Tracking the movements of free-ranging marine mammals is important for ecological and life history research as well as monitoring the post-release survival and behavior of stranded animals that have undergone rehabilitation. Although the range of VHF transmitters is less than that of satellite transmitters, VHF transmitters are much less expensive and are very useful for monitoring coastal species such as harbor seals (Phoca vitulina) by aerial survey. The battery life of radio transmitters can be maximized by duty-cycling or manipulating the pulse rate of transmitters and potentially last several years or more. However, radio tags attached to the hair coat of phocid seals are often dislodged due to moult, degeneration of the hair shafts at the attachment site, or other factors. Radio tags attached to flippers commonly tear out of the flipper webbing. Radio transmitters implanted subcutaneously should last much longer and result in better post-release monitoring and improved biological data.
Previous surgical implantation in pinnipeds has been hampered by excessive tissue reaction and subsequent rejection of implanted materials. Pinnipeds are reputed to generally react poorly to suture materials and have a high rate of wound breakdown following surgery. However, the use of improved materials for implants, monofilament suture materials, and good surgical technique may limit wound breakdown and improve post-surgical healing.
Four recently-weaned and one adult harbor seal were implanted with subcutaneous radio transmitters. The instruments used are summarized in Table 1. All harbor seals were anesthetized approximately 10 minutes after atropine (0.02 mg/kg IM) pre-medication by either masking with isoflurane or by giving 0.8 mg/kg IV of a 1:1 combination of tiletamine and zolazepam (Telazol®). All animals were maintained with isoflurane for the duration of the surgery. All transmitters were sterilized in glutaraldehyde for at least 45 minutes and then rinsed with sterile saline prior to implantation. An 8 to 10 cm transverse skin incision was made approximately 10 to 20 cm caudal to the distal scapula and 10 to 15 cm lateral to the dorsal midline, on the left thoracic flank of each animal. The subcutaneous fat layer (blubber) was dissected down to the fascial layer and a small pocket was undermined using a finger in the cranial aspect of the incision. The transmitter was used to undermine the rest of the tissue in order to place it entirely retrograde under the blubber layer, with only the distal end visible in the incision site. The long external antenna in the single transmitter was placed by using 5 mm diameter laparoscopic forceps to grasp the distal end of the antenna and tunnel normograde from the incision site along the flank of the animal to a second 1 cm long incision approximately 25 cm away. The end of the antenna was then removed from the forceps and the forceps were retracted. Closure of all wounds included a simple continuous pattern of 3-0 PDS II suture material to close the deep blubber and fascial layer, occasionally tacking down to the underlying muscle. A continuous vertical mattress pattern of 2-0 PDS II suture material was used to oppose the blubber layer. A continuous, everting pattern was used to close the subcuticular tissue with 3-0 PDS II suture material. Skin was closed using 2-0 PDS II and an interrupted cruciate pattern. Cyanocrylate glue was placed over the skin edges and skin sutures. Stainless steel staples were used to close the incision in the adult harbor seal and no glue was used in this animal.
Wound healing was unremarkable in the animals that were implanted with cylindrical Telonics transmitters which were encapsulated with a physiologically compatible wax coating. In these animals, incision sites remained clean, dry and very minimal post-operative swelling was observed. Skin incisions appeared to heal within approximately 10 days after surgery. However, wound breakdown was evident in both animals that were implanted with Advanced Telemetry Systems (ATS) transmitters which were encapsulated in a biologically inert resin. Both animals had mucopurulent discharge from the wounds and opening of the skin incisions. The animal with the external antenna developed a large fluid pocket over the incision site that was filled with mucopurulent exudate. The animal also developed a marked leukocytosis. Treatment included removal of superficial suture layers to allow for drainage and repeated irrigation of the surgical site with chlorhexidine. Systemic antibiotic therapy was also instituted. However, incision sites did eventually heal in both of these animals and the radio tags were not rejected.
From this preliminary study, it appears that wax-coated tags may result in less tissue reaction than resin-encapsulated tags. However, due to the small sample size, other factors including possible contamination during or immediately following surgery may have been responsible for the wound breakdown and can't be discounted. The long subcutaneous antenna external to the transmitter may exacerbate poor wound healing and may not be appropriate in phocid seals. Subcutaneous radio transmitter implantation without significant post-surgical complications is possible in phocid seals. This technique may lead to the acquisition of longer-term telemetry data to increase our understanding of phocid biology and improve our ability to successfully monitor seals after release from rehabilitation programs.
Table 1. Summary of types of radio-transmitters implanted subcutaneously in five rehabilitated harbor seals (Phoca vitulina)