FISHERY BULLETIN: VOL. 78, NO. 3 



and the weekly mean rose about 5° C. As men- 

 tioned previously, a plankton bloom also occurred 

 during this period. The rapid rise in water tem- 

 perature and the onset of plankton blooms were 

 the primary factors associated with the rapid rise 

 in mortality rate of the yearling prawns. Abnor- 

 malities in appearance and behavior of the prawns 

 were noted soon after their arrival at Henderson 

 Inlet in June. The prawns became covered with 

 fouling organisms, particularly the hydroid, 

 Obelia sp., and the suctorian protozoan, Ephelota 

 gemmipara . Heaviest fouling occurred on 

 periopods and ventral edges of the cephalothorax, 

 but the gills were unaffected. Within 2 wk many 

 prawns were dying while survivors seemed 

 lethargic and did not feed or molt. The entire stock 

 was given a formaldehyde/malachite green dip (25 

 to 0.1 ppm ratio for 8 h), but beneficial effects 

 lasted only a few days. 



On 2 July, 49 surviving yearling prawns from 

 one clam-fed replicate at Henderson Inlet were 

 removed from the experiment and placed in a vinyl 

 coated wire mesh cage (0.9 x 0.9 x 0.5 m) that was 

 placed on the bottom of the Inlet, into water that 

 was colder (Figure 2), less lighted, and out of the 

 influence of the surface plankton bloom. Within 2 

 d the prawns became active and began molting 

 although many still had .some fouling organisms. 

 By September, only four mortalities had occurred 

 in the cage (92% survival), while prawns in the 

 surface net pens had suffered lOO'^ mortality. 

 Colder water, reduced light, and the possibility of 

 being out of the surface plankton bloom could ex- 

 plain this increase in survival. There are no 

 natural populations of prawns in the shallow in- 

 lets of southern Puget Sound (i.e., those inlets to 

 the west of Henderson Inlet). Berkley ( 1930) noted 

 that late larval and postlarval prawns in British 

 Columbia were commonly found inshore at depths 

 of 3.7-5.5 m. The adverse conditions encountered 

 in surface waters during this study produced ex- 

 tensive mortalities and could explain the absence 

 of natural prawn populations in the shallow inlets 

 of southern Puget Sound. 



Survival of yearling prawns at Clam Bay after 

 10 mo (5 June 1974-28 March 1975) was 78.6% for 

 clam-fed treatments and 66.7% for unsupplement- 

 ed dietary treatments (Figure 7). The difference 

 in survival was significant (y^ = 9.48, df = 1, 

 P<0.005). 



Temperature fluctuations and plankton blooms 

 at Clam Bay were associated with increases in 

 mortality. Over half of the total mortalities oc- 



curred in a 2-mo period (August-September) in 

 which the weekly mean temperature rose to a high 

 for the year ( 14.2° C) and the lowest transparency 

 value occurred (Figures 2, 3). In general, yearling 

 prawn mortality coincided with rapidly decreas- 

 ing water transparency and increasing water 

 temperature and not the absolute value of Secchi 

 disc readings. 



Both treatment groups at Clam Bay grew at 

 essentially the same rate during the June to Au- 

 gust period. Thereafter the clam-fed prawns were 

 significantly heavier it = 10.98, df = 2,539, 

 P<0.00) and longer (Z' = 3.17, df = 2,539, P<0. 001) 

 than the unsupplemented prawns (Figure 8). 



Despite the fact that no food was given, the un- 

 supplemented group had a positive growth rate 

 throughout the experiment and a fairly high sur- 

 vival (66.7% ). About 607^ ofprawns sampled at the 

 termination of the experiment had materials in 

 their stomachs, including unidentifiable brown 

 matter, various algae (brown filamentous forms 

 and diatoms, particularly Riddulphia sp.), and 

 fragments of exoskeleton from either the prawns' 

 exuviae or naturally occurring crustaceans in the 

 net pens. The prawns had apparently maintained 

 themselves on net fouling and/or pelagic or- 

 ganisms and through consumption of dead prawns 

 or molted exuviae. 



Net fouling organisms were preyed upon by 

 prawns in both treatment groups. Commonly seen 

 net fouling organisms (mussels, tunicates, and 

 bryozoans) were noticeably absent from the nets 

 containing pravvns after 10 mo of immersion. The 

 only major fouling organisms remaining were 

 hydroids, Obelia sp., and small entroprocts (both 



CLAM BAY 



• • Clom processing woste 



O O Unsupplemented 



■■%-:...M^o o- 



May Jun Jul Aug Sep Oct Nov Dec Jon Feb Mar 



Figure 8. — Seasonal mean weight of yearling spot prawns 

 reared at Clam Bay compared with a British Columbia wild pop- 

 ulation (Butler 1964). 



786 



