AbStraCt.-Variation in skulls of 

 415 dusky dolphins, Lagenorhynchus 

 obscurus, was studied based on 37 

 standard cranial measurements and 

 meristic variables and 28 non- 

 metrical characters (NMC) by using 

 both bivariate and multivariate 

 analyses. Geographic variation was 

 analysed in mature skulls from cen- 

 tral Peru (7v=189), Chile (2V=22), 

 New Zealand (JV=47) and southwest- 

 ern Africa (iV=40). Advanced fusion 

 in the frontal-supraoccipital suture 

 is the most reliable (95^ efficiency) 

 cranial criterion of sexual maturity; 

 distal fusion in premaxillary and 

 maxillary cannot be used. Sexual di- 

 morphism, verified with /-tests and 

 analysis of covariance, was encoun- 

 tered in only 6 of 37 measurements 

 and in none of the NMC characters. 

 Although skull size is the same, 

 males have a wider rostrum and a 

 longer temporal fossa than females. 

 Analyses of covariance and /-tests 

 indicated highly significant differ- 

 ences between geographic groups. A 

 discriminant analysis successfully 

 classified 96.5%, 91.7%, and 90.9% 

 of skulls from respectively the South- 

 east Pacific (Peru and Chile pooled), 

 New Zealand, and SW Africa. Such 

 differentiation is considered of sub- 

 specific level. All methods indicated 

 higher morphological similarity 

 within the Peru/Chile and SW Af- 

 rica/New Zealand grouped pairs than 

 between the pairs. Most strikingly, 

 skulls from New Zealand and SW 

 Africa are on average 3.1cm (8.5%) 

 shorter than these from Peru and 

 Chile, concordant with differences in 

 body length. New Zealand specimens 

 differ from SW African animals by 

 smaller tooth size and a greater 

 number of teeth. Nine cranial vari- 

 ables revealed significant variation 

 between dusky dolphins from cen- 

 tral Peru and northern Chile, but 

 the degree of heterogeneity should 

 be re-evaluated with a larger sample 

 from Chile. NMC characters con- 

 firmed the trends mentioned but 

 resolving power was limited; their 

 use is not recommended as the prin- 

 cipal method for subspecific dis- 

 crimination. The Peruvian dusky 

 dolphin is thought to represent the 

 most ancestral (plesiomorph) form 

 from which the others were derived 

 through dispersion via the east- 

 flowing Westwind Drift. 



Manuscript accepted 24 June 1993. 

 Fishery Bulletin 91:754-774 1 1993). 



754 



Geographic variation and sexual 

 dimorphism in the skull of the 

 dusky dolphin, Lagenorhynchus 

 obscurus (Gray, 1 828) 



Koen Van Waerebeek 



Centra Peruano de Estudios Cetologicos (CEPEC) 

 Asociacion de Ecologia y Conservacion 

 •casilla 1536, Lima- 18, Peru 



The dusky dolphin, Lagenorhynchus 

 obscurus (Gray, 1828), is mainly en- 

 countered in temperate coastal wa- 

 ters off Peru, Chile, Argentina, New 

 Zealand, and South Africa (Gaskin, 

 1968; Webber, 1987; Van Waerebeek, 

 1992, a and b). Records, either speci- 

 mens or sightings, are also available 

 from Namibia 1 (Rose and Payne, 

 1991) and Angola 2 . 



This species was originally sug- 

 gested to have a continuous circum- 

 polar distribution (Brownell, 1965; 

 Hershkovitz, 1966; Morzer Bruyns, 

 1971); however, owing to its coast- 

 dwelling habits many authors favor 

 the notion of a disjunct distribution 

 and existence of several isolated 

 stocks (Brownell, 1974; Anonymous, 

 1975; Leatherwood and Reeves, 1983; 

 Webber, 1987; Webber and Leather- 

 wood, 1990). Nevertheless, evidence 

 to support this hypothesis is lacking. 

 Possible pelagic occurrence of dusky 

 dolphins in the southern oceans and 

 off many oceanic islands remains un- 

 known. Specimen records are docu- 

 mented from the Campbell, Auck- 

 land, and Chatham Islands around 

 New Zealand (Gaskin, 1968; Baker, 

 1983; Webber, 1987), the Falkland Is- 

 lands (British Museum of Natural 

 History specimens; Brownell, 1965) 



and from Gough Island (40°20'S, 

 10°00'W) on the mid-Atlantic Ridge'. 

 Presence of dusky dolphins off south- 

 ern Australia and Tasmania (re- 

 viewed by Webber, 1987; Van Waere- 

 beek, 1993) and off the Kerguelen 

 Islands (Robineau, 1991) could not 

 be confirmed. Kasamatsu et al. 4 re- 

 ported a single sighting from 

 south of Madagascar but no photo- 

 graphs were taken and both the un- 

 usual (for L. obscurus) sea surface 

 temperature (25°C) and offshore lo- 

 cation solicits some caution. The 

 sighting of a group of Lagenorhyn- 

 chus sp. in deep tropical waters of 

 the South Pacific, for instance, re- 

 sulted in considerable debate before 

 they were identified as L. australis, 

 despite the availability of excellent 

 photographs (Leatherwood et al., 

 1991). 



Incidental fishing mortality occurs 

 throughout the range of dusky dol- 

 phin and it is heavily exploited in Peru 

 (Read et al., 1988; Van Waerebeek and 

 Reyes, 1990, 1993) and, locally, off 

 northern Chile (Van Waerebeek and 



'P.B. Best, South African Museum, pers. 

 commun. February 1991. 



-P.B. Best. South African Museum, unpubl. 

 photograph, identification confirmed by author. 



'P.B. Best, South African Museum, unpubl. 

 photographs, identification confirmed by the 

 author. 



'Kasamatsu, F, G. Joyce, P. Ensor. and J. 

 Mermoz. 1990. Current occurrence of Cetacea 

 in the southern hemisphere; results from the 

 IWC/IDCR southern hemisphere minke whale 

 assessment cruises. 1978/79-1987/88. Doc S( ' 

 42/015 presented to the Int. Whal. Comm. Sci. 

 Committee, 77 p. 



