LASKER: FIELD CRITERIA FOR SURVIVAL 



any food in their intestine (Arthur 1956; Berner 

 1959), this seems to be a result of rough handling 

 due to capture in a plankton net and subsequent 

 preservation with Formalin^ which causes almost 

 all anchovy larvae to defecate (Kjelson et al. 1975). 

 In the laboratory a high proportion of anchovy 

 larvae which had never seen a food particle will 

 strike at and ingest them provided there is a high 

 enough concentration of the right size food par- 

 ticles. Hunter and Thomas (1974) have shown that 

 the rate of larval anchovy feeding increases with 

 increasing food density. 



Despite the success of O'Connell and Raymond 

 (1970) and Kramer and Zweifel (1970) who were 

 able to rear anchovy larvae using micronauplii 

 concentrated in wild plankton samples, the quan- 

 tities needed by the larvae seemed inordinately 

 higher than the concentration of nauplii reported 

 by Beers and Stewart (1967) for the euphotic zone 

 of the California Current. It was possible, of 

 course, that concentrations of nauplii exist in 

 dense aggregations. Beers and Stewart (1970a) 

 reported that nauplii concentrate in or immedi- 

 ately above chlorophyll maximum layers off the 

 California coast. However, no concentrations have 

 been identified which are high enough to support 

 anchovy larvae (e.g., 1/ml). On the other hand, 

 Lasker et al. (1970) showed that anchovy larvae 

 would feed and grow on a diet of the dinoflagellate 

 G. splendens. The fact that blooms of a variety of 

 phytoplankters are known to occur in the Califor- 

 nia Current, particularly in the spring, suggested 

 that phytoplankton cells were more likely to 

 provide the particle size and cell density essential 

 to survival and growth of first-feeding anchovy 

 larvae. For these reasons, inshore chlorophyll 

 maximum layers were selected as possible fruitful 

 areas to investigate for larval feeding. 



METHODS 



To determine areas in the sea where high con- 

 centrations of living particles might be present, a 

 pump was used to bring water on board from 

 known depths. The hose from the pump was 

 lowered below the surface by means of a metered 

 winch. The water was pumped through a Turner 

 fluorometer in the ship's laboratory to measure 

 chlorophyll a and other fluorescing substances. 

 Chlorophyll a was extracted from water samples 



"Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



taken at each station from different depths and 

 the fluorescence profile adjusted to reflect only 

 chlorophyll a (Kiefer and Lasker 1975). 



A school of approximately 700 sexually mature 

 northern anchovies, maintained in the aquarium 

 of the Southwest Fisheries Center, La Jolla 

 Laboratory, produce about 1,000 fertilized eggs 

 per day. The number of eggs can be increased by 

 injections of gonadotrophins to stimulate massive 

 spawning in individual fish (Leong 1971). With 

 temperature control of development (Lasker 

 1964), larvae in first-feeding condition can be 

 made available whenever desired. Thus, 

 preliminary to a cruise, development of embryos 

 and larvae can be accelerated or retarded by 

 temperature manipulation to ensure that on each 

 day of the cruise there will be at least several 

 hundred larvae ready to feed. 



Prior to each of the cruises described here, fer- 

 tilized eggs at the same stage of development 

 were sorted into liter jars containing seawater 

 previously filtered through a 5-/Am pore size Cuno 

 Aquapure filter, and the jars were immersed in 

 water baths at suitable temperatures. For trans- 

 port to the ship insulated chests were used, and on 

 board a temperature-controlled room was con- 

 tinually adjusted between 13° and 18°C to insure 

 that feeding larvae would be available on specific 

 days. Recent studies have shown that newly feed- 

 ing larvae have only about 2V2 days after the yolk 

 sac is absorbed to get suflRcient food or they will 

 die (Lasker et al. 1970). 



Experiments to determine if samples of 

 seawater contained suitable food for anchovy lar- 

 vae were done in cylindrical, 8-liter jars wrapped 

 with dull black cardboard and set on black plastic. 

 Above the jars a bank of four "daylight" fluores- 

 cent lights were suspended which illuminated the 

 surface of the jars at approximately 2,152 lux. 

 When a sample of seawater was brought into the 

 ship's laboratory it was permitted to warm slowly 

 to room temperature. Larvae were added to 5 liters 

 of seawater by pouring them from the incubation 

 jars. Dilution of the 5 liters by the larval incuba- 

 tion water was corrected by concentrating the 

 contents of an additional liter of seawater to a few 

 milliliters with fine mesh netting and by adding 

 the concentrate to the whole. 



A gentle air stream was directed onto the sur- 

 face of the water in each experimental vessel to 

 ensure mixing of the water. Control experiments 

 on shipboard with cultured organisms indicated 

 that this had little effect on the larvae. 



455 



