HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEI IN LA JOLLA BIGHT 



10000 



5 10 10 20 30 40 50 60 



DISTANCE OFFSHORE, km 



Figure 12. — The horizontal, offshore distribution of 

 Pleurobrachia postlarvae on 28 August 1969 (circles) and 29-30 

 May 1970 (triangles and squares). The profile indicated by the 

 triangles is off Scripps Institution and that indicated by squares 

 is off Encinitas, about 20 km to the north. Note that both dis- 

 tributions are plotted to the same scale of distance as in Figvu^e 

 11 and that the scale of abundance is broken in order to account 

 for absence data. The vertical bar represents the range of values 

 for the replicate samples, and solid symbols indicate that the 

 range is less than the size of the symbol; open symbols without 

 any vertical bar are single observations. 



samples with absences. Shifts were found in the 

 offshore locations of the highest ctenophore abun- 

 dances at different positions along the coast (Fig- 

 ure 11). All sampling over the six lines of stations 

 was completed between 0700 and 2300 h, 6 

 November. At the northernmost station at Del 

 Mar, highest values were closest to shore and de- 

 creased over tenfold by 6.4 km. Off Torrey Pines, 

 Scripps Institution, Pacific Beach, and Point Loma 

 the observed maxima were located between 3.2 

 and 6.4 km. The exceptional case was the max- 

 imum abundance observed beyond 10 km off 

 Ocean Beach. The surface tides associated with 

 these six lines of stations were slack flood tide at 

 Del Mar and Ocean Beach, slack ebb at Scripps 

 Institution, flood at Pacific Beach and ebb at Tor- 

 rey Pines and Point Loma (refer to the inset of 

 Figure 5 for the times and heights of tides). The 

 nearshore maximal abundance at Del Mar may 



represent the slack flood tide onshore movement of 

 water and ctenophores, while the ebb and slack 

 ebb tides at Torrey Pines and Scripps Institution, 

 respectively may have caused offshore movements 

 of surface water and ctenophores such that the 

 maximal abundances occurred at 3.2 km. After 

 sampling the outer stations off Scripps Institu- 

 tion, a strong southerly wind about 10 m/s gener- 

 ated short period swells 1-2 m high. Increased 

 wind stress and turbulence may have altered the 

 current pattern south of Point La Jolla and added 

 considerable variation to the expected pattern of 

 the distribution. The presence of high abundances 

 of salps in the net tows at stations off Ocean Beach 

 and Point Loma, which were not present north of 

 Scripps Institution, indicated that the water to the 

 south was different in faunal composition than the 

 normal coastal assemblage. On 28 August 1969 

 and 29-30 May 1970, the offshore distributions 

 indicated a tenfold decrease in abundance in the 

 first 10 km from shore (Figure 12). The maximal 

 abundances between 1 and 2 km were associated 

 with slack flood or flood tides nearing slack flood. 

 The secondary peak at 5 km on 28 August oc- 

 curred during sampling on midebb tide, and it may 



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 STA. 2 

 STA. 3 

 STA. 4 

 STA. 5 

 BETWEEN STA. 4-5 



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1970 



1971 



Figure 13. — Seasonal variation in abundance of Pleurobrachia 

 postlarvae from 8 March 1970 to 2 June 1971 at stations located 

 1.6 km from shore. The solid line connects the medians of each 

 sample date. Note the break of scale to account for absence data. 

 Each type of symbol represents a different station, except for 

 those sample dates prior to May 1970. Refer to the text for details 

 of the field sampling. 



309 



