What we're doing & why

Remote sensing

Continuous water temperature

Water temperature data are collected on all of the sentinel lakes year around. We deploy data sensors that collect temperature data every hour within the water column. The depth of these sensors varies from lake to lake. For some lakes, these data are collected at only a single depth, while other lakes have long chains of temperature sensors deployed at different depths throughout the entire water column. These sensor chains allow us to determine when and how long a lake stratifies, how strong the stratification is, and other water temperature related metrics.

Because most fish species have specific temperature habitat requirements, these data are important in understanding how these habitats are changing at both short- and long-term timescales. Additionally, temperature metrics relate to many other physical, chemical and biological processes that occur in lakes and help us gain greater knowledge about what is going on in Minnesota's lakes

Continuous water levels

Sentinel Lake water levels are measured using underwater pressure transducers. These sensors measure the depth of the water based upon the pressure of the surrounding water. These measurements are then rectified to a known lake level staff gauge reading by Sentinel Lakes staff and passed on to staff in the DNR Ecological and Water Resources Division for rectification to sea level.

Weather stations

We deploy weather stations at the Tier One Sentinel Lakes. These stations collect air temperature, relative humidity, wind direction, wind speed, gust speed, and dew point. To supplement these data, we also use data downloaded from the nearest airport weather station for each lake. These weather data help inform the relationship between air temperature and water temperature, how weather affects lake stratification, and other atmosphere-lake interactions.

Additional sampling

Trophic level measurements

Additional trophic levels are sampled at various time intervals and analyzed by topic experts working at our collaborating partners, the Minnesota Pollution Control Agency and DNR Ecological and Water Resources division.

  • Macroinvertebrates
  • Plants
  • Phytoplankton
  • Water chemistry
  • Zooplankton

Fish sampling


A juvenile fish survey is designed to sample younger fish on Sentinel lakes that are not yet vulnerable to standard fisheries survey gears (gillnets and trap nets) because they are too small to be effectively captured. Typical fish sampled during a juvenile fish survey include Largemouth and Smallmouth Bass, Yellow Perch, Bluegill, and Rock Bass. We use a boat electrofishing unit which requires a boat driver and usually 2 netters stationed in the front of the boat to scoop up stunned fish. Fish are stunned when a generator on board submits an electrical charge into the water to temporary stun fish. Captured fish are counted, measured, weighed and some structures may be removed for aging like scales or ear stones (otoliths). Young fish are sensitive to changes in water temperature and in fact the water temperature during their first year of life predominately determines their length heading into the winter with warm years generally resulting in longer fish. Juvenile fish surveys occur in October because fish growth for that year has been completed.


A near-shore survey is designed to sample many of the non-game fish species found in a Sentinel Lake and helps give a complete picture of the fish community. Typical fish sampled during a nearshore survey include Minnows (Cyprinidae), Darters (Percidae), and young Sunfish. A nearshore survey uses two unique gear types that are not employed during a standard survey: Backpack electrofishing and the 50 foot seine. Backpack electrofishing employs a much smaller version of the electrofishing equipment used in DNR electrofishing boats and is worn on the back of one of the crew members. Crew members wear waders and walk in the shallow water (0 to 3ft deep) along the shoreline at each sampling site. The equipment emits an electrical charge into the water that temporarily stuns fish and allows them to be netted yet does not penetrate the waders of the crew members. The 50 foot seine is a 50 foot length of mesh netting mounted to a 5 foot pole on each end. The seine is dragged through the shallow water (0 to 3 feet) along the shoreline by two crew members using the poles at each end as a handhold. After a set distance the seine is pulled to shore, and the captured fish are counted and identified.


A pelagic (open water) fish survey is designed to sample fish that inhabit the open and deep water portions of a Sentinel Lake. Typical fish sampled during a pelagic fish survey include Cisco (Tulibee), Lake Whitefish, and Lake Trout. The pelagic fish survey employs two gear types: vertical gillnets and hydroacoustic surveys. Hydroacoustic surveys use a powerful signal similar to a boat depth finder that identifies the number and relative size of fish. The hydroaccoustic survey is usually conducted at night on day 1. Vertical gillnets are set on day 1, allowed to fish overnight, and lifted on day 2. The vertical gillnet is stretched from the lake bottom all the way to the surface of the water using buoys and anchors. Captured fish are counted, measured, weighed and some structures may be removed for aging like scales or ear stones (otoliths). We also record the depth each fish was captured so that we can assign fish species to the marks seen using the hydroacoustic survey and estimate how many Cisco are in the lake and what depth they are inhabiting. This depth is important because it reflects the available pelagic habitat for these cold water fish with adequately low water temperatures and high dissolved oxygen. Pelagic coldwater fish can become stressed and sometimes die when temperatures get too high or oxygen too low resulting in fish kills.

Standard surveys

A standard survey is designed to sample the gamefish population in a Sentinel Lake. The standard survey employs two primary gear types: gill nets, and trap nets. Typical fish sampled during a standard survey include Walleye, Northern Pike, Bluegill, Black Crappie, Yellow Perch, and Lake Trout. The gillnets used in standard surveys are 250 feet long, 5 feet high nylon mesh stretched along the bottom of the lake at predetermined sampling sites. The net is held to the bottom with an anchor at each end, and marked on both ends with buoys. Fish attempting to swim through the net become entangled, usually by the gill covers, fin spines, or even teeth. The net is set on day 1, allowed to fish overnight, and lifted on day two. Captured fish are counted, measured, weighed and some structures may be removed for aging. The trap nets used in a standard survey have two main parts: a long lead of relatively fine mesh that is anchored to shore on one end, and a trap made up of several compartments with progressively smaller funnel shaped openings between them leading towards the back end of the trap. Fish swimming along the shoreline are blocked by the lead mesh, turn and follow it away from shore into the trap, and are funneled through the progressively smaller openings until they are in the back of the trap. Once inside the trap most fish are not able to navigate their way back out of the funnels they swam through. Like gillnets, trap nets are set on day 1, allowed to fish overnight, and lifted on day 2. Captured fish are counted, measured, weighed, and released.

Data management

Data processing and storage

Because the Sentinel Lakes Program is a long-term scientific data collection project, it is important to maintain a level of data standardization and quality control. We are continuously ensuring the methods we use to collect and process the data are consistent. For many of the datasets, these tasks are the responsibility of the agencies and divisions we are partnered with. Within the program itself, we have developed protocols for collecting and processing water temperature, weather, and certain fish metrics. These tasks include automatic QC tests, that detect and flag any anomalous data points, visual QC tests involving manually flagging any remaining anomalous data points, and organizing the data into a standardized format that is easily used by the data users.

Currently, the majority of the datasets are stored by the agency or division that is responsible for collecting them. We are working towards the goal of creating a portal where Sentinel Lakes data are easily accessible to anyone who is interested in using them for scientific or management uses.

Tool development

To ensure the data processing tasks are run consistently efficiently, we are developing tools that allow anyone to complete them without knowledge of the underlying programming code. We are also developing tools that help visualize and analyze the data, providing new perspectives and ways to interact with the data.

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