Table 1. — Growth and survival of juvenile Pandal us platyceros during three tests. 



'Terminated at 60 days 



^Includes net-fouling organisms. 



^Includes salmon mortalities, uneaten fish feed (Oregon moist pellets), salmon feces, and net-fouling organisms 



three 30-day sampling periods. Carapace lengths' 

 and individual wet weights (nonblottedi were 

 measured to the nearest 0.5 mm and 0.01 g, re- 

 spectively. In all experiments growth data were 

 analyzed by one-way analysis of variance and sur- 

 vival by chi-square tests. 



In Experiment B, prawns were held in two net 

 pens measuring 1.2 x 1.8 x 1.8 m and constructed 

 of knotless nylon web (6.8-mm stretched measure 

 mesh I. Each pen was vertically divided into three 

 equal compartments (6.5 m-^ of substrate each) 

 with only the outer two being stocked with 

 prawns. The prawns were stocked at a density of 

 30.8/m^ of immersed substrate. The net pens were 

 covered with black plastic to prevent bird preda- 

 tion and to reduce light intensity. Two dietary 

 treatments were evaluated, mussel meat and salm- 

 on. Each treatment consisted of two replicates 

 and was fed exclusively on one of the two diets. 

 Feed was to excess every other day. 



A sample of 50 prawns/replicate (100/ 

 treatment) was measured for length and weighed 

 during each of eight sampling periods, except dur- 

 ing the last three periods in which all survivors 

 were measured and weighed. The prawns were 

 sampled at the beginning of the experiment and 

 32, 60, 88, 146, 205, 292, and 365 days later. 



In Experiment B, in addition to evaluating the 

 diets, we studied the net cleaning ability of the 

 prawns and the value of the organisms on the net 

 (i.e., the net fouling organisms) as a supplemen- 

 tary food source. Samples of net fouling organisms 

 were taken from inside and outside of a compart- 

 ment containing prawns (test) and inside and out- 

 side of one not containing prawns (control). The 

 nets were selected at random, and each sample 



' Carapace length is defined as the distance from the base of the 

 eyestalk to the posterior middorsal edge of the carapace. 



consisted of all the organisms on the net within the 

 area of two 20-cm-diametei" circles; one circle was 

 at 0.25-m depth, and the other was at 1 .0-m depth. 

 The material collected was identified, enumer- 

 ated, and measui'ed volumetrically. The nets were 

 sampled during November 1975 and March 

 1976. 



In Experiment C, juvenile prawns were stocked 

 in a net pen with coho salmon, Oncorhynchus 

 kisutch, (age-group 0) that averaged 20 g each. A 

 single net pen (without dividers) having a sub- 

 strate area of 10.8 m^ was used. Prawn density was 

 9.3/m^ of substrate, and salmon density was 82'm'' 

 of water. The salmon were fed Oregon moist pel- 

 lets at 37f body weight/day; however, no feed was 

 provided for the prawns other than what they 

 could scavenge. All the prawns were measured at 

 each of seven periods: at the beginning of the ex- 

 periment and 15, 33, 60, 89, 146, and 206 days 

 later. 



Care was taken to standardize culture condi- 

 tions such as lighting, substrate type, and water 

 temperature within each experiment. This was 

 not practical between experiments because of in- 

 herent differences between laboratory and net pen 

 work. 



Stocking density differed between experiments, 

 but its impact on growth and survival (agnostic 

 behavior and feeding dominance) was minimized 

 by distributing an excess of food throughout the 

 rearing enclosure. In several years of behavioral 

 observations we have rarely seen overtly agres- 

 sive or cannibalistic behavior in spot prawns. 



Results and Discussion 



Mussel-fed juvenile prawns had the best survi- 

 val and growth rate of all the prawns raised in the 

 laboratory tanks (Experiment A); 74% of the 



887 



