Harbor seal-pebble sacks-SST

Published: 11 April 2025| Version 1 | DOI: 10.17632/wcph8bdgt7.1
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Description

In the northeast Pacific Ocean, the harbor seal exhibits its southernmost distribution in waters off the Baja California Peninsula, which includes nine islands and archipelagos and some sites on the coast. A decrease in abundance at the coastal colonies was reported for 2016, when pebble extraction at some of those beaches increased. The present study analyzed seal counts in aerial photographs taken in 2016, 2018, 2019, and 2022, evaluating them as a function of both the sea surface temperature (SST) obtained from from the US National Oceanic and Atmospheric Administration (NOAA) climate data record, as taken from its polar-orbiting satellite (ftp.oceans.ncei.noaa.gov/pub/data.nodc/pathfinder/Version5.3/L3C) for the 50 km around each colony and the occurrence of pebble extraction. We found a consistent decrease in the number of seals over time for all colonies off the Baja California Peninsula in the southern California Current Large Marine Ecosystem, coinciding with an increase in SST that could indicate lower habitat suitability. However, the decline of seal numbers on the Todos Santos, San Jeronimo, San Benito, Natividad, and San Roque islands was slower, possibly because these are more productive sites. Additionally, the disappearance of many seal colonies on the coast could result from disturbances caused by pebble extraction. We used aerial photographs to count sacks containing pebbles. The present study provides evidence of the effect of ocean warming trends and disturbance on harbor seal distribution and abundance.

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Aerial surveys were conducted using a Cessna aircraft at an altitude of 100 to 200 m above the coastline of the mainland and the islands off the Baja California Peninsula, from the Coronado Archipelago to Asuncion Island and, on the mainland, from Todos Santos Bay to Asuncion Bay. When a harbor seal colony or pebble extraction activities were detected, we took photographs of the location continuously with a Canon EOS 50D camera equipped with a Canon 100-400 mm telephoto zoom lens and recorded the GPS position. We collected data in 2016, 2018, 2019, and 2022, during the molting season, when the highest number of seals is expected to occur at the colonies. We analyzed the aerial photographs, using the ImagePro Plus 6.0 software (Media Cybernetics Inc., 2006), to count the harbor seals. To analyze the trends identified, we applied Bayesian regression models, using the integrated nested Laplace approximation with Gaussian prior distribution (INLA) (Rue et al., 2009). To ascertain whether the temporal trends presented by harbor seal counts, namely the relative importance of the colonies, were related to changes in the marine environment, we analyzed the SST trends and compared the slopes obtained with the derivatives obtained via the linear regression conducted on the harbor seal counts. We obtained the SST values for the study area from the US National Oceanic and Atmospheric Administration (NOAA) climate data record, as taken from its polar-orbiting satellite (ftp.oceans.ncei.noaa.gov/pub/data.nodc/pathfinder/Version5.3/L3C), for the period 2008–2022, with an original daily spatial resolution of 0.05 degrees (with only nocturnal data used). The database was processed using the R environment (R Core Team, 2023). We used polygons to analyze the SST information obtained within a radius of 50 km from each seal colony. The SST time series covered thirteen years (2008–2022). To analyze the effect of pebble extraction (PE) on the harbor seal counts along the western coast of the Baja California Peninsula, we used aerial photographs to record extraction activities (sacks containing pebbles). We measured the volume of the sacks used for pebble collection on the beaches and counted them using the DotDotGoose software version 1.6 (Ersts, 2023). The counts were then georeferenced using Microsoft Access and mapped in R.

Institutions

  • Universidad de las Americas Puebla
  • Universidad Autonoma de Baja California
  • Centro de Investigacion Cientifica y de Educacion Superior de Ensenada

Categories

Animal Ecology, Conservation of Biodiversity

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