Bottlenose Dolphins (Tursiops truncatus)
by Juan Diego Pacheco-Polanco

The common bottlenose dolphin is a large, robust dolphin with a short, stubby beak, clearly demarcated from the melon. The tall and falcate dorsal fin is centrally positioned along the back, being relatively thick and blunt at the tip. The flippers are moderately long and slender. The body is uniformly gray above, fading to paler gray on the flanks and paler still, or in some cases pinkish on the belly. The flippers and tail flukes are usually dark gray. Golfo Dulce bottlenose dolphins are social animals, commonly seen in pods of 2 to 15 individuals that might change in time. 
Bottlenose dolphin' Kernel density distribution in Golfo Dulce, Costa Rica. 

We are able to distinguish two ecotypes of bottlenose dolphins in Golfo Dulce and  Costa Rica, based on in their distribution, external morphology and prey types: there is an offshore form, usually found in the mouth of the Golfo Dulce, within or close to the 200 m isobath. This ecotype is morphologically shorter, darker and slimmer than the coastal form. The inshore-coastal eco-type is generally found at the sill and inner basin of the gulf. The coastal ecotype is usually seen foraging at the mouth of the rivers draining into the inner basin of the gulf, bottlenose dolphins use the tidal fronts associated with the tidal cycles and localized at the mouth of these rives to catch prey. One important prey item is the needlefish (Tylosurus acus pacificus). We consider these areas to be a critical foraging habitat for them. 
Needlefish (mexfish.com)

Being closely related with continental waters, this population of bottlenose dolphins are
prone to suffer from adverse effects derived from the various human activities near their preferred habitat, such as coastal development, tuna feed lots, agriculture and especially pesticides (DDT, PCBs, DDE, BHC, Dieldrin, Aldrin, Endrin, etc), as well as forest clear cutting, all present in Golfo Dulce as threats to this coastal population. If not properly mitigated, the current situation might even result in a relictual distribution or the disappearance of this key predator from such important coastal-marine area.        

Coastal Pantropical Spotted Dolphins 
(Stenella attenuata graffmani)
by Lenin Oviedo

The coastal form of the pantropical spotted dolphin (Stenella attenuata graffmani) is the
ecological analogue of the Atlantic spotted dolphin in the coastal environs of the Eastern Tropical Pacific.  This eco-morph is subdivided into four genetically different management units, with one specific for Costa Rica. Coastal pantropical spotted dolphins are larger and stockier than their oceanic counterparts, their spotting pattern is also more defined and notorious. The home range of this dolphin includes shallow inshore habitats, where the species organized themselves in small groups (12-20 individuals), to very deep (close to shore) open oceanic waters (200 m), where they aggregate in  big groups up to 500 individuals.

Coastal pantropical spotted dolphins' Kernel distribution in Golfo Dulce.

If there is a representative dolphin from Costa Rica’ Pacific, that would definitely be the pantropical spotted dolphins. The occurrence of the species is so important and widespread along the Pacific coast, that it supports most of the small scale whale-watching operations. Spotted dolphins in Golfo Dulce live sympatrically with bottlenose dolphins. This species use the inner basin as an important foraging area. Based on behavioural evidences and group size distribution, we hypothesize that spotted dolphins inhabiting Golfo Dulce, could be so closely affiliated with the structure of their inshore pelagic habitat, that they might have started to diverge genetically from con-specific elsewhere. Research to determine population size, group structure, residency rate as well as emigration/immigration patterns are in progress.

Humpback whales (Megaptera novaeangliae) 
by María Gabriela Silva-Hernández

It is common that when you mention the word “whale”, people imagine the majestic humpback whale, with long black and white pectoral fins or flippers, which can usually be observed in the surface of the water, along with great displays of jumps and splashes (breaching). All these unequivocal characteristics have played an important role in gaining the attention of scientists, environmental collectives and individuals, making this species the best known cetacean species among baleen whales. Humpback whales, called this way due to the prominent hump on their backs, can reach up to 56 feet and weigh up to 40 tons, with flippers of about one-third of its body length. They usually have distinctive knobs or tubercles on their rostrum, on their lower jaw and on the edges of their flippers.

This species has a wide distribution, from the poles to the tropics, in almost all oceans,
and they are known for their long migrations, close to a 10,000 miles round-trip; feeding in cool waters and breeding in warmer areas during the winter time of the poles. Humpbacks
use a social technique, known as bubble-netting, for catching their prey (mainly krill and small schooling fish). The males are famous for singing the longest and most complex songs in the animal kingdom, during their migration and prolonged presence in breeding areas; the songs can vary slightly between individuals of the same population, between years and even more between different populations. 

M. novaeangliae was one of the most persecuted and hunted baleen whales during the commercial whaling era, but latest studies show that populations are recovering well.

Humpback whales' relative density distribution in Golfo Dulce, white dots represents encounters with calves within the group.
Golfo Dulce provides a good nursing and breeding spot for at least two populations of M. novaeangliae (Southeast Pacific and Northeast Pacific). It is possible that some spatio-temporal overlapping occurs between both sub-populations in this area. We have been carrying out field work focused on photo-identification and acoustics, and we are looking forward to starting DNA sampling soon in order to get a better understanding of the stock identity and potential genetic exchange.