FISHERY BULLETIN: VOL. 74, NO. 2 



would not seem to be constant in the heterogene- 

 ous marine environment, but would more likely 

 vary in intensity both temporally and spatially. 

 Predation may only be a dominant factor in un- 

 usual years and/or on a small-scale basis. 



Other members of the plankton community 

 undoubtedly feed on the same food organisms as 

 C. magister and competition may become an im- 

 portant factor when these food organisms become 

 sparse. One potential competitor was tentatively 

 identified as C. oregonensis. Its larvae are very 

 abundant in the inshore area and cooccur wdth 

 those of C magister. Both species are morphologi- 

 cally similar and pass through the same number 

 of larval stages, except that the larvae of C. 

 magister become increasingly larger with de- 

 velopment. There are studies showing the an- 

 tagonistic effects of a mutually shared food re- 

 source. Brooks and Dodson (1965), in a study of 

 two species of freshwater Daphnia, concluded 

 that the larger species was more efficient in col- 

 lecting both small and large particles and would 

 competitively exclude the smaller species as long 

 as size dependent predation was of low intensity. 

 Conversely, Schoener (1969), in a theoretical 

 study, concluded that large predators ate an 

 equal or a greater range of food compared to the 

 smaller ones as long as food was at some upper 

 level. But, as food abundance was reduced, the 

 optimal predator size shifted towards the smaller 

 predator. Similar situations could conceivably 

 occur and explain why C. magister larvae were 

 less numerous in 1971. The interactions of 

 hierarchies of predators and their prey involving 

 temporal and spatial changes in densities and 

 size fi'equencies can be exceedingly complex. 



Hypothesis 4: Oceanic currents and 

 multiple environmental effects 



Planktonic organisms have limited means of 

 locomotion and consequently are subject to the 

 vagaries of oceanic currents. Changes in the 

 strength or timing of these currents can be ulti- 

 mately responsible for the success or failure of 

 larval populations and their adult stocks (Coe 

 1956). The transport of entire larval stocks out of 

 their normal environment can have catastrophic 

 results for annual recruitment. 



During the winter-spring larval period of C. 

 magister, the major nearshore oceanographic fea- 

 ture is the northerly intrusion of the Davidson 

 Current along the Oregon-Washington coast and 



its reversal in March- April. The strength and du- 

 ration of the Davidson Current are critical factors 

 in the initiation, development, and persistence of 

 seasonally dominant plankton communities. 

 Southern neritic zooplankton species appear 

 abundant off the Oregon and Washington coasts 

 during fall and winter and are believed to be car- 

 ried by the northerly surface drift (Cross and 

 Small 1967; Miller 1972; Frolander et al. 1973). 

 Frolander (1962) observed widespread anomalous 

 conditions off the Washington coast during Feb- 

 ruary 1958, compared to the previous year. Lower 

 plankton volumes and a change in plankton 

 species were associated with an increase in the 

 surface temperatures, a decrease in dissolved in- 

 organic phosphate, and unusual weather during 

 the anomalous February. These events were be- 

 lieved to be the result of southerly offshore waters 

 moving into the coastal area to a larger extent 

 that year. 



Superimposed upon the nearshore currents 

 with their characteristic water properties, a dom- 

 inant modifying process results from precipita- 

 tion and river runoff. A band of low salinity oc- 

 curs all along the North Pacific coast. Little 

 information is available on the effect of the heavy 

 river runoff on the endemic plankton populations 

 in the neritic zone, but some studies have been 

 done concerning the effect of the Columbia River 

 plume on the physical processes and biota over its 

 range of influence (Anderson 1972). The Colum- 

 bia River effluent flows north along the coast of 

 Washington during the winter in response to the 

 prevailing southwesterly winds (Barnes et al. 

 1972). Hobson (1966) and Anderson (1972) ob- 

 served that chlorophyll and productivity at the 

 surface of the plume and am.bient waters were 

 higher than nearby oceanic waters due to the in- 

 creased stability of the water column providing 

 an environment where phytoplankton could ac- 

 cumulate. The major influence of the Columbia 

 River plume on phytoplankton development is be- 

 lieved to be in the timing of events. Phytoplank- 

 ton populations can develop 3-5 wk earlier in the 

 plume due to the increased stabilization. Hein- 

 rich (1962, 1968) stated that the seasonal cycle of 

 phytoplankton communities are less balanced in 

 the neritic zone and that the phytoplankton popu- 

 lations in this area can vary depending on the 

 timing and differential growth of relative copepod 

 species. Shifts in weather patterns create corres- 

 ponding changes in nearshore currents resulting 

 in the intrusion and displacement of endemic 



370 



