ANNUAL FLUCTUATIONS IN BIOMASS OF TAXONOMIC GROUPS OF 

 ZOOPLANKTON IN THE CALIFORNIA CURRENT, 1955-59 



J. M. COLEBROOK 1 



ABSTRACT 



Year-to-year fluctuations in the abundance of the zooplankton of the California Current region, from 

 1955 to 1959, have been studied. The abundance of zooplankton was measured in terms of the biomass 

 of each of 17 major taxonomic categories (generally Class or Order). Principal components analysis was 

 used to produce concise descriptions of the major elements of the fluctuations in the abundance of the 

 categories in each of 14 areal subdivisions of the survey area. Considerable coherence with respect to 

 annual changes was found both between the taxonomic categories and between the areas. The 

 principal common element in the fluctuations could be associated with a marked increase in the 

 temperature of the surface waters which occurred in 1957 and persisted through 1958 and 1959. A less 

 pronounced but still quite clear common element in the fluctuations could be associated with year-to- 

 year fluctuations in the amount of coastal upwelling in the area. 



Since 1949, the regular surveys conducted by the 

 California Cooperative Oceanic Fisheries Inves- 

 tigation (CalCOFI) program have yielded infor- 

 mation about a variety of physical, chemical, and 

 biological parameters (see, e.g., Marine Research 

 Committee 1957). For the CalCOFI survey cruises 

 during January, April, July, and October for each 

 of the years from 1955 to 1959, samples of 

 zooplankton were analyzed to provide estimates of 

 the biomass for each major taxonomic category 

 within the zooplankton (Isaacs et al. 1969). 



These data were generously made available to 

 the author by J. D. Isaacs to provide material for a 

 study of year-to-year changes in the abundance of 

 the major components of the zooplankton. As 

 stated by Isaacs et al. (1969), "Selection of the 

 years 1955 through 1959 for analysis of biomass 

 distribution was dictated by interest in the 

 occurrence and nature of patterns of seasonal and 

 annual variability among the functional groups of 

 zooplankton. During this time, yearly mean 

 temperatures above the thermocline shifted up- 

 ward from the relatively cold years of 1955 and 

 1956 to the relatively warm years of 1958 and 

 1959." 



The object of the study described in this paper is 

 to describe the annual changes, from 1955 to 1959, 

 in the abundance of the zooplankton of the 

 CalCOFI survey area in as much detail as is 



'Institute for Marine Environmental Research, Plymouth, 

 England. 



available from the survey data in order to discover 

 whether observed changes can be associated with 

 environmental fluctuations. 



MATERIAL 



The details of the procedures for deriving 

 biomass estimates .have been described by Isaacs 

 et al. (1969), who also give the reasons for the 

 selection of the particular set of taxa (listed in 

 Table 1). It was their intention to provide 



TABLE 1. — A list of the taxa from CalCOFI cruises for which 

 biomass estimates are available. They are listed in alphabetical 

 order and a code used in Figures 7 and 10 is given. 



Manuscript accepted October 1976. 

 FISHERY BULLETIN: VOL. 75, NO. 2, 1977. 



estimates of the "nutrient quality" of the standing 

 crop of zooplankton as well as an index of 

 "trophodynamic complexity." The categories were 

 chosen to represent the quality and quantity of 

 zooplankton as food for fish rather than as 

 indicators of variability of the zooplankton as 

 such. 



The collection method for the standard 

 CalCOFI plankton samples has been described in 



357 



