Systematic assessment of the increasing presence of white sharks in Atlantic Canadian waters

This study presents the first systematic assessment of trends in white shark Carcharodon carcharias presence in Atlantic Canadian waters (ACWs) using 2 standardized acoustic monitoring arrays—the Halifax and Cabot Strait lines—deployed by the Ocean Tracking Network between 2014 and 2023. We evaluated annual changes in the probability that acoustically tagged white sharks (n = 260) migrate from US waters to these arrays and the extent of their seasonal detection period in ACWs. Our analysis revealed an increase in the probability of white sharks migrating into ACWs, starting around 2019-2021. The probability of migrating to the Halifax line increased 2.4-fold [1.8, 3.3] (median [95% credible interval]), from 0.18 [0.13, 0.24] to 0.44 [0.38, 0.49], and to the Cabot Strait line increased 3.7-fold [2.4, 6.2], from 0.08 [0.05, 0.11] to 0.28 [0.24, 0.33], between 2018 (n = 63) and 2022 (n = 159). We also observed a 1.4-fold [1.2, 1.8] increase in the seasonal detection period in ACWs, extending from 48 d [40, 58] during 2014-2018 (n = 13) to 70 d [63, 78] in 2023 (n = 76). While our analysis robustly detected an increase in white shark presence in ACWs, limited data in the early years introduced uncertainty in quantifying the amplitude of the migration probability trend. The reported numbers should be interpreted carefully. The rapid timespan of this increase highlights the need to investigate causal mechanisms structuring white shark presence in ACWs, which is essential for the management of the Northwest Atlantic population.