Preliminary report on bycatch fish species collected from the Tokyo Submarine Canyon, Japan (2024)

Abstract

An ichthyofaunal list of bycatch species was compiled, the fish captured by bottom gill-nets set at approximately 300 m depth in the Uraga Suido Channel central Japan. Fragmentary ichthyofaunal lists are available for this area; these lists have focused on chondrichthyans or commercial actinopterygians, but voucher specimens have not been prepared for museum storage. An initial list of the fish fauna was compiled with vouchers, and seven species not previously recorded from the channel are reported. Most of these species belong to the Class Actinopterygii; Apristurusplatyrhynchus (Tanaka, 1909), Beryxdecadactylus Cuvier, 1829, Hoplostethusjaponicus Hilgendorf, 1879, Sebastesiracundus (Jordan & Starks, 1904), Scalicusamiscus (Jordan & Starks, 1904), Atrobuccanibe (Jordan & Thompson, 1911), and an unidentified species of the eelpout family Zoarcidae. The taxonomic identity of the eelpout and the biogeography of the Uraga Suido Channel are considered. Further research is required to resolve outstanding faunistic issues, but live collections will likely end when the aging fishers who provide the specimens retire. At that point, existing museum collections will become increasingly important for future research. Examination of a collection that may have been previously deposited in the Chiba Prefectural Museum will be essential.

Keywords: deep sea, distributional boundary, Sagami Sea, western Pacific

Introduction

The deep ocean is a frontier for ichthyological exploration. The fish fauna of the deep sea is much less well known than that of shallow coastal zones. Efforts to conserve deep-water fish faunas are essential in the face of threats from anthropogenic disturbance, such as seabed mining (Cuyvers et al. 2018). These efforts are hampered by a dearth of data on deep sea fish diversity.

The Tokyo Submarine Canyon has an unusual hydrography. The shoreward end is located at the mouth of Tokyo Bay (in the narrow sense, defined below). About 31 million people live around the shores of the bay’s basin (Tokyo Bay Environmental Information Center 2017). The maximum water depth in the Uraga Suido Channel at 5–7 km offshore is about 700 m (Fig. ​(Fig.1).1). The canyon drops off rapidly into the Sagami Trough (> 1000 m deep), and then plunges deeper to the Japan Trench (8020 m maximum depth) (Kato et al. 1985).

Knowledge of the fish fauna in the submarine canyon is fragmentary. Obara et al. (2008) compiled a list of chondrichthyans, which are represented by 41 species belonging to eight orders. Some checklists have included deep-sea species of commercial interest, such as the splendid alfonsino Beryxsplendens Lowe, 1834 and the blackthroat seaperch Doederleiniaberycoides (Hilgendorf, 1879) (Kohno et al. 2011; Kudo 1997, 2011). The non-commercial ray-finned species occurring in deep water have been little studied, and the few reports available have not provided essential information on voucher specimens, so the records cannot be re-verified.

Data for the ichthyological surveys in the deep waters of the channel have been obtained by examining the bycatch in commercial bottom gill nets. These nets are used to catch the Japanese spider crab, Macrocheirakaempferi Temminck, 1836, and the Japanese lobster, Metanephropsjaponicus (Tapparone-Canefri, 1873). The nets have been deployed by a single fishing boat, the Chougorou-maru, since the 1980s. The first author of this report was able to sail on this boat with a TV crew, and was provided with bycatch specimens as the nets were hauled onboard. The specimens captured included some of the first records from the Uraga Suido Channel. Here, we provide a checklist of the fishes of the Tokyo Submarine Canyon with voucher specimens and photographs. We discuss ichthyofaunal issues in the region.

Materials and methods

Results and discussion

Based on examinations of our voucher specimens (56 individuals) and the photographic images, we identified 36 species in 25 families and 13 orders (Table ​(Table1;1; Figs ​Figs22–5). The collection included rare chondrichthyans, such as the frilled shark Chlamydoselachusanguineus Garman, 1884 and the goblin shark Mitsukurinaowstoni Jordan, 1898, which were previously reported by Yano et al. (2007) and Obara et al. (2008). The four species, M.owstoni, Lophiomussetigerus (Vahl, 1797), Doederleiniaberycoides (Hilgendorf, 1879), and Eopsettagrigorjewi (Herzenstein, 1890), were identified from their database photographic images only (Fig. ​(Fig.5);5); no voucher specimens were deposited in the museums. The compilation included one agnathan, 13 chondrichthyans species (Figs ​(Figs2,2, ​,5),5), and 21 actinopterygian species (Figs ​(Figs33–5). Among the actinopterygians, the family Macrouridae was the most speciose (four species). Other families were represented by one or two species.

We found seven species (Table ​(Table1)1) that had not been included in the previous reports on the fish fauna of the Uraga-suido Channel (Kudo 1997, 2011; Senou et al. 2006; Obara et al. 2008; Kohno et al. 2011): Apristurusplatyrhynchus (Tanaka, 1909), Beryxdecadactylus Cuvier, 1829, Hoplostethusjaponicus Hilgendorf, 1879, Sebastesiracundus (Jordan & Starks, 1904), Scalicusamiscus (Jordan & Starks, 1904), Atrobuccanibe (Jordan & Thompson, 1911), and an unidentified species of the eelpout family Zoarcidae (see below). Miya and Aizawa (1995) briefly reported on the ichthyofauna of the same region based on specimens collected with the fishing gear that we used. This work is reported in an abstract for the “Symposium on Taxonomy, Ecology, and Stocks of Elasmobranchs” held at the Ocean Research Institute, University of Tokyo on November 27–28^th, 1995. However, neither a species list nor details of voucher specimens were provided, and no publication emerged subsequently. Some of the specimens collected in this 1995 study may have been deposited in the Natural History Museum and Institute, Chiba Prefecture to which they have belonged. Examination of previous collections that may have been deposited in the Chiba Prefectural Museum and other museums will be increasingly important and essential, because live collections will likely end when the aging fishers who provide the specimens retire.

The fishers who provided our specimens are aging, and no successors are likely to take over their operations. Collections will likely cease when the fishing closes down and hence, museum holdings will become increasingly important for faunistic studies.

The specimen that we identified as Sebastesiracundus (Fig. ​(Fig.4B)4B) has a black inner surface in the mouth. Nakabo and Kai (2013) indicated that this coloring is characteristic of Sebastesflammeus (Jordan & Starks, 1904). However, other morphological traits, such as the patterns of tooth bands, are indicative of S.iracundus. A fishing writer also mentioned the same confusion, that is, it is difficult to identify the two species captured from this area based on the external morphology (Shiina 2019). These two species were regarded as conspecifics by Balanov et al. (2004), but genetic differences between the two entities have been reported subsequently (Orr and Hawkins 2008). The Uraga Suido Channel is the southern distribution boundary for these species (Nakabo and Kai 2013), and we provide the southernmost record of S.iracundus.

The unidentified eelpout (Fig. ​(Fig.4J)4J) in our collections may be a species shown as “Andriashevianatsushimae” by Nishiguchi et al. (2009). However, the name is not available according to Art. 8 of the International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature 1999). There are inconsistencies in the original description (the number of vertebrae in the holotype) and morphological details are inadequate (Hatooka 2013; Shinohara and Takami 2014). Based on further observations of the external and internal morphology of additional specimens collected from Suruga Bay and Sagami Bay, Shinohara and Takami (2014) reported that the undescribed species probably belongs to an undescribed genus. The taxonomic study of this new entity is ongoing.

Acknowledgements

We deeply appreciate H Tejima, A Tejima, and S Takei for kind cooperation to ride on their fishing boat together and to give us the bycatches for making specimens. We also thank J Matsuzawa, T Nagano, N Kawada, T Kanazawa, R Namiki, K Minomo, K Izumori, T. Moriyama, and other staff belonging to the TV Tokyo, and the Kyodo Television for their help our research and provisions of their movies to register to the image database, H Kohno and A Ishikawa for helping us to process specimens, and T Kudo for comments on ichthyofaunal issues of the Tokyo Bay including the Uraga Suido Channel.

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References