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Table 2 Summary of key constraints (categorized by abiotic, anthropogenic, environmental/habitat, and biotic) associated with using electronic tags to study animals in freshwater systems, existing technical solutions, and additional technical needs that need to be addressed to improve equipment performance and science outcomes

From: Tracking animals in freshwater with electronic tags: past, present and future

Category Condition Constraint Existing technical solutions Additional technical needs
Abiotic Ice cover in lentic systems In some regions ice cover for part of the year impedes access the water When ice conditions are safe animals can be tracked through the ice with radio receivers on foot (skiing, snowshoeing) or driving (for example, snowmobile) Little information on acoustic performance under ice but some evidence of noise from cracking or shifting reducing detections (Mike Parsley, personal communication)
Modifies signal properties (for example, via reflections, noise) Acoustic receivers can be deployed below ice but must be moored so that they are not dragged during ice breakup (for example, do not use surface buoys [26]) Understand the performance of acoustic telemetry (especially for logging and automated positioning) under the ice [27]
May create problems during breakup by moving or damaging equipment   
Abiotic Ice dynamics in lotic systems Winter ice dynamics pose problems for gear deployed in water due to potential damage (for example, PIT antennas, hydrophones [28]) Radio telemetry is favoured for tracking [29, 30] For PIT and acoustic systems there is need for antenna and receiver designs that are not subject to ice damage
Loss of reception associated with noise and physical obstruction (especially acoustic telemetry [31]) Flatbed PIT antennas reasonably robust to ice processes [32]
  In small streams, use mobile PIT antennas through the ice [33]
Abiotic Shallow water Exceptionally shallow water (for example, 0 to 30 cm or even ephemeral systems, like billabongs) can expose underwater hydrophones and make manual acoustic tracking nearly impossible [34] Radio telemetry and mobile PIT antennas can be used in shallow water Limitations due to physics and physical constraints unlikely to be overcome with technology
Care must be taken not to disturb or chase animals being tracked
Abiotic Deep water Deep water attenuates radio signals [35, 36] and makes use of radio transmitters and PIT tags ineffective for positioning (in the latter case by virtue of the current difficulty of deploying PIT antennae at depth, though technically feasible if linked in situ with waterproofed logger and power supply) Acoustic telemetry performs well in deep water but positioning accuracy will depend on depth Unlikely that technical developments will improve performance of radio and PIT systems in deep water
Can use CART tags [22]
A downrigger device has been developed that enables tracking of radio-tagged fish at depth, but has limited reception zone [37]
Low-frequency radio tags can improve performance in deep water [35] as can use of more powerful batteries or stronger transmitter output [36]
Abiotic Thermocline Thermoclines can impair the performance of acoustic transmitters [38, 39], but radio transmitters are unaffected (aside from the fact that thermoclines often occur at depth) Hydrophones can be placed both above and below thermoclines to improve reception, or at the same level as the fish being tracked if they do not frequently cross the thermocline Unlikely that technical developments will entirely address this issue with acoustic tags
Rarely is performance so degraded that acoustic detections are not possible, but may impede ability to achieve fine-scale positioning
Abiotic High flows, currents and turbulence High water flows in riverine systems or from lentic currents (creating noise on hydrophones) and associated turbulence and entrained air (even in low flows) can impede the performance of acoustic telemetry systems [40] Increasing the density of acoustic receivers can improve detection probability [40] There is need for technical developments further refining code detection and noise filtering in acoustic telemetry systems deployed in high flows, currents and turbulence
Fixed acoustic receiver station deployment innovations enable their placement in large rivers [41] Shielding of hydrophones may help to reduce noise (Dale Webber, unpublished data)
Radio telemetry is unimpeded by such conditions  
Fixed PIT systems may be difficult to maintain in high flows (get washed out)  
Abiotic Turbidity High turbidity (especially if associated with suspended matter) can impair performance of acoustic systems [42] Acoustic systems usually perform well except in extreme turbidity Need to develop alternative ways of positioning animals with sensors that do not require light levels (that is, PSATs)
Makes use of light sensors to position animals ineffective (for example, loggers or PSATs) Radio and PIT are much less affected
Abiotic Conductivity High conductivity can severely attenuate VHF and HF radio signals in particular but can also reduce performance of acoustic telemetry systems [34] The extent of radio signal attenuation varies by radio frequency such that choice of lower frequencies (for example, 30 MHz) can improve performance [43, 44] Innovations in the marine realm for devices that function well in high salinity (and thus high conductivity) potentially benefit freshwater research
Conductivity presumably also influences PIT system performance [45]
Compared with radio, acoustic performance is relatively unaffected by high conductivity [33, 46]
Can use CARTs [22]
Abiotic Salinity Saline environments have very high conductivity and make VHF and HF telemetry unusable (as above) In saline environments, PSATs based on corrosive releases that enable tags to be jettisoned can be used Development of PSAT releases that work in freshwater would provide new tracking opportunities
High salinity has an effect on how much sound energy water absorbs [47], which is relevant for acoustic tracking Corrosive releases do not work in low-salinity environments; limits use of PSATs in inland systems
  Most inland systems would not have problems with radio attenuation or acoustic energy absorption due to salinity
Abiotic Air-water interface Some animals move in and out of water frequently (for example, turtles) despite being primarily aquatic, which leads to varied signal strength for radio tags and no signal for acoustic tags when in the air Most studies of animals at the interface use radio telemetry although satellite and GPS tags can also be effective Unlikely that technical solutions will address these issues
Acoustic tags can still be used to quantify timing and duration of out of water activities (for example, basking in turtles)
CART technologies can be used
Anthropogenic Human activity and vandalism In areas with human activity, some telemetry infrastructure is conspicuous and can attract vandals [48] Acoustic receivers, if deployed at depths and using gear that the public cannot easily reach, are usually safe from vandals Development of less conspicuous deployments for radio and PIT gear, such as autonomous underwater antennas and receiving systems (as is common in acoustic telemetry)
Swim-through PIT loops, with the top edge above the water surface should not be deployed where boats occur Most fixed-radio and PIT tag antenna arrays (unless in controlled access areas) are subject to vandalism even when using ‘vandal-resistant’ gear (for example, fixed and locked boxes [48])
  Miniaturization of power systems and autonomous receivers has helped to deploy receivers out of sight
Labeling the equipment with information on content, purpose and value of equipment may reduce likelihood of vandalism in some areas
Anthropogenic Human infrastructure and construction materials Human infrastructure (for example, fishways, dams) are often studied using electronic tags but the construction materials (for example, concrete, metal), operations (for example, entrained air, high flows), and electromagnetic activity (for example, from hydro wires) can interfere with radio, PIT and acoustic systems Underwater radio antennas can be used in areas near electrical infrastructure to reduce interference [49] Continued development of equipment that is robust and shielded from electromagnetic activity
Background radio noise over a wide range of frequencies (for example, communication systems and electrical devices) Many efforts have been devoted to developing acoustic systems that can work around infrastructure Development of improved radio telemetry coding systems
  PIT arrays work well in such areas but need to be cautious around metal surfaces  
  Use existing coding systems to avoid background interference  
  Prior to starting a study, identify frequencies that are prone to high levels of interference in your area  
Anthropogenic Noise Boat traffic, waves, rain and biological noise can all make detection of acoustic signals difficult Radio and PIT unaffected unless electrical interference from vessels (including electromagnetic noise interference from tracking boat [22) Improvements in coding to improve performance when signal to noise ratio is not optimal for acoustic systems
For example, rainfall can produce or increase noise levels of 15 to 25 dB in the water column [47] Much less biological noise in freshwater than marine systems, so not a problem in most situations Conducting detailed system-performance tests under different conditions needed to determine acoustic system efficiency (for example, in a shipping canal)
Environment or habitat Remote areas Many inland regions are remote, creating challenges for tending and operating electronic equipment [48, 50] Solar panels and special battery banks have provided power in remote locations for fixed stations (mostly radio and PIT systems [50]) Need for technological improvements to address challenges with maintaining systems in winter or damp environments (for example, better weatherproofing of equipment [48])
    Satellite connections enable data to be retrieved and receivers to be configured from afar (already exists for radio [51] and recently developed for acoustic systems with a surface cable and GPS or cell interface) More remote communication options to enable servicing from afar
    Aircraft-mounted antennas can be used for radio tracking animals over great distances in remote locales [52, 53] Batteries that last longer or systems that require less power
Environment or habitat Habitat configuration Geomorphological configuration of habitats (for example, type and configuration of banks, fjords, roughness of substrate, type of substrate) can influence performance of both acoustic [40, 54] and radio telemetry [55] Increasing density of acoustic receivers can improve detection probability in some areas (for example, such as near banks in rivers or areas with high levels of substrate roughness [56]) Need for additional studies to better understand how habitat configuration influences detection of radio and acoustic telemetry signals
Manual radio tracking can be confusing around some habitats (for example, rock walls) such that practice is needed to become efficient at locating tags
Environment or Habitat Cryptic habitats Some animals make use of cryptic habitats that make tracking difficult (for example, crayfish and mammals can use burrows [57]; some fish hide under rocks [37]) due to signal attenuation and deflection; acoustic telemetry ineffective in such situations Use tags that are as powerful as possible (for example, high-output radio tags) An intrinsic challenge that cannot be easily overcome with technology
Radio telemetry and PIT tracking possible if animals are not too deep into cover Efforts should focus on training of team members to track in such environments
Biotic Macrophytes Macrophytes can impede sound when using acoustic telemetry [34] and can foul external telemetry transmitters [58] Unclear if specific frequencies or other characteristics of acoustic telemetry systems would improve performance in dense macrophyte beds Need for rigorous testing of acoustic telemetry gear in and around macrophytes to identify ideal frequencies for local conditions
Radio telemetry performs well in dense macrophytes
Use internal tag implantation in animals that are in regions with lots of macrophytes
Biotic Animal damage Vermin of various types (mostly mammals such as muskrats or insects such as ants) can chew through wires for cabled systems (including cabled acoustic arrays [59]) or damage-sensitive electronics [48] Wires can be protected with sheaths and various pest control strategies can be used (for example, pesticides in land-based fixed stations) Further developments such that cabled systems are not needed
External tags can be designed to minimize chewing by mammals (for example, was a problem for box turtles [60]) Development of strategies for camouflaging and protecting wires and making gear sealed to prevent entry of insects
Biotic Biofouling Algae and molluscs (for example, Dreissena spp.) in particular can build up on underwater equipment, such as fixed hydrophones or underwater antennas, reducing performance [61] Underwater equipment such as hydrophones and antennas can be cleaned (for example, by divers) or various biofouling materials (for example, special paints [61]) can be used Need for biofouling paints and materials that are minimally toxic
Fouling of external tags can reduce performance and burden tagged animals [62] Tags (and antennas) can be placed internally [62] Need for methods of geopositioning animals without light
Fouling of light sensors makes use of geopositioning tags in freshwater difficult   
  1. CART, combined acoustic radio transmitter; GPS, global positioning system; HF, high frequency; PIT, passive integrated transponder; PSAT, Pop-up satellite tag; VHF, very high frequency.