RENSEL and PRENTICE: FACTORS CONTROLLING GROWTH AND SURVIVAL 



same overall size and depth as the pens for the 

 juveniles but were made of a larger mesh size 

 (stretched measure, 9.0 mm) and not divided into 

 chambers. Polyvinyl chloride pipe frames were 

 placed in the pen bottoms to maintain the pen's 

 shape. The total immersed substrate available to 

 the prawns was 11.5 m^/pen. Each pen was 

 stocked with 112 prawns for an initial density of 

 9.7 prawns/ m^. 



The prawns were divided into treatment groups 

 based on age and diet. Juvenile prawns were fed 

 raw meat of the blue bay mussel, Mytilus edulis. 

 Yearling prawns were divided into two diet treat- 

 ments. A "clam-fed" diet consisted of frozen pro- 

 cessing waste from the geoduck, Panope generosa, 

 which was fed without limit every other day after 

 old food was removed. In a second treatment, "un- 

 supplemented," the prawns were not fed but for- 

 aged on organisms growing on or drifting into the 

 net pens. Dead prawns and exuviae were collected 

 from each treatment every other day. All treat- 

 ments were replicated three times at both test 

 sites. 



All surviving prawns were measured for length 

 and weight except juveniles whose weights were 

 estimated from carapace lengths using the for- 

 mula log W = 2.93148 log L = 3.07787, where: L = 

 length in millimeters and W = weight in grams 

 (Butler 1964). Initially, the carapace of each 

 juvenile prawn was measured to the nearest 0.1 

 mm with an ocular micrometer. Carapace length 

 of yearlings was measured with calipers to the 

 nearest 0.5 mm. Starting in October, the juvenile 

 prawns were also measured with calipers. A top 

 loading balance was used to obtain individual wet 

 weights (nonblotted) of the prawns to the nearest 

 0.01 g. 



RESULTS AND DISCUSSION 

 Environmental Data 



Salinity at Henderson Inlet and Clam Bay 

 ranged from 28.4 to 31.0%o, being within the range 

 reported by Butler ( 1964) for wild prawn popula- 

 tions. Dissolved oxygen (DO) peaked in May (11.0 

 ppm at Henderson Inlet and 9.0 ppm at Clam Bay) 

 and gradually fell to a minimum (5.0 ppm) in Sep- 

 tember at both sites. This low value at both sites 

 only lasted a few hours during some tidal cycles. 

 We believe these DO levels, because of their short 

 duration and lack of stress on salmon in adjacent 

 net pens, were adequate for the prawns and never 



caused stress. Bottom temperatures at Henderson 

 Inlet were always higher than those at Clam Bay 

 (Figure 2). 



Light influences the growth of crustaceans. 

 Forster (1970) reported significantly higher 

 growth rates for juvenile prawns, Palaemon ser- 

 ratus, held in total darkness when compared with 

 those held in other light conditions. A similar 

 phenomenon has been reported with the Ameri- 

 can lobster, Homarus americanus (Conklen 1975). 

 In our study, juvenile and yearling prawns avoided 

 brightly illuminated areas and stopped feeding 

 when the black covers were removed. This sudden 

 increase in light intensity and the presence of an 

 observer could have affected growth and survival, 

 but no quantitative information was obtained. 



18 



o 



° 16 



UJ 



a: 



Z3 14 



)- 

 < 



S 12 



Q. 



US 10 



Y- 



Henderson Inlet 05nn below surfoce 



Clam Boy 0.5m below surface 



Henderson Inlet I.Om 



above bottom 



May Jun Jul Aug Sep Oct Nov Dec 



Figure 2. — Weekly mean water temperatures for Henderson 

 Inlet and Clam Bav. 



At Henderson Inlet, maximum water transpar- 

 ency, as measured with a Secchi disc, was from 0.5 

 to 4.2 m (5-d mean) less than at Clam Bay (Figure 

 3). Seasonal fluctuation in water transparency at 

 both sites was caused by plankton blooms and 

 runoff. 



Growth and Survival of Juveniles 



Between-site comparison of juvenile prawn 

 growth was terminated in late November when 

 Clam Bay juveniles were significantly heavier ( t = 

 3.61, df = 2,147, P<0. 001) (Figure 4) and longer ( t 

 = 3.35, df = 2,147, P<0.002) than those at Hen- 

 derson Inlet. Growth monitoring continued at 

 Clam Bay until March 1975. 



Water temperature is an important factor af- 

 fecting the growth of the spot prawn, and Wickins 

 ( 1972) indicated that the optimum was at 18° C in 

 the laboratory During the period from July to 

 September the maximum water temperatures at 



783 



