Daniel M. Palacios et al. 2012. Cetacean distribution and relative ...

48 PALACIOS et al.: CETACEAN DISTRIBUTION AND ABUNDANCE IN COLOMBIAbetween oceanographic stations at a cruising speed of 10 knotsfor Malpelo (18.5km/h) and 11 knots (20.4km/h) forProvidencia. An observer searches for cetaceans from theship’s flying bridge 10m above the water line using 7x50mmhand-held binoculars. Unlike the SWFSC surveys, sightingsare not closed on, so only cetacean groups that occur near theship can be identified and counted. For this reason, searcheffort is only conducted under acceptable viewing conditions(no fog or rain, and in Beaufort sea states of 3 or less).Dive and Seascape charter tripsFundación Malpelo and Fundación Yubarta have also placedmarine mammal observers on 39 chartered trips to theoffshore islands of Malpelo and Gorgona between 2004 and2008. Twenty-four of these were dive trips organised by thecompany Embarcaciones Asturias and 17 were dedicatedresearch trips conducted under the auspices of ConservationInternational’s ‘Eastern Tropical Pacific Seascape Program.’One additional dive charter trip in 2008 organised byFundación Cabo Mar covered the coastal segment from CaboMarzo-Buenaventura. All trips used M/V Maria Patricia,which has a cruising speed of 8 knots (14.8km/h).Observations took place during transit between theoriginating port (Buenaventura) and the island destinationsas well as during transit between destinations. Searching wasconducted by one to three observers from the vessel’s topdeck at a height of 4m above the waterline under acceptableviewing conditions (no fog or rain, and in Beaufort sea statesof 3 or less). The vessel was diverted to approach sightingswhen conditions allowed during the Seascape trips but notduring the dive trips. Sightings collected at the dive sites (i.e.at anchor or while circumnavigating the islands) wereremoved from the data set to avoid introducing bias by islandassociatedcommunities at these biologically rich localities.Sports-fishing charter tripsCetacean observations were collected on coastal sportsfishingtrips organised by Fundación Cabo Mar between2000–2001 and 2004–2008. During these periods, 20 dailytrips were conducted along one of three coastal routes:Buenaventura-Bahía Solano, Bahía Solano-Cupica andCupica-Cabo Marzo. An additional route between thelocality of El Valle and offshore waters was covered on oneoccasion. Two fiberglass boats with outboard engines wereused: M/V La Cotizada (23ft long, two 40hp engines)between 2000 and 2005; and M/V El Gran Blanco (32ft, two115hp engines) between 2006 and 2008. Two observerslocated near the boat’s bow collected sighting data at a heightof 2.2m and 2.7m above the water line, respectively for eachboat. Travel speed was variable, ranging between 12 and30km/h depending on navigation conditions.DATA ANALYSISSighting categoriesIn the field, sightings were classified under 38 differentidentification categories including subspecies, species andhigher taxonomic levels such as genus, family, etc. Foranalysis purposes, several of these categories were pooledinto more manageable or meaningful species groupings asfollows. Species having several recognised subspecies orsighting categories (see Dizon et al., 1994) were pooled intoa single species. Thus, a single ‘pantropical spotted dolphin(Stenella attenuata)’ category was created from threecategories: offshore; coastal and unidentified subspecies.Similarly, a ‘spinner dolphin (Stenella longirostris)’ categorywas created by pooling four categories: eastern; whitebelly;Central American and unidentified subspecies. Conversely,sightings possibly belonging to several related species werecombined into a single category when field identification tospecies level was not possible due to inconspicuousbehaviour and/or lack of easily distinguishable features. Thiswas the case for unidentified sightings in the genusMesoplodon, for which a ‘mesoplodont whales (Mesoplodonspp.)’ category was created by pooling sightings ofBlainville’s beaked whale (M. densirostris) with sightingsidentified to the genus level only.A single-species category was created by pooling confirmedsightings of the target species with sightings belonging eitherto the target species or to a related species when the two aredifficult to separate in the field. This was done only in caseswhere biogeographic evidence indicates that the presence ofthe related species in the study area is unlikely or extremelyrare. Thus, a ‘Bryde’s whale (Balaenoptera edeni)’ categorywas created by pooling sightings positively identified asBryde’s whales (i.e., when the auxiliary ridges on the head ofthe animal were clearly seen) with those identified as ‘eithersei (B. borealis) or Bryde’s whales’ (i.e. when a very closeexamination at the animal’s head was not obtained andtherefore identification remained uncertain). Positiveidentification of the sei whale in the field can be very difficult,but no confirmed sightings of this species have been madeanywhere in the eastern tropical Pacific during the longstandingsurveys by the SWFSC. In a similar manner, a ‘dwarfsperm whale (Kogia sima)’ category was created by poolingsightings positively identified as dwarf sperm whales withthose identified as Kogia sp. In this case, it is possible thatsome of the Kogia sp. sightings belonged to the pygmy spermwhale (K. breviceps), although only one confirmed sightingof this species has been reported south of 24°N by theextensive SWFSC surveys.Sightings classified under nine ‘unidentified’ categories:unidentified beaked whale; unidentified rorqual(Balaenoptera sp.); unidentified dolphin; unidentified smallwhale; unidentified large whale; unidentified cetacean;unidentified whale; unidentified small delphinid andunidentified medium delphinid, were not used in this study.In this manner, 20 sighting categories are reported here: 19individual species and the genus Mesoplodon.Encounter ratesSpecies encounter rates were computed for each data sourceseparately (except for the Siben and Odyssey expeditions,which lacked effort data), based on the sightings recordedwhile search effort was being conducted. These are reportedas group sightings per unit search effort in Appendices 1–5using the original measurement unit recorded (i.e. kilometersor hours). Since the methodology for group size estimationwas variable among the different sources (see section on‘Biases and caveats’), average group size was not used in thecalculation of relative abundance to avoid introducing furtherbias. For ease in presentation and discussion, the estimatedencounter rates for all sources are summarised in Table 3 in