ABSTRACT
The main objective of the present study was to describe the characteristics regarding interactions between Guiana dolphins, Sotalia guianensis and seabirds in feeding associations in two distinct areas of the Lagamar estuary, Brazil. Boat-based surveys directed towards photo-identification studies of S. guianensis were conducted in the Cananéia Estuary (CE) (25 degrees 01' S and 47 degrees 55' W) from July 2004 to March 2008, as well as in the Paranaguá Estuarine Complex (PEC) (25 degrees 24' S and 48 degrees 24' W) from April 2006 to February 2008. On all occasions when seabirds were observed engaging in multi-species feeding associations with S. guianensis, data on species involved and their numbers were gathered. From 435 observed groups of S. guianensis in the CE, 38 (8.7%) involved interactions with seabirds. In the PEC, from the 286 observed groups, 32 (11.2%) involved the mentioned interactions. The following seabirds were observed in feeding associations with S. guianensis: Fregata magnificens, Sula leucogaster, Phalacrocorax brasilianus, and Sterna sp. In the CE, S. leucogaster was more commonly observed in feeding associations with Guiana dolphins (chi2 = 22.84; d.f. = 3, p < 0.05), while in the PEC no differences were reported when comparing seabird species (chi2 = 5.78; d.f.=3, p = 0.1223). In the CE, feeding associations were significantly more frequent in inner waters (subset A0; chi2 = 9.52; d.f. = 2, p < 0.05), and in winter (chi2 = 12.46; d.f. = 1, p < 0.05). Within these events, 44.7% of the association groups were composed by more than one seabird species. Seasonality in feeding associations was also observed in the PEC (chi2 = 4.76; d.f. = 1, p < 0.05), with same patterns observed in the CE. Interactions were more frequent in inner waters of the Laranjeiras bay, PEC (chi2 = 11.65; d.f. = 2, p < 0.05). Within these events, 74.2% of the association groups were composed by more than one seabird species. Water transparency, prey and seabird abundance and distribution, cetacean group size, and the life cycle of prey and seabirds are listed as the main factors addressing multi-species feeding associations in the Lagamar estuary.
Subject(s)
Birds/physiology , Dolphins/physiology , Ecosystem , Feeding Behavior , Animals , Birds/classification , Brazil , Dolphins/classification , Population Density , SeasonsABSTRACT
The aim of the present study was to describe the characteristics of Guiana dolphins (S. guianensis) group size and composition in the Paranaguá Estuarine Complex (25 degrees 15'-25 degrees 36' S and 48 degrees 02'-48 degrees 45' W), Paraná State, as well as to verify the existence of relationships between a given environmental variable (water depth, salinity, transparency and temperature) and group size. An area of around 124 km(2) was surveyed by boat from April 2006 to February 2008 in the following subsets of the estuary: Canal do Superagui (approximately 28 km(2)), Pinheiros bay (approximately 34 km(2)), part of Laranjeiras bay, which included the Guaraqueçaba sub-estuary (approximately 38 km(2)), and part of the Mixture Section of the Paranaguá Estuarine Complex (approximately 24 km(2)). In 45 survey days, a total of 147 hours were spent observing 286 groups of S. guianensis. Groups varied from two individuals to aggregations of up to 90 individuals (mean +/- SD: 11.5 +/- 14.4 individuals). Solitary individuals were seen only on five occasions (1.7% of all observations). Groups with calves (n = 247) represented 86.4% overall and were more common in all sub-areas when compared to groups without calves. Groups without calves were found in all sub-areas and were usually smaller and composed of up to 12 individuals. A usual group composition of 70% of adults and 30% of calves was observed considering all sub-areas and seasons. No correlations were found between group size and water temperature (Spearman's rank test, r = 0.0059, p = 0.9338), transparency (Spearman's rank test, r = 0.0597, p = 0.9333), depth (Spearman's rank test, r = 0.1421, p = 0.4698), and salinity (Spearman's rank test, r = -0.1938, p = 0.0665). While Guiana dolphin groups were seen in the entire surveyed area, group size varied significantly among the three main surveyed sub-areas (Kruskal Wallis, H2,196 = 29.85, p = 0.0000). Groups were larger in Laranjeiras bay and smaller in Canal do Superagui. The physical environment, risk of predation, seasonal distribution and abundance of prey are the main possible factors driving larger groups in inner and protected waters.