PEARCY: MICRONEKTON AND MACROZOOPLANKTON OFF OREGON 



the upper 140 to 300 m are summarized by 

 Gushing (1971) for upwelling regions of the 

 world. The average biomass of zooplankton col- 

 lected within 120 km of the Oregon coast (Table 

 4) is within the range of values given by Gush- 

 ing, after conversion to displacement volume per 

 1,000 m^ and to grams carbon per square meter. 



Zooplankton standing stocks off Oregon can also 

 be compared with those reported by the Galifornia 

 Gooperative Oceanic Fisheries Investigations 

 (GALCOFI) which used 0.25-0.55-mm mesh in 

 nets towed obliquely from 140 m to the surface. 

 Zooplankton displacement volumes near the Ore- 

 gon coast accord with values of Reid et al. (1958) 

 and Reid (1962) greater than 400 cm3/l,000 m^ for 

 July and August 1955 from Point Gonception, 

 Galif , to northern Washington, and with Thrail- 

 kill's (1956) values of 100-900 cm3/l,000 m^ for 

 1949 and 1950 off Oregon and northern Galifor- 

 nia. Smith's (1971) median displacement vol- 

 umes for pooled areas within 100 miles of shore 

 between Point Gonception and San Francisco 

 Bay, Galif., are 200-400 cm3/l,000 m^ during 

 April-July 1951-60, with decreased volumes 

 south of Point Gonception. Median displacement 

 volumes for Oregon (either on an annual or a 

 summer basis, Tables 2 and 4) are appreciably 

 lower than Smith's values for northern Galifor- 

 nia. This difference may be ascribed to differ- 

 ences between vertical and oblique tows, mesh 

 size, or annual differences in standing stocks. Or, 

 a real trend may exist for the nearshore zoo- 

 plankton standing stocks to increase in the 

 Galifornia Gurrent system between Oregon and 

 northern Galifornia, a trend that may be attrib- 

 uted to the more intense upwelling — and hence 

 higher productivity — that occurs off northern 

 Galifornia (Bakun 1973). 



Zooplankton volumes within 120 km of Oregon 

 are several times those given by McAllister 



Table 4. — Dry weight of Oregon zooplankton converted to dis- 

 placement volumes and grams carbon. 



"Conversion based on data of Ahlstrom and Thrailkill (1963, Table 7): wet 

 weight plus interstitial water ("displacement volume) « 0.06 = dry weight. 



tC was estimated to be 50% of the dry weight (see Omori 1969, 

 Table 5). 



tCalculated using Gushing s (1971) conversion of 0.065 x displacement 

 volume = gC. This conversion assumes that displacement volumes do not in- 

 clude interstitial water, but according to the data of Ahlstrom and Thrailkill 

 (1963, Table 7) an average of 42% of the wet weight of mixed zooplankton is 

 interstitial water. 



(1961) and LeBrasseur (1965) for oceanic areas of 

 the Gulf of Alaska (0-150 m vertical tows with a 

 0.45-cm diameter net, 0.35-mesh), even after 

 their catches are adjusted for the relatively low 

 catching power of their net (McAllister 1969; 

 LeBrasseur and Kennedy 1972). Average vol- 

 umes at weather station "P" (lat. 50'^N, long. 

 145°W) were more similar to those at the station 

 >120 km off the Oregon coast. Increased produc- 

 tivity associated with coastal upwelling along 

 Oregon, therefore, enhances the average zoo- 

 plankton standing stocks out to about 120 km 

 from shore several times above the stocks farther 

 offshore or upstream in the North Pacific Drift 

 (see also Reid 1962). The width of this zone of 

 high zooplankton standing stocks appears to be 

 considerably less than the 200-500 km reported 

 by Gushing (1971) for the region off northern 

 Galifornia. 



Seasonality of Standing Stocks 



Seasonality in the biomass of zooplankton, 

 with maxima in the summer and minima in the 

 winter, has been reported in the Galifornia Gur- 

 rent system off central Galifornia (Lasker 1970; 

 Smith 1971) and in waters off the Oregon- Wash- 

 ington coast (Peterson 1972). Yet there was lim- 

 ited evidence for differences in macrozooplankton 

 standing stocks between the two seasons in Ore- 

 gon waters. Thus seasonality of standing stocks 

 appeared to be more pronounced for micronekton 

 than macrozooplankton, or for carnivores than 

 herbivores. This may be because the high vari- 

 ability of macrozooplankton catches (Figure 8) 

 makes important seasonal changes difficult to 

 detect. Also the months selected for the two 

 seasons may not match the periodicity of natural 

 cycles. Another possible explanation is that the 

 seasonality in catches of common animals such as 

 Euphausia pacifica and Calanus spp. may be less 

 than that in small herbivores with shorter life 

 spans and generation times. Small copepods such 

 as Pseudocalanus, Oithona, and Acartia, which 

 were not sampled adequately with my nets, are 

 known to be abundant in Oregon-Washington 

 waters in the summer, especially in upwelled 

 waters along the coast (Frolander 1962; Gross 

 1964; Peterson 1972; Peterson and Miller 1975). 



Inshore-Offshore Variations 



Largest standing stocks of macrozooplankton 

 and micronekton (grams per square meter but not 



77 



