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, RETENTION OF RADIO TRANSMITTERS 265 orado squawfish and razorback suckers from which radios were removed were compared to those of nonimplanted fish that had been tagged (Carlin dangler) and subsequently recaptured from 1979 to 1987. Only fish recaptured from the Green Riv- er basin whose lengths fell within size ranges of radio-implanted individuals were used for this comparison. Results and Discussion Fourteen transmitters were recovered from im- planted fish: 12 had been retained from 3 months to 8 years in 10 Colorado squawfish, and 2 were retained for 1 year in 2 razorback suckers (Table 1). Average retention time was 2.2 years for Col- orado squawfish and 1 year for razorback sucker implants. Such long-term retention of surgical im- plants is particularly noteworthy, because rapid expulsion of implanted transmitters has been re- ported for channel catfish Ictalurus punctatus (Summerfelt and Mosier 1984; Marty and Sum- merfelt 1986) and rainbow trout Salmo gairdneri (Chisholm and Hubert 1985). Extended retention of transmitters is consistent with radiotelemetry results. Both Colorado squaw- fish and razorback suckers have been routinely tracked 5-6 months (Tyus 1985, 1987). Of 112 Colorado squawfish tracked by U.S. Fish and Wildlife Service personnel, 5 have been followed 12-18 months (Tyus et al. 1987). No expelled transmitter had been documented through 1985. Individuals of both species can obviously retain implanted transmitters for protracted periods, and potentially for life. Long-term monitoring is es- pecially important for study ofrare fishes, for ad- equate samples may not be obtainable otherwise. -Long-term retention of radio transmitters noted here is in marked contrast to laboratory work per- formed on ictalurids and salmonids with paraffin and silicone-coated dummy transmitters. Channel catfish implanted by Summerfelt and Mosier (1984) and held in ponds demonstrated a 71 % transin- testinal expulsion of paraffin-treated capsules through the anus in only 112 d. Marty and Sum- merfelt (1986) documented a rapid, 53% expulsion of silicone-rubber and paraffin-coated dummy transmitters in 23 d: 14 transmitters were expelled through the anus, 22 through the incision, and 1 through the body wall. Chisholm and Hubert (1985) noted 59% expulsion of paraffin-coated dummy transmitters from rainbow trout main- tained in aquaria for 175 d. These transmitters were presumably expelled through the anus. Re- jection of dummy transmitters in these studies may TABLE I.-Retention time of surgically implanted ra- diotransmitters recovered from Colorado squawfish and razorback suckers, Green River, 1980-1987. Transmitter casing and fish number Months retained Colorado squawfish Acrylic 3055" 4001 36 37 Polystyrene 4018b 96 Polycarbamate 480 4017 3175 3054 3055" 4018b 0165 0064 4007 41 15 I3 12 12 12 5 3 3 Razorback sucker Acrylic 3077 4022 12 12 "Fish 3055 was implanted twice: May 1983 and May 1984. b Fish 40 I 8 carried two transmitters during one 12-month period (May I985-May 1986). be related to surgical techniques, transmitter coat- ings, transmitter size, holding stress, or species differences. My experience allows comments on the first three of these possibilities. For transmitter implantation, I used a lateral incision (Tyus and McAda 1984) through heavy ventrolateral musculature, and noted no trans- mitter expulsion. Marty and Summerfelt (1986) used a ventral midline incision and noted hernia- tion, rupture, and dummy-transmitter expulsion through the linea alba. A more lateral incision may better protect the wound from internal pressure of viscera or implant; ripe females are particularly likely to lose transmitters through midline inci- sions (Marty and Summerfelt 1986). Abrasion of the wound or sutures on tank or pond bottoms may also be a factor with ventral incisions that could be avoided with more laterally placed im- plantations. Ziebell (1973) implanted transmitters in seven channel catfish by lateral incision and successfully tracked them for 3 months. All transmitters recovered in this study were surrounded by connective tissue and had to be excised or probed free. Growth of tissue onto and apparently into the beeswax coating encapsulated the transmitters and isolated them from the vis- cera. The transmitter capsules were usually fixed