closures ( Figure 1 , Table 2). The largest amount of 

 organisms occurred at the shallower depths of the 

 nets as in Moring and Moring's (19751 study of 

 salmon net pens at the same site. Mussels, asci- 

 cliaiis. and t uliicolous polychactt's, Spirarhis sp., 

 c(iiiti'il)utc'(l lln' most fouling iti the control cham- 

 bers (without prawns). Except for a few Spirorbis 

 sp.. the net pen chambers with spot prawns were 

 completely clean. Little algal growth was present 

 due to the reduction of light by the black plastic 

 covers. 



Rearing prawns and salmon in the same net pen 

 (Experiment C) proved encouraging. After 6'j mo 

 of culture the growth of these prawns (Figure 2) 

 exceeded that of the monoculture Experiment B 

 (P<0.01t and that reported for a natural popula- 

 tion (Butler 1964). Survival was 93' f and not sig- 

 nificantly different (P >0.95) from that of Experi- 

 ment B. 



There was no evidence of adverse salmon/prawn 

 interaction. The types of food available to the 

 prawns when reared with salmon included: dead 

 fish, uneaten fishfood pellets, fish feces, and net- 

 fouling organisms. The relative contribution of 

 each was unknown. 



A limiting factor to stocking juvenile prawns in 

 commercial salmon net pens is the requirement 

 that the prawns must be large enough to prevent 

 them from going through meshes of the net. 

 Smaller "nursery" nets of reduced mesh size could 

 be hung inside the main salmon nets until the 

 prawns reach a suitable size (about 4 g, or 3 mo of 

 age). 



Several advantages might accrue from using a 

 scavenger, such as the spot prawn, as a companion 

 crop in salmon culture. In Experiment B a reduc- 

 tion in net fouling was seen in net pens with 

 prawns (Table 2. Figure 1). This reduction will aid 

 salmon culture because it would allow greater 

 water circulation within the enclosure, thus in- 

 creasing dissolved oxygen levels and flushing of 



CO 

 5 10 



< 

 cr 



Q_ 



o 



8 



S^ 6 

 UJ 



Ijj 

 < 



UJ 



> 



< 



DIET 



A Experiment C (voriety of foods) 

 • Experiment B (mussel) 

  Experiment B (salmon) 



British Columbia 

 natural population 

 (Butler 1964) 



X 



_L 



3 6 9 



NUMBER OF MONTHS 



12 



FlClKK 2. 



-Growth of juvenile Pandali/s plalyrcros in net pens 

 compared with a natural population. 



metabolic wastes from the pens. A reduction in net 

 maintenance cost might also be realized. Experi- 

 inents B and C demonstrated that fish in the pres- 

 ence of net-fouling organisms was an acceptable 

 food for the prawn (Figure 2); the utilization of 

 dead salmon by a scavenger would be a valuable 

 conversion of an otherwise unused protein and 

 would reduce the labor needed to remove salmon 

 mortalities from the system. 



Further experiments are needed to determine 

 proper stocking densities of prawns and salmon to 

 maximize growth rates, survival, and to make op- 

 timum use of the net cleaning activities of the 

 prawns. Further, while our studies showed that a 

 single diet fed to prawns in the laboratory was not 

 adequate for rapid growth, other studies (Kelly et 

 al. 1976) have shown that combination raw diets 

 could produce adequate growth. These combina- 

 tion diets need to be evaluated in the net pen 

 system. 



T.AHl.K 2. — Displacement volumes i milliliters) of fouling or- 

 ganisms on the inside and outside surfaces of net pens with and 

 without Pandcihis platycerog from July 1975 to March 1976. All 

 pen chambers were clean at the start of the experiment. Sample 

 area was 314.2 cm^ of vertical mesh. 



Literature Cited 

 Butler, T. H. 



1964. Growth, reproduction, and distribution of pandalid 

 shrimps in British Columbia. J. Fish. Res. Board Can. 

 21:1403-1452. 

 KKLLY, R. O. a., a. W. H.\SKI.TINK. .-\Nn E. E. EliKRT. 



1976. Mariculture potential of the spot prawn, Pandalus 

 platyceros Brandt. Aquaculture 10( 1 1:1-16. 

 Mahnken, C. V. W. 



1975. Status of commercial net pen farming of Pacific 

 salmon in Puget Sound. In J. W. Avault, Jr.. and R. Miller 



889 



