prawns survived, and they had a final mean 

 weight of 3. 14 g (Table 1 ). The prawns fed salmon 

 were significantly smaller (P<0.01) than the 

 mussel-fed group. However, the growth of prawns 

 in both the mussel and salmon diet groups equaled 

 or exceeded the growth reported for a natural 

 population in British Columbia (Butler 1964). 



Growth was similar between oyster waste and 

 no food diet groups (Table 1), but survival was 

 significantly different (P<0.01), 64'/f for oyster 

 waste and 269^ for no food supplement. Prawns fed 

 oyster wastes or receiving no food grew at a slower 

 rate than the prawns in the other two diet groups; 

 this portion of the study was terminated after 60 

 days. 



The poor growth of prawns fed oyster wastes 

 contrasts with the good growth of lobster //o/??a/7/.s 

 cifucricanus fed algae and oyster wastes in a sew- 

 age enriched raceway system (Mitchell 1975). In 

 that study intermediate organisms that fed on the 

 solid wastes of oysters were also available as a food 

 source for juvenile lobsters. In our study, raw 

 unfiltered seawater was used, but cleaning the 



tanks twice weekly prohibited the establishment 

 of intermediate organisms. 



Prawns in the net pens (Experiment B) grew 

 significantly faster than those in the laboratory 

 (Experiment A) for both mussel- (P<0.01) and 

 salmon-fed (P>0.01) treatments (Table 1). 

 Further, there was no significant difference in the 

 growth (P>0.10) or survival (P>0.25) of prawns 

 fed mussel or salmon in the pens as there had been 

 in the laboratory tanks. 



In the net pens, the presence of net fouling or- 

 ganisms as an additional food source for the 

 prawns could explain the improved growth over 

 that seen with the same basic diets used in the 

 laboratory. Net fouling organisms could have pro- 

 vided nutritional requirements that were 

 deficient in the basic diets as provided in the 

 laboratory. 



Prawns in both diet groups were observed re- 

 moving organisms that were on both the inside 

 and outside surfaces of the net pens using their 

 second periopods. The amount of net fouling or- 

 ganisms was reduced by the prawns in these en- 



FlGURE 1. — Webbing of a three-chambered net pen showed reduced fouhng in the right and left chambers (spot prawns present) 



compared with the center chamber without prawns. 



888 



