FISHERY BULLETIN: VOL. 69, NO. 4 



generally in excess of 12 to 15 (lU and PO4-P 

 above 1 (jlM. It is possible, however, that a nu- 

 trient requirement of the diatoms may not be met 

 because of the lack of "conditioning compounds" 

 or some similar mechanism as postulated by 

 Barber et al. (1970). Temperature is of doubt- 

 ful significance as many of the abundant species 

 in the "blooms" enumerated by Ryther et al. 

 (1970) and Strickland et al. (1969) might be 

 expected to grow equally well at both winter and 

 summer surface temperatures. 



Thus, at this time of year the food chain lead- 

 ing to the anchovy probably consists of an inter- 

 mediate zooplankton step. Villanueva et al. 

 (1969) found stomach contents of anchovies 

 collected 6 to 8 June during this study in an area 

 off Punta San Juan and San Nicholas to be pri- 

 marily zooplankton. Since no measure of the 

 anchovy standing stock was made, it is not pos- 

 sible to estimate the predation of the anchovy 

 on the zooplankton, but it is possible with our 

 data to evaluate the importance of zooplankton 

 grazing on phytoplankton as a mechanism for 

 preventing their "blooming." The standing 

 stock of zooplankton in the Peruvian coastal 

 waters is generally high (Reid, 1962) although, 

 as pointed out by Gulland (1970), there is pro- 

 bably a marked degree of seasonal and geograph- 

 ical variation in their abundance. Gushing 

 (1969) noted an inverse correlation marked by a 

 short lag between anchovy egg numbers and 

 zooplankton abundance off the Peru coast during 

 the spawning .season. He implicated the spawn- 

 ing fish as either the direct or indirect causative 

 agent for this. In either case it was suggested 

 that the low zooplankton stocks and hence their 

 reduced grazing pressure at the end of the 

 spawning season allows for another cycle of 

 biological production. Initially this would be of 

 principally primary production during late sum- 

 mer-early fall followed by an increase in sec- 

 ondary production in the fail. The latter would 

 be available to the juvenile anchovies. While 

 this is simply speculation at the moment, hope- 

 fully it will become clearer when more data on 

 the seasonal variation, including small-scale var- 

 iations, of plankton populations are tabulated 

 (see Gulland, 1970). 



A striking feature of the zooplankton popula- 



tions we observed off Peru was the great ab- 

 solute and relative abundance of ciliates. The 

 ciliates may be essential elements for the utili- 

 zation of the small phytoplankton species pre- 

 sent, and, if preyed upon in turn by the zooplank- 

 ton, may represent still an additional trophic 

 level and lengthening of the "food chain" be- 

 tween the primary producers and the harvestable 

 anchovy. The average ciliate organic carbon 

 level over the euphotic zone was about an order 

 of magnitude greater than that found for 12 

 equidistantly spaced stations from lat 10° N to 

 12° S along long 105° W in the eastern tropical 

 Pacific (Beers and Stewart, 1971). Ciliate 

 abundance off Peru was similar to the average 

 estimated for a site 1 mile off the coast of La 

 Jolla, Calif., from weekly samples over a 5-month 

 period in the spring and summer of 1967 (Beers 

 and Stewart, 1970). However, oflf La Jolla the 

 tintinnids accounted for almost three-quarters 

 of the ciliate biomass. Also, just 5 to 6 miles off 

 the California coast the average ciliate abun- 

 dance (as organic carbon) over the same period 

 had decreased to a level about one-quarter of that 

 recorded for the present set of stations which 

 were generally between 10 and 20 miles off the 

 coast. 



Despite their prominence in the zooplankton 

 populations the standing stock of ciliates as or- 

 ganic carbon in the euphotic zone was an average 

 of only 3.2 9f of the phytoplankton standing crop 

 (6 stations with productivity data). The aver- 

 age daily phytoplankton production over the 

 euphotic zone at these six sites was 49 mg C/m^. 

 The ciliate carbon was only approximately 5% 

 of the new phytoplankton crop being added daily. 

 An estimate of the fraction of the daily primary 

 production that might be consumed by the cili- 

 ates can be made assuming the ciliates require 

 three times their bodily carbon per day. Lab- 

 oratory culture studies of pelagic ciliates (un- 

 published) have suggested that tintinnids may 

 be dividing every 1 or 2 days and that the doub- 

 ling time may be even shorter for the oligotrichs 

 (see also Beers and Stewart, 1970) . Values for 

 possible ciliate consumption of new phytoplank- 

 ton production ranged from 5'^i (Station 93) 

 to 24'; (Station 87), averaging 15^^. Other 

 microzooplankton consumption would, on the 



872 



