Species composition and distribution of eelpouts (Zoarcidae, Perciformes, Actinopterygii) in the northwestern Sea of Okhotsk in summer
Introduction
Zoarcidae are one of the most widely represented families of benthic fish in the North Pacific (Anderson, 1994, Borets, 1997, Mecklenburg et al., 2002, Hatooka, 2013) in terms of species composition and abundance. In the northern Sea of Okhotsk, 42 nominal species of eelpouts have been recorded, which accounts for about 15% of the list of all fish species encountered in this region (Fedorov et al., 2003). Nevertheless, the actual species composition, distribution, and ecology of eelpouts inhabiting the northern Sea of Okhotsk are largely unknown. The published lists of Zoarcidae for this area are mostly inconsistent and contain a still undetermined number of synonyms along with the valid species (Borets, 1997, Fedorov et al., 2003, Fedorov, 2004, Anderson and Fedorov, 2004, Parin et al., 2014), thus, indicating the ongoing taxonomic issues. Difficulties with field identification of zoarcids result from their high phenotypic polymorphism (Balanov and Kukhlevskii, 2011, Saveliev et al., 2014b, Mecklenburg and Anderson, 2015), which complicates the selection of the reliable diagnostic characters. The extent of morphological variation in the overwhelming majority of eelpout species is unknown, the boundaries of the ranges of some species are understood inadequately, and misidentifications are common. These issues are a significant challenge in eelpout studies. To date, sampling has been carried out sporadically and inconsistently, without understanding eelpouts taxonomy. As a consequence, many earlier studies on eelpouts of the Sea of Okhotsk are significantly outdated and can only be partially used for few taxa (Popov, 1931, Taranets, 1937, Schmidt, 1950, Lindberg and Krasyukova, 1975, Toyoshima, 1985, Borets, 1990, Borets, 1997).
The cited summaries and catalogues provide only approximate information on the ranges of species; for some of them, the only indication is “throughout”, which makes it impossible to establish precise boundaries of the ranges, as well as to assess the distribution and density of the species in a certain area. For most of the northern Sea of Okhotsk eelpouts, only bathymetric ranges are known (Dudnik and Dolganov, 1992, Borets, 1997, Sheiko and Fedorov, 2000, Fedorov et al., 2003, Hatooka, 2013). The geographical distribution is shown more in detail for few of the species (Shuntov, 1965, Ivanov, 2002, Badaev, 2012). Very limited reliable data on the taxonomy and distribution of some of the eelpout species from the area under consideration is available to date (Anderson, 1982, Anderson, 1994, Chernova, 2000, Nazarkin and Chernova, 2003, Nazarkin, 2010a, Nazarkin, 2010b, Nazarkin and Fedorov, 2007, Chernova and Nazarkin, 2020).
Detailed studies of the northern Sea of Okhotsk, accompanied by total assessment of the composition and abundance of all demersal fishes, were carried out during groundfish trawl surveys in the late 1980s (Dudnik and Dolganov, 1992, Borets, 1990, Borets, 1997). These data, along with previously published, mostly sparse and incomplete, reports, still remains a reference for evaluating the qualitative and quantitative composition of the northern Sea of Okhotsk eelpout fauna (Fedorov et al., 2003). Judging from the species list in Borets (1990), based on trawl surveys conducted in the northern Sea of Okhotsk shelf and upper slope in 1988, the author faced serious challenges with the field identification of eelpouts because the published data differs substantially from that obtained in our study.
In the summer of 2013, benthic fishes were collected in the northwestern Sea of Okhotsk bottom trawl survey within nearly the same area and depths as in 1988 (Borets, 1990). When comparing the published list of Zoarcidae (Borets, 1990) with that compiled by us, we found the former to be almost entirely inconsistent. The aim of this work is to describe the species composition and spatial distribution of eelpouts in the northwestern Sea of Okhotsk outer shelf and adjacent upper continental slope in summer.
Section snippets
Environmental features of the study area
The Sea of Okhotsk is a 1.5 × 106 km2 marginal sea adjacent to the northern Pacific Ocean. The main topographic feature of the northwestern Sea of Okhotsk is a relatively broad (200–300 km) shelf with typical depths of 100–200 m that occupies about 40% of the basin area (Moroshkin, 1968, Ohshima and Martin, 2004, Shcherbina et al., 2004). The Sea of Okhotsk is the coldest region in the North Pacific. The wind-driven water circulation in the sea is generally cyclonic (Fig. 1). Pacific water
Species list
Bothrocara hollandi (Jordan & Hubbs, 1925) (Fig. 2, Fig. 4)
Japan-Sea eelpout
Material: MIMB 41538, 9 spec., 57°49,8′ N, 152°10,6′ E, 227 m, 3 Aug. 2013.
Asiatic boreal. Reported from waters north of the eastern Yellow Sea and throughout the Seas of Japan and Okhotsk (including waters off the Kuril Islands) to off the southeastern Kamchatka Peninsula, at depths of 140–1950 m. Mesobenthal. Numerous. A total of 150 individuals 140–280 mm TL were recorded from depths of 154 to 301 m, most frequently
Conclusion
A total of 19 species were encountered in the outer shelf and upper slope of the northern Sea of Okhotsk in the summer of 2013. A rich eelpout fauna was revealed even in the area of coldest waters. The composition of this fauna is more different from that of the continental slope and less different from the fauna of the rest of the less cooled shelf area. A study of the bathymetric distribution of species revealed a fairly clear vertical zonation and a pronounced pattern of species assemblage
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank to Aleksander S. Sergeev and Vladimir S. Danilov (MagadanNIRO) for their great assistance during expedition as well as the crew of the stern trawler Akvaresurs. Andrey A. Balanov (NSCMB FEB RAS) helped with laboratory identification of some specimens. Evgeniy Shvetsov (NSCMB FEB RAS) corrected the text of the draft manuscript. We are grateful to Dr. D. M. Eric Anderson (SAIAB) and Dr. Catherine W. Mecklenburg (CAS) for their helpful comments.
Funding: This study was partially funded by
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