Autonomous wave gliders as a tool to characterize delphinid habitats along the Florida Atlantic coast

Abstract

As climate change and anthropogenic activities continue to impact cetacean species, it becomes increasingly urgent to efficiently monitor cetacean populations. Continuing technological advances enable innovative research methodologies which broaden monitoring approaches. In our study, we utilized an autonomous wave glider equipped with acoustic and environmental sensors to assess delphinid species presence on the east Florida shelf and compared this approach with traditional marine mammal monitoring methods. Acoustic recordings were analyzed to detect delphinid presence along the glider track in conjunction with subsurface environmental variables such as temperature, salinity, current velocity, and chlorophyll-a concentration. Additionally, occurrences of soniferous fish and anthropogenic noise were also documented. These in-situ variables were incorporated into generalized additive models (GAMs) to identify predictors of delphinid presence. The top-performing GAM found that location, sound pressure level (SPL), temperature, and chlorophyll-a concentration explained 50.8% of the deviance in the dataset. The use of satellite environmental variables with the absence of acoustic variables found that location, derived current speed and heading, and chlorophyll-a explained 44.8% of deviance in the dataset. Our research reveals the explanatory power of acoustic variables, measurable with autonomous platforms such as wave gliders, in delphinid presence drivers and habitat characterization.

Keywords: Autonomous gliders; Delphinids; Generalized additive models; Habitat characterization; Marine mammal monitoring; Passive acoustic monitoring.

Figure 1. Glider track along the Florida Atlantic coast.

The glider track is colored by date with the start date (indicated by the green arrow) represented with the lightest color and the end date (indicated by the red arrow) represented with the darkest color. Bathymetry is represented by grey solid contour lines. The transition from the continental shelf to the slope is demarcated by the first 200 m isobath contour. The contour interval is 200 m.

Figure 2. Glider sensor payload on the tow-body.

Images of the glider sensor payload on the tow-body during the 2019 east Florida shelf survey showing the Remora acoustic recorder, the YSI EXO1 CTD, the glider passive acoustic monitoring system (PAM) and the C3 Turner fluorometer.

Figure 3. Smooth plots displaying the results of the top-performing GOM assessing the relationship between delphinid presence and environmental variables.

(A) SPL (dB), (B) chlorophyll-a (RFU), and (C) temperature (°C). Tick marks on the x-axis represent data observations and gray shading represents the 95% confidence interval. The y-axis represents the relative effect of each variable on the presence of delphinids, with values above zero indicating increased presence and numbers below zero representing decreased presence. The number associated with each variable on the y-axis represents the effective degrees of freedom (EDF) for each smooth term, indicating the complexity of each variable’s relationship with delphinid presence, 1 being the least complex.

Figure 4. Weekly maps of delphinid presence along the glider track as a function of date and location ID for the glider-only GAM (GOM).

Each image represents one week of the glider deployment, with the color scale indicating presence, with darker colors indicating higher detected presence and lighter colors indicating lower detected presence. SA, PI and CC indicate the location of St. Augustine, Ponce Inlet and Cape Canaveral, respectively.

Figure 5. Smooth plots displaying the results of the top-performing satellite-only GAM (SOM) assessing the relationship between cetacean presence and environmental variables.

(A) Chlorophyll-a (mg m⁻³); (B) current heading (degrees north); (C) current speed (m s⁻¹). Tick marks on the x-axis represent data points and gray shading represents the 95% confidence interval. The y-axis represents the relative effect of each variable on the presence of delphinids, with values above zero indicating increased presence and numbers below zero representing decreased presence. The number associated with each variable on the y-axis represents the effective degrees of freedom (EDF) for each smooth term, indicating the complexity of each variable’s relationship with delphinid presence, 1 being the least complex.

Figure 6. Weekly maps of delphinid presence along the glider as a function of the date and location ID for the satellite-only GAM (SOM).

Each image represents one week of the glider deployment, with the color scale indicating presence, with darker colors indicating higher detected presence and lighter colors indicating lower detected presence.