NOTES 



THE SIZE AT SEXUAL MATURITY OF 



BLUE KING CRAB, PARALITHODES 



PLATYPUS, IN ALASKA 1 



The blue king crab, Paralithodes platypus, is similar 

 in size and general morphology to the better known 

 and commercially more important red king crab, P. 

 camtschatica. Unlike the red king crab, which is 

 generally distributed throughout most of coastal 

 Alaska, the blue king crab only occurs in small 

 isolated populations. The principal fisheries for blue 

 king crab occur in the eastern Bering Sea, where most 

 populations are associated with offshore islands; 

 minor, but locally important, fisheries occur south of 

 the Alaska Peninsula, where populations are found in 

 a number of widely scattered, enclosed bays. 



Harvest of both species of king crab is restricted 

 to males larger than minimum size. For red king crab, 

 the minimum size is set at the average size of a male 3 

 yr after reaching sexual maturity in an attempt to 

 assure that each male will have at least one oppor- 

 tunity to mate before becoming available for harvest 

 (North Pacific Fishery Management Council 1981). 

 For blue king crab, however, the size at maturity is 

 not well known, and in some areas the minimum size 

 limit is set at the same size as red king crab. 



In this paper we estimate the size at maturity of 

 female and male blue king crab in each of four pop- 

 ulations. For females, the size at sexual maturity is 

 based on the change in the presence of eggs or egg 

 remnants on the pleopod setae as a function of size. 

 For males, the size at maturity is based on chela 

 allometry, using a new computer technique to es- 

 timate the size at which chela growth increases rela- 

 tive to carapace growth. 



Materials and Methods of Collection 



Samples of blue king crab were collected from each 

 of four populations: St. Matthew Island, Pribilof 

 Islands, Olga Bay, and Prince William Sound (Fig. 1). 

 Sampling methods differed somewhat between pop- 

 ulations. The populations from St. Matthew Island 

 and Pribilof Islands were sampled with bottom trawls 

 on cruises conducted by the National Marine 



Fisheries Service during the months of June and July 

 of each year from 1976 to 1981. Sampling depths 

 ranged from 30 to 180 m. The Olga Bay population 

 was sampled by scuba divers and with hand operated 

 ring nets in March, June, and October 1980 and 

 January 1981. Sampling depths ranged from 1 to 50 

 m. The Prince William Sound population was sam- 

 pled with bottom trawls in September 1980 and with 

 commercial king crab pots in September 1979 and 

 December 1 980. Sampling depths ranged from 80 to 

 150 m. 



Carapace length of both sexes and the major 

 (righthand) chela height of males were measured to 

 the nearest 1 mm using sliding jaw calipers (see 

 Wallace et al. 1949 for definitions of these mea- 

 surements). Reproductive condition of females was 

 classified as either 



virgin — no eggs or egg remnants attached to the 



pleopod setae, 

 attached eggs — eggs attached to the pleopods, or 

 hatched eggs — egg remnants, consisting of egg mem- 

 branes and egg funiculi, attached to the pleo- 

 pod setae. 



Female Size at Maturity 



Female blue king crab mate and extrude eggs quite 

 soon after every adult molt (a pathological exception 

 to this is discussed in Somerton and Macintosh 2 ); 

 therefore, females can be classified as mature or im- 

 mature based on the presence or absence of eggs or 

 egg remnants on the pleopods. Using this classifica- 

 tion criterion, the percent of females that were ma- 

 ture was calculated for each 3 mm size interval. 

 Percent mature is plotted against carapace length in 

 Figure 2. 



The size at 50% maturity (SM50) was chosen as an 

 appropriate measure of the size at maturity [see 

 Somerton (1981) for a discussion of the strengths 

 and weaknesses of this particular measure]. SM50 

 was estimated for each population by fitting a logistic 

 equation to percent mature by size, using weighted 

 nonlinear least squares (Somerton 1980a), then 



'Contribution No. 625, College of Fisheries, University of Wash- 

 ington, Seattle, WA 98195. 



2 Somerton, D. A., and R. A. Macintosh. In prep. Reproductive biol- 

 ogy of the blue king crab, Paralithodes platypus. Center for Quantita- 

 tive Science in Forestry, Fisheries, and Wildlife, University of 

 Washington, Seattle, WA 98195. 



FISHERY BULLETIN: VOL. 81, NO. 3, 1983. 



621 



