Age Class Structure of Spawning Populations of Two Endangered Endemic Striated Spined Loaches in Lake Biwa River System

Jpn. J. Environ. Entomol. Zool. 29（4）： 135 － 141 （2018）環動昆第 29 巻第 4 号：135 － 141 （2018） Original Article Age class structure of spawning populations of two endangered endemic striated spined loaches in Lake Biwa river system Kiyohito Morii1)* and Koh-Ichi Takakura2) 1) Graduate School of Environmental Science, The University of Shiga Prefecture, 2500, Hassaka-cho, Hikone-City, Shiga 522-8533, Japan 2) Environmental Science, The University of Shiga Prefecture, 2500, Hassaka-cho, Hikone-City, Shiga 522-8533, Japan （Received: March 28 , 2018；Accepted: October 20, 2018） Abstract Population dynamics models with age structure have often been applied to many organism species. Knowledge related to the age class structure, which is fundamentally important for such models, is expected to contribute to the effective conservation of endangered species. Two endemic spined loaches in the Lake Biwa river system, Cobitis magnostriata and C. minamorii oumiensis, have narrowed their distributions. Today, they are regarded as endangered species. For this study, to estimate their age structures, we surveyed body lengths of fish of the two species at a spawning site in Takashima, Shiga Prefecture for three years (2015–2017). A series of surveys captured 601 C. magnostriata individuals (488 male, 113 female) and 148 C. minamorii oumiensis individuals (60 male, 88 female). We applied models with different numbers of cohorts, k (1–7) for each sex and species and selected the best-fitted model among them. Models with k = 4 and k = 2 were selected respectively for C. magnostriata males and females. Models with k = 3 and k = 6 were selected respectively for C. minamorii oumiensis males and females. Considering the largest number of age classes in females of C. minamorii oumiensis and some studies related to this and related species, it is probable that females of this species have often failed to spawn and have repeatedly re-run to the spawning site, at least during these several years. Keywords: age class structure, Cobitis, conservation, population ecology, reproduction, spawning Introduction an age class structure is expected to contribute greatly to the efficient conservation of endangered species, even if it is rather Age class structure is an important determinant of incomplete. population dynamics, as emphasized in some prominent The Cobitis striata species complex (Cypriniformes, mathematical models such as the Leslie model (Leslie, 1945). Cobitidae) is a group of freshwater fishes in Japan with five Population dynamics models considering the age class species and 11 subspecies (Nakajima, 2017). The taxonomy in structure, such as the Leslie matrix model and Lefkovitch this species complex has been confused, although some matrix model, have often been used for the resource estimation regional races were reported (Minamori, 1955; Saitoh and of commercially important species in fisheries and for Aizawa, 1987) before the current taxonomy (Nakajima, 2012). sustainable analysis of endangered species (Ariga et al., 2014; All of them are listed as Critically Endangered, Endangered, or Watanabe et al., 2014). Determination of the age class structure Vulnerable (CR, EN or VU) on Japan’s red list (Ministry of the requires great effort. Therefore, it should generally be difficult Environment, 2015). Therefore, conservation of these for many species, especially for endangered species of which species/subspecies is a necessity. Nevertheless, the life history the population size has already become smaller. However, such of Cobitis striata species complex remains unclear. Surveys of *Corresponding author: [email protected] - 135 - Morii and Takakura spawning populations have not been conducted even the numbers of age classes. Based on those results, we qualitatively, but some anecdotal reports have been submitted demonstrated that C. minamorii oumiensis deserve deep for C. striata striata Nakajima, 2012 and C. minamorii concern for the future persistence of the population. minamorii Nakajima, 2012 (Saitoh, 1990). Both C. magnostriata Nakajima, 2012 and C. minamorii Materials and Methods oumiensis Nakajima, 2012 are endemic species distributed throughout the Lake Biwa river system of Japan. Before Study site Nakajima (2012) described C. magnostriata and C. minamorii We conducted a series of surveys at a fallow paddy field in oumiensis as new species, the former and latter species were Shin-Asahi, Takashima, Shiga Prefecture, Japan. Adults of both designated respectively as large race and Biwa-small race in C. magnostriata and C. minamorii oumiensis were found at the reports. They have mutually similar morphology (Saitoh, this site (Nakano et al., 2015). At the site was a ditch of ca. 1984), but C. magnostriata is larger than C. minamorii 230 m total length and 30–200 cm width. The depth was ca. 15 oumiensis in body length and egg size (Minamori, 1956). cm at most parts of the ditch and ca. 30 cm deep at the deepest Saitoh and Matsuda (1990) reported that adults of C. part. Details of the study site were presented by Morii et al. magnostriata inhabit Lake Biwa and spend most of their life (2018b). there, except for the spawning season. During May–June, they migrate to and spawn at small and permanent water areas Collection of adult fish and body length measurement connected to the lake, such as ditches and side streams. Saitoh Adult fish were collected at the study site using three and Matsuda (1990) also reported that adults of C. minamorii collection methods: hand-netting, traps targeting immigrating oumiensis inhabit ditches connecting Lake Biwa and the lake, individuals into the study site, and traps targeting emigrating and that they run to and spawn at temporal water areas such as individuals from the site. Both C. magnostriata and C. paddy fields during June–July: the two species spawn at water minamorii oumiensis live most of the year in Lake Biwa. The areas of different types. A recent report (Nakano et al., 2015), exception is the spawning season, in which sexually mature however, has described that both species spawned individuals migrate to ditches or paddy fields. Therefore, sympatrically at a site. individuals captured in this study are either sexually mature Earlier reports of the literature (Saitoh, 1993; Saitoh and individuals or newborn juveniles that were produced by them. Matsuda, 1990) suggested that C. magnostriata and C. Surveys were conducted while water flowed in the ditch: minamorii oumiensis were distributed throughout the coast of during May 9 – July 4 in 2015, May 10 – July 29 in 2016, and Lake Biwa. The distribution ranges of both species have April 22 – July 23 in 2017. All three collection methods were declined to the present day (Saitoh, 1993). Particularly, the used once a week in 2015. In 2016 and 2017, however, they distribution range of C. minamorii oumiensis appears to be were done once every two weeks in May–June and once a extremely smaller because all surveys of this species published week in the other duration. We conducted a total of 55 during this decade (Nakano et al., 2015; Morii et al., 2018a, hand-nettings on each survey day. The locations for 2018b) were conducted at only one site. No other spawning hand-netting were scattered. They covered the entire ditch site is known. Some conservation activities have been homogeneously. Hand-nettings were conducted by one person conducted at the site (Nakajima, 2017), but our field survey using a hand-net (38 × 38 cm, 1 mm mesh size). The body strongly suggests that C. minamorii oumiensis is still length was measured by placing the fish in an acrylic narrow endangered (Morii et al., 2018a). Therefore, clarifying the tank with a scale (AK-1, Mitani Tsurigyogu, Kohnosu). actual situation of the reproduction of both species is an urgent In addition to hand-netting, a small trap net (mesh size 4 task. The age structure is expected to contribute to our mm) was placed, respectively, at the inlet and outlet of the understanding of the current reproductive situations in the two ditch. At 18:00–19:00 of the day before the hand-netting species. survey, these traps were placed to capture adult fish migrating This study was conducted to clarify the age structure of from the canal into the ditch. Adults were collected in the traps spawning populations of both species in the field. We used at 8:00–9:00 of the next day. We covered the trap entrances hand-netting and trapping to collect adult fish to investigate the with a coarse net (mesh size 40 mm) to prevent Japanese size distributions for the respective species. Then we estimated common catfish, Silurus asotus Linnaeus, 1758, from entering - 136 - Age class structure of spawning populations of two endangered endemic striated spined loaches in Lake Biwa river system morphological characteristics. Multiplex polymerase chain reaction (PCR) using species-specific primers was used, targeting the mitochondrial DNA sequence. Details of molecular identification were explained in our earlier work (Morii et al., 2018b). Statistical analysis We estimated the number of cohorts in the spawning populations of both species based on the size–frequency distributions of captured individuals. To do so, we estimated the number of components (single normal distributions) of the observed size frequency distribution. We regarded it as the Fig. 1 Small trap net used to collect fish that emigrated from number of age classes. Such estimation has often been done in the study site with drainage water. some fishes (Carlson et al., 2011; Frost, 1945). The component number was estimated from the model the traps and preying on the fish. selection. Models with k (= 1–7) components were fitted to Using traps, we collected individuals of the two species that each observed size frequency distribution. A model with the fell from the study site when the water of the ditch at the site least Akaike’s information criteria (AIC) was selected as the was drained on July 4, 2015, July 2, 2016, July 29, 2016, June best-fitted model.

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