Population trend
Trend
Commerson's Dolphin seems to be the most abundant species of the genus Cephalorhynchus (Dawson 2009) although much of its range has not been surveyed and there are only a few estimates of abundance. Leatherwood et al. (1988) conducted aerial surveys in the northeastern Strait of Magellan, Chile, in JanuaryâFebruary 1984 and estimated 3,211 (standard error (SE) = 1,168) dolphins for that area at that time. Venegas and Atalah (1987) estimated abundance in the same area between 12â14 May 1987 as 313 (SE = 99). Later, Venegas (1996) estimated abundance there in late December 1989 as 718 ± 196 individuals. Another survey conducted in June 1996, in the same area, produced an estimate of 1,206 (95% confidence interval (CI) 711â2,049) individuals (Lescrauwaet et al. 2000). Apparently, none of those estimates was corrected for availability or perception bias. Unfortunately, it is difficult to reach any conclusion about population trend due to the differences in methods and analytical approaches used in the various studies.
In Argentine waters, a series of aerial surveys carried out from the mid 1990s to mid 2000s resulted in an estimate of 40,000 individuals between the coastline and the 100 m isobath between 43°S and 55°S (Pedraza 2008). This estimate was derived from three sets of summer surveys different strata as follows: northern Chubut (seven flights between 1994 and 2000), northern Santa Cruz (four flights between 1994 and 1997) and southern Patagonia (i.e. southern Santa Cruz and Tierra del Fuego) (two flights in 2001). The resulting density estimates were 0.052 dolphins/km² (coefficient of variation (CV) = 43.3%), 0.716/km² (CV = 56.05%) and 2.071/km² (CV 27.35%), respectively (Pedraza 2008). There was a latitudinal gradient in density with hundreds of individuals near the northern border of the range and at least 15,000 in Tierra del Fuego (Pedraza 2008). More recently, eight scientific cruises along the Patagonian shelf during austral summer and fall (November-April), 2009-2015, recorded sightings of 88 schools (212 individuals) of Commersonâs Dolphins in 8,535 km surveyed. The Commersonâs Dolphin sightings were all less than 60 km from shore. Fitted models indicated overall abundance of 21,933 individuals CV = 74%, 95% CI = 6,013â80,012) (Dellabianca et al. 2016). The abundance estimates by Pedraza (2008) and Dellabianca et al. (2016) are in the same order of magnitude. However, given the differences in methods, areas surveyed, and level of precision reported by Dellabianca et al. (2016), no conclusion concerning trend in abundance is possible.
Marine mammal and seabirds surveys around the Falkland (Malvinas) Islands found Commersonâs Dolphins to be one of the most frequently observed cetaceans, particularly in inshore waters, with 336 Commersonâs Dolphins recorded, on 100 occasions, during at-sea surveys carried out monthly between February 1998 and January 2001 (White et al. 2002).
The status of the Kerguelen Islands subspecies is less clear than that of the South American subspecies. As of 1985, there had been more than 100 reported sightings, the largest group of about 100 dolphins having been seen near the edge of the shelf (Goodall 1994, Robineau 1989). A seasonal pattern has been documented in the occurrence of Commersonâs Dolphins in the Golfe du Morbihan area, suggesting movements to and from this shallow, semi-enclosed bay. Since 1985, many more observations have been reported in the Golfe du Morbihan, as Commersonâs Dolphins are commonly encountered from boats from the permanent research station that operate within the bay, and a few observations have been reported from other coastal and shelf areas around Kerguelen (V. Ridoux, unpubl. data). Recent capture-mark-recapture analyses based on photo identification studies of dolphins in the Golfe du Morbihan estimated this local population at 69 ± 13 individuals in 2013 (Paul Tixier, pers. comm). No similar figure exists for the rest of the C. c. kerguelensis distribution. The Kerguelen subspecies is restricted in range and is therefore probably very small in number and relatively vulnerable to any anthropogenic threats.