Estimating spatial mixing within the St. Lawrence Estuary beluga population

Inter-individual variability in habitat preferences affect local abundance and residency times. Within a population range, this can lead to sectors having a continuous flow of unique individuals, while others are used by the same set of resident individuals. These patterns of habitat use by individuals, referred to here as individual spatial mixing, can have important implications for ecological and evolutionary processes. To estimate individual spatial mixing, we propose comparing local individual diversity against abundance indices. To test this approach, we compare abundance estimates for the endangered St. Lawrence Estuary (SLE) beluga population derived from 35 systematic aerial surveys taken over 18 years against individual diversity estimates from capture-recapture (photo-identification) data obtained over the same period. Divergences between relative abundance and diversity across sectors within the SLE, suggest that several sectors with low abundance are used by a relatively high diversity of individuals. This was the case notably for the Saguenay Fjord, where 1.8% of the population was detected on average during summer surveys, but where 41% of the individuals were observed based on photo-identification data. This suggests that individual spatial mixing was high in this sector. Conversely, low mixing was observed in other sectors such as the eastern central portion of the summer distribution. In these sectors individual diversity was lower than expected based on abundance estimates, suggesting that the same individuals were repeatedly observed during surveys. By comparing spatial patterns of diversity and abundance we suggest it is possible to gain information about individual spatial mixing within populations. We discuss these results in the context of building a simulation model (3MTSim) to estimate the distribution of local stressors such as anthropogenic noise in the SLE beluga population.