MULLIN and CONVERSE: EUPHAUSIID AND ZOOPLANKTON BIOMASSES 



euphausiid to small zooplankton biomass differed 

 significantly in the direction opposite to that pre- 

 dicted. 



Interannual Variation in 1962-67 off 

 Oregon 



We also examined similar hypotheses using 

 data provided by W. G. Pearcy (see Pearcy 1976) 

 for a repeatedly sampled transect from the Ore- 

 gon coast seaward to 530 km, since this region is 

 closer to the geogi-aphic center of the Pacific 

 whiting fishery than was our California, north- 

 ern area. The data set includes 89 values for dry 

 weight biomasses of euphausiids and copepods 

 from nocturnal samples from April to October 

 1962-67. By analogy to our treatment of Cali- 

 fornia data, we defined inshore as the innermost 

 100 km, and offshore as the remainder of the 

 transect; we therefore divided this set into sub- 

 sets, 1962-65 (56 total samples, 34 inshore) and 

 1966-67 (33 samples, 20 inshore). 



To compare results from this Oregon transect 

 to one transect within the "southern" area, we 

 chose CalCOFI hne 93 (see Figure 1), and sub- 

 divided samples from it into similar subsets: 

 1960-65 (33 samples, 17 inshore) and 1966-69 

 (23 samples, 14 inshore). The du'ect comparison 

 of biomass off Oregon to that on hne 93 means 

 little, since the methods differed (for instance, 

 we measured "small zooplankton" and Pearcy 

 measured copepods), but we could compare the 

 patterns of variation shown on the two tran- 

 sects. 



On line 93, biomass of euphausiids was signifi- 

 cantly less offshore than inshore (as was true for 

 the enth'e southern area), and the biomass of 

 small zooplankton tended similarly, though the 

 difference was nonsignificant. Neither the small 

 zooplankton nor the euphausiid biomass, nor 

 their ratio, differed between 1960-65 and 1966- 

 69 — conclusions which also characterized the 

 entire southern area (Fig. 3). The Oregon 

 transect revealed a similar spatial pattern (sig- 

 nificant decreases of biomass offshore for both 

 copepods and euphausiids) and a similar lack of 

 temporal change (no significant changes for the 

 entire transect, or its inshore portion, in copepod 

 biomass, euphausiid biomass, or their ratio) be- 

 tween 1962-65 and 1966-67. 



Therefore, the patterns off Oregon were very 

 similar to those off southern California, and 

 failed to indicate a change in the biomass of 

 euphausiids concurrent with the whiting fishery 

 off Oregon. 



Expected Response of Euphausiid 

 Biomass to the Pacific Whiting 

 Fishery 



Detection of a change in the biomass of 

 euphausiids which could be caused by the start of 

 the whiting fishery depends on 1) the magnitude 

 of the change in the predatory impact on eu- 

 phausiids due to change in the stock of whiting, 

 2) the rapidity with which the community re- 

 adjusts to such changes, and 3) the variability 

 among the available samples in which change is 

 to be detected. Francis (1983) calculated the food 

 consumption by the virgin and exploited whiting 

 stocks in several standard regions of the North 

 American west coast; these estimates can be ap- 

 plied to the areas we sampled by making various 

 assumptions about the more detailed geogi'aphy 

 of the effect of the fishery on the whiting stock. 

 The variances of the gi'oups of samples we an- 

 alyzed give an estimate of the "noise" against 

 which this "signal" must be detected, and, as 

 discussed above, we can use the biomasses of 

 copepods to correct for long-term variability. 



Since we were unable to detect a change in 

 biomass of euphausiids attributable to the Pacific 

 whiting fishery, we did a simple calculation to 

 determine how many samples, with the same 

 variances as the samples we did analyze, we 

 would have had to analyze to detect an expected 

 change at the 0.05 probability level, if 1) only 

 the change in whiting stock affected the biomass 

 of euphausiids, or 2) if changes in the biomass of 

 small zooplankton were used to normalize the 

 euphausiid biomass for nonfishery effects (i.e., 

 using a simphfied euphausiid/small zooplankton 

 biomass ratio). 



First, we assumed that the biomass of 

 euphausiids was in equilibrium before the start 

 of the fishery in 1966, such that the sum of mor- 

 tality and growth was zero and immigration 

 equalled emigration, and that subsequently a 

 biomass of euphausiids simply accumulated, pro- 

 portional to the decrease in whiting predation, 

 without change of the population parameters. 

 We averaged the biomasses of euphausiids from 

 all samples in a given area from 1960 to 1965, and 

 then calculated the expected biomass two years 

 after the start of the fishery from the change in 

 consumption by whiting in that area, using the 

 estimates of Francis (1983) corrected for the 

 fraction contributed by euphausiids to whiting 

 gut contents (Alton and Nelson 1970; Livingston 

 1983; Rexstad and Pikitch 1986), for the dry/wet 

 weight ratio, and for the volume represented by 



639 



