Environmental Niche Overlap between Common and Dusky Dolphins in North Patagonia, Argentina

Abstract

Research on the ecology of sympatric dolphins has increased worldwide in recent decades. However, many dolphin associations such as that between common dolphins (Delphinus delphis) and dusky dolphins (Lagenorhynchus obscurus) are poorly understood. The present study was conducted in the San Matías Gulf (SMG) ecosystem (North Patagonia, Argentina) where a high diet overlap among both species was found. The main objective of the present work was to explore the niche overlap of common and dusky dolphins in the habitat and temporal dimensions. The specific aims were (a) to evaluate the habitat use strategies of both species through a comparison of their group attributes (social composition, size and activity), and (b) to evaluate their habitat preferences and habitat overlap through Environmental Niche modeling considering two oceanographic seasons. To accomplish these aims, we used a historic database of opportunistic and systematic records collected from 1983 to 2011. Common and dusky dolphins exhibited similar patterns of group size (from less than 10 to more than 100 individuals), activity (both species use the area to feed, nurse, and copulate), and composition (adults, juveniles, and mothers with calves were observed for both species). Also, both species were observed travelling and feeding in mixed-species groups. Specific overlap indices were higher for common dolphins than for dusky dolphins, but all indices were low, suggesting that they are mainly segregated in the habitat dimension. In the case of common dolphins, the best habitats were located in the northwest of the gulf far from the coast. In the warm season they prefer areas with temperate sea surface and in the cold season they prefer areas with relatively high variability of sea surface temperature. Meanwhile, dusky dolphins prefer areas with steep slopes close to the coast in the southwestern sector of the gulf in both seasons.

Fig 1. Study Area.

(a) Map showing the location of the study area and latitudinal limits of the sympatric area of common dolphins (Delphinus delphis) and dusky dolphins (Lagenorhynchus obscurus) in the Patagonia Shelf Ecosystem. (b) Map showing systematic surveys (transects) conducted between 1995 and 2011 in the study area.

Fig 2. Sightings of common dolphins (Delphinus delphis), dusky dolphins (Lagenorhynchus obscurus) and mixed-species groups.

(a) Warm season and (b) cold season. Study period (1983–2011). Maps also show means SST gradients obtained from AVHRR (NOAA) satellite images of each study season.

Fig 3. Group size distributions.

Frequency distributions (percentages) of group size categories for (a) common dolphins (Delphinus delphis), (b) dusky dolphins (Lagenorhynchus obscurus) and (c) mixed-species groups of common and dusky dolphins. The number in parentheses indicates the sample size.

Fig 4. Social composition of groups.

Percentages of occurrence of social categories for common dolphins (Delphinus delphis), dusky dolphins (Lagenorhynchus obscurus) and mixed-species groups recorded in each season. (A+J) adults and juveniles, (MwC) mothers with calves, (A+J+MwC) mixed combination of mothers with calves, juveniles and adults. The number in parentheses indicates the sample size.

Fig 5. Predominant group activities.

Percentages of occurrence of predominant group activities for common dolphins (Delphinus delphis), dusky dolphins (Lagenorhynchus obscurus) and mixed-species groups. Data were only obtained from systematic nautical surveys. The number in parentheses indicates the sample size.

Fig 6. Averaged Maxent Models.

Maps of mean habitat suitability resulting from MaxEnt modeling of common dolphins (Delphinus delphis, “a” and “b”) and dusky dolphins (Lagenorhynchus obscurus “c” and “d”) within the study area for each study season.

Fig 7. Response curves of dolphin habitat suitability to environmental variables.

Averaged response curves of the two most important environmental variables resulting from Maxent modeling of common dolphins (Delphinus delphis) and dusky dolphins (Lagenorhynchus obscurus) localities. (a) Response curve of common dolphin habitat suitability to Standard Deviation of Sea Surface Temperature (SST-SD) in the cold season; (b) response curve of dusky dolphin habitat suitability to Seabed Slope in the cold season; (c and d) response curves of common and dusky dolphin habitat suitability to distance to the coast in the warm and cold season respectively (curves of both species are plotted together in order to facilitate visual comparisons); (e) response curve of common dolphin habitat suitability to Sea Surface Temperature (SST) in the warm season; (f) response curve of dusky dolphin habitat suitability to Depth in the warm season.