Marine mammal distribution in Ecuador: surveys aboard a ship of opportunity as a means of monitoring relative abundance

Abstract

Five marine mammal surveys between 2008 and 2011 were conducted aboard the Buque de Investigación Orion (the research vessel for the Oceanographic Institute of the Ecuadorian Navy) within oceanic waters adjacent to mainland Ecuador and the Galápagos Islands. The surveys dedicated extensive time in deep, offshore waters where cetaceans were not densely present. Sightings of 12 species were compared with an earlier survey aboard the B/I Orion in 2001 as well as with a subset of published data from three NOAA STAR (Stenella Abundance Research) surveys between 1999 and 2003. Additionally, a small boat, near-shore survey, was conducted during June 2010 among andnear the Galápagos Islands. Encounter rates ranged annually from 0.012 cetacean/km to 0.027 cetacean/km. The highest encounter rate aboard the B/I Orion took place during the April 2009 survey. In order to compare sightingrates between the B/I Orion and NOAA platforms, the average effective half-strip widths were used to determine encounter rates per area effectively searched. A zonation within the study region was observed between odontocete andbalaenopterid cetaceans as well as between striped (Stenella coeruleoalba) and short-beaked common dolphins (Delphinus delphis). Several methodological aspects of surveys and geographical features that may influence encounter rates and subsequent abundance estimates are discussed. This study demonstrates that vessels of opportunity provide a valuable means of studying open-ocean and coastal distributions of marine mammals. Possible methodological improvements, such as the use of high-power binoculars, that could increase the absolute number of sightings, the efficiency of these opportunistic surveys, and improve the sighting rates of more evasive species are discussed.