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The Michigan DNR Fisheries Division initiated a statewide Status and Trends Program (STP) in 2002 to assess fish populations and aquatic habitat in Michigan's inland waters. A key component of the STP is a network of fixed (index) sites in streams and rivers across the state that are rotationally sampled through time to assess temporal trends in and describe the regional-scale dynamics of salmonid and centrarchid populations. Fixed site surveys employ standardized methods and provide high-quality data on many of the state's most renowned fisheries.
The data collected from STP fixed site surveys include population parameters such as total abundance estimates and abundance estimates by age or size groups, age-specific growth, and survival for important species including steelhead, coho salmon, brown trout, brook trout, and smallmouth bass. Serving this information in a timely and efficient manner facilitates management of Great Lakes tributary and inland resources. It also enlightens biologists, anglers, interest groups, and the public about important processes influencing fish populations and current trends in abundance, growth, and survival of important sport fishes in streams and rivers throughout Michigan.
Data presented are from two types of electrofishing surveys, mark-recapture surveys for salmonids and single-pass surveys for smallmouth bass. Abundance summaries include total population abundance and abundance estimates for specific size groups or age-classes of fish. Mean length at age values are average lengths of fish of specific ages from these surveys (which generally occur around August). Survival values are calculated as the estimated number of age(x) fish from this year's survey divided by the estimated number of age(x-1) fish from the previous year's survey, with the result expressed as a percentage. Note that survival values can be highly variable when abundance estimates are low, and biased if substantial immigration into or emigration from the survey reach occurs. Survival estimates were capped at a maximum of 100% to avoid estimates greater than 100% due to immigration.
Since abundance of fish populations in streams can vary synchronously over time1 in response to regional-scale flow2 and climate conditions, map-based displays enable viewers to better see the spatial extent of trends in fish population parameters. Map-based displays can be accessed using the "Map" view. The colored dots on the map-based display indicate the degree to which parameter values from the most recent survey for the species at a particular site compare to average values calculated from surveys conducted at that site since 2002, when STP surveys were initiated. Note: Each time a new parameter ("Trend type" option) is selected, be sure to hit the "Update Map" button to refresh the map based on values for the chosen parameter.
Graphs and tables of the survey data can be obtained in the "Data" view by using the dropdown menus to select the river or rivers, fish species, and trend type of interest. The year slider just below the graph can be used to adjust the time period covered in the graph, and the data are shown below. Information in the details view can be exported as a PDF or Excel file using links shown in the upper right portion of the "Data" view screen.
The number of fish captured each year can vary not only due to changes in the population, but also because of variability in capture efficiency resulting from weather and stream conditions, sampling gear, and experience of personnel completing the survey. This variability can effect abundance estimates, and associated estimates of survival and mean length at age. The population characteristics conveyed through this web application are estimates and should not be construed as exact values. Users of the data should consider all potential sources of variability, and note that sample sizes and measures of variance around estimates were excluded for display purposes. Those who wish to use the data for formal scientific investigation and other purposes beyond the scope of this web application should contact Fisheries Division for additional information and guidance.
1 Zorn, T. G., and A. J. Nuhfer. 2007. Regional synchrony of brown trout and brook trout population dynamics among Michigan rivers. Transactions of the American Fisheries Society 136:706-717.
2 USGS current water data for Michigan: daily streamflow conditions. Online at:
http://waterdata.usgs.gov/mi/nwis/rt
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