Assessing the distribution of Atlantic spotted dolphins in the Azores

Abstract

The Atlantic spotted dolphin (Stenella frontalis) is one of the seasonal species sighted every year in the Azores. We aim to analyze its distribution using data collected between 2009 and 2011 off the south coast of São Miguel during whale watching tours. The Atlantic spotted dolphin was the fourth most sighted species in this study, making up 12.43% of all cetacean sightings (3090 sightings of 18 different species). Each year they were encountered between June and December, with 75% of sightings occuring between July and September, coinciding with the period when common dolphin sightings decrease in the area. In 2009 there were also some records in January and February, corresponding with the unusual higher temperatures registered during those months. 69.8% of sightings occurred with a sea surface temperature between 21 and 23.5ºC. 71.35% of sightings were recorded in areas fewer than 900m deep. During the three year study period, groups often comprised of several hundred individuals. In 2009 group size increased to more than 1000 individuals. Most of the groups (74.22%) were formed by a mixture of adults, juveniles and calves.

Full text

Laura González1,2, Miranda van der Linde1 , Clara Sardà1 & Jesús Torres-Palenzuela2
1 Futurismo Azores Whale Watching, Ponta Delgada, São Miguel, Azores.
2 Department of Applied Physics, Vigo University, Vigo, Spain.
Corresponding author: lauragonzalez1986@hotmail.es
Data of dolphin sightings were collected off the south coast
of São Miguel during whale watching tours between 2009 and
2011. We used GEBCO Bathymetry data set downloaded from
http://www.bodc.ac.uk, and sea surface temperature data
gathered from http://www.myocean.eu.
• 384 sightings between 2009 and 2011 (168, 90 and 126
respectively). The 4th most sighted species of the study period.
•Recorded very year between June and December, mostly
between July and September (75%). Some records in January
and February of 2009 (Fig.1A).
•Encountered when sea surface temperature was within
16.9ºC and 24.5ºC, having 69.8% between 21ºC and 23.5ºC
(Fig.1B).
The Atlantic spotted dolphin is more frequent in summer
months, according with the higher sea surface temperature.
They were not recorded under 16.8 ºC.
The 2009 winter sightings were probably due to the unusual
high sea surface temperature.
Laura González
Whale watching companies are increasingly providing
useful sources of information, even with some limitations, to
understand the ecological patterns of marine mammals.
In this study we want to assess the temporal distribution of
Atlantic spotted dolphins (Stenella frontalis) off São Miguel
(Azores), looking as well at their preferences in depth and sea
surface temperature with data gathered between 2009 and
2011 during whale watching tours.
Most of the groups recorded were families with calves.
•71% of the sightings were recorded in depths under 900 m (Fig.2).
Rate of sightings of Atlantic spotted dolphin increases in
summer months, just when common dolphin sightings
decrease. It would be interesting to assess if there is some
niche competition between these species (perhaps feeding
competition?) as they seem to live in the same area at
different times.
•Groups were formed mainly by adults, juveniles and calves
(74.22%) and usually they included several hundred dolphins,
and even more than 1000 in 2009.
A
B
Fig.1. Sightings of the Atlantic spotted dolphin off São Miguel, Azores.
A) Temporal distribution. B) Sea surface temperature sightings.
Fig.2. Sightings of Atlantic spotted dolphins over the bathymetry map.
ISLANDS
DEPTH (m)