LOUGH: LARVAL DYNAMICS OF DUNGENESS CRAB 



planktons. Nearshore modifying processes can 

 change the character of these communities 

 setting forth new interactions among the 

 populations. 



Anomalous hydrographic and meteorological 

 conditions were observed along the Oregon coast 

 during the winter of 1971 in the present study. Its 

 effect on the plankton populations to date only 

 have been investigated in relation to C. magister 

 larvae. To what extent did the relaxation of on- 

 shore transport of surface waters during January 

 and February with subsequent increased trans- 

 port in March 1971, compared to the same period 

 in 1970, affect the dynamics of the C. magister 

 larval population? The circumstantial evidence 

 suggests that heavy mortality of the larvae oc- 

 curred in 1971. However, the difference in larval 

 abundance for the 2 yr may not be real if the 

 larvae were quite localized in their alongshore 

 distribution and moved out of the study area. 

 Sampling was not conducted in other areas for 

 those years to fully answer this point. In addition, 

 the late zoeal stages were undersampled both 

 years leaving a gap in our knowledge of their true 

 numbers, distribution, and condition. Assuming 

 that a mass mortality of larvae did, in fact, occur 

 in the study area, what are the most likely en- 

 vironmental mechanisms? Did the decreased on- 

 shore surface water transport in early winter of 

 1971 relative to 1970 allow more larvae to be car- 

 ried offshore that year where food abundance was 

 lower, etc.? Any larvae swept off the shelf area 

 that survived would still probably be beyond suc- 

 cessful recruitment to the adult nearshore popu- 

 lation. Did the greater onshore transport of sur- 

 face waters during late winter of 1971 move the 

 bulk of the larval population closer to shore into a 

 suboptimal environment too early in their de- 

 velopment? What is the effect of the increased 

 precipitation and river runoff during the winter 

 of 1971 that reduced nearshore salinities? Was a 

 phytoplankton bloom initiated earlier in the sea- 

 son and how did it affect populations of other 

 planktonic organisms utilized as food for C. 

 magister larvae? Chamberlain (1961) commented 

 that, for crab larvae feeding indiscriminately on 

 both algae and zooplankton, a phytoplankton 

 bloom initially may retard zoeal development; 

 however, following the increase of the herbivore 

 population, more nutritionally adequate food is 

 available and would accelerate larval develop- 

 ment. Do the low-salinity Columbia River plume 

 and other river effluents effectively act as bar- 



riers against northerly alongshore transport of 

 larvae? The lower temperatures and salinities in 

 1971, particularly in the nearshore area, coupled 

 with adverse biological pressures, i.e., increased 

 predation, may have had a synergistic effect on 

 larval mortality. Many alternatives are open in 

 marine ecosystems where stochastic processes 

 prevail producing innumerable permutations. 

 The indirect effect of physical variables on larval 

 food organisms and predator-prey relations can 

 be extremely complex and important. Subtle 

 changes in these relations may have an ac- 

 cumulative effect on a larval population already 

 in a stressed condition and near the point at 

 which recovery diminishes. 



Answers to these questions remain conjectural 

 and may only be sought through further com- 

 prehensive and detailed studies. However, in con- 

 clusion, there is no substantial evidence from this 

 study that the colder winter of 1971 caused a 

 delay in the initial appearance and developmen- 

 tal schedule throughout the larval period of C. 

 magister. The generally poor appearance of the 

 early zoeal larvae collected during the 1971 sea- 

 son suggests that whatever factor(s) responsible 

 for the apparent mortality appeared to have an 

 immediate effect on these stages. The first few 

 zoeal stages may be the critical period in the 

 early life history of C. magister where the 

 greatest mortality occurs ultimately determining 

 future year class strength. 



RECOMMENDATIONS FOR 

 FUTURE RESEARCH 



Studies to date have provided a broad overview 

 of knowledge concerning the initial timing, 

 abundance, and dispersal of C magister larvae in 

 relation to major oceanographic events off the 

 central Oregon coast. First approximation esti- 

 mates of length of larval life, mortality, and feed- 

 ing have been achieved, but we are still lacking 

 detailed insight into the dynamics of the larvae- 

 plankton-environment matrix. This study points 

 out our limited knowledge and understanding of 

 the physical and biological mechanisms affecting 

 the dispersal and subsequent survival of C. 

 magister larvae. An understanding of these pro- 

 cesses is necessary for an understanding of the 

 stability and long-term productive potential of 

 the Dungeness crab as a fishery resource in the 

 Pacific Northwest. By studying processes control- 

 ling the dispersal and survival of the larvae, we 



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