McFarlane and King: Migration patterns of Squalus acanthias 



359 



Figure 1 



Tagged spiny dogfish were recaptured along the North American coast from Mexico to Alaska through to the eastern Pacific (Japan! and 

 grouped by the areas indicated. The inset shows the west coast of Canada, and release areas for tagged spiny dogfish are denoted by rect- 

 angular boxes. Tagging occurred in the Strait of Georgia, northern British Columbia, and the west coast of Vancouver Island. 



Petersen disc tag, a modified elongated tag was developed 

 (McFarlane and Beamish, 1986). In 1979, approximately 

 30% of the tags deployed were modified elongated disc 

 tags. Since then, only the modified elongated tag has been 

 deployed. The tag consists of two elongated plastic discs 

 with rounded ends (Fig. 2A). The application of the tag is 

 similar to that of a Petersen disc tag (Wydoski and Emery, 

 1983). Each disc is attached to the fish by two pins, which 

 are inserted through holes drilled in each disc 2.5 mm from 

 each end. The disc is fastened to the fin of a dogfish with 

 titanium pins made specifically for this study from grade 4, 

 commercially pure (TiVOA) titanium wire (Fig. 2, A and B). 

 Each pin is 7.6 cm long and 0.99 mm in diameter. 



The applicator consists of a pair of hypodermic needles 

 attached to a plexiglas handle (Fig. 2Cl in a way that allows 

 for the insertion of the pins with the exact spacing required 

 (McFarlane and Beamish, 1986). The discs, on each side of 

 the fish, are attached just below the anterior base of the 

 first dorsal fin. During the tagging operation, the hypoder- 

 mic needles are pushed through the base of the fin, the two 

 pins are inserted through one disc and into the hypodermic 

 needles, and the applicator is withdrawn, leaving the pins 

 and disc attached to the fish. The second disc is placed over 

 the pins and secured by bending the end of each pin 180° to 

 form a small circle, with the free end of the pin resting un- 

 der the bent portion of the pin that projected from the hole 

 in the disc. The discs are loosely affixed and bent outwards 

 to follow the contour of the fish. In 1988, another modifica- 

 tion to the tag was initiated in the Strait of Georgia. A more 

 flexible plastic was used as the tag material in one third of 

 the tags used that year. 



Barbless hooks were used to capture most fish; however, 

 barbed hooks and bottom trawls also were used. In most 



cases fish were held in tanks on the vessel and only fish 

 that appeared healthy were tagged. Fish were anaesthe- 

 tized with MS 222 (tricaine methane sulfonate) prior to 

 tagging and were measured for total length (nearest mm) 

 from the tip of the snout to the tip of the upper lobe of the 

 caudal fin when held in a horizontal position. Most tagged 

 fish also were held in shipboard tanks to ensure recovery 

 prior to release. A reward was paid for recaptured fish. 

 Capture locations were recorded for major areas (Fig. 1). 

 All returned fish were measured for length (nearest mm) 

 and their sex was determined. 



Movement between major areas was described as the 

 percentage of recaptured fish originating from each release 

 area. Because the number of fish recaptured will decline 

 with time at liberty, we initially assessed the proportion of 

 total fish recaptured (by release area) as a function of time 

 at liberty to select an appropriate timeframe to use as a 

 focus for reporting the percentage of recaptured fish. 



It is important to note that these percentages do not 

 refiect differences in fishing effort between major areas. In 

 order to compare long-term movements between areas, it 

 was necessary to standardize tag returns to effort, exploita- 

 tion rate, or catch. Accurate effort data and estimates of ex- 

 ploitation rates are unavailable for the areas of tag returns. 

 Catch (metric tons It] ) data for spiny dogfish were available 

 for the areas in which the majority ot tagged fish were 

 recaptured (Table 1 ). The use of catch to standardize recov- 

 eries between areas is valid only if population abundances 

 are approximately equal between areas. This is true only 

 for the Strait of Georgia and Puget Sound and west coast 

 of Vancouver Island (Saunders, 1989; Ware and McFarlane, 

 1995). It is likely that the abundance off the Washington 

 State coast is similar to that off the lower west coast of 



