FISHERY BULLETIN: VOL. 87, NO. 3, 1989 



northern California (Nelson 1985). The commer- 

 cial catch of pandalid shrimp in Oregon increased 

 markedly at the time the Pacific whiting fishery 

 began; from this coincidence, Bailey et al. (1982) 

 and Francis (1983) suggested that the shrimp no 

 longer eaten by whiting were available to fisher- 

 men, even though these commercial shrimp are 

 only a minor portion of the whiting's diet. 

 [Livingston and Bailey (1985) pointed out, how- 

 ever, that most of the increase in shrimp catch 

 was due to increased fishing effort on shrimp 

 rather than to increased catch per unit effort; 

 hence it is not certain that the shrimp population 

 has increased.] Nevertheless, the result and 

 evidence of zooplanktivory summarized above 

 stimulated an analogous question — whether the 

 biomass of euphausiids increased in summer 

 samples of zooplankton from the California Cur- 

 rent off northern Cahfornia in 1966-69, relative 

 to summers of earher years. 



The trophic dynamics of the Pacific whiting, 

 and the implications of the fishery, have been 

 calculated from a simulation model by Francis 

 (1983). His results indicated that the fishery may 

 have reduced the (calculated) virgin whiting 

 stock by about 21% without changing the annual 

 production significantly because the production/ 

 biomass ratio increased. This conclusion implies 

 that any indirect impact of the fishery on the 

 whiting's food resources should be less than the 

 change in the whiting's biomass. Francis also 

 reasoned that the geogi'aphical distribution of 

 food consumption by whiting would shift 

 towards central California from northern Cali- 

 fornia and Oregon. 



Since the climate of the Cahfornia Current is 

 known to be correlated with interannual changes 

 in the biomass of zooplankton, any change in 

 euphausiid biomass must be scaled against the 

 biomass of other zooplankton which would (pre- 

 sumably) be affected by chmatic change but not 

 directly affected by the removal of whiting. To 

 test the assumption that the biomasses of euphau- 

 siids and smaller zooplankton respond similarly to 

 chmatic change, we reexamined data from the 

 late 1950s, when there was a major El Nino. 



We then determined the biomass of euphausi- 

 ids and other pelagic shrimps (a major whiting 

 food) relative to small zooplankton (not eaten by 

 adult whiting) before and after the initiation of 

 the fishery, and in a northern area closer to the 

 fishery compared to further south, and then 

 tested for significant differences. We also ana- 

 lyzed a published set of data from the California 

 Current off cental Oregon (Pearcy 1976), since 



this was closer to the center of impact of the 

 Pacific whiting fishery than were the samples 

 available to us. Finally, we calculated whether, 

 given the variances observed in the zooplankton 

 samples we analyzed, we should have been able 

 to detect a change in euphausiid biomass owing 

 to partial removal of a major predator by the 

 fishery. 



METHODS 



The California Cooperative Oceanic Fisheries 

 Investigations (CalCOFI) yielded samples of 

 zooplankton from the California Current from 

 the mid-1940s to the present. Though the 

 CalCOFI net is not a perfect sampler of eu- 

 phausiids and other large, active, pelagic 

 shrimp, samples taken at night (when most of 

 these species migrate into surface waters) do 

 contain euphausiids. From 1951 through 1968, 

 the standard net was of mixed silk mesh — a 

 large, forward portion of 0.55 mm and a small, 

 rear portion of 0.25 mm — towed from the sur- 

 face to a depth of 140 m. In 1969, the standard 

 was changed to a net of uniform 0.505 mm nylon 

 mesh, towed to 210 m (target depth). Though 

 these procedures have been intercalibrated 

 (Smith 1974; Hewitt 1980), care in interpretation 

 of differences between pre-1969 and 1969 sam- 

 ples is necessary. 



We divided nocturnal, summer (June-Octo- 

 ber) CalCOFI samples into the following space/ 

 time blocks or categories (Fig. 1): North of Mon- 

 terey (CalCOFI line 70), 1960-65—41 samples 

 (24 inshore); south of Dana Point (CalCOFI line 

 90), 1960-65—172 samples (103 inshore); north 

 of Monterey, 1966-69 — 47 samples (25 inshore); 

 and south of Dana Point, 1966-69 — 116 samples 

 (73 inshore). "Inshore" samples, treated as a 

 separate subset because of the inshore nature of 

 the whiting fishery, thus constituted 53-63% of 

 the samples in each block. Twenty-one other 

 samples, in which no euphausiids were found, 

 were excluded from statistical tests; at least two 

 such samples occurred in each space/time block. 

 The samples were from the following CalCOFI 

 cruises (designated "aabb", where aa = year and 

 bb = month): 6007, 6010, 6107, 6110, 6210, 6407, 

 6507, 6509 (southern area only), 6606, 6607, 6608 

 (southern area only), 6610, 6907, 6908, and 6910. 

 We made analogous divisions into space/time 

 blocks in the CalCOFI data set from the late 

 1950s and the set from Oregon. 



Each sample consisted of formalin-preserved, 

 unsorted zooplankton captured at one station by 



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