Biomasses of Euphausiids and Smaller Zooplankton 



in the California Current — Geographic and 



Interannual Comparisons Relative to 



the Pacific Whiting, Merluccius productus, Fishery 



M. M. Mullin and A. Conversi 



ABSTRACT: We examined data on size-fraction- 

 ated zooplankton biomasses from the California 

 Current in summer to 1) verify that euphausiid and 

 smaller zooplankton biomasses varied in similar 

 ways geographically and interannually, and 2) test 

 for increase in euphausiid biomass after 1966, con- 

 current with initiation of a fishery on Pacific whit- 

 ing (a major predator of euphausiids), and distinct 

 from general, interannual changes. We accom- 

 plished purpose 1, but were unable to detect a sig- 

 nificant effect attributable to the Pacific whiting 

 fishery. 



On the scale of years to decades and thousands of 

 square kilometers, variability in the biomass of 

 zooplankton, or of taxonomic categories within 

 the zooplankton, can result from physical and 

 chemical causes, from biotic interactions (espe- 

 cially in closed, manipulated systems such as 

 lakes) or from some combination of these. In 

 nonmanipulated systems, a plausible hypothesis 

 is that variability results from a change in the 

 physical processes influencing the area (the ulti- 

 mate cause) plus ecological responses or read- 

 justments of the populations present (the proxi- 

 mate cause). For the zooplankton of the open 

 ocean, it is difficult to assess, by examination of 

 case histories, the relative roles of physical and 

 biological processes, because manipulation on 

 the scales of interest is extremely difficult. 



The California Current is known to vary inter- 

 annually in transport and in physical proper- 

 ties — in aggregate, the Current's climate — the 

 most extreme warming and decrease in south- 

 ward flow being called El Nino (Wooster and 

 Fluharty 1985). Con-elated with (and in some 

 sense probably caused by) these changes are 

 variations in the biomass of zooplankton (Wick- 

 ett 1967; Reid 1962). These changes are coherent 

 through a large area — when zooplankton bio- 

 mass is anomalously large or small in one area 



M. M. Mullin and A. Conversi: Institute of Marine Re- 

 sources. A-018, University of California, San Diego, La 

 Jolla, CA 92093-0218. 



within the region, it tends to be large or small in 

 all areas during the same year (Chelton et al. 

 1982). Further, several major taxonomic groups 

 of zooplankton have similar interannual changes 

 (Colebrook 1977). 



By examining the timing of maximal zooplank- 

 ton biomass relative to that of maximal south- 

 ward flow, Roesler and Chelton (1987) concluded 

 that off northern California interannual varia- 

 tions in biomass are caused by variations in 

 direct advection of biomass from more northern 

 regions (where biomass is high); off Baja Cali- 

 fornia, variations in advection of nutrients from 

 the north, translated via the food chain into 

 zooplankton biomass with some lag, are more 

 important. 



Even in the presence of natural interannual 

 variability, rapid development of a major com- 

 mercial pelagic fishery is an anthropogenic ma- 

 nipulation which might cause detectable change 

 in the biomass and/or composition of zooplank- 

 ton. Pacific whiting, Meriuccins prodndus, (also 

 called Pacific hake) is one of the dominant fish in 

 the California Current (Smith 1978). Euphausi- 

 ids, especially Euphausia pacifica and Thysa- 

 noessa spinifera, make up > 70% of the weight 

 of gut contents of whiting, especially fish <45 cm 

 long, aged 3-4 years (Livingston 1983). 

 Euphausia pacifica is a vertical migrator, at 

 least much of the year (Brinton 1967; Brooks and 

 Mullin 1983). Thysanoessa spinifera apparently 

 remains in the upper 150 m at all times (Young- 

 bluth 1976). Another prey of whiting, especially 

 offshore, is the pelagic shrimp, Sergestes similis, 

 (Alton and Nelson 1970), which has a nocturnal 

 distribution similar to that of £■. pacifica (Omori 

 and Gluck 1979). Whiting guts are most full of 

 food in the evening and early night (Livingston 

 1983), when the fish tend to be dispersed near 

 the surface (Bailey et al. 1982). 



In 1966 a foreign (later joint-venture) fishery, 

 conducted from spring through fall, began re- 

 moving considerable quantities of whiting in 

 coastal regions off Washington, Oregon, and 



Manuscript accepted September 1988. 

 Fishery Bulletin. U.S. 87: 633-644. 



633 



