Abstract.- Over eOOO male snow 

 crabs were tagged during a 6-year 

 period in Conception Bay, Newfound- 

 land, in order to estimate the in- 

 crease in size at the time of molting. 

 Ninety-two animals were recaptured 

 which had usable information on 

 growth increments. Based on the 

 amount of growth, we hypothesized 

 that 20 of these had molted once 

 while the remainder molted twice. 

 Two lines of evidence support this in- 

 terpretation. First, animals in the 

 group presumed to have molted 

 twice were at liberty on average 

 twice as long as those presumed to 

 have molted once. Second, a regres- 

 sion line fitted to data on single- 

 molters predicted the size after two 

 molts in close agreement with a 

 regression line fitted to data on dou- 

 ble-molters. A nonlinear regression 

 model was developed to estimate the 

 parameters of the relationship be- 

 tween post- and pre-molt sizes using 

 the combined data set for single and 

 double molters. The method was also 

 generalized to account for a quadra- 

 tic relationship between post- and 

 pre-molt size. For crabs in the size 

 range 80-110 mm carapace width, 

 the predicted size after molting in 

 mm is equal to 7.398 + 1.038 x pre- 

 molt size. A similar study conducted 

 in Bonavista Bay, Newfoundland, 

 yielded growth information for 18 

 animals. The molt increments appear 

 similar to those observed from Con- 

 ception Bay. 



Growth per Molt of Male Snow 

 Crab Chionoecetes opilio from 

 Conception and Bonavista 

 Bays, Newfoundland 



David M. Taylor 

 John M. Hoenig 



Science Branch, Department of Fisheries and Oceans 



PO Box 5667, St John's, Newfoundland AlC 5X1, Canada 



Manuscript accepted 29 .liine 1990 

 Fishery Bulletin, U.S. 88:7,53-7(3(1. 



The snow crab Chionoecetes opilio 

 has supported extensive commercial 

 fisheries on both the Atlantic and 

 Pacific coasts of North America since 

 the 1960s (Elner and Bailey 1986). 

 Snow crabs are also commercially ex- 

 ploited in Japan. Only the males are 

 harvested in North America because 

 females never attain commercially 

 acceptable sizes. Efforts to manage 

 the resources have been hampered by 

 a lack of detailed life-history informa- 

 tion. This is because the crabs are 

 found in deep water (50-700 m) and 

 are thus difficult to study. 



Information on growth of snow 

 crabs is required for effective man- 

 agement for at least three reasons: 

 (1) for incorporation in a yield-per- 

 recruit model, (2) to forecast the bio- 

 mass available to the fishery from 

 size-specific, pre-season biomass esti- 

 mates, and (3) to interpret size-fre- 

 quency distributions. Although gi'owth 

 can be studied in the laboratory, there 

 is no guarantee that the observed 

 growth will reflect what happens in 

 free-living populations. Consequent- 

 ly, there is a need to estimate gi-owth 

 parameters from field data. 



There have been some tagging 

 studies of snow crab growth, but the 

 reported results were either of a 

 preliminary nature or were unsatis- 

 factory due to tag retention prob- 

 lems (McBride 1982, Taylor 1982, 

 Bailey and Dufour 1987). Improve- 

 ments in tagging methods (Hurley et 

 al. In press) have made field studies 

 feasible. 



Growth of Crustacea is often esti- 

 mated by studying two components: 

 the increase in size at the time of 

 molting (molt increment) and the 

 timing of molting (either the inter- 

 molt period or the proportion molting 

 in a given season). Recent work by 

 Moriyasu and Mallet (1986) and 

 O'Halloran and O'Dor (1988) has pro- 

 vided a method for estimating the 

 proportion of the population that will 

 molt in a given time period. In this 

 paper, we concentrate on the prob- 

 lem of estimating the size-specific 

 molt increment of snow crabs in Con- 

 ception and Bonavista Bays, New- 

 foundland, from mark-recapture data 

 consisting of the size at the time of 

 tagging, the size at recapture, and 

 the time at liberty. 



Conception and Bonavista Bays are 

 deepwater bays on the Northeast 

 coast of Newfoundland (maximum 

 depth 295 m in Conception Bay; 412 

 m in Bonavista Bay). Commercial 

 fishing grounds for crabs exist at 

 depths exceeding 180 m. The bottom 

 type in these areas is predominantly 

 mud or muddy sand with mean bot- 

 tom temperatures ranging from 

 - 1.3 to 0.5°C. Since the mid 1970s 

 both areas have experienced high 

 levels of commercial crab fishing 

 effort (Taylor and O'Keefe 1987). 

 While aggregations of crabs may be 

 found at depths <180 m, the deeper 

 commercial fishing grounds appear 

 to have the soft substrate necessary 

 for snow crab molting. 



753 



