KOSLOW: FEEDING SELECTIVITY OF NORTHERN ANCHOVY SCHOOLS 



diameter (505 /u.m mesh) net with a digital meter. 

 Zooplankton samples were obtained successfully 

 on four cruises conducted in the spring, summer, 

 and fall of 1975-76. All sampling was conducted 

 in daylight hours. Samples of the northern 

 anchovy from the schools were taken by a com- 

 mercial purse seiner on all but one cruise (April 

 1976) for positive species identification, analysis 

 of their size composition, and examination of gut 

 contents. 



The sampling scheme varied slightly on sever- 

 al of the cruises: 1) The data from April 1976 

 represent the results of four replicate tows taken 

 in the wake of the school and three control tows, 

 rather than the two replicates taken for each set 

 of tows on other cruises. 2) On the last cruise of 

 October 1976, the concentration of plankton was 

 measured before and after a school passed 

 through a single patch of water. The control tows 

 were taken first, directly in front of the school; 

 the second set was obtained after the school had 

 passed through the same area. 3) On the first 

 cruise of August 1975, the 0.5 m diameter net 

 was used alone. However, no large zooplankters 

 were found in the samples from this cruise, and 

 those collected were well within the net's range of 

 maximal efficiency, as determined by comparison 

 of catches from this net and the larger, 1 m net 

 on subsequent cruises. 



Analysis of the plankton samples consisted 

 primarily of determining the size-frequency compo- 

 sition of the zooplankton in the wake of the school 

 as compared with its composition in control tows. 

 I selected for analysis dominant species from the 

 major taxa of zooplankton occurring in the sam- 

 ples (i.e., copepods, chaetognaths, cladocerans, 

 and larvaceans (Table 1)). Species were also se- 

 lected on the basis of size, so that representatives 

 of the smallest and largest commonly occurring 

 zooplankters in each set of samples were enumer- 

 ated. Following Cassie (1968), aliquot size was 

 determined to count 20-50 organisms/size cate- 

 gory; size categories with actual counts <10 were 

 lumped with the adjacent size category. Copepods 

 were enumerated by life history stages, other 

 organisms by body length. To facilitate compari- 

 son, results were converted whenever possible to 

 micrograms carbon (/xg C) using conversions 

 obtained from the literature for Calanus (Mullin 

 and Brooks 1976), microcopepods (Landry 1976, 



Table l. — Plankton biomass and genera enumerated from 

 control samples of plankton tows taken around northern 

 anchovy schools in the nearshore zone of the Southern California 

 Bight. 



Moderate. 

 ^High. 

 ^Low. 



1978; Bartram et al.^), and Sagitta (Reeve 1970; 

 Sameoto 1971). The total zooplankton biomass in 

 the plankton samples was determined from dis- 

 placement volumes of the samples taken with the 

 0.5 m diameter net; these values were converted 

 to milligrams carbon per cubic meter (mg C/m^) 

 (Wiebe et al. 1975). 



RESULTS 



Characteristics of 

 Northern Anchovy Schools 



The estimated biomass of the five schools 

 studied ranged from 25 to 200 t (Table 2). The 

 length of the schools (the dimension normal to 

 the school's motion) varied by a factor of 4 (55- 

 200 m). The breadth of the schools (the dimension 

 parallel to the school's axis of motion) was gen- 

 erally less than their length and varied by less 

 than a factor of 2 (30-55 m). The breadth of a 

 feeding school, as a function of the number offish 

 from front to back, is critical to the degree the 

 school depletes the plankton. The lesser variabil- 

 ity in the breadth of the schools may result from 

 behavioral regulation of this parameter, which 

 determines the relative difference in feeding con- 

 ditions encountered from front to back of the 

 school. However, these data are inadequate to 



"Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



"Bartram, W. C, D. M. Checkley, and J. F. Heinbokel. 1976. 

 Further use of a deep tank in the study of the planktonic food 

 chain. IMR Rep., IMR Ref 76-7, p. 157-166. Institute of 

 Marine Resources A-018, University of California, San Diego, La 

 Jolla, CA 92093. 



133 



