BEERS ET AL.: PLANKTON AND UPVVELLING OFF PERU 



and stock abundance, and primary production 

 levels (Guillen and Izaguirre de Rondan, 1968), 

 appears to be highest during the summer period. 

 Anchovy abundance near the coast is greatest 

 during spring and summer, the time of the most 

 active fishing (Sanchez, 1966). 



Strickland et al. (1969) and Ryther et al. 

 (1970) as well as others have pointed out the 

 relatively large variation in upwelled patches 

 at the surface. During the present cruise ex- 

 tensive majjping both to the north and south 

 of Callao failed to show the very high pigment 

 patches (chlorophyll a, >10 jUg/liter) reported 

 by Strickland et al. (1969) in March-April 1966. 

 Further, the two patches we followed did not 

 have the visually brown coloration described by 

 Strickland et al. (1969) for patches with phyto- 

 plankton in bloom conditions. The phytoplank- 

 ton population of high-chloi'ophyll, brown-water 

 patches has been found to be principally diatoms 

 whereas the dominant forms in "blue" water 

 areas were generally small flagellates (coccolith- 

 ophorids and "monads"). In the regions where 

 Rojas de Mendiola et al. (1969) reported the 

 diet of anchovies to be phytoplankton, the crop 

 was principally diatoms and these were the main 

 forms, although not necessarily the same species, 

 recognized in the gut contents of the fish. Even 

 though the cell size of many of the diatom species 

 reported from the brown-water patches is small 

 they are often chain-formers which would pro- 

 vide a size and/or configuration that could be 

 more easily retained on the gill rakers of the 

 anchovy. 



The size of most plant cells in the populations 

 found during the present study is relatively small 

 compared with the chain-forming diatoms and, 

 therefore, these may not be efficiently utilized 

 by the anchovy if they pass through their filter- 

 ing mouthparts. If this size limitation is im- 

 portant, then it is plausible to suggest the an- 

 chovy will prey on larger organisms such as 

 many of the zooplankters which in turn have 

 grazed the small algal cells. At the time of year 

 of this study juvenile anchovies (i.e., peladilla) 

 of length 65 to 70 mm and of age 4 to 6 months 

 are generally abundant, having been si)awned 

 during the spring and early summer months 

 (see Sanchez, 1966). Rojas de Mendiola 



(1959)" and Rojas de Mendiola et al. (1969) 

 have shown the stomach content of this size class 

 of fish to be mainly zooplankton and have sug- 

 gested a preference for this food. It would ap- 

 pear this could possibly be due to a lack of the 

 diatoms or other appropriate-sized plant cells. 

 At such times as this when the diatoms are not 

 abundant the food chain leading to the anchovy 

 would necessarily be lengthened by at least one 

 extra trophic level and therefore "efficiency" of 

 utilization of primary production in terms of pro- 

 duction of anchovy carbon would be lessened. 

 Growth rate (i.e., increase in length) of ancho- 

 vies during these winter months is less than half 

 that at other times of the year (Sanchez, 1966). 

 This may be a reflection of the lower abundance 

 of their "food" at this time of year and the need 

 for expending a greater amount of energy to 

 obtain the same or a lesser amount of food. 



While some of the diatoms that have been 

 reported in bloom proportions at other times of 

 the year (e.g., Ryther et al., 1970) were present 

 during this study, the absolute abundance of 

 diatoms was low and they were a relatively un- 

 important component of the total phytoplankton 

 crop. This could be the result of a low rate of 

 production for these forms or may suggest they 

 are being kept down by grazing. 



Obvious possible limiting variables of the 

 chemical and physical environment to diatom 

 production do not seem to be relevant here. Even 

 though these observations were made at the 

 winter solstice, light at a location so near the 

 Equator is probably not limiting to production, 

 although at the time of this study the near- 

 coastal region of Peru was often obscured by a 

 persistent cloud cover. And, in fact, photosyn- 

 thesis was saturated at an irradiance about 20 % 

 of that at the surface. Basic nutrient require- 

 ments of the diatoms should be met with the 

 levels present. Silicate-Si at four of the five sta- 

 tions Vi-here productivity was measured was 

 8 to 10 ixg at. /liter or greater through the 

 eu])hotic zone. Nitrate + nitrite-N levels were 



° Rojas de Mendiola, R. 1959. Breve informe los 

 habitos alimenticios de la anchoveta (Evgrnvlis ringens 

 Jennys) en los anos 1954-1958. A report presented to 

 the Cia. Administradora del Guano, 30 April 1959. (Un- 

 published manuscript.) 



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