Table 1 .—Average prawn abundance per observer per 5 m depth increnrient observed in the depth range 

 20-124 m during the 4 days of observations, 6-9 October 1986. A = afternoon dive, E = evening dive, 

 numbers in brackets = no. of ascents, no. of observers, < > = dead or dying. 



'No observations were made above 65 m on the evening of 9 October because of the entanglement of Pisces in submerged 



rope. Recorder equipment failure prevented inclusion of one of the observer's counts of prawn on some of the dives. 



^Live prawn abundance was estimated >€ m'^, but for purposes of analyses, 6 m'^ x 3 m' visibility was assumed, 



giving 18 prawn per meter of water depth 

 3Dead prawn abundance was estimated 

 giving 6 dead prawn per meter of water depth 



'Dead prawn abundance was estimated >2 m"', but for purposes of analyses, 2 m ^ x 3 m' visibility was assumed. 



parasitism (S. Bower^). Dead prawns, sometimes 

 consisting only of exoskeleton remains, were ob- 

 served at a density >2 m"^ in some areas. Ap- 

 parently healthy prawns were concentrated in the 

 depth range of 75-80 m at densities >6 m"^. Dead 

 and dying prawns were also observed during both 

 dives on 9 October. By comparing the estimated 

 numbers of dead and living prawns observed in a 

 vertical transect, it was conservatively estimated 

 that approximately 25% of the prawn population 

 observed may have died during this 24-h period, with 

 observations taken over 9 ascents along about 1.5 

 km of shoreline (Table 1). 



Prawn was the only species observed to be dying. 

 Apparently healthy amphipods, munids, and flatfish 

 were observed around the dying prawns. However, 

 coincident with the onset of prawn mortality, the 

 vertical distribution of pink shrimp separated into 

 two groups, one above and one below the depth 

 range of prawn mortality (Fig. 1). No dead pink 



IS. Bower, Pacific Biological Station, Nanaimo, B.C., V9R 5K6, 

 pers. commun. October 1986. 



shrimp were observed. Only amphipods were ob- 

 served eating the dying or dead prawns. 



Discussion 



Our observations indicate that the tolerance of 

 prawn to low DO2 levels may be less than that re- 

 ported from laboratory experiments. The tolerance 

 of prawn in a sealed chamber (10°C, SO^/oo salinity) 

 to low DO2 levels has been experimentally ex- 

 amined by Whyte and Carswell (1982). Under their 

 experimental conditions, prawns exhibited a reduced 

 metabolic rate at DO2 levels below approximately 

 2.5 mL • L"^ and died at approximately 0.35 mL 



• L"^ They did not determine how long prawns 

 would survive at dissolved oxygen levels below 1 mL 



• L"^ since their experimental design included only 

 a fixed amount of oxygen. Our study suggests that 

 minimum tolerance occurs at around 1 mL  L~^ 

 since at levels below this, death occurred. Oxygen 

 stress in munids has been reported (Burd 1983) to 

 coincide with a loss of equilibrium similar to the 

 disoriented locomotor behavior we observed for 



605 



