Potential causes for the stranding of Macrohectopus branickii (Dybowsky, 1874) specimens along the Angara River: strong currents, artificial lighting, or ice melting

Abstract

Macrohectopus branickii (Dybowsky, 1874) is the world's only freshwater pelagic amphipod native to Lake Baikal. The habitat of this amphipod is limited to the pelagic zone of Lake Baikal (with periodic nocturnal migrations to the littoral part of the lake). Until now, this amphipod had never been recorded in the rivers flowing into Lake Baikal or in the Angara River flowing out of it. However, on 16 May 2022, near the Taltsy Museum, 20 km from the source of the Angara River, a massive accumulation of Macrohectopus individuals was washed ashore. The reasons for this phenomenon are still unknown. Considering the uniqueness of M. branickii within the world amphipod fauna and its importance for the Lake Baikal ecosystem, the determination of the factors contributing to this phenomenon is of great importance. In this paper we have presented some hypotheses that could explain the appearance of Macrohectopus on the shores of the Angara River. The first hypothesis is that the amphipods could not cope with the current at the source of the river and were carried down the river, and since they are negatively affected by natural light (at the beginning of the morning), they became trapped and died. The second hypothesis is that the Macrohectopus were attracted and disoriented by the artificial light emanating from the nearby villages, before being carried by the current. The third hypothesis relates to the time frame in which this event was observed. This peculiar phenomenon was observed in mid-May, following the melting of the ice on Lake Baikal. Accordingly, Macrohectopus could have been feeding on the lower surface of the ice, or alternatively they could have been frozen in the ice as the ice floes were transported downstream. Each of the hypotheses we have presented does not necessarily exclude the others, but on the contrary may complement them. In this paper we do not exclude the possibility that there are other explanations for this phenomenon. However, if massive washing ashore of Macrohectopus occurs regularly, further research is needed, taking into account, among other things, the influence of washed ashore individuals on the food spectrum of aquatic organisms and waterfowl of the Angara River.

Keywords

• amphipods
• alan
• artificial light at night
• daily vertical migrations
• feeding under ice
• lake baikal