Kernel density distribution of each of the variables used in this study to define the occurrence of both blue whale calls in the Southern Ocean.

<p>Open circles represent each presence and absence of blue whales while barplots are for factor variables. SBACC is the distance of acoustic station from the nearest southern boundary of ACC (km), StationDur is station duration (hrs), lChl.a is log chlorophyll-a (mg m^-3), Distance is the distance from the nearest Antarctic shore (km), WindSp is wind speed (m s^-1), and WindDir is wind direction (°).</p

Box and whisker plots showing the Antarctic blue whales vocalization rate trends for the Z- and D- calls detected over the summer months during the IWC SOWER surveys.

<p>The box represent the first quartile to the third quartile (the interquartile range), and the segment inside the box is the median. The whisker delineates 1.5 times the interquartile width, and the closed circles are observations that are outside the range covered by the whisker.</p

Monthly variations of the six satellite-derived environmental predictors used in models for the summer (December–February) of 1997 to 2009 in areas surveyed by the IWC SOWER cruises in the Southern Ocean (38–78°S, -180-180°E).

<p>(a) Log transformed blended chl-a concentrations (mg m^-3), (b) Sea surface temperatures (°C), (c) Sea surface heights (m), (d) Wind speed (m s^-1), (e) Wind direction (degrees), and (f) Wind stress (N m^-2). Note that not all environmental variables were acquired for all months of our study period due to satellite data unavailability.</p

Partial effects of the different predictors on D-call rates of blue whales using the RF model.

<p>Plots indicate the marginal effect on blue whale call occurrence (y-axes) by each predictor variable (x-axis).</p

RF model performance for blue whale occurrence based on AUC.

<p>RF model performance for blue whale occurrence based on AUC.</p

False negative rates at different thresholds for Z- and D-calls using the 1997 (left) and 2009 (right) detection templates.

<p>False negative rates at different thresholds for Z- and D-calls using the 1997 (left) and 2009 (right) detection templates.</p

Relative importance of the different predictors in influencing the acoustic behaviour of blue whales based on RF model results.

<p>Relative variable importance is measured by residual sum of squares and expressed relative to the maximum.</p

RF model performance for blue whale call rates based on RMSPE and Spearmen’s rho.

<p>RF model performance for blue whale call rates based on RMSPE and Spearmen’s rho.</p

RF model plots of partial effects of the different predictors on the occurrence of blue whale D- and Z-calls.

<p>Y-axis is the partial effect of each predictor on occurrence (in logit-scale).</p

Partial effects of the different predictors on Z-call rates of blue whales using the RF model.

<p>Plots indicate the marginal effect on blue whale call occurrence (y-axes) by each predictor variable (x-axis). Contribution of each variable to the model given below the function. Y-axes are different across all plots. Scale of x-axes is different across each predictor variable.</p