RICHARDSON: SPAWNING BIOMASS AND EARLY LIFE OF NORTHERN ANCHOVY 



3000- 



CALIFORNIA 



278 



km FROM COAST 



3000 



OREGON 



2000 



O 



z 



1000 



93 130 167 



km F ROM COAST 



203 



240 



278 



Figure lO. — Abundance of Engraulis mordax eggs and larvae 

 with distance from the coast off California and Oregon. Upper 

 panel based on data collected off Point Arguello, Calif., in 

 January, April, and July 1964 (Smith and Duke text footnote 13). 

 Lower panel data collected off Oregon and Washington in July 

 1975. 



The plume may provide an optimal environment, 

 in terms of stability and productivity particularly 

 within 100 km or so from the river mouth (Ander- 

 son 1972; Barnes et al. 1972), to insure good feed- 

 ing conditions and enhance survival of first feed- 

 ing larvae. Such an environment may not exist in 

 the less productive ambient oceanic water or the 



highly productive but too cold and dynamic coastal 

 upwelling zone. Unfortunately data on type and 

 availability of potential food items in the plume, 

 which would help validate or refute this 

 hypothesis, are not available. 



LARVAL TRANSPORT AND 

 JUVENILE NURSERIES 



Because of the obvious southward transport of 

 larvae away from the spawning center off Oregon 

 and Washington and the later occurrence of 

 juveniles in Oregon bays and rivers where spawn- 

 ing is apparently rare or unsuccessful, questions 

 arise about return mechanisms. The deep (bottom 

 third of water column) northward flowing coun- 

 tercurrent that develops in late summer beneath 

 southward flowing surface waters (Huyer et al. 

 1975) could provide a mechanism for reduction of 

 southward transport if larvae utilize it by migrat- 

 ing vertically Unfortunately we have no depth 

 distribution data on the larvae related to the depth 

 of the shear layer between currents off Oregon to 

 demonstrate this. However, if northern anchovy 

 larvae come to the surface at night to gulp air and 

 conserve energy, as off California (Hunter and 

 Sanchez 1977), then southward and offshore 

 transport would be enhanced, at least at night. 

 Changing wind patterns in the fall from northerly 

 to southwesterly (Wyatt et al. 1972) result in a 

 shift in surface currents from southward to 

 northward, cessation of upwelling, and an onshore 

 drift of surface waters which may contribute to a 

 northerly onshore movement of juveniles. Also, 

 northern anchovy spawned later in the season 

 may not be transported as far south as those 

 spavined earlier and thus would not have to travel 

 as far to return to northern bays and rivers. 



An additional offshore spawning center to the 

 north would help explain recruitment of juveniles 

 to the Oregon rivers but there is no evidence for 

 the existence of one, as discussed earlier. 



The southward transport of larvae may provide 

 an avenue of gene flow from the northern to cen- 

 tral subpopulation. 



OTHER SPAWNING CENTERS 



Whether the area off the Columbia River is the 

 primary or only spawning center for northern an- 

 chovy in the northern subpopulation is unknown. 

 No evidence of spawning to the north exists at 

 least in offshore waters. Also, no evidence is avail- 



873 



