DJTRODUCTION 



Knowledge of the vertical distribution of pilchard eggs and larvae 

 is of interest from two standpoints: first, in adding to the general knowl- 

 edge of the early life of this species in the seaj and second, in defin- 

 ing the vertical range of its early stages so that regular surveys of the 

 spawning grounds may be designed to sample all strata in v;hich eggs and 

 larvae are to be found. In reference to the second point it v;as desired, 

 if possible, to relate vertical distribution to physical characteristics 

 of the sea, so that the depth to which nets ;must be sent might be predicted. 

 It was felt that the concentration of organisms at a given depth would 

 be controlled chiefly by temperature, density, and light intensity. The 

 analyses described in this paper have been designed to discover and de- 

 fine the relationships between these three variables and the vertical dis- 

 tribution of pilchard eggs and larvae, 



METHODS 



The field technique employed throughout the experiment was to make at 

 each station successive net hauls, each sampling a layer of water below that 

 of the preceding haul. In 1939, hauls were approximately horizontal. In 

 19U1, however, oblique hauls were adopted to preclude the possibility af 

 missing concentr,ations of lg,rvae in the gaps between hauls. Each oblique 

 haul extended diagonally from the top of the layer to be sampled by the next 

 haul, to the bottom of the layer sampled by the preceding haul. The "sur- 

 face" haul was made with an open net, while for the others the net was low- 

 ered closed to the desired level, hauled for from 15 to 20 minutes, closed, 

 and brought to the surface. 



Closing nets used vfere identical, except for dimensions and materials, 

 with the one described by Leavitt (1935), and the tripping device was the 

 one figured by him in a later paper (Leavitt, 1938, fig. 2). Sizes of nets 

 and materials used are given in the list of stations at the end of this paper. 



Temperatures were taken in 1939 v\rith reversing thermometers, and in 19U1 

 with a bourdon-type bathythermograph modified from the one described by 

 Spilhaus (19U0). 



Y7ater flov/ was measured in the 1939 hauls by a meter consisting of 

 an impeller and counter. '*3tray angle" (departure of the tovdng wire from 

 the vertical) was measured v;ith a pendulum protractor. 



PERIOD AND AREA COVERED 



Stations were located within a radius of 120 miles of San Diego, Cali- 

 fornia, and were in an area somewhat to the south and east of the areas of 

 heaviest pilchard spawning as indicated by other surveys, Four series of 

 hauls were made in April, 1939, and SSven in April, May, and June, 19l;l. 

 Exact locations, dates, and depths fished are given in the list at the end 

 of the paper. 



182 



