Sea lamprey (Petromyzon marinus) have had dramatic negative impacts on fish populations in the Great Lakes [1, 2]. Beginning in the 1950s, state, provincial, and federal agencies have worked to suppress sea lamprey populations to facilitate restoration of native Lake Trout (Salvelinus namaycush) populations. Effective, targeted control of nuisance species requires an understanding of the species’ life history and movements. Sea lamprey are a semelparous, adfluvial species; after hatching in spring, larval sea lamprey (ammocoetes) spend 4–6 years buried in stream sediments before they metamorphose into parasites, migrate downstream, and spend 12–18 months in lakes before migrating back into streams to spawn. Control efforts, including selective lampricides and barriers to block migrations, focus on the larval and spawning phases, as the parasitic juveniles are too dispersed to target [2, 3].

Continuing issues with permitting and nontarget mortality associated with use of lampricides and stream barriers have motivated a search for alternative control methods. Alternative methods such as male sterilization and trapping with pheromone attractants have been developed to target spawning adults and limit reproductive success [4, 5]. The life stage that has received the least attention for control is the outmigrating juvenile stage [1, 2]. As with spawning adults, the migratory behavior of this life stage makes them vulnerable to capture at a single in-stream location. Metamorphosing juvenile sea lamprey were targeted in the 1940s and 1950s using dams and inclined screen traps [2, 6, 7], but these efforts were discontinued following the discovery of selective chemical lampricides and limited research on this life stage has occurred since then. A better understanding of the spatial and temporal patterns of outmigration is needed to develop efficient methods to target this life stage.

PIT tags have been used to track in-stream movement and migration patterns of many fish species [8–10]. PIT tags are small, individually coded capsules that range from 8 to 32 mm in length. Tags are implanted in individual fish and can be read with a handheld scanner or remotely when the fish passes an electromagnetically charged antenna [11, 12]. Antennas can be designed to encompass cross sections of a stream and detect tagged individuals that pass the antenna array without physically recapturing the fish [13, 14]. Antennas can be constructed to withstand high discharge events and periods of high debris loading that would be likely to dislodge or destroy other sampling gear [12, 15, 16]. These characteristics make PIT antenna systems ideal for monitoring juvenile sea lamprey outmigrating during the fall and spring.

When using implanted tags to track movement or behavior of individual fish, it is important to ensure tags are retained and do not affect survival or behavior. In prior studies, sea lamprey ammocoetes [17] and juvenile Pacific Lamprey (Lampetra tridentata) [18, 19] had high survival and tag retention when tagged and held at temperatures between 8 and 23 °C. However, the tagging procedures in these studies required dissecting microscopes, sutures, parafilm bandages, and multiple needles and lacked the simplicity needed for large-scale field deployment. In response, a less-invasive procedure [20] was tested on juvenile Pacific Lamprey using a single incision, a 9-mm FDX PIT tag inserted by hand, and no sutures or bandages. Lamprey were held between 9 and 18 °C and showed similarly high survival and tag retention as the studies with more complicated procedures [20].

No prior studies have been conducted with tagged metamorphosing juvenile sea lamprey or have tagged and held juveniles under natural conditions. Although lamprey survival tends to increase when held at lower water temperatures [19, 20], survival has not been tested at temperatures as low as the 5 °C which is thought to be the upper threshold for inducing outmigration by juvenile sea lamprey [6]. Additionally, survival and tag retention has not been tested for juvenile lampreys tagged using the simple procedure with the larger 12-mm HDX PIT tags [12]. We tested the survival and tag retention of metamorphosing juvenile sea lamprey implanted with 12-mm HDX PIT tags using the less-invasive tagging procedure [20] and held at water temperatures that would be experienced during their natural outmigration, to (1) determine whether tagging affected short- and long-term survival and (2) quantify tag retention for juvenile sea lamprey.

Our results demonstrate that metamorphosing juvenile sea lamprey can be tagged with 12-mm PIT tags using a field-amenable, simple tagging method [20], and exhibit high survival and tag retention. The sea lamprey used in this experiment were collected at different times, sourced from three different streams, held for different lengths of time before and after tagging, and varied widely in length, yet overall survival at the end of an extended observation period was nearly 93%. By demonstrating that survival and tag retention can be high regardless of these different conditions, our results are applicable to a wide range of future studies in which these conditions are likely to vary as well.

Survival in our experiment was similar to that seen in other experiments where outmigrating juvenile Pacific Lamprey were tagged and held in cool water [17, 19, 20]. Tagged Pacific Lamprey held at 8 and 9 °C exhibited slightly lower overall mortality than we observed at 5.2 °C; however, this difference was minimal when compared with the dramatic increase in mortality observed for Pacific Lamprey held at temperatures between 12 and 23 °C [19, 20]. Most mortalities observed during other experiments with tagged juvenile lampreys resulted from fungal infection, which appeared to be a severe issue in several cases [17–20]. Only one tagged and three untagged sea lamprey died in our experiment as a result of fungus. We did not treat sea lamprey to inhibit fungal growth in this experiment and assumed fungal infections were a product of captive conditions. If tagging does not affect the rate of fungal infection in a natural environment, lamprey could be released 24 h after tagging to reduce this source of mortality. Further studies should investigate survival of lamprey released immediately after tagging, which would reduce stresses due to handling, transportation, and captivity.

A greater proportion (3 of 8; 38%) of tagged lamprey <140 mm died during our experiment than larger lamprey (8 of 142; 6%). Though our sample of <140 mm lamprey was small, other lamprey tagging studies have suggested thresholds similar to this value. Mueller et al. [19] also used 12-mm tags in juvenile Pacific Lamprey and recommended a threshold of 120 mm; however, their surgical process utilized a needle to inject the tag through a small puncture in the skin that was then closed by a single suture. Although this process appears to allow smaller individuals to be tagged, the process may be too complicated for use in the field and a 135-mm threshold was suggested when following the simpler tagging procedure to implant 9-mm tags [20]. Low survival was observed for larval sea lamprey with an average length under 120 mm even with 8-mm tags (40% survival) and 9-mm tags (10% survival; [22]); however, recent research demonstrated substantially higher survival rates (97% field; 94% laboratory) for larval Pacific Lamprey implanted with 8.4 mm × 1.4 mm FDX PIT tags [23]. Schreck et al. [18] suggested 150 mm as a minimum size using 8-mm PIT tags; our findings suggest this may be conservative as 82.3% of lamprey in our study <150 mm survived with larger tags (12 mm × 2.12 mm).

Demonstrating high survival for juvenile sea lamprey implanted with the 12-mm HDX tag provides managers and researchers with a tool for understanding downstream migration behavior of sea lamprey where monitoring larger open channels is required. Previous field studies of PIT tagged lamprey have been limited to the detection of ammocoetes or lamprey that had begun to metamorphose using portable detection equipment that can be slowly and repeatedly passed over an area [17, 22]. Although shorter lamprey can be successfully tagged with 8.4-mm PIT tags [23], detection range is generally reduced as tag size decreases [12]. To effectively detect outmigrating sea lamprey, studies should utilize the largest tags possible to maximize the probability of detection when using stream-spanning wire arrays. Antenna arrays for larger HDX tags also do not require the rigid, watertight enclosures necessary for the FDX tags allowing for easier installation at a lower cost.

PIT telemetry has proved to be a valuable tool for management of downstream migrating salmon smolts and has provided a wealth of information about this critical early-life period, much of which remains unknown for sea lamprey. For example, PIT telemetry has been used to determine system-wide, reach-specific, and seasonal survival [24, 25], temporal and spatial movement patterns [26], and timing of movement and survival relative to stream conditions and hydropower operations [27, 28]. Ultimately, having the capability to monitor downstream movement in natural, open, stream channels can provide a better understanding of when and how sea lamprey outmigrate. This tool will allow managers to understand where and when invasive sea lamprey outmigrants in the Great Lakes are most vulnerable to capture and assess risk of mortality during outmigration for sea lamprey in their native range.

Demonstrating the use of larger HDX tags as a viable option for PIT telemetry for these small fish provides another monitoring tool for researchers who might find the FDX PIT systems too costly or technically prohibitive but wish to maximize detection range. Given that juvenile sea lamprey typically migrate during periods of high flow and often heavy debris load (leaves, snow, ice), antenna arrays can be damaged or completely destroyed. A cheaper alternative for antennas allows monitoring in open stream channels where equipment damage is more likely.

The overall high level of survival and tag retention in this study indicates that future studies can successfully use metamorphosing juvenile sea lamprey implanted with 12-mm HDX PIT tags to better understand their downstream movement patterns and timing of outmigration. Currently in the Great Lakes, no control effort targets the downstream migratory phase, in large part due to the protracted migratory period. Understanding when downstream movement occurs and how this movement relates to environmental cues would allow managers to target their effort during periods of higher likelihood of capture to maximize trapping benefits relative to cost of operations.