Using spatial capture-recapture methods to estimate long-term spatiotemporal variation of a wide-ranging marine species

Determining population status to inform mitigation of anthropogenic
threats requires statistical approaches that investigate spatial and
temporal variation. In the face of climate change, it is increasingly
important to differentiate between changes in population size and
redistributions of populations. This is especially true for wide-ranging
species such as the blue whale. An abundance of eastern North Pacific blue
whales has previously been estimated using (non-spatial) closed
capture-recapture and distance sampling methods, but the estimates show
opposite and diverging trends over the last 30 years. Evidence that the
distribution has been expanding could explain the apparent disparity, due
to the confounding effects of spatial variation in sampling and the
changing distribution. To investigate this, we apply, for the first time,
spatial capture-recapture (SCR) methods to blue whale photo-identification
data from small boat surveys to estimate abundance. The study area was
defined as the length of the continental USA coastline, extending
approximately 100 km offshore. Average annual effort from 1991 to 2023 was
97 days, resulting in 7,358 sightings of 1,488 unique individuals. We find
significant support for non-linear spatiotemporal variation. In all years,
there were higher densities at lower latitudes, but there were notable
decadal cyclical fluctuations in the number of animals using the study
area. This large variation in the numbers of animals using these waters
motivates further study into the relationship with environmental changes.
Our results are an important step in spatially-explicit modelling of
observational blue whale data, which highlight the value of including
spatial and temporal data and are relevant to any marine mammal species
monitored using photo-identification.