Evaluation of loss rate of coded-wire tags implanted into adipose eye tissue of masu salmon Oncorhynchus mason and effect on growth of tagged salmon

摘要

In Japan, fin-clipping has long been used to evaluate the effectiveness of masu salmon Oncorhynchus masou enhancement programs and to obtain biological information, such as migration, growth and survival rate of fish. However, with the exception of the adipose fin, some fins of salmonids regenerate after a relatively short time and their mutilation affects the survival of fish after stocking. Furthermore, the limited combinations of fin-clips do not allow for many codes and thus limits the number ofvariables that can be evaluated. In North America, a method of marking fish by implanting a coded-wire tag (CWT) has established itself as a reliable alternative to fin-clipping. Coded-wire tags for salmon are almost always implanted into the nasal cartilage of the snout or nose, with adipose fin-clipping often serving as an external flag. When fin-clipped salmon are caught by commercial or sport fishermen, the tags can be easily recovered from the snouts of the discarded heads. In Japan, however, salmon heads are valuable as foods and they cannot be recovered from fish in fish markets. This cultural difference between North America and Japan, therefore, necessitates novel approaches to implanting CWT. It was reported that adipose eye tissue in masu salmon should be a suitable location for CWT tagging both because it would be possible to remove this tissue in the fish market without reducing the fish's commercial value and because it has a high tag retention rate in an artificial rearing experiment. It was unknown, however, whether this high tag retention rate could be maintained after CWT-implanted masu salmon were stocked into natural streams and the ocean. In the present study, approximately 30 000 tagged masu salmon smolts were stocked to investigate the tag loss rate and effect on growth for recovered adults. Masu salmon smolts for stocking were reared at Kumaishi Research Branch of the Hokkaido Fish Hatchery, located in south-western Hokkaido (Fig. 1). From 26-28 October 1999, the fish were marked by removal of the adipose (Ad) and left ventral (LV) fins with a sharp pair of surgical scissors, to provide quick and easy flags for visual identification in the field. These fish were reared in two concrete ponds (3.5 X 24.6 X 0.4 m) until stocking. From 10-13 April 2000, half-length CWT (0.5 mm long) were implanted into the fish, according to the procedures described by Nagata et al. After fish were anesthetized, CWT were implanted into adipose eye tissue (transparent post-ocular tissue), whichis located behind the eyes of smolts using four Mark IV CWT injectors (Northwest Marine Technology Inc., Shaw Island, WA, USA). Fork length and body weight of smolts at the time of tagging was 14.3 ± 0.8 cm and 29.9 ± 4.8 g, respectively (N= 295, mean±SD). Average tagging speed was 390 fish/h/machine and 30 507 fish were tagged. Every day before stocking, all dead fish in the ponds were collected and checked for tag retention using a wand detector (Northwest Marine Technology Inc.). On 12 May, 7 days before stocking, 293 live smolts were sampled randomly from the ponds to examine tag retention. On 19 May, all fish were released from the ponds at Kumaishi Research Branch.