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Edith Gould 



Diane Rusanowsky 



Donna A. Luedke 



Northeast Fisheries Center Milford Laboratory 

 National Marine Fisheries Service, NOAA 

 212 Rogers Avenue 

 Milford, CT 06460 



VERTICAL DISTRIBUTION AND 



MASS MORTALITY OF 



PRAWNS, PANDALUS PLATYCEROS, IN 



SAANICH INLET, BRITISH COLUMBIA 



Prawn, or spot prawn, Pandalus platyceros Brandt, 

 1851, British Columbia's largest shrimp species, is 

 extensively fished in Canada and is of considerable 

 economic importance (Noakes and Jamieson 1986). 

 The species ranges from California to Alaska and, 

 being largely associated with rocky terrain, is fished 

 with traps in many of the region's coastal inlets 

 (Butler 1980). 



This study reports a fortuitous observation of 

 catastrophic mortality of prawn in Saanich Inlet, 

 noted during a series of observations on the vertical 

 distribution of prawn on the walls of this fjord using 

 a submersible. These observations are important 

 because, under the circumstances involved, these 

 mobile benthic organisms had ample opportunity to 

 avoid the apparent rapid intrusion of lethal environ- 

 mental conditions by moving upwards, and thereby 

 remaining in a favorable environment. 



Well-documented sudden mass deaths of adult 

 marine invertebrates in subtidal environments have 

 usually been associated with man-induced environ- 

 mental perturbation, such as an oil spill, pollutant 

 discharge, entrainment of organisms into a lethal 

 environment (e.g., dredge or power plant cooling 

 water intake), or the entrapment of benthic organ- 

 isms by some lethal environmental event (Tulkki 

 1965). However, the selective high mortality of one 

 or only a few species in a subtidal community, with 

 no associated physical habitat perturbation and with 

 apparent opportunity for escape, has been infre- 

 quently described in documented catastrophic mor- 

 talities (Brongersma-Sanders 1957; Swanson and 

 Sindermann 1979; Levings 1980a, b; Tunnicliffe 

 1981; Burd and Brinkhurst 1984, 1985; Renaud 

 1986). It is known that species differ in their rela- 

 tive tolerances to environmental stress (e.g., Renaud 

 1986), but for subtidal invertebrates, the proximity 

 to lethal conditions of the majority of a population 

 for extended time periods has not been generally 

 noted. This study shows that prawn may occur close 

 to lethal environmental conditions, and that abrupt 

 mortality results if lethal water conditions sudden- 

 ly intrude. In certain locations, such mortality may 

 be more frequent than previously recognized and 

 may justify a unique exploitation strategy. 



Materials and Methods 

 This study was conducted between 6 and 10 



FISHERY BULLETIN: VOL. 86, NO. 3, 1988. gQl 



