FISHERY BULLETIN: VOL. 72, NO. 2 



more ships would better enable separation of the 

 total variability into variations from 

 time-dependent physical effects, true spatial dif- 

 ferences, and replicate sample error. It is indeed 

 discouraging that confidence limits for the mean 

 of replicate samples could not be reduced below 

 about one-half to twice the mean, even with a t 

 value based on 90 degrees of freedom. Zooplank- 

 tologists may continue to be plagued with the ina- 

 bility to reduce field sampling variability much 

 below this level, given reasonable time and man- 

 power limitations and no significant changes in 

 sampling methodology. Because of their large size 

 and lack of rapid escapement, postlarvalP. bachei 

 are as easy to sample accurately as any zooplank- 

 ter is likely to be. 



Seasonal changes in abundance of P. bachei 

 postlarvae observed in La Jolla Bight during my 

 study (Figure 13) agree with the earlier work of 

 Esterly (1914) off San Diego and work by Parsons 

 et al. (1970) in the Strait of Georgia, British Co- 

 lumbia (the values reported in the Strait of Geor- 

 gia work are numbers of Pleurobrachia plus 

 Philidium per cubic meter). These two studies 

 showed that seasonal maxima occurred in July or 

 August; high densities were from June to Sep- 

 tember and lower values and absences were ob- 

 served from October to March. Esterly (1914) 

 noted that P. bachei were more abundant at tem- 

 peratures above 18°C than below; they were espe- 

 cially abundant at about 19°C in August. He also 

 noted that although P. pileus and P. bachei are 

 similar in morphology, their distributional pat- 

 tern and temperature optima are widely different. 

 In the Atlantic P. pileus was abundant at lower 

 temperatures during the year (<15°C); in the 

 Pacific the reverse seemed to be the case. 



Seasonal studies of P. pileus in Wellington 

 Harbor, New Zealand (Wear, 1965) and the North 

 Sea region (Russell, 1933; Fraser, 1970; Greve, 

 1971) show that it differs from P. bachei in the 

 season of maximal abundance. In Wellington 

 Harbor P. pileus was dominant in the winter 

 plankton, and it was the most variable plank- 

 tonic species. P. pileus was absent in February- 

 March, rare (1-10/20-min tow) in April-May, and 

 December-January, common (20-100/tow) in 

 June and September- November and abundant 

 (500-1,000/tow) in July- August (note that this is 

 the winter in New Zealand). Critical temperature 

 for the occurrence of P. pileus was between 15° 

 and 16°C. When the temperature fell below this 

 level, P. pileus occurred in great abundance; in 



early summer at temperatures above 16°C they 

 were rare or absent. In the North Sea off 

 Plymouth, P. pileus occurred in a bimodal sea- 

 sonal distribution with early summer 

 (May-June) and fall (October) maxima (Russell, 

 1933). In the North Sea near Helgoland P. pileus 

 occurred with a May- June maximum at 10-15°C 

 and a less distinct fall peak (Greve, 1971). 

 Long-term mean seasonal distributions in the 

 Scottish North Sea showed a clear November 

 maximum with a less distinct secondary mode in 

 June (Fraser, 1970); however, the month of the 

 seasonal maximum can be as early as 

 July-August in "abnormal" years compared to 

 the expected fall maximum of normal years. 

 Highest numerical abundance of postlarval P. 

 pileus in the North Sea was on the order of 

 10-20/m3 (Fraser, 1970; Greve, 1971). This is 

 about the same as the maximum of 40/m^ I found 

 for P. bachei postlarvae, but through most of the 

 year the population of P. bachei was dominated 

 by numbers of larvae and eggs. Contrary to the 

 annual or biannual spawning patterns of P. 

 pileus in the North Atlantic (Fraser, 1970), P. 

 bachei produced eggs throughout the year except 

 for spring and some summer months. 



Important differences exist between P. pileus 

 and P. bachei in addition to the pattern of sea- 

 sonal distributions and the surface temperature 

 at the season of maximum abundance. Patterns 

 in the seasonal co-occurrence of Bero'e with 

 Pleurobrachia and the parasitism of each 

 Pleurobrachia species are different for P. pileus 

 and P. bachei. In the North Sea, P. pileus 

 occurred in patterns of seasonal abundance which 

 were 180° out of phase with the abundance of 

 Beroe (Russell, 1933; Greve, 1971). In La Jolla 

 Bight abundances of P. bachei and Bero'e sp. gen- 

 erally increased and decreased in phase without 

 time lags. The seasonal patterns for the co- 

 occurrence of Bero'e with P. pileus and P. bachei 

 suggest that Bero'e and other predators may over- 

 exploit P. pileus temporarily to decrease the 

 population abundance seasonally, whereas Bero'e 

 and P. bachei appear to co-occur in a less intense 

 predator-prey association. In the North Sea, P. 

 pileus were parasitized by nematodes (Greve, 

 1971) and cercaria of Opechona, a trematode 

 (Fraser, 1970). In La Jolla Bight, P. bachei were 

 parasitized by H. mediterranea. Farther to the 

 north Hyperoche mediterranea is replaced by H. 

 medusarum (Bowman, 1953), and P. bachei is 

 parasitized by this species in waters off northern 



322 



