Current technology allows the placement of sonic or archival or satellite tags inside the fish body cavity or on its back to record detailed information on fish movements in relation to their environment. Information obtained from electronic tags is intended to improve our knowledge on fish behaviour and their vulnerability to the various fishing gears.
Sonic tags are miniature radio transmitting devices that are surgically implanted into the peritoneal cavity of the tagged tuna.
Sonic tags send a coded radio signal that could be detected and stored into a receiver station if the tagged fish is swimming into the detection range of the receiver (often around 800 meters). Some sonic tag models could also record depth of the tagged tuna and send this information to the receiver. Receiver memory needs to be downloaded to get the informations.
These tags are used to detect the presence/absence of tuna in the vicinity of FADs (Floating Aggregrated Devices) equipped with listening stations.
A green conventional tag indicates that the fish also carries a sonic tag.
Unlike archival tags, sonic tags do not externally protruding stalk/antenna, so can not be identified in this manner.
For a sonic tag, the reward is USD 50.
Example of sonic tags and of a sonic receiver on a FAD
Archival tags are miniature computerised data recorders that are surgically implanted into the peritoneal cavity of a tuna. Archival tags measure a range of parameters, including water pressure, water temperature, light intensity and the internal body temperature of the tagged tuna.
Two different size classes of archival tag are used:
- the larger LTD-2310, LAT2810 (Lotek Wireless, Newmarket, Canada) and the Mk9 (Wildlife Computers, Redmond, USA) which are surgically implanted into fish 60 cm and larger;
- the smaller LTD-2410, LTD-2510 (Lotek Wireless, Newmarket, Canada) which are implanted into fish 40 cm and larger.
Depth, fish and sea water temperatures and ambient light are recorded each minute for LTD-2310, LAT2810 and Mk9. The LTD-2410 and LAT-2510 have limited memory capacity (128 Kb and 512 Kb, respectively) and to extend the period of sequential records of all data, the tags are programmed to record every 4 minutes.
The data is downloaded in a computer once the tag has been recovered. These tags are used to study the vertical and horizontal movements of tuna.
For a archival tag, the reward is USD 250.
Example of archival tags and surgery on a yellowfin tuna for archival tag insertion
Fish are captured for archival tagging during pole-and-line operations during the day, and also at night by using hand lines or rod-and-reel techniques when tied up to a FAD. Smaller bigeye and yellowfin (< 70 cm FL) are prioritized for tagging during pole-and-line fishing as fish condition is not compromised by the fishing technique. Larger-sized fish (> 70 cm FL) are generally caught with rod-and-reel or hand line at night and lifted from the water using a purpose-built sling, to minimize injury or stress.
Tuna selected for archival tagging are placed in a smooth vinyl tagging cradle or left in the vinyl landing sling if greater than 10 kg. The eyes are immediately covered with a wet artificial chamois cloth, a sea water hose inserted in its mouth to gently irrigate the gills, and the hook removed. If fish condition is judged suitable, an electronic tag(s) is surgically implanted.
V-shape tagging cradle re-designed for archival tagging
Implantation involved the insertion of the Betadine-rinsed tag into the body cavity through a small incision (3 cm) made with a knife-blade, which for yellowfin and bigeye tuna was closed using a dissolvable suture after insertion. The tagging operation lasts between 50 seconds and 2 minutes. Fish are then closed using three stainless steel staples delivered by a 3M 35W surgical staple gun.
Each fish is also marked with a conventional dart tag placed below the second dorsal fin. An orange or green conventional tag indicates that the fish also carries an archival tag.
The archival tag located in the peritoneal cavity, can be further identified by the stalk/antenna protruding externally from the peritoneal cavity.
Satellite tags record the same information than archival tags but they do not need to be recovered. They are fixed on the fish back and after a pre-set time, they will detach from the fish, float to the surface and transmit the recorded information to a satellite (Argos system). The recorded data are then forwarded to the researcher. Constraints of this ideal looking technology are the big tag size (only large tuna could handle the tag) and the need to aggregate the data to be sent to the satellite (the obtained information is less detailed than in the archival tag case).
Example of satellite tag
Electronic tags provide detailed information on fish movements in relation to their environment. The comprehensive information obtained from electronic tags is intended to improve our knowledge on fish behaviour and their vulnerability to the various fishing gears.