Chondrichthyan fishes in the Arctic Ocean and adjacent seas - do we know our species?

Abstract

The basal prerequisite for managing species and understanding ecosystems is correct species and population identification. Without knowing the species, we cannot with accuracy detect changes in distribution or abundance due to climate change, fisheries, diseases or any other types of impact. This thesis gives the first complete overview of chondrichthyan fishes (sharks, skates, rays and chimaeras), recorded in the Arctic Ocean and adjacent seas. A total of 49 species were found to occur, and the most specious regions were the Bering Sea, the Norwegian Sea and the Barents Sea, also known as the Arctic Gateways. The Bering Strait is an effective zoogeographic barrier, separating the Pacific from the Atlantic species. In particular, skates are prone to misidentifications. Species complexes are known to occur, and the group is commonly lumped as “Skates & Rays” in fishery statistics or in scientific publications. By use of genetic methods (DNA barcoding), the skate species reported from the northern Northeast (nNE) Atlantic was investigated. A total of 12 species were found to occur in the area, and three of those were new for the region. Skates are commonly encountered in the nNE Atlantic, but ~95 % are the smaller-sized Amblyraja radiata. Larger skates may be landed and they are necessarily a mix of the remaining 11 species, some of which are considered threatened. There were strong indications of the disappearance of Leucoraja fullonica, as well as over-reporting of the critically endangered common skate complex formerly known as Dipturus batis. Amblyraja radiata was suspected to be cryptic due to large differences in size-at-maturity across its distributional range. However, the mitochondrial gene cytochrome c oxidase subunit 1 (COI) did not support this hypothesis. To further elucidate this question, a higher resolution method was applied, and 10 microsatellite markers were investigated from specimens across the North (N) Atlantic. Pairwise fixation indexes (FST) indicated that there may be three major clusters in the N Atlantic: Northwest Atlantic, Mid Atlantic (Greenland) and NE Atlantic. However, more rigorous tests revealed high physical mixing of individuals over a large geographical area. The microsatellite results did not support the hypothesis of A. radiata being cryptic. Although the population structure of A. radiata could not be fully explained, baselines for occurrence of chondrichthyan fishes are established, and a new identification guide for skates in the nNE Atlantic is provided. Hopefully, this will pave the way for a better management of the skates by improving data quality.