JAMIESON and PHILLIPS: OCCURRENCE OF CANCER CRAB MEGALOPAE 



was the same size as that on the bongo gear. The 

 neuston sampler was towed at 4 kn approximately 

 10 m from the side of the vessel to reduce hull 

 avoidance by the larvae. Tow duration was usually 

 15 minutes but was shortened when crab larvae 

 occasionally became very abundant at dawn and 

 dusk. 



Bongo gear was a modified SCOR design (Mason 

 et al. 1984), having a mouth opening of 25 cm (each 

 side) with the outboard (left) net of 500 ^a Nitex and 

 the inboard (right) net of 230 ^ Nitex. A General 

 Oceanics flowmeter was mounted in the mouth of 

 each net. Sampling procedure followed that de- 

 scribed by Smith and Richardson (1977*). At stations 

 <100 m water depth, the bongo was fished in an un- 

 dulating fashion from the bottom to the surface in 

 order to filter a standard volume of water (about 

 300 m^). 



The epibenthic sled (Phillips and Mason 1986) used 

 in inshore waters had a 60 cm mouth opening and 

 1 mm Nitex netting; it was towed for 10 minutes 

 at 2-3 kn. Nine tows were made over bottoms of 

 unknown characteristics. The modified beam trawl 

 (Gunderson and Ellis 1986), also used in inshore 

 waters, had a 3 m mouth opening, a 7 mm mesh net, 

 and a 3 mm mesh cod end; it was towed for 10 

 minutes at about the same speed as the sled. For 

 both gear types, distance towed was calculated by 

 radar triangulation to reference points on land. 



Data Analysis 



All plankton samples were preserved initially in 

 a 4% formaldehyde solution of saltwater. In the 

 laboratory, settled volume was determined, and 

 general composition of the plankton noted. Samples 

 were then sieved, and Cancer larvae picked by hand 

 and returned to a 2% formaldehyde solution before 

 their identification. Cancer magister megalops are 

 readily identifiable by their larger size (Trask 1970; 

 Lough 1975), but the currently used key (Lough 

 1975) was not always effective in separating C. ore- 

 gonensis and C. jyroductus. Presence of lateral spines 

 is a subjective criterion, and morphological dimen- 

 sions and counts of setae were two variable to 

 distinguish species. These smaller megalops were 

 finally concluded to be C. oregonensis after rearing 

 hundreds of larvae to the juvenile stage and finding 

 no C. productus. 



With beam trawl samples, the catch was sorted 



*Smith, P. R., and S. L. Richardson. 1977. Manual of methods 

 for fisheries resource survey and appraisal. Part 4. Standard tech- 

 niques for pelagic fish egg and larval surveys. Southwest Fish. 

 Cent., Natl. Mar. Fish. Serv., NOAA, Adm. Rep. No. 77-11. 



on deck to remove juvenile crabs, and the megalopae 

 were preserved as above. Species composition of the 

 catch was noted. 



For the bongo tows, the volume of water filtered 

 was used to calculate a haul factor (Mason et al. 

 1984), which accounted for tow depth and allowed 

 expression of the data as the integrated number of 

 organisms beneath 10 m^ of sea surface. 



For neuston tows, relative abundance was ex- 

 pressed as area swept rather than as volume filtered 

 and again expressed as number of animals under 10 

 m^ of sea surface. 



Calculations of larval abundance are necessarily 

 conservative, and direct comparisons between dif- 

 ferent gear types are not presently possible, given 

 our current understanding of gear efficiency and lar- 

 val catchability, which varies with both depth and 

 time of day. Numbers of larvae reported here are 

 thus directly comparable only within each gear type 

 used. For bottom gear, numbers of crabs were cal- 

 culated with no consideration of gear efficiency. 



Molt Staging 



State of development within the megalopal stage 

 was determined for all Cancer megalopae collected 

 at a station, or 25 randomly selected individuals of 

 each species if the number collected exceeded 25. 

 The sequence of epidermal changes occurring dur- 

 ing this intermolt period has been described for 

 Dungeness crab by Hatfield (1983), and her proce- 

 dures and staging criteria were applied for both 

 species. Whole megalopae were stained with 0.25% 

 Fast Green dye in either water or polyvinyl lacto- 

 phenol for 2-18 hours, and then the second maxil- 

 lipeds were removed and mounted. Megalopae were 

 identified to 1 of the 13 intermolt stages recognized 

 by Hatfield (1983); these were then combined to 

 form 3 groups: early (stages 1-4), middle (stages 

 5-8), and late (stages 9-13) megalopae. Temporal 

 durations of these three groups in laboratory studies 

 were 5.8, 15.4, and 6.3 days, respectively (Hatfield 

 1983). Corresponding durations for C. oregonensis 

 are unknown. Stages were grouped into the three 

 categories described for simplification of analysis; 

 many of Hatfield's stages were of <48-h duration, 

 and this was considered too fine a resolution for our 

 purposes. 



RESULTS 



Water Temperature 



Water temperature at the surface ranged from 



529 



