RENSEL and PRENTICE FACTORS CONTROM.ING GROWTH AND SURVIVAL 



prawns continued to increase in weight at a rela- 

 tively constant rate until the termination of the 

 study. The wild population showed a decrease in 

 growth rate during the period of late October to 

 the end of January which was followed by a growth 

 rate which approximated that of the cultured 

 group. Water temperature and decreased avail- 

 ability of food are among the many factors which 

 could account for the decreased growth in the wild 

 populations. 



At termination of comparative testing in No- 

 vember, total mortality of juvenile prawns was sig- 

 nificantly greater (x^ = 67.2, df = 1, P<0.05) at 

 Henderson Inlet than at Clam Bay (Figure 6). The 

 experiment at Clam Bay was continued to 10 

 March 1975, when survival was 79%. 



We partially attribute the high mortality rate 

 and the reduced growth of juvenile prawns at Hen- 

 derson Inlet from July to September to plankton 

 blooms. At the time of stocking in early July, water 

 transparency was depressed by a plankton bloom 

 (Figure 3). A 20% mortality occurred during the 

 first 2 wk of July followed by a decrease in the 

 mortality rate after the bloom subsided (Figure 6). 

 During the same period prawns at Clam Bay 

 showed an estimated 2% mortality as determined 

 from semidaily pen examination. 



Clam Bay 

 Henderson Inlet 



Jul Aug Sep Oct Nov Dec Jan Feb Mar 

 1974 1975 



Figure 6. — Survival of juvenile spot prawns from July 1974 to 

 March 1975 at Henderson Inlet and Clam Bav. 



The mortality of juvenile prawns at Henderson 

 Inlet increased again in early September during 

 blooms of the dinoflagellates, Ceratium sp. and 

 Peridinium sp. Salmon mortalities also increased 

 in adjacent net pens during this period. Prawn 

 mortalities at Henderson Inlet decreased with the 

 end of the intense plankton blooms in the fall 

 (Figures 3, 6). 



Substantial mortalities of Pacific salmon reared 

 in net pens in British Columbia have also been 



associated with algae blooms. Kennedy et al.^ 

 suggested that the algae promoted the production 

 of suffocating mucus or physically damaged gills 

 through contact with sharp diatom spicules. At 

 Henderson Inlet the prawns' gills were noticeably 

 blackened and had unidentifiable matter on the 

 lamellae that may have been mucus or deterio- 

 rated dinoflagellates. 



At Henderson Inlet, during the first 2 wk only, 

 several dead prawns had single lesions — dark in 

 the center and often surrounded by an area of 

 reddish tissue. These lesions were not observed on 

 the prawns at Clam Bay. Lightner and Lewis 

 ( 1975) found the cuticular injuries from handling 

 of penaeid shrimp resulted in bacterial septicemic 

 diseases. Handling could partly explain the le- 

 sions and initial losses of juvenile prawns at Hen- 

 derson Inlet because the net pens were pulled to 

 the surface frequently to remove old food and dead 

 prawns. This procedure was not followed at Clam 

 Bay where water transparency allowed examina- 

 tion of the prawns in place. 



Growth and Survival of Yearlings 



No yearling prawns at Henderson Inlet survived 

 in either dietary treatment after the first 2.5 mo 

 (Figure 7). In early June, maximum surface tem- 

 peratures increased from 11.8° to 21.9^ C in 1 wk. 



^Kennedy, W. A., C. T. Shoop, and \V. Griffioen. 1975. Pre- 

 liminary experiments in rearing Pacific salmon ( 1973 parr). 

 Environ. Can., Fish. Mar Serv,, Tech. Rep. 541, 17 p. Pac. Biol. 

 Stn., Nanaimo, B.C. V9R 5K6. 



100 



80 



5 eo 



Z 

 LlJ 

 O 



(T 

 Ul 



a. 



40 



20 



FIGURE 7. — Survival of yearling prawns held in floating net 

 pens at Henderson Inlet and Clam Bay. 



785 



