HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEI IN LA JOLLA BIGHT 



AUGUST SePTEMBER OCTOBER NOVEMBER DECEMBER JANUARY FEBRUARY MARCH APRIL 



Figure 8. — Distribution of water temperature during the seasonal study at stations 



located 1.6 km from shore. 



wind velocities during the study were: southwest- 

 erly at 3 m/s from 23 to 26 May, northwesterly at 

 5 m/s from 27 to 28 May, and northwesterly at 3 

 m/s from 29 May to 1 June. The range on any 

 given day was 0-7 m/s. No clear pattern of the 

 effect of wind velocity on drogue trajectory was 

 observed. 



From these studies the limited data for 

 "near-surface waters" indicated predominantly 

 northerly flow near the coast with some counter- 

 clockwise rotational motion in La Jolla Cove and 

 off Oceanside. Some data also showed small scale 

 onshore-offshore motion associated with the sur- 

 face tidal cycle. South of Point La Jolla the cur- 

 rents on one occasion indicated southerly flow and 

 little east^west motion associated with tidal cy- 

 cles. The larger scale rotational motion off Ocean- 

 side was not associated with a promontory and a 

 submarine canyon complex and remains to be ex- 

 plained by other means. 



The water temperature in the upper 50 m for the 

 period from May 1970 to June 1971 was measured 

 by BT casts at stations 1-5. Since the stations 

 were located in water of different depths, only the 

 upper 20 m values were represented by averages 

 for all stations. Data at 30 m were from stations 2, 

 3, and 5; data at 50 m were only from station 5. 

 Thermal stratification began in May and June 

 and reached maximal development in August and 

 early September (Figure 8). The 12.5°C isotherm 

 rose to the surface in January, and at this time the 

 smallest gradients were found. Note that the an- 

 nual temperature range at the sea surface was 

 almost identical to the range of temperature in 

 mid- August between the surface and 20 m. 



The vertical distribution of P. bachei on 3-6 

 November 1969 (Figure 9) showed three main fea- 

 tures: 1) very low abundances below 50 m for those 

 profiles which sampled that deep, 2) the pattern of 

 vertical distribution indicates that P. bachei 



NUMBER PER m' 

 10 10 O 10 10 10 10 10 10 10 10 



5-1600 '" -6-0535 



3rd 4 th 



2115- 0100- 

 2350 0320 



4th 

 0905- 

 1153 



4 th 

 1402- 

 1612 



4lh 

 2100- 

 2345 



5th 

 0120- 

 0500 



5th 



0900- 



1145 



5th 

 1253- 

 1530 



5lh 6 th 



1615- 0155- 

 1853 0350 



NOVEMBER 



Figure 9. — The vertical distributions of Pleurobrachia bachei 

 and temperature during the study of 3-6 November 1969. The 

 scale of numerical abundance is given at the top, and the scale of 

 temperature and the time interval required to sample each 

 profile are given at the bottom of the figure. Note that the depth 

 is given with a change of scale below 100 m. The hatched lines 

 below each profile indicate the sea bottom and the numbers at 

 the last sample depth give the approximate numbers of 

 ctenophores per square meter of sea surface. The dashed line 

 between successive profiles connects the centers of gravity of the 

 distributions. 



occurred nearer the surface during the day and 

 deeper at night, and 3) the extent of the "vertical 

 migration" as measured by diel vertical displace- 

 ment of the center of gravity of the population was 

 less than 20 m. The modal class of ctenophore sizes 

 at all depths was 8 mm with a range from 4 to 12 

 mm. Neither larvae nor eggs were found in these 

 bongo net samples. Vertical separation of differ- 

 ent size classes of ctenophores was slight, the 20- 

 to 40-m depth interval consisting of a modal size 

 class at 9 mm and the 0- to 5-m interval consist- 

 ing of a modal size class at 6 mm. Note that the 

 relatively small vertical movements of P. bachei 

 enabled these animals to spend part of the day in 

 or above the thermocline near 17°C and part of the 



307 



