Dolphin Group Size Research Articles

Clymene dolphins (Stenella clymene) in the eastern tropical Atlantic: distribution, group size, and pigmentation pattern

The Clymene dolphin (Stenella clymene) is endemic to warm Atlantic Ocean waters and is one of the least known delphinids. We reviewed existing published data (primarily strandings, captures, and bycatch) and unpublished sightings to examine the distribution, habitat, group size, seasonality, and pigmentation patterns of Clymene dolphins within the eastern tropical Atlantic (ETA). Following photographic verification, 84 confirmed and 8 probable at-sea sightings of ETA Clymene dolphins were analyzed. The documented records ranged from ∼19°N in central Mauritania to 14°26′S in southern Angola, confirming 14 ETA range states. At-sea sightings occurred in water depths of 437–5,000 m and at distances from shore of 21–937 km, corresponding with a continental slope and oceanic habitat. Sightings within the wider ETA region had year-round occurrence. Group size ranged from 3–1,000 animals, with 60.9% of groups comprising ≤ 50 animals. Photographic examination revealed a complex and variable pigmentation pattern in ETA Clymene dolphins, which contrasts with the simple tripartite pattern often described for this species. In particular, most ETA animals had a dark gray lateral stripe extending diagonally along the flank from the beak to the genital area and a conspicuous dark gray eye–flipper stripe of varying intensity and thickness. The at-sea sightings documented here significantly extend current knowledge of the distribution, ecology, and appearance of the Clymene dolphin.

Quantifying the effect of boat disturbance on bottlenose dolphin foraging activity

Assessments of anthropogenic impacts on marine wildlife often concentrate on large-scale displacement. However, changes in the activity patterns of animals that do not flee could also affect their energy balance. Increasing boat traffic raises concerns for exposed marine mammals. Understanding risk perception is critical for effective mitigation and management, but it is hard to disentangle the effect of noise, physical presence of boats, and context. We used passive acoustic techniques to quantify how boat disturbance affected bottlenose dolphin foraging activity, and characterized the conditions influencing responses. To account for potential masking effects of boat noise on foraging vocalizations (buzzes), we developed a novel procedure to estimate the relationship between buzz detection probability and noise levels in particular 1/3 octave bands. Bayesian hierarchical modeling was then used to assess the effect of boat presence on buzz occurrence, as well as potential interactions with noise level, location, year, day, hour, dolphin group size, and boat type and number. Our results indicate that boat presence was associated with a short-term 49% reduction in foraging activity, but there was no relationship with noise level. Differences between sites and between years suggested a variable susceptibility depending on foraging conditions. This effect increased for increasing number of boats and depended on boat type. This is the first study conclusively showing that boat physical presence, and not just noise, plays a large role in disturbance.

Interactions between short-beaked common dolphin (Delphinus delphis) and the winter pelagic pair-trawl fishery off Southwest England (UK)

During offshore and onshore studies (2004 to 2009), the interactions between pair-trawls and short-beaked common dolphins (Delphinus delphis) were studied to better understand the impact of bycatch. A ‘hotspot’ area where pair-trawls overlapped with high dolphin abundance was identified. We made comparisons between boat-based data collected in absence and presence of pair-trawlers. The relative abundance and group-size of dolphins was significantly higher in the presence of pair-trawlers. Dolphins were observed associating with towing and hauling procedures. Significantly, more carcasses occurred in areas with hauling-activity than those without. Body-temperatures obtained from carcasses found near operating pair-trawlers indicated that bycatch mostly occurred at night. During necropsy studies, difficulties were encountered in identifying the fishing-gears responsible. Strandings data highlighted that the number of dead stranded dolphins was probably much higher than previously reported and there was a significant difference in the age and gender-composition of carcasses. Mature/sub-adult males appeared at greater risk from entanglement in pair-trawls offshore, whilst females with young appeared more vulnerable to inshore gillnets. Our findings show that the overlap between pelagic fisheries and the common dolphin hotspot is causing direct mortality through bycatch and, together with recent range-shifts, may have contributed to a localised decline of this species in this winter hotspot since 2007. Key words: Bycatch, common dolphin, Delphinus delphis, pair-trawl fishery, gillnets, strandings, English Channel, conservation, abundance, trawling, mortality.

River dolphins as indicators of ecosystem degradation in large tropical rivers

Human stressors are currently impacting both the Amazon and Orinoco river basins and these are likely to increase. However, there is a lack of standardized monitoring programs to track these human stressors in most of the countries that overlap these basins, and no clear ecological indicators have been identified. In this study we investigated the relationships between measures of ecosystem degradation and river dolphins as potential ecological indicators. The presence of human stressors and their distance from the areas surveyed were used to provide an estimate of ecosystem degradation. We tested three ecological indicators of freshwater ecosystem degradation using river dolphins: (i) density of river dolphins, (ii) mean group size of dolphins, and (iii) dolphin sighting rates. We found a strong negative relationship between measures of habitat degradation and river dolphin density estimates in selected locations of the Amazon and Orinoco. Therefore, we suggest that river dolphins are good candidates as ecological indicators, flagship and sentinel species for monitoring the conservation status of large tropical rivers in South America. We suggest that further effort should be directed toward collecting reliable data on human stressors, creating collaborative networks for compiling existing data, and documenting and monitoring current trends in freshwater ecosystem degradation and indicator species in the Amazon and Orinoco basins with the goal of targeting areas for recovery or sustainable management.

Population Size of Indo-Pacific Humpback Dolphins (Sousa chinensis) at Khanom, Thailand

Indo-Pacific humpback dolphins (Sousa chinensis) were estimated with the Mark-Recapture Model from a boat-based photo-identification survey at Khanom, Nakhon Si Thammarat, Thailand from July 2008 - June 2009. Indo-Pacific humpback dolphins were encountered at a water depth of 1.1 - 7.5 m, a distance off shore of 21 - 1,021 m and water transparency of 67 - 256 cm. Groups and solitary Indo-Pacific humpback dolphins were sighted 39 times. They were extensively viewed in the coastal zone between Nang Kham bay to Thong Ching bay. There were 34 sightings of groups and five sightings of solitary individuals. A total of 49 individuals were identified and catalogued. There were 33 identified adults, eight juveniles and eight calves. The group size of humpback dolphins varied from 2 - 20 individuals. The group size of adults ranged between 2 - 12 dolphins. The juveniles group size ranged from 0 - 6 dolphins. There were no groups observed consisting solely of juveniles. Groups without juveniles were seen 8 times. The number of calves per group ranged between 0 - 4 individuals. The percentage and number of calves per group were significantly lower than the percentage and number of juveniles. The mean size of groups containing calves was larger than non-calf groups. The estimated abundance of this population from the Jolly-Seber method was 49 individuals.

Ecological aspects of marine tucuxi dolphins (<i>Sotalia guianensis</i>) based on group size and composition in the Cananéia Estuary, southeastern Brazil

Observations on group size and composition of marine tucuxi dolphins, Sotalia guianensis, found in the Cananéia estuary (25°03'S, 47°55'W), southeastern Brazil, were conducted from May 2000 to July 2003. Correlations among four environmental variables (water temperature, salinity, transparency and depth) and group size were investigated, as well as on the way foraging and feeding behavioral patterns related to group formation in S. guianensis. The surveyed area comprised approximately 132km² and field efforts were conducted in 87 days. A total of 378 groups were recorded. Group size (mean ± SD: 12.4 ± 11.4 individuals) varied from lone individuals, rarely seen, to aggregations of up to 60 individuals. No significant differences were found regarding group size among seasons. Groups with calves (89.7% of all observed groups) were more frequent than groups without calves. Feeding associations with marine birds were common in winter when waters were clearer. A positive correlation was only reported when investigating group size and water depth (Spearman's rank test, r=0.4716, p=0.0000). Marine tucuxis usually form large aggregations (20 to 60 individuals) when engaged in feeding activities in deeper waters. Groups were larger (mean ± SD: 18.3 ±15.0 individuals, n=90) in the main estuary entrance, where water is deeper, and smaller in inland shallower waters (mean ± SD:7.8 ± 6.9 individuals, n=47). Because of these common observed aggregations, this area of the estuary has been used as a dolphin watching site since the late 1990s without specific regulations. Guidelines are urgently needed to conduct dolphin watching tours as calves are seen year-round and uncontrolled tourism may disrupt important social interactions.

Bottlenose dolphin (Tursiops truncatus) presence and incidental capture in a marine fish farm on the north-eastern coast of Sardinia (Italy)

On the north-eastern coast of Sardinia, from November 2004 to January 2006, the first attempt in the Mediterranean basin to obtain information on encounter rate, group size and incidental capture of bottlenose dolphins in a marine fish farm was assessed, combining direct observations from fish farm boats with photo-identification studies. During 15 months of research, 79 d (65.3% of the total monitored days) were spent in direct observation of 146 groups of bottlenose dolphins around the fish farm cages. There was a peak in bottlenose dolphin presence during winter. Photographs were taken in the fish farm area during 79 encounters on 61 different days (totalling 34 marked individuals). The regular occurrence of some dolphins suggests individual preferences for the fish farm area. The incidental bottlenose dolphin capture observed in large, loose predator nets (1 dolphin per month) is cause for concern, as it is questionable whether or not the bottlenose dolphins in the area can sustain incidental capture of this magnitude. The information gained from this study shows the necessity for further regulations to be established, both in the use of predator nets and management of marine fish farms.

Hawaiian spinner dolphins and the growing dolphin watching activity in Oahu

Hawaiian spinner dolphins (Stenella longirostris) feed offshore at night on mesopelagic prey and move to protected shallow waters in early morning to rest. We hypothesized that this behaviour would make them particularly sensitive to anthropogenic factors in these rest areas and would affect their overall behaviour and their population frequency. To test our hypothesis we collected data in a known rest area along the north-west coast of the island of Oahu during August of 2001, 2002 and 2003. Using land- and water-based surveys, we evaluated dolphin group size, counted boats, kayaks and swimmers in the vicinity of the dolphins (<40 m) and reported any changes in dolphin behaviour. Our results demonstrated a stable dolphin habitat frequency over the three summers, which emphasizes the critical value of this rest area for this spinner dolphin population. The results also showed an increase in dolphin-oriented activities, but we could not draw any definitive conclusion on their real impacts on spinner dolphin ecology and ethology.

Whistles of Bottlenose Dolphins (<I>Tursiops truncatus</I>) in the Sado Estuary, Portugal: Characteristics, Production Rates, and Long-Term Contour Stability

This study focuses on the whistle characteristics and production patterns of bottlenose dolphins (Tursiops truncatus) resident in the Sado Estuary, Portugal. Recordings and observations were conducted inside the estuary and in adjacent coastal waters using single hydrophones between 1987 and 2000. In the groups sampled, the mean number of whistles emitted per minute per animal was 0.28. The acoustic characteristics of a sample of 735 whistles were measured and compared with data from other Tursiops populations, showing that, in a pattern of overall similarity, the whistles recorded in the Sado are relatively long and the frequency range used is relatively wide. Mean peak frequency was 9.2 kHz. About 30% of the whistles were stereotyped, and remarkable stability was found in several contours over a 12-year period. No relation was found between dolphin group size and whistle rate, suggesting some restriction in production; and no relation was found between dolphin group size and the emission of different contours. Significant variation was found between episodes of simple travel and more aroused activities in terms of the production of whistles in general and also of different whistle contours.

Occurrence and behavioral patterns of the spotted coastal dolphin Stenella attenuata (Cetacea: delphinidae) in the Gulf of Papagayo, Costa Rica

Dolphins are characterized by a significant behavioral versatility, which allows them to respond to environmental seasonality. Seasonal variation in dolphin behavior in tropical waters is not well known. Stenella attenuata graffmani is a resident dolphin in the clearly defined seasonal Gulf of Papagayo, Costa Rica, and we studied if dolphin group size, occurrence and behavioral patterns were associated with season and time of day in the gulf. Using strip transects we surveyed two locations for three consecutive years. School size ranged from 1 to 50 individuals, mean group size was 10.16 (SD = 9.61) individuals. Overall, foraging activities were the most frequent, followed by social interactions and travel. From 6:00 AM to 9:00 AM we mostly observed social interactions, followed by feeding-socializing (9:00 AM-12:00 PM) and feeding exclusively (12:00 PM-3:00 PM). Social activities intensified afterwards (3:00 PM-6:00 PM). Behavior and gulf seasonality were associated (chi2 = 90.52, gl = 6, p<0.05, n = 99). In the dry season (December-April) feeding predominated over other activities, but socializing was more frequent in the early rainy season (May-July). Larger groups (mean 12 dolphins) forage actively; smaller groups (mean 6 dolphins 6.51 +/- 5.12) foraged more passively. Seasonal variation in dolphin activities are likely to be associated with food availability, as observed in the high number of groups involved in foraging behaviors, and a high investment in foraging activities during the dry season.

Parallel influence of climate on the behaviour of Pacific killer whales and Atlantic bottlenose dolphins

AbstractThe grouping behaviour of animals is governed by intrinsic and extrinsic factors which play an important role in shaping their social organization. We investigated the influence of ocean climate variation on the grouping behaviour of two widely separated populations of cetaceans, inhabiting north Atlantic and north Pacific coastal waters. The group size of both bottlenose dolphins in the Moray Firth, UK, and killer whales in Johnstone Strait, Canada, varied from year to year in relation to large‐scale ocean climate variation. Local indices of prey abundance were also related both to climate indices and predator group sizes. The cetaceans tended to live in smaller groups when there was less salmon available in both areas which seem to occur 2 years after a lower phase of the North Atlantic and Pacific Decadal Oscillations. These findings suggest that, even in highly social mammals, climate variation may influence social organization through changes in prey availability.

Bottlenose Dolphins (&lt;I&gt;Tursiops truncatus&lt;/I&gt;) Increase Number of Whistles When Feeding

We examined the hypothesis that dolphins increase their rate of sound production during feeding events to recruit new individuals. We recorded 135.5 min of underwater sounds from bottlenose dolphins (Tursiops truncatus) near Isla del Coco, Costa Rica. Data were collected from eight feeding groups and three nonfeeding groups. We classified sounds as whistles, click trains, or pulse bursts. The number of whistles per min per dolphin was higher in feeding groups than in nonfeeding groups. More whistles than click trains or pulse bursts were produced when dolphins were feeding. On the other hand, there was no difference in the proportion of each sound type produced when dolphins were not feeding. New dolphins joined the feeding events for which we recorded dolphin sounds. Results supported the hypothesis that dolphin group size increases in response to an increase in the number of whistles by conspecifics; however, confounding factors, such as the use of specific feeding calls, need to be accounted for to support the increased sound-rate hypothesis.

Hawaiian spinner dolphins aggregate midwater food resources through cooperative foraging

To test the hypothesis that spinner dolphins in Hawaii may actively aggregate their prey through cooperative foraging, a 200-kHz multi-beam sonar (Simrad MS2000) was used to observe 323 groups of spinner dolphins foraging within a midwater, micronekton sound-scattering layer off Oahu. Strong cooperation was observed in groups of 8–14 pairs of spinner dolphins. The dolphin group size was highest at midnight when the density of prey was highest and was significantly higher in Makua Beach, where the prey density was higher, than Electric Beach, where the prey density was lower. Cooperative groups of dolphins aggregated their food resources, apparently using their preys’ avoidance behavior to create distinct, high-density patches in the prey. Prey aggregation was strongly stereo-typed, regardless of the distribution of the scattering layer. Dolphins swam around the edge of a 28–40 m diameter circle at least 5 times, concentrating prey within this area before pairs of dolphins on opposite sides of the circle swapped positions in the circle, swimming through the high density prey ’donut’ they had formed. The hypothesis that nocturnal animals aggregate prey in midwater could not have been tested without the three-dimensional information on prey distribution and dolphin geometry provided by the multi-beam.

Interactions between marine predators: dolphin food intake is related to number of sharks

Dolphins and sharks feed at times on the same food; however, the influence of these interactions on the feeding success of either predator has not been measured. I employed underwater video to record bottlenose dolphins Tursiops truncatus and silky sharks Carcharhinus falciformis feeding on the same school of fish, and for the first time measured food intake of free-ranging dolphins. Regression analyses showed that dolphin food intake diminished as the number of feeding sharks increased, but was unrelated to the number of dolphins feeding, size of the prey clump or duration of feeding events. The number of dolphins increased at the beginning of a feeding event in the presence of sharks but not in their absence. This increase apparently provided a benefit to dolphins since the number of sharks feeding was negatively related to the number of dolphins feed- ing. Other studies have indicated that risk of shark predation influences dolphin group size and habi- tat use. This study indicates that interspecific contests over food influence dolphin food intake and perhaps also dolphin group size.

Group Characteristics, Site Fidelity And Seasonal Abundance Of Bottlenosed Dolphins (Tursiops Aduncus) In Jervis Bay And Port Stephens, South-Eastern Australia

Social organisation and abundance of bottlenose dolphins (Tursiops aduncus) in Jervis Bay (JB) and Port Stephens (PS), NSW, were investigated through behavioural/photo-identification surveys between May 1997 and April 2000. Mean group size was significantly larger at JB (12.3 � 0.87, n =167) compared to PS (6.8 � 0.37, n = 218). At both sites, groups were significantly larger when calves were present. Group size varied with activity, being smallest when feeding and largest when socialising. While mean group size of feeding dolphins did not vary between sites, travelling and socialising groups were significantly larger in JB. Site fidelity was assigned based on sighting rates and presence across seasons. Sighting rates varied significantly between areas, but the proportion of dolphins categorised as residents, occasional visitors and transients did not. Minimum abundance by season, based on mark-resighting of recognisable individuals, ranged from 61 � 3.2 to 108 � 7.1 in JB and 143 � 8.1 to 160 � 8.1 in PS. Differences in group size at the two sites may relate to social factors and/or human impacts, while differences in abundance may be associated with habitat size and complexity. The lack of dolphin matches between areas suggests that they represent distinct populations.

Fine-scale habitat selection of foraging bottlenose dolphins Tursiops truncatus near Clearwater, Florida

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 222:253-264 (2001) - doi:10.3354/meps222253 Fine-scale habitat selection of foraging bottlenose dolphins Tursiops truncatus near Clearwater, Florida Mark C. Allen1,*, Andrew J. Read3, Jocelyn Gaudet2, Laela S. Sayigh1 1Department of Biological Sciences, and 2Department of Earth Sciences, University of North Carolina at Wilmington, 601 S. College Road, Wilmington, North Carolina 28403, USA 3Duke University Marine Laboratory, Nicholas School of the Environment, 135 Duke Marine Lab Road, Beaufort, North Carolina 28516, USA *E-mail: allenmc1@aol.com ABSTRACT: Previous studies have proposed that seagrass habitats, by supporting diverse and abundant fish assemblages, are preferred by foraging dolphins in coastal systems. To test this hypothesis, we (1) examined the fine-scale behavior of bottlenose dolphins in relatively pristine and developed inshore sites near Clearwater Harbor, Florida, USA, and (2) used an otter trawl to sample potential fish prey in non-seagrass and seagrass habitats. In the pristine site, dolphins preferred dredged channel and spoil-island habitats, while least preferring shallow seagrass habitats. In the developed site, foraging dolphins preferred the natural channel habitat, but exhibited little further selection. Therefore, the hypothesis that dolphins prefer seagrass habitats while foraging was rejected. Feeding frequency was significantly associated with diel state, with foraging peaking at dawn and decreasing throughout the day. Dolphin group size was negatively correlated and nearest neighbor distance positively correlated with feeding frequency. Analysis of trawl data focused on the pinfish Lagodon rhomboides which dominates the diets of dolphins in west Florida. The relative abundance of pinfish was significantly greater and standard lengths significantly less in seagrass than in non-seagrass habitats. Dolphins therefore forage in non-seagrass habitats where fish prey is both larger and perhaps more available. Although seagrass habitats support greater abundance of smaller fishes, they also provide a structural refuge which obscures fishes both visually and possibly acoustically. Thus, the importance of seagrasses to the health of coastal dolphins is probably indirect, as seagrasses support fish populations on which dolphins rely. KEY WORDS: Tursiops truncatus · Foraging strategy · Prey availability · Diel patterns · Pinfish Lagodon rhomboides Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 222. Online publication date: November 05, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

OCCURRENCE, PHOTO‐IDENTIFICATION AND PREY OF PACIFIC WHITE‐SIDED DOLPHINS (LAGENORHYNCUS OBLIQUIDENS) IN THE BROUGHTON ARCHIPELAGO, CANADA 1984–1998

AbstractThis study summarizes occurrence of Pacific white‐sided dolphins (Lagenorhyncus obliquidens) in the Broughton Archipelago on the west coast of Canada, from October 1984 through December 1998. Dolphins were detected on 472 d. The annual percent of total occurrence rose from 0.4% in 1984 to 19% in 1994 and then declined to 2% in 1998. Seasonal occurrence peaked from 1 October through January. Dolphin group size ranged from 2 to 1,000; the most common range was 11‐50. While unreported for the Broughton Archipelago prior to 1984, the species is represented by teeth distributed throughout the past 2,000 yr of First Nations midden sediment, suggesting sporadic long‐term occurrence. Increased water temperature from the 1937–1984 mean of 8.6°C, to the 1985–1998 mean of 9.3°C and increased abundance of two fish populations in the study area are considered potential factors in the recent increase in occurrence. Of the 675 naturally marked dolphins that were photo‐identified, 214 were resighted. A pair of dolphins was photographed swimming in tandem, fourteen months apart. Tight groups, of five or fewer extensively scarred dolphins with extremely falcate dorsal fins were seen within every aggregation of over 50 animals, suggesting the existence of all‐male associations. Prey species were collected from 25 encounters with feeding dolphins; they included herring (Clupea harengus), capelin (Mallotus villosus), and Pacific sardine (Sardinops sagax). Predation on eulachon (Thaleichthys pacificus) is suspected. Unreported for the Broughton Archipelago, the capelin sampled in this study may belong to the Bering Sea population. Pacific sardines returned to commercial viability on the British Columbia coast in 1997 after a 60‐yr population collapse. Dolphin frequency of occurrence declined following introduction of underwater acoustic deterrent devices into the study area.

Using simultaneous counts by independent observers to correct for observer variability and missed sightings in a shore-based survey of bottlenose dolphins, Tursiops truncatus

Simultaneous counts by independent shore-based observers have been used to generate revised population estimates for gray and bowhead whales, but no similar technique has been applied to shore-based dolphin surveys. Shore-based whale surveys generally rely on a single observation site from which migrating whales are counted as they pass in one direction over a period of weeks to months. Shore-based dolphin surveys, however, typically use multiple observation sites over a much shorter time period (hours) in order to avoid double counting individuals as they change direction. This paper reports on a new technique to correct for observer variability and missed sightings for coastal bottlenose dolphin surveys conducted at Myrtle Beach, South Carolina, USA. Comparisons were made between concurrent counts by 39 pairs of independent shore-based observer teams. A model was developed to revise observer estimates in which the number of observed dolphin groups was multiplied by a correction factor to estimate the true number of groups, and this number in tum was multiplied by the mean group size to determine the total number of dolphins. The true number of dolphin groups was estimated using a modified Petersen mark-recapture estimate, stratified by group-size category. The mean proportion of groups missed by observers was negatively correlated with reported group size: 32.7% for groups of 1-2 dolphins; 16.5% for groups of 3-4 dolphins; and 9.9% for groups of &gt;4 dolphins. A variability factor was also calculated to determine a confidence interval for the average number of dolphins per group, based on the mean percent difference between paired observer teams, stratified by group size. The model was used to calculate revised estimates for shore-based bottlenose dolphin surveys conducted in South Carolina in 1994 and 1995. The original uncorrected abundance estimates were increased by a factor of 1.14 and 1.19 respectively, comparable to similar calculations from shore-based surveys of gray whales. However, the estimated confidence interval of± 38% of the revised estimates is approximately four times the magnitude found in the gray whale studies. This difference is primarily due to the large observer variability for estimated dolphin group size and can be reduced using various revisions of survey design and methodology. Ideal conditions for this technique include elevated observer posts and accurate estimates of the proportion of the population within visual range of the coastline. This study demonstrates that shore-based dolphin surveys are a potentially efficient census technique and an attractive low cost alternative to aerial and boat surveys.

Group size and seasonal pattern of occurrence of humpback dolphins Sousa chinensis in Algoa Bay, South Africa

The occurrence of humpback dolphins Sousa chinensis and their group size in Algoa Bay, South Africa, are described for the period May 1991–May 1994. The mean group size of dolphins was seven (ranging between 3 and 24). Group size was not affected by diurnal, tidal or lunar variability. Solitary, large individuals were commonly observed and they occasionally joined groups of other humpback dolphins. The number of sightings of humpback dolphins per unit effort and their group size varied seasonally, increasing in summer and again in late winter. These observations follow a regular seasonal fluctuation in water temperature, coincide with the dolphins’ reproductive seasonal cycle and could be related to seasonal changes in the abundance and distribution of their prey.

Feeding Associations between White-Fronted Terns and Hector's Dolphins in New Zealand

I investigated the relationship between White-fronted Terns (Sterna striata) associating with Hector's dolphins (Cephalorhynchus hectori) while feeding in the near-shore environment of New Zealand. Associations were observed only in the austral spring and early summer. At this time up to 15.7% of all dolphin groups were accompanied by terns. The birds usually joined surface-feeding dolphins with single terns apparently hovering over single dolphins. Associated tern numbers were positively correlated with dolphin group size and increased significantly with the duration of the association. Terns were more often associated with significantly larger dolphin groups and under calmer sea conditions. Water clarity, however, appeared to have little influence in the birds' decision whether or not to associate with a group of foraging dolphins. The occurrence of feeding associations appeared to be restricted by the occurrence of small, inshore moving fish species. The associations are likely to constitute facultative commensalism by the birds and may be particularly advantageous during breeding when energetic demands are high.