LASKER: FIELD CRITERIA FOR SURVIVAL 



having the right diameter for feeding, a minimum 

 concentration of perhaps between 25 and 50 

 cells/ml was needed. 



The Effect of Temperature on Feeding 



The chlorophyll maximum layers along the coast 

 were characterized by temperatures between 14° 

 and 15°C, which is lower than the optimum 

 temperature for feeding and growth in anchovy 

 larvae (16°C). Because shipboard experiments 

 were done at temperatures higher than those 

 found in the chlorophyll maximum layers it was 

 desirable to determine if the minimum particle 

 count at which first-feeding larvae were stimu- 

 lated to feed was in any way reduced by lower 

 temperatures. Figure 4 illustrates the results of 

 experiments which show that at 14°C, the food 

 particle count must be higher than 20 cells /ml 

 before significant feeding can occur over an 8-h 

 period. At the higher temperature tested, 18°- 

 19°C, food particle counts of between 5 and 20 

 particles/ml may stimulate feeding. However, 



le-is-c _,-' 



6 splandens CELLS PER MILLILITER 



Figure 4.-The effect on larval anchovy feeding of different 

 concentrations of food at two temperatures. Each experiment 

 began with 100 larval anchovies which were in first-feeding con- 

 dition and was terminated after 8 h. See text for details. 



during the shipboard experiments, particle counts 

 of 5-20 cells/ml did not stimulate larval feeding 

 even at the higher temperatures (15°-19°C) of the 

 ship's laboratory (Table 1). This discrepancy may 

 be due to the different kinds of food particles 

 available to the larvae, as well as to other factors 

 related to a larva's inability to capture certain 



particles as opposed to others. For example, when 

 Chaetoceros sp. was present in any of the samples, 

 anchovy larvae did not feed on this phytoplankter, 

 owing probably to the spiny nature of this chain- 

 forming diatom, despite the considerable lengths 

 (longer than 37ju,m) of the chains. In the Seal Beach 

 surface sample taken on 21 March 1974, Chaetoceros 

 sp. and other chain-forming diatoms made up over 

 30% of the longer than 37-jam category. This result 

 was confirmed at a station off Imperial Beach, 

 Calif, (lat. 32°34.0' N; long. 117°10.5' W) on 11 

 April where a dense bloom dominated by Thalas- 

 siosira sp. (37 chains/ml) was found. Chlorophyll a 

 at the surface was 4.8)u,g/liter and slightly higher, 

 5.1)u.g/liter, at a depth of 7 m. Feeding by anchovy 

 larvae on this organism was virtually nil. Thus, the 

 composition of the stock of phytoplankton appears 

 to be an important factor in the initial feeding of 

 anchovy larvae. 



The observations described above indicate that 

 chlorophyll measurements alone, and the indica- 

 tion of a strong chlorophyll maximum layer are not 

 by themselves sufficient criteria for establishing 

 the existence of good conditions for the feeding of 

 anchovy larvae. A cruise of the RV David Starr 

 Jordan was made back to the San Onofre station 

 on 22 and 23 April 1974. A sharp chlorophyll 

 maximum layer was discovered there once again 

 and was found to extend seaward for at least 14 

 km (Figure 5), yet shipboard larval anchovy feed- 

 ing was negative. Subsequent microscopic 

 examination of the water from these layers in- 

 dicated that cryptomonads of about 10 ixm in 

 diameter dominated the samples in concentrations 

 of 3,400-7,200 cells/ml, a size too small to be fed 

 upon by anchovy larvae. 



50 - CHLOROPHYLL-O ( |l« / lit*' 1 



Figure 5.-Chlorophyll maximum layers off San Onofre, Calif., 

 22-23 April 1974. 



459 



