The Mussels That Came in From the Cold: Long-Term Effects of the Population Collapse in the 1960s May Explain Low Abundances of Boreal Mussels in the Subarctic Despite the Warming.

Boreal species are generally stressed by climate warming at the southern limit of their distribution and are expected to thrive at the poleward limit. Blue mussels were the heralds of climate warming in the Arctic, appearing in Spitsbergen a quarter of a century ago after the absence of a millennium. Here we tested the hypothesis that warming has a positive effect on subarctic mussels, too. We gathered and analyzed two datasets from the Murman Coast (Russia), 1000 km southeast of Spitsbergen across the Barents Sea. The first dataset contained data on the age structure of a mussel population monitored almost annually in 2001-2020, while the second dataset contained all comparable historical data on mussel biomass: 173 estimates of mean biomass at 62 sites along the entire coast collected in 1933-2020. Analysis of the first dataset showed that temperature had a positive effect on recruitment and survival. However, no positive effect was found in the analysis of the long-term dynamics. The abundance of mussels was high until the 1960s and then dropped tenfold. The collapse coincided with the climate cooling. The populations partially recovered in the subsequent unprecedentedly warm period, but no direct relationship between the recovery and temperature was found. We speculate that the subarctic mussels could not fully recover due to long-term consequences of the collapse. The Allee effect in their depleted populations might have limited recovery for decades. Besides, the fitness of blue mussel populations is currently undermined by competition and hybridization between the two cryptic species, the native