Not to be dted without prior reference to the author
International Council for C.M.1993/K:47 the Exploration of the Sea Shellfish Committee Ref. B,D,G,H,J,N
Inshore survey for Snowerab, Chionoecetes opilio, in West Greenland.
by
• Michael Andersen Greenland Fisheries Research Institute Box 570 DK-3900 Nuuk/Godthaab Greenland
Introduction:
During 1991 an exploratory fishery for snowcrab, Chionoecetes opilio, was conducted in West Greenland. Major effort was direeted to inshore areas. Previously only one small vessel had conducted some eommerdal fishing for this resouree, in a restricted area • around Nuuk/Godthaab (NAFO Div. 1D). Only a few investigations have been performed on this species in Greenland (Smidt 1965, Johansen 1974).
The purpose of the present fishery was to get an impression of the distribution of the crabs, and to elucidate the possibilities of starting a larger scale commerdal fishery.
Materials and methods:
Localities and seasons:
The target area was inshore areas between 62°30'N and 70° N (figure 1), with a depth of more than 200 meters.
The fishery was started in May and terminated in December 1991. Sampling was carried 2 out in lune (Nuuk (NAFO Div. 1D», August (Sisimiut (NAFO Div. 1B», and September (Disko Bay (NAFO Div. lA». For the rest of the period, the fishery was performed as a commercial fishery in the areas with highest densities.
Fishing gear:
A small shrimp trawler, the M/TR 'Hans' (188,9 GRT) was chartered and slightly rebuild.
The primary fishing gear used, was pots of japanese conieal type, rigged with 70 mm (knot to knot) mesh, set in fleets of 30 to 50. Distance between pots was 20 fathoms (approx. 40 meter). • Standard size of pots was an upper diameter of 65 cm, a lower diameter of 120 em, and a height of 55 cm. In addition some larger pots were used, having the same upper diameter, a lower diameter of 127 em and a height of 65 cm. Henee, the angle of the side of the pot was the same as in the standard model. These 'targe' pots were set randomly among the standard pots.
Pots were mainly baited with maekerel and squid, but other bait (eod, redfish and Greenland halibut) was used oeeasionally.
A fjord was always fished with at least four fleets and soak time varied between 20 and 48 hours.
Catch handling:
• All crabs caught were separated by sex, and carapax length (CL) was measured on a measure-board to the nearest mm below. Some individuals were set aside for a simultaneous measurement of CL on measure-board, CL and carapax width (CW) by slide caliper, and weighing.
Data handling:
In order to make a rough estimate of the population size, it was tried to calculate the relation between the catch and the density on the bottom. Miller (1975) compared density on the bottom, estimated from bottom photography, with the number of crabs caught in pots similar to the ones used in the present fishery. He found that the number of crabs per pot responded to the number of crabs per 4108 m 2 in average. 3 . . At this early stage of the fisheI)', the only possible way of getting an estimate of abundance, seemed to be calculating under the assumption that the area of influence is
a circle around the pot. \Vith an area 4100 m 2 , this circle has a radius of approx. 36 meters. The distance between the pots on a line was 40 meters, so neighbouring pots would have a high degree of overlap. It was therefore decided that a fleet of pots would be fishing in an area of
2xRxDx(n-l)+1tR2
where: R = radius of the circular area of influence, D = distance between pots on a line, and n = number of pots on a line.
• As the survey was done as a trial fishery, with commercial gear, the estimates of population size are only performed on legal sized male crabs. Legal size in Greenland is, at present, 90 rnrn carapax length.
Because calculations are carried out with figures that, for a large part, are estimates and assumptions, statistical analysis have not been performed on data related to abundance estimates.
Results:
174 fleets were set, with a total of 6446 pots. The highest catch was 17,6 crabs/pot, and only about 30% of the lines gave catches of 5 crabs or more per pot. These lines were • concentrated in three main areas; two fjords south of Nuuk, the fjord complex south of Sisimiut and in some parts of Disko Bay. These areas are shown in figure 1.
Although a substantial part of the pots gave no catch at a11, it was a striking feature that none of the "areas" (fjords) were completely devoid of snowcrab. A few attempts were made offshore, but catches were low.
Except for one locality in Nuuk, where snowcrab was caught at depths between 100 and 200 meters, best catches were obtained in depths between 300 and 600 meters. The deepest fisheI)' was at 857 meters in Disko Bay, where the catch was 3.7 crabs/pot. With the exception of the sma11 locality in Nuuk, catches of crabs at depths less than 200 meters were mostly made up by the toadcrabs, Hyas araneus and Hyas coarctatus, which have no commerdal value. 4 Table I shows the mean catch in numbers of legal sized crabs per pot in the 'good' localities, I?ean density, the area in sq.km., and the total abundance. Because of lack of proper seacharts, areas offishing grounds were calculated as the surface area of the fjord, minus 10 %. The last 10 % is the area that is estimated to be outside the depth range of snowcrabs in Greenland.
Length distribution:
Length distributions in the three commercially interesting areas are shown in figure 2. Differences in mean length between areas are evident. Mean CL in Nuuk is 106 mm, in Sisimiut 112 mm, and in Disko Bay 118 mm.
• Length-weight relations:
Figure 3 shows the length-weight relation, based on 717 male snowcrab. Data are pooled for all areas.
A linear regression on the equation logWEIGHT=loga+bxlogLENGTH
gives -9.0645 for the intercept and 3.27 for the slope. R 2 is 0.98.
Different methods of measuring:
A linear regression on CL vs CW from 350 male crabs (measured by caliper) has an • intercept of 3.4 and a slope of 0.9610 (R2 = 0.99).
For CL measured on measure-board vs CL measured by slide caliper, a regression analysis on data from 167 male crabs shows an intercept of -0.1 and a slope of 1.0024 (R2 = 0.99).
Discussion:
Snowcrabs were caught in a11 fjords, but catches varied substantia11y between different fjords. Three larger areas yielded catches that were better than average, namely two fjords south of Nuuk (64°N), the fjord complex south of Sisimiut (67°N), and parts of Disko Bay (69-700 N). 5 The densities of snowcrabs in the West Greenland fjords are apparently at a lower level than on the fishing grounds in Canada. Miller (1975) found densities in Eastern Canada
between 3.i and 7.1 males> 90 rnrn / 500rn 2. The densities encountered during the survey did not encourage further development of a fishery directed at snowcrab in the areas. There is however, still some interest in the resource, pending developrnent of world market prices.
Investigations in depth distribution in the areas are irnpeded by the topography of the fjords. Many of the fjords have very steep slopes and a fairly level bottom. It is thus impossible to get good data from a wide variety of depths in the same fiord. However, it does seem that Chio1Zoecefes opilio in Greenland mainly occurs at greater depths than in Canada, as also stated by lohansen (1974). • The increase in mean length from South to North is not due to occurrence of higher amounts of large crabs only, as it is seen that the proportion of smaller males in the catch, decreases from South to North. Another survey, performed injune 1992 (data not presented here) showed the same difference in length distributions between Sisimiut and Disko Bay. This rules out the possibility that the difference is caused by fishing in different seasons.
The results from the sirnultaneous measuring experiments, shows that far all practical purposes there is no difference between the two methods of measuring length, and that data are directly comparable to investigations where size is measured as carapax width.
References:
• lohansen, P., 1974: Length-weight key, carapace length-width key, and occurrence at West Greenland of Chio1Z0ecefes opilio. ICES C.M. 1974 / K:6.
Miller, R.l., 1975: Density of the commercial spider crab, Chio1Zoecefes opilio, and calibration of effective area fished per trap using bouom photography. l. Fish. Res. Bd. Can. 32: 761-768.
Srnidt, E., 1965: Is a Greenland crab fishery possible ? ICES C.M. 1965 / No. 116. 6
Table I: Mean catch in numbers of legal sized (>90 mm CL) male Chionoecetes opilio per pot, densities of legal sized males per 500 m2 , area of fishing ground, and abundance in five localities where mean catch were more than five large crabs per pot.
Locality Mean no. Density Area Abundance I 2 2 I per pot no./500m km
Ameralik (Nuuk) 7.03 1.42 246.1 697,629
Buksefjord (Nuuk) 11.85 2.00 62.0 248,409
Sisimiut 6.98 1.27 539.4 1,366,537
S. coast of • Disko 1sl. (Disk) 7.66 1.31 800 1 2,096,000 Ata Sound (Disk) 7.05 1.20 188.4 450,345
• 7
66°N I------~~~r+_------
•
Figure 1: Map of survey area, showing localities with average catch of more than 5 crabs per pot. ••
20
8 ,. Disko Bay n=3268 p
•r c • '0 n t
80 85 , 0 0 • 0 5 , .0 •• 5 .20 , 25 '20 • 25 • 40 , 45 , 50
20
Sisimiut '5 n=1756 p •r c n• '0 t
..
o 0 .. , 00 , 0 .. , , 0 '20 , ~ 0 , 3 5 , 4 0 , 4" , .. 0 , 5" '0 " " '2" 20
, .. Nuuk • n=2409 p
•r c '0 •n t
..
o 00 0 .. .00 •0" , '0 ,, .. , 20 , 2 .. , 30 , 3 .. , 4 0 , 4" , .. 0 , " .. C8/8Il8X Iength (5 mm ~) Figure 2: Lengtti frequericies (5 mm groups) of the catch of male ClzioTzoecetes opiÜo in three different localities in West Greenland. •
9
1600
1500
HOO +
1300
1200
W e 1 100 i g 1000 h t 900 I n 800 g r a 700 m m 600 e + s 500
400 +
300
200
+ • 100 ~""'~-~------""'+. 'I- •••••• + ...... 0 40 50 60 70 80 90 100 1 10 120 130 140 150 160 Carapax length (mm)
Figure 3: Length-weight relations for 717 male Chionoecetes opilio from West Greenland. Dashed lines indicate 95 % confidence limits.