Van Waerebeek: Variation in skull morphology of Lagenorhynchus obscurus 



755 



Guerra, 1986 5 ; Guerra et al., 1987). Population identi- 

 fication is needed so that breeding units can be man- 

 aged separately. Cranial measurements have been used 

 to discriminate populations in several delphinids in- 

 cluding the common dolphin, Delphinus delphis (Evans, 

 1975, 1982; Heyning and Perrin*), bottlenose dolphin, 

 Tursiops truncatus (Ross, 1977; Walker, 1981 7 ), tucuxi, 

 Sotalia fluviatilis (Borobia and Sergeant, 1989), 

 pantropical spotted dolphin, Stenella attenuata, and 

 spinner dolphin, S. longirostris (Perrin, 1975, a and b; 

 Perrin et al, 1979; Schnell et al., 1982 H , 1985; Douglas 

 etal., 1984). 



In addition to skull measurements, non-metrical cra- 

 nial characters have also been employed in population 

 studies: harbor porpoises, Phocoena phocoena ( Sjovold, 

 1977; Kinze, 1985, 1990 9 ), spotted and spinner dol- 

 phins (Perrin et al, 1982 10 ; Yablokov et al, 1983) and 

 common dolphins (Perrin et al, 1988", 1989). The re- 

 solving power of non-metrical methods has been diffi- 

 cult to assess, however, because the identities of the 

 dolphin stocks on which the technique was applied 

 were debatable and results somewhat equivocal (Perrin 

 et al., 1988", 1989). Also, non-metrical methods are 

 more prone to observer bias and drift in character- 



"'Van Waerebeek, K. , and C. Guerra. 1986. Review of the distribu- 

 tion and status of the Burmeister's porpoise in Chile. Cetacean sur- 

 vey in II region of Chile, August-September 1986. Institute de 

 Investigaciones Oceanologieos, Universidad de Antofagasta, casilla 

 170, Antofagasta, Chile. Unpubl. Rep., 13 p. 



6 Heyning. J. E.. and W. F. Perrin. 1991. Re-examination of two forms 

 of common dolphins (genus Delphinus) from the eastern North Pa- 

 cific; evidence for two species. Dep. Commer., NOAA, Natl. Mar. 

 Fish. Serv., Southwest Fish. Sci. Cent., P.O. Box 271, La Jolla, CA 

 92038. Admin. Rep. LJ-91-28, 37 p. 



7 Walker, W. A. 1981. Geographic variation in morphology and biol- 

 ogy of bottlenose dophins tTurswps) in the eastern North Pacific. 

 Dep. Commer.. NOAA, Nat. Mar. Fish. Serv., Southwest Fish. Sci. 

 Cent., P.O. Box 271, La Jolla, CA 92038. Admin Rep. LJ-81-03C, 

 54 p. 



s Schnell, G. D., M. E. Douglas, and D. J. Hough. 1982. Geographic 

 variation in morphology of spotted and spinner dophins iStenella 

 attenuata and S. longirostris) from the eastern tropical Pacific. Dep. 

 Commer., NOAA, Natl. Mar. Fish. Serv, Southwest Fish. Sci. Cent., 

 P.O. Box 271, La Jolla, CA 92038. Admin. Rep. LJ-82-15C, 213 p. 



''Kinze, C. C. 1990. Non-metric analysis of harbour porpoises 

 {Phocoena phocoena) from the North and Baltic seas: implications 

 for stock identity. Unpubl. Paper SC/42/SM35 presented to the IWC 

 Sci. Committee, June 1990. 



"'Perrin, W. F.. A. V. Yablokov. and V. L. Cass. 1982. Preliminary 

 report on the use of non-metrical skull characters to discriminate 

 populations of pelagic dophins. Dep. Commer., NOAA. Natl. Mar. 

 Fish. Serv, Southwest Fish. Sci. Cent., P.O. Box 271, La Jolla, CA 

 92038. Admin. Rep. LJ-82-14, 32 p. 



"Perrin. W. F., A. V. Yablokov, J. Barlow, and M. V. Mina. 1988. The 

 use of non-metrical skull characters for analyzing geographical varia- 

 tion in the common dolphin, Delphinus delphis. Part 1: Selection of 

 characters. Dep. Commer.. NOAA, Natl. Mar. Fish. Serv, Southwest 

 Fish. Sci. Cent., P.O. Box 271, La Jolla, CA 92038. Admin. Rep. LJ- 

 88-20, 24 p. 



state evaluation over time, especially if large samples 

 are involved (e.g., Perrin et al., 1982 10 , 1989). 



Of the six extant species of Lagenorhynchus, geo- 

 graphic variation has been examined for the three 

 northern hemisphere species: Pacific white-sided dol- 

 phin, L. obliquidens (Walker et al.. 1986), Atlantic 

 white-sided dolphin, L. acutus, and whitebeaked dol- 

 phin, L. albirostris (Mikkelsen, 1991). The purpose of 

 this paper is to quantify sexual dimorphism and indi- 

 vidual and geographical variation in skulls of L. 

 obscurus from Peru, Chile, southwestern Africa (in- 

 cluding South Africa, Namibia and Angola), and New 

 Zealand utilizing material currently available in sci- 

 entific collections. Evidence for separate dusky dolphin 

 populations, as presented below, should urge agencies 

 responsible for marine resource management in afore- 

 mentioned coastal states to take special steps to en- 

 sure the conservation of L. obscurus on a stock by 

 stock basis, rather than to evaluate the status of the 

 species on a global scale. 



Material and methods 



Skulls were obtained by the author from animals re- 

 cently killed in fishery operations as well as from mu- 

 seum specimens with widely variable degrees of docu- 

 mentation. Of more than 300 cranial specimens of L. 

 obscurus from central Peru, collected along a coastal 

 strip between Huacho (11°10'S) and San Andres 

 (13°50'S) in the period 1984 to 1990, 239 skulls were 

 adequate for study. The majority of these were ob- 

 tained from freshly butchered dusky dolphins at the 

 wharves of Pucusana (12°30'S) and Cerro Azul 

 (13°00'S); the rest were retrieved from decomposed re- 

 mains at local fish-offal dumps or were beach-cast. 

 When available, biological data, including sex, stan- 

 dard body length (SL) and reproductive status, were 

 recorded for each specimen. Skulls were deposited at 

 the Centra Peruano de Estudios Cetologicos, Pucusana, 

 Peru; the Zoologisch Museum of the University of 

 Amsterdam; the Koninklijk Belgisch Instituut voor 

 Natuurwetenschappen (Brussels); and at the U.S. Na- 

 tional Museum of Natural History. Washington D.C. 

 Surveys to collect stranded and by-caught specimens 

 in northern Chile were carried out in collaboration 

 with colleagues from the Instituto de Investigaciones 

 Oceanologicas, Universidad de Antofagasta. Anto- 

 fagasta, Chile (Van Waerebeek and Guerra, 1986"; 

 Guerra etal., 1987). 



The remainder of cranial material was located at 

 the following institutions: the American Museum 

 of Natural History, New York; British Museum (Natu- 

 ral History), London; Canterbury Museum and Can- 

 terbury University, Christchurch, New Zealand; 



