

Figure 3. — Dungeness crab egg sample showing empty egg cases representing egg mortalities alongside 



viable eggs from the same egg mass (24. 5x). 



Discussion 



Various workers have attributed mortalities 

 (Johnson et al. 1974; Lightner et al. 1975; Nilson 

 et al. 1975; Fisher et al. 1975) in cultured crusta- 

 ceans to epibiotic fouling. These reports suggest 

 that death may be caused either by mechanical 

 interference in larval molting or restriction of 

 gaseous exchange across the egg or gill mem- 

 brane. The fouling organisms may also consume a 

 great deal of the available oxygen from the envi- 

 ronment. The dramatic effect of this condition may 

 be seen in Figure 5 where the moderately fouled 

 egg case is entirely intact, yet the embryo is at- 

 rophied and nonviable. 



Infestation with fouling organisms presumably 

 does not begin until the eggs are oviposited. Al- 

 though heavy fouling may occur, few mortalities 

 are observed in the early developmental periods. 

 Fouling on the eggs held in rearing tanks pro- 

 gressed as the eggs developed. The progression 

 was an increase in the number or filament length 

 of any one type of the organisms or the addition of 

 other types of organisms. By the second and third 

 developmental categories, mortalities were regu- 

 larly encountered where fouling occurred. 



The samples obtained from regions II and III 

 showed the heaviest epibiotic fouling, as well as 



the highest levels of mortality. In comparison, re- 

 gion V showed the least extensive fouling and the 

 fewest mortalities. This suggests that there is a 

 relationship between epibiotic fouling and egg 

 mortality. 



Closer examination of the histograms in Figure 

 4 reveals a possible trend of mortalities and foul- 

 ing progressively decreasing from region II to re- 

 gion V. Although the number of samples obtained 

 from region I may not be conclusive evidence, they 

 suggest that the trend may not continue south of 

 San Francisco Bay. The region VI data show a 

 slight reversal of the trend although mortalities 

 and fouling are still comparatively low. 



The mortalities observed in regions II and III 

 are particularly relevant when the coastal crab 

 catch over the last 25 yr is considered. Figure 6 

 shows a general coast-wide decline in Dungeness 

 crab catch commencing in 1958. In 1965, the 

 northern fishery areas began a strong recovery, 

 whereas the San Francisco area remained at low 

 level. During this decline, the catch of the San 

 Francisco fishery dropped from 8y2 million pounds 

 to less than 1 million pounds where it has re- 

 mained. 



Several studies have investigated the potential 

 impact of overfishing on the Dungeness crab popu- 

 lation. Poole (1962) and Cordier (1966) showed 



204 



