Acknowledgments 



I would like to thank M. Nelson, J. E. Vande- 

 vere, and H. L. Wilhelm who in some cases ini- 

 tially located the whales described and Kenneth 

 S. Norris for reading and commenting upon the 

 manuscript. 



Literature Cited 



Baldridge, a. 



1972. Killer whales attack and eat a gray whale. J. 

 Mammal. 53:898-900. 

 Fay, F. H. 



1963. Unusual behavior of gray whales in summer. Psy- 

 chol. Forsch. 27:175-176. 



GiLMORE, R. M. 



1954. The return of the gray whale. Sci. Am. 192(1):62- 



67. 

 1960. A census of the California gray whale. U.S. Fish 



Wildl. Serv., Spec. Sci. Rep. Fish. 342, 30 p. 

 1968. The gray whale. Oceans Mag. l(l):9-20. 

 HOUCK, W. J. 



1962. Possible mating of grey whales on the northern 

 California coast. Murrelet 43:54. 



HuBBS, G. L. 



1959. Natural history of the grey whale. Proc. XVth Int. 

 Congr. Zool., p. 313-316. 

 Ingles, L. G. 



1965. Mammals of the Pacific States; California, Oregon 

 and Washington. Stanford Univ. Press, Stanford, 506 p. 

 MOREJOHN, G. V. 



1968. A killer whale — gray whale encounter. J. Mammal. 

 49:327-328. 

 Rice, D. W., and A. A. Wolman. 



1971. The life history and ecology of the gray whale 

 {Eschrichtius robustus). Am. Soc. Mammal., Spec. Publ. 

 3, 142 p. 

 Sauer, E. G. F. 



1963. Courtship and copulation of the gray whale in the 

 Bering Sea at St. Lawrence Island, Alaska. Psychol. 

 Forsch. 27:157-174. 



Tomilin, a. G. 



1937. Kity Dal'nego Vostoka (The whales of the Far East). 

 [Engl, summ.] Uch. Zap. Mosk. Gos. Univ., Ser. Biol. 

 Nauk 13:119-167. 

 Walker, T. J. 



1971. The California gray whale comes back. Natl. Geogr. 

 Mag. 139:394-415. 



Alan Baldridge 



Hopkins Marine Station of 

 Stanford University 

 Pacific Grove, CA 93950 



NET FILTERING EFFICIENCY 



OF A 3-METER 



ISAACS-KIDD MIDWATER TRAWL 



The errors associated with quantitative sampling 

 of open ocean populations of zooplankton and 

 epipelagic nekton have received considerable 

 attention. Net selectivity, net sampling efficiency, 

 and patchiness have been examined by Barkley 

 (1964), Murphy and Clutter (1972), and Wiebe 

 and Holland (1968), respectively. Studies of the 

 error caused by avoidance have been summarized 

 by Clutter and Anraku (1968) and further 

 advanced by Barkley (1972). Aron and Collard 

 (1969) have reported on the effects of net speed 

 on catch. Extrusion of organisms through the net, 

 the degree of mesh retention, and the effects of 

 net clogging have been summarized by Vannucci 

 (1968), and a review of filtration problems has 

 been presented by Tranter and Smith (1968). 



Somewhat less effort has been directed toward 

 problems encountered in sampling the mid- 

 water fish fauna. Harrison (1967) reported on the 

 reliability of trawl data, the bias that may result 

 from using various types of gear, and the prob- 

 lems associated with sampling mesopelagic fishes. 

 These fishes are commonly sampled with an 

 Isaacs-Kidd Midwater Trawl (IKMT) (Isaacs and 

 Kidd, 1953) and results of such sampling, which 

 include considerations of net performance, have 

 been reported by Pearcy and Laurs (1966), Gibbs 

 et al. (1971), Friedl (1971), Backus (1972) Krue- 

 ger and Bond (1972), and others. 



Net performance is critically dependent on the 

 filtering efficiency of the net. Filtering efficiency 

 is a measure of the total volume of water fil- 

 tered by the net and enables a better quantitative 

 estimate to be made of the actual population 

 density of organisms sampled. Pearcy and Laurs 

 (1966) reported a filtering efficiency of 85% for 

 a 2-m IKMT. To the authors' knowledge, no 

 comparable figure has been published for the 

 3-m IKMT. This paper investigates the efficiency 

 of this larger net. 



Methods 



In conjunction with studies of macroplankton 

 and midwater fishes of an area off Bermuda 

 called Ocean Acre (Brooks, 1972), experiments 

 were conducted in January 1973 to determine 

 the net filtering efficiency of a 3-m IKMT. 



618 



