Rediscovery of a presumed extinct species, Salvelinus profundus, after re-oligotrophication

Lake Constance (47° 38' N, 9° 22' E) is a deep (max. depth 251m) and large (surface area 536 km2) postglacial lake in Central Europe. Originally, it harboured two charr species – Salvelinus umbla and S. profundus. The first is a medium-sized, colorful, winter spawning charr, which is widespread across Central European lakes, the second a small, pale, summer spawning, deepwater charr, which is endemic to Lake Constance (Schillinger 1901, Kottelat and Freyhof 2007). S. profundus has exceptionally large eyes and the upper jaw strongly overlaps the lower jaw. These traits are both considered adaptations to its life in the deep where it mostly feeds on profundal benthos (Schillinger 1901). During the second half of the last century, Lake Constance became eutrophic, resulting in oxygen-depletion in deep waters (IGKB 2004). The anoxic conditions harmed the development of eggs by profundal spawning fish (Baer et al. 2017). Thanks to strict management interventions, the lake has returned to an oligotrophic state, and oxygen is again available in the water column down to the greatest depth (IGKB 2004).

康斯坦茨湖（Lake Constance，47°38' N，9°22' E）是位于中欧的冰川后大型深水湖泊，最大水深达251米，表面积为536平方千米。该湖原本栖息有两种红点鲑属物种：乌尔巴红点鲑（Salvelinus umbla）与深水红点鲑（S. profundus）。其中前者为中等体型、色彩艳丽的冬季产卵红点鲑，广泛分布于中欧各类湖泊中；后者则是体型小巧、体色浅淡的夏季产卵深水红点鲑，为康斯坦茨湖特有物种（Schillinger 1901, Kottelat and Freyhof 2007）。深水红点鲑具有异常硕大的眼部，且上颌显著覆盖下颌，这两项特征均被认为是对深水栖息环境的适应性演化，该物种主要以深水底栖生物为食（Schillinger 1901）。上个世纪后半叶，康斯坦茨湖出现富营养化现象，导致深水区出现缺氧状况（IGKB 2004）。缺氧环境对深底产卵鱼类的卵发育造成了损害（Baer et al. 2017）。得益于严格的管理干预措施，该湖现已恢复至贫营养状态，水体从表层至最大水深区域重新获得溶解氧（IGKB 2004）。