 3.06-10.92 7 Feb. 41 - 4.78-10.43 19 Mar. 43 - 4.69-9.72 27 Apr. 29 - 4.30-8.91 25 May 6 - 4.88-9.18 1998 24 Jun. 33 - 2.29-7.89 18 Jul. 20 - 3.05-8.53 2 Aug. 23 - 3.27-8.53 21 Nov. 53 - 3.36-9.47 *1 The locations of set-nets 1 and 2 are shown in Fig. 1. *2 The date in parentheses pertains to set-net 2. *3 The data in parentheses pertain to set-net 2. compared the sex ratios each season. Statistical anal- most fish (more than 91%) were smaller than 3 cm yses were conducted using the Chi-square test. SL at depth of 20-30 m in June-August surveys. The The samples used in the study have been deposit- mode of the body size distributions changed from ed at the Lake Biwa Museum (LBM), Kusatsu, Shi- 1-1.99 cm SL in June to 2-2.99 cm SL in July and ga, Japan (No.1210055406-1210055429). August, then to 3-3.99 cm SL in October at depths of 30 and 40 m. Water temperature distributions are Results shown in Fig. 3. Distinct thermoclines were detected at depths of less than 20 m in June-August, with the In shore and offshore surveys from early summer to water temperature colder than 12℃ at depths below autumn 20 m. The thermocline dropped to 18-26 m depth by Cottus reinii were caught deeper than 30 m from October, when water temperatures under 12℃ oc- June to October, not being taken at all at 5 m in June curred deeper than 24 m. and July, 2 and 10 m in August, and 10 and 20 m in October (Table 1). Fish were scarce at 10 m depth in Year-round in shore and offshore surveys by set-net June and July, while a large number of fish (>50) was and bottom trawl-net caught at 20 m in June-August and at 30 and 40 m in At the lake shore, C.