Oviduct-inserted transmitters have shown promise for determining precise location of spawning in fishes. Use of traditional manual tracking to locate expelled oviduct transmitters is laborious and accurate estimates of time of transmitter expulsion require frequent surveys. We tested the feasibility of using oviduct-inserted transmitters with positional telemetry to estimate time and location of spawning in lake trout (Salvelinus namaycush). Three assumptions were tested: (1) oviduct transmitters remain within fish until spawning, (2) oviduct transmitters are expelled with the eggs during spawning, and (3) time and location of oviduct transmitter expulsion can be accurately determined.
In the laboratory, 39 of 44 (89%) lake trout retained an oviduct transmitter until end of the spawning
period and all premature transmitter expulsions occurred before maturation. Natural spawning in the laboratory was not feasible; however, of 35 ripe trout that retained transmitters, 31 (89%) expelled their transmitter with eggs when subjected to manual stripping. Ability to position transmitters with a telemetry array at known spawning sites in Lake Huron (North America) was poor when oviduct transmitters were placed in the substrate compared to transmitters suspended 1 m above substrate - 78% of transmitters in substrate could not be positioned. However, in simulations, time and location of spawning were determined with reasonable accuracy by double-tagging trout with one transmitter that is expelled with the eggs during spawning while another transmitter remains in the fish. Accuracy of estimated time and location of transmitter spatial separation varied with distance traveled from spawning site and swimming speed, and was dependent on transmission delay.
Our results satisfied the three assumptions of oviduct tagging and suggested that oviduct transmitters can be used with positional telemetry to estimate time and location of spawning in lake trout and other species. In situations where oviduct transmitters may be difficult to position once expelled into substrate, pairing oviduct transmitters with a normal-sized fish transmitter that remains in the fish is recommended, with spawning inferred when the two tags separate in space. Optimal transmitter delay will depend on expected degree of spawning site residency and swim speed.