Biology and fisheries for the spot prawn (Pandalus platyceros, Brandt 1851)

Abstract

In this dissertation I investigate aspects of the biology and fisheries for the spot prawn (Pandalus platyceros Brandt 1851). I clarified several basic biological issues, including the geographic range and the length of the planktonic phase of the life cycle. I also improved estimates of the fecundity, growth rate, survivorship and longevity for this species, with reference to stocks in Washington State.I studied bycatch in spot prawn trawl and pot fisheries, and showed that trawl fisheries had very high levels of bycatch, and likely caused severe damage to benthic habitats. Pot fishery bycatch was much smaller. Partly as a result of these studies, trawl fisheries for spot prawns were closed coast wide by 2003.I developed three reliable polymorphic microsatellite markers for spot prawns and used them to genotype 525 individuals from 10 locations, mostly within Washington State. However, I was unable to detect any statistically significant differences between samples, probably due to a lack of power caused by the small number of loci and their low level of polymorphism. I did use the markers successfully to analyze the parentage of samples from 14 broods of eggs taken from wild ovigerous females. This showed no evidence for multiple mating in this species.Finally, I modeled the population dynamics of the spot prawn stock off the Washington outer coast. I estimated gear selectivity for pots and trawls and a stock synthesis model was fitted to individual vessel catch per unit effort indices from commercial logbooks and length frequency data. The results showed that the dynamics of this stock are driven mainly by recruitment, which varied by a factor of 5 over the study period. Heavy fishing in 1997 to 2002 combined with low recruitment in 1997 to 1999 reduced the stock to less than 50% of its virgin level. However, some recovery was subsequently seen due to fairly large recruitments of 2 year old prawns to the fishery in 2001 and 2002, combined with reduced catches associated with the closing of the trawl fishery. Nevertheless, current catch quotas still appear to be higher than this stock will support sustainably.