Implications for nature conservation
The Gullmar Fjord has been impacted by bottom trawling and other disturbances since the 1900s. At about the midpoint of the fjord, close to the bay of Skår, one of Sweden’s busiest ferry services has been running since the late 1960s, operating the largest car-driven car ferries in the country. During the day these ferries ply back and forth across the fjord every 20 min. This causes constant turbulence of the bottom sediments on the slopes and, consequently has at least locally, a considerable impact on the fjord’s ecosystem. This impact, together with bottom trawling for shrimps in the habitat for D. velifer might have caused the decline and local extinction of the species. Furthermore, at some periods during later part of the 1970s and early 1980s there were some periods of low oxygen levels in the deep trench of the fjord, which must have added stress to the organisms living there. The collections of D. velifer from the 1940s were in the innermost and relatively shallow sections of the fjord, in which the bottom environment could have been comparatively less affected, at least for some time, and be more stable. It is unknown if such a stability still exists in places, which may imply that a population of D. velifer could still be present. This would then be in parallell with that of the long-absent aeolid nudibranch Flabellina borealis (Odhner, 1922), which was recently rediscovered in the fjord (Lundin and Malmberg, 2016).
The two historical records of single specimens of D. velifer in the Oslo Fjord in southeastern Norway and in the Hardanger Fjord on the southwest may represent a link from the Gullmar Fjord in Sweden to the northwestern Norwegian coast. The Bunnfiorden area has a threshold at 60 m, and an inner deep trench that goes down to 150 m. It is adjacent to parts of the city of Oslo and its southeasters suburbs. The water in the deep trench is of poor quality, hence we can hardly expect any D. velifer to have survived here since the early 1900s. The locality at Sunde, in the Hardanger Fjord on the southwestern coast of Norway is close to the mouth of the fjord, and thus it has better environmental conditions.
Although we cannot report living specimens of Dendronotus velifer from the Gullmar Fjord now, this study is the first that summarises all findings of this species and the first that uses morphological, molecular and ecological evidence to assess the taxonomic position of the predominantly Arctic D. velifer. Such an integrative assessment is important for conservation since there is an ongoing discussion on establishing a national marine park within the Gullmar Fjord. The relatively recent record of D. velifer in the Gullmar Fjord would certainly strengthen the arguments in favour of a marine park to protect the whole area.