861 



Preliminary study on the use of 



neural arches in the age determination of 



bluntnose sixgill sharks iHexanchus griseus) 



Gordon A. McFarlane 



Jacquelynne R. King 



Mark W. Saunders 



Pacific Biological Station 



Fisheries and Oceans Canada 



Nanaimo 



Bntish ColumbiaV9R 5K6, Canada 



Email address (for G A McFarlane) McFarlaneSiq^pacdfo-mpogcca 



Conventional structures used for 

 age determinations of teleost 

 fishes (e.g. fin rays, otoliths, scales) 

 cannot be used for elasmobranchs 

 in which these structures are car- 

 tilaginous. However, vertebral cen- 

 tra with systematic deposits of 

 calcium phosphate, have been used 

 for age estimation in a number of 

 elasmobranch species such as the 

 tiger shark iGaleocerdo cuvier), 

 scalloped hammerhead {Sphy7-7ia 

 lewini), and several smoothhound 

 sharks (Mustelus spp.) (Cailliet, 

 1990). Cailliet et al. (1983) pro- 

 vided preparation techniques for 

 enhancing and examining vertebral 

 bands in these elasmobranchs. 

 Unfortunately, the vertebral cen- 

 tra of a number of sharks, in- 

 cluding bluntnose sixgill sharks 

 iHexanchiis griseus), are too poorly 

 calcified to provide age information 

 (Cailliet, 1990). Previous attempts 

 to age sixgill sharks by using ver- 

 tebral centra have been unsuc- 

 cessful (Ebert, 1986). Sharks with 

 poorly calcified vertebral centra 

 tend to be either deep-water spe- 

 cies or from relatively primitive 

 families (Cailliet, 1990). However, 

 systematic deposits of calcium 

 phosphate have also been found 

 in chondrocranium, jaws, visceral 

 arches, fin cartilage, claspers, neu- 

 ral and haemal arches (Clement, 

 1992). 



Bluntnose sixgill sharks (hereafter 

 referred to as simply "sixgill sharks") 

 are one of the largest and most primi- 



tive species of elasmobranchs. They have 

 a world-wide distribution and in the 

 north-east Pacific Ocean range from 

 the Aleutian Islands to Baja, California 

 (Hart, 1973). Because they are deep-wa- 

 ter inhabitants occupying depths up to 

 2500 m along the outer continental shelf 

 and upper slope waters (Compagno, 

 1984; Ebert, 1994), little is known about 

 their biology. Sixgill sharks are about 

 65-70 cm at birth and the maximum 

 length recorded is 482 cm (Castro, 

 1983 ). They are ovoviviparous and have 

 reported litter sizes ranging from 22 to 

 108 (Compagno, 1984; Ebert, 1992). The 

 estimated size-at-maturity for females 

 is 396 em (Ebert, 1992), although cap- 

 ture of mature sixgill sharks is rare. It 

 is likely that the young inhabit inshore 

 waters (Compagno, 1984). 



Off the west coast of Canada, an 

 experimental fishery for sixgill sharks 

 was initiated in the early 1990s but 

 was terminated because of conserva- 

 tion concerns. Recently, there has been 

 a renewed interest in a fishery, both 

 commercial and sport, for the species. 

 Incomplete knowledge regarding the 

 biology and life history of sixgill sharks 

 remains a concern. Given the continu- 

 ing interest in a fishery (commercial 

 and sport) and potential conservation 

 concerns, we investigated the use of 

 neural arches as an alternate body 

 structure to use for the age determina- 

 tion of sixgill sharks. We describe the 

 staining technique used and the bands 

 (annuli) observed. We suggest that our 

 technique represents a possible avenue 

 for developing an aging technique for 



various elasmobranchs, particularly 

 those with poorly calcified vertebral 

 centra. 



Methods 



From May through September 1994, as 

 part of a co-operative industry-govern- 

 ment sixgill shark tagging program, 

 259 sixgill sharks were captured with 

 hook and line gear off the west coast 

 of Vancouver Island, British Columbia, 

 Canada. Fishing occurred within five 

 main areas: Kyoquot Sound (50°00'N 

 and 127°20'W), Esperanza Inlet (49° 

 45'N 127°00'W), Nootka Sound (49° 

 25'N and 126°40'W), Tofino Inlet (49° 

 05'N and 125°40'W), and Barkley 

 Sound (48°50'N and 125°20'W). A 

 sample of ten sharks was obtained 

 for age determination research. Total 

 length (TL, cm) and sex were recorded 

 for each shark. 



A portion of the vertebral column 

 containing 15-20 vertebrae, including 

 the neural and haemal arches, was 

 removed just posterior to the head 

 and immediately frozen. In the labora- 

 tory the vertebral column section was 

 thawed. The connective tissue and the 

 outer layer of cartilage were removed 

 from each vertebra and neural arch 

 (Fig. lA) by soaking them in bleach 

 for 15 minutes and rinsing in distilled 

 water. Some teasing away of tissue was 

 required. For each set of vertebrae, five 

 neural arches were separated from the 

 vertebral centra (Fig. IB). A single dor- 

 soventral cut was made in each neural 

 arch to expose the inner portions (Fig. 

 IB). The silver nitrate staining tech- 

 nique for revealing calcium deposits 

 in vertebral centra of other shark spe- 

 cies (Cailliet et al., 1983) was modified 

 and applied to the neural arches. Each 

 arch was soaked in 150 mL of 1% silver 

 nitrate while exposed to wide spectrum 

 light (320-400 nm). Soak times in silver 

 nitrate varied according to the size of 

 the arch, but all soak times lasted at 

 least one hour. The degree of staining 

 was assessed on an initial arch for each 

 shark. If required, subsequent arches 

 were restained in the silver nitrate so- 

 lution and removed at 15-30 minute in- 

 tervals after removal of the first arch. 



Manuscript accepted 27 May 2002. 

 Fish. Bull. 100:861-864 (2002). 



