COMPARISON OF FOREGUT CONTENTS OF Sergestes similis 

 OBTAINED FROM NET COLLECTIONS AND ALBACORE STOMACHS 



David C. Judkins and Abraham Fleminger^ 



ABSTRACT 



Sergestes similis, an oceanic shrimp, was taken at a number of locations in the eastern North Pacific, 

 principally in the California Current region. The contents of foreguts from shrimp caught by net 

 during the day and night and those of shrimp eaten by free-swimming fish were compared. In all three 

 categories of foreguts the predominant prey were adult specimens of the larger, common calanoid cope- 

 pods which typically inhabit the upper 200 to 300 m in the California Current region. However, be- 

 cause the diversity of calanoid species and the numbers of fish scales, calanoids, and euphausiids were 

 appreciably greater in the foreguts of net-caught S. similis than in fish-caught samples, it appears likely 

 that S. similis feeds in collecting nets under tow. 



Sergestes similis Hansen is an abundant pelagic 

 shrimp endemic to North Pacific waters of bo- 

 real-temperate influence (Pearcy and Forss, 

 1969; Judkins, unpublished data). Examina- 

 tion of its stomach contents indicates it is pre- 

 daceous and feeds primarily on copepods and 

 euphausiids (Renfro and Pearcy, 1966). 



There is an expanding body of evidence that, 

 in the freshwater environment, predation by 

 planktivores is size selective and determines, in 

 part, the composition of zooplankton communi- 

 ties (Brooks and Dodson, 1965; Brooks, 1968; 

 Dodson, 1970 ; Hall, Cooper, and Werner, 1970) . 

 Size-selective feeding by an abundant oceanic 

 carnivore such as S. similis may play an im- 

 portant role in limiting the abundance of an 

 array of prey species within a size range and, 

 hence, in determining the composition of the 

 zooplankton community within its habitat. The 

 first step in determining the impact of S. siynilis 

 as a predator in the community is to identify and 

 to enumerate the prey species it utilizes. 



In this report we identify and enumerate the 

 foregut contents of S. similis from net tows and 

 albacore stomachs taken at a number of local- 

 ities in the eastern North Pacific. To determine 

 if the results were affected by feeding in the 



^ Scripps Institution of Oceanography, University of 

 California, San Diego, La JoUa, CA 92037. 



net or by diurnal changes in feeding intensity 

 and diet, comparisons were made between three 

 categories of specimens: day-net samples, night- 

 net samples, and fish-stomach samples. 



MATERIALS AND METHODS 



About two thirds of the 270 foreguts with 

 contents examined in this study were obtained 

 from 5. similis collected by nets, principally 

 Isaacs-Kidd midwater trawls, over several sea- 

 sons (Table 1). The remaining one third were 

 obtained from troll-caught albacore taken in 

 July-August 1968 (Laurs and Nishimoto, per- 

 sonal communication). With the exception of 

 one net tow taken in the Gulf of California all 

 of the collections were made in the northeast Pa- 

 cific Ocean between lat 31° and 53° N (Table 1). 



Carapace lengths (measured from the tip of 

 the rostrum to the dorsal mid-point of the pos- 

 terior margin) of the net-caught shrimp ranged 

 from 6.5 to 17.0 mm, with a median of 11.3 mm. 

 Carapace lengths of fish-caught shrimp ranged 

 from 6.3 to 12.5 mm, with a median of 8.3 mm. 

 Because many fish-caught shrimps were partially 

 digested, it was necessary to estimate their cara- 

 pace lengths from the lengths of their foreguts 

 (Judkins, unpublished data). 



The foreguts were removed intact from the 

 specimens with fine forceps and placed on glass 



Manuscript accepted September 1971. 



FISHERY BULLETIN: VOL. 70, NO. I, 1972. 



217 



