FISHERY BULLETIN: VOL. 73, NO. 3 



Vertical Distribution of Anchovy Larvae 



At San Onofre on 8 April the chlorophyll 

 maximum layer was due, in part, to a high density 

 of G. splendens. Larvae on board ship fed freely in 

 the water from this layer. Judging from the larval 

 feeding and the size and density of the food par- 

 ticles, this chlorophyll maximum layer should have 

 been an ideal place for first-feeding larvae to be 

 found. To test this, plankton tows were made at 

 three depths: within the chlorophyll maximum 

 layer at 16 m, just above the layer at 10 m, and at 

 the surface (see Figure 3). The standard length of 

 anchovy larvae from these tows was measured, 

 and the degree of eye pigmentation noted (full 

 pigmentation indicating a visually competent 

 larva). The results, given in Table 3, show that 

 there was a distinct difference in vertical dis- 

 tribution between the number of first-feeding 

 larvae that could see as opposed to yolk-sac larvae 

 which lack eye pigmentation and could not see. 



The surface water contained 2,100 anchovy 

 larve 71,000 m\ Larvae without eye pigmentation 

 outnumbered sighted larvae two to one. At the 

 10-m stratum above the chlorophyll maximum 

 layer there were 40,000 anchovy larvae/ 1,000 m^ 

 with larvae capable of seeing outnumbered by 

 eight to one. In the chlorophyll maximum layer 

 4,900 anchovy larvae/ 1,000 m^ were present but 

 larvae that could see were about as numerous as 

 those which could not. Although it may be coin- 

 cidental, the possibility that larvae were actively 

 seeking out areas with food cannot be dismissed. 



Criteria for Successful First-Feeding 

 by Anchovy Larvae 



It is evident from the data presented in this 

 report that the following environmental criteria 

 must be met before first-feeding anchovy larvae 

 can feed successfully in the ocean. Phytoplankton 

 aggregations with over 20 cells/ml must be 

 available at the same time or within 2V2 days after 

 the larvae are ready to feed. Individual phy- 



toplankton cells must be about 40]u,m in diameter. 

 Successful feeding is dependent on food density so 

 that the higher the concentration of cells, the bet- 

 ter the feeding. Monotypic algal blooms are re- 

 sponsible for some chlorophyll maximum layers off 

 the California coast and first-feeding anchovy 

 larvae were found to be living within them. Only 

 some phytoplankters stimulate feeding and sup- 

 port growth of anchovy larvae; for example G. 

 splendens is known to support growth while 

 anchovy larvae would not feed on Chaetoceros sp. 

 or Thalassiosira sp., spiny and/or chain-forming 

 diatoms. Finally, it could not be demonstrated that 

 micronauplii or other microzooplankton contribute 

 significantly to larval anchovy survival during the 

 first week of larval life. Beers and Stewart (1967) 

 reported that in December 1965, the inshore sta- 

 tion off San Diego (their station I) contained a 

 maximum of only 30 organisms /I iter in the 35 to 

 103-ju,m size class. Of these organisms, copepod 

 nauplii and post-nauplii together numbered 7- 

 9/liter, two orders of magnitude lower than that 

 required by anchovy larvae to survive, i.e., 

 1,000/liter (O'Connell and Raymond 1970). 

 However, it is reasonable to assume that under 

 special circumstances suitable concentrations of 

 micronauplii might serve as a food source for 

 first-feeding anchovy larvae. Nonliving particles 

 larger than 37 jxm were not seen and may be in- 

 significant in the nutrition of first-feeding 

 anchovy larvae because of their low concentration 

 in anchovy spawning areas. 



It is important to emphasize the transient na- 

 ture of good feeding conditions. There was a large 

 number of larvae present at depth at the San 

 Onofre station on 8 April 1974, capable of taking 

 advantage of the subsurface bloom of Gym- 

 nodinium. Furthermore, spawning had been ex- 

 tensive in the entire water column as indicated by 

 the large number of anchovy eggs caught during 

 the same tows (Table 3). Earlier I indicated that a 

 wind storm obliterated the chlorophyll maximum 

 layer at San Onofre on 9 April 1974, and that 8-h 

 shipboard experiments showed the larvae were 

 unable to capture enough food on 10 and 11 April 

 to fill or partially fill their intestines. If my con- 

 tention is correct, then a large proportion of the 

 larvae which were present as eggs or yolk-sac lar- 

 vae on 8 April were doomed to die from lack of food 

 after the storm on 9 April because of the dilution 

 and dispersion of suitable larval food organisms. 



Although this investigation was confined to 

 first-feeding anchovy larvae, the technique 



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