Marine fishes from the northwest Atlantic Ocean were analysed to determine whether barcoding was effective at identifying species. Our data included 177 species, 136 genera, 81 families and 28 orders. Overall, 88% of nominal species formed monophyletic clusters based on >500 bp of the CO1 region, and the average bootstrap value for these species was 98%. Although clearly effective, the percentage of species that were distinguishable with barcoding based on the criterion of reciprocal monophyletic clusters was slightly lower than has been documented in other studies of marine fishes. Eelpouts, sculpins and rocklings proved to be among the most challenging groups for barcoding, although we suspect that difficult identifications based on traditional (morphology based) taxonomy played a role. Within several taxa, speciation may have occurred too recently for barcoding to be effective (e.g. within Sebastes, Thunnus and Ammodytes) or the designation of distinct species may have been erroneous (e.g. within Antimora and Macrourus). Results were consistent with previous work recognizing particularly high levels of divergence within certain taxa, some of which have been recognized as distinct species (e.g. Osmerus mordax and Osmerus dentex; and Liparis gibbus and Liparis bathyarcticus), and some of which have not (e.g. within Halargyreus johnsonii and within Mallotus villosus). The results from this study suggest that morphology-based identification and taxonomy can be challenging in marine fishes, even within a region as well characterized as Atlantic Canada. Barcoding proved to be a very useful tool for species identification that will likely find a wide range of applications, including the fisheries trade, studies of range expansion, ecological analyses and population assessments.

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DNA-based identifications have been employed across broad taxonomic ranges and provide an especially useful tool in cases where external identification may be problematic. This study explored the utility of DNA barcoding in resolving skate species found in Atlantic Canadian waters. Most species were clearly resolved, expanding the utility for such identification on a taxonomically problematic group. Notably, one genus (Amblyraja) contained three of four species whose distributions do not overlap that could not be readily identified with this method. On the other hand, two common and partially sympatric species (Little and Winter skates) were readily identifiable. There were several instances of inconsistency between the voucher identification and the DNA sequence data. In some cases, these were at the intrageneric level among species acknowledged to be prone to misidentification. However, several instances of intergeneric discrepancies were also identified, suggesting either evidence of past introgressive hybridization or misidentification of vouchered specimens across broader taxonomic ranges. Such occurrences highlight the importance of retaining vouchered specimens for subsequent re-examination in the light of conflicting DNA evidence.

The demersal fish fauna at 900-1800 m depths off Nova Scotia, Canada, is described using data from exploratory bottom trawl surveys conducted in November 1994 and March 1995 by a commercial fishing trawler. Approximately 25 metric tons (39 000 specimens) of demersal fish belonging to at least 82 species were caught, 30% of which had not previously been recorded from the area. However, more than half the catch consisted of the two species Centroscyllium fabricii (black dogfish) and Coryphaenoides rupestris (roundnose grenadier). Catches were higher in the shallower depth strata fished and cluster analysis showed that depth was the primary factor determining species composition of catches. It is suspected, however, that the vessel fished less effectively at depths greater than about 1500 m, contributing to the reduction in catch quantities at these depths. The importance in catches of large bodied species, particularly Chimaeriformes and sharks, contrasts with results from surveys in adjacent areas. This likely reflects the greater fishing power of the vessel/gear used in present surveys rather than real differences in faunal composition. Catches of mesopelagic species during these surveys, and during an earlier deepwater trawling survey in this area, are also described.

Two unusual adult specimens of the barracudina Arctozenus risso were caught in midwater trawls off the Flemish Cap in April 2016. These specimens displayed atypical traits including lack of teeth and gill rakers and fragile jaw bones; a combination of morphological examination and mtDNA sequencing confirmed species identification. Both specimens exhibited ripening ovaries and testes, confirming simultaneous hermaphroditism. These fish represent the first records of late‐stage loss of features in the genus Arctozenus and may indicate a relationship to spawning in larger specimens.