Survey of Bottom Substrate Composition and Fish Habitat Value at Nett Lake

Transcription

1 Survey of Bottom Substrate Composition and Fish Habitat Value at Nett Lake Prepared by: Frank G. Stone, U.S. Fish & Wildlife Service Ashland FRO May 2006

2 The objective of this project was to characterize and map bottom substrate composition and fish habitat value at Nett Lake. This information will provide data critical to the Bois Forte Natural Resources Department for the assessment of wild rice production, fish habitat and the long term management of Nett Lake. This information will also fulfill management recommendations specified in the Bois Forte Integrated Resources Management and Aquatic Resources Management Plans. Methods: During May 9-10, 2006, I conducted an assessment of the common substrate types found in Nett Lake. A systematic mid-lake sampling method as well as a continuous line transect circumventing the lake, were established to determine substrate types. Using an air-boat (supplied by the Bois Forte Natural Resources Department), substrate types were GPS referenced and identified into one of five categories: silt/muck/clay sand (partial size <0.25") gravel (partial size from 0.25" up to 2.5") cobble (partial size from 2.5" up to 9") bolder/bedrock (partial size >10") Air-boat supplied by the Bois Forte Natural Resources Department. Substrate types were identified by prodding the lake bottom with a long aluminum pole (12 by 1⅜ ) and feeling for the texture and vibrations that resulted. At each sampling point, the pole was plunged into the substrate several inches and at two different areas (approximately 20 feet apart). If additional verification was needed to identify a substrate type, a Ponar Dredge was used to pull up a sample of the soil. A Ponar Dredge was used when needed to verify substrate composition. 2

3 Systematic Mid-Lake Sampling: A total of 109 reference points were provided by the Bois Forte Forestry Department for this survey (refer to adjacent image). Using a GIS program, each point was evenly spaced (approximately 1,584 feet or.3 mile). The resulting positions were then transferred into a handheld GPS recorder. This unit was used by the boat operator to locate each specific sampling point. The midlake survey began in the southwest corner of Nett Lake and was concluded in the upper northeast corner. All 109 reference points were identified except for the northeastern portion of the lake were safe boat navigation was of a concern. Those areas (point numbers 1, 11 and 12) were arbitrarily assigned a classification of silt/muck/clay. Continuous Line Transect Sampling: Following the natural contour of the lake, the boat operator maintained a path within feet of the shoreline. Where thick vegetation was not present, the boat was often maneuvered within 10 feet of shore. An aluminum pole was again used to probe the substrate. For this portion of the survey, clear water transparency significantly helped to determine the type of substrates present. As the boat continued to follow the shore line, a GPS point was recorded for each change in the substrate type. Starting from the Nett Lake boat landing/picnic area and proceeding north, one complete circle of the lake and Spirit Island were made. Using an aluminum pole to probe the substrate (from boat dock). 3

4 Results: Systematic Mid-Lake Sampling: Of the 109 sample points we assessed during this survey, 100% of the samples were composed of silt/muck/clay. Number of Sample Points Silt/Muck/Clay Sand Gravel Cobble Bolder/Bedrock Continuous Line Transect Sampling: During this portion of the project, continues observations were made for any substrate changes throughout the entire circumference of the lake. Boat speed was maintained at a slow pace that allowed for easy site identification and prodding with the aluminum pole. As a result of this effort it was determined that the predominate substrate type was silt/muck/clay. Only three areas near the boat landing were found that contained other substrate types (Table 1). Table 1. GPS reference points and map showing the only gravel, cobble or bolder substrate types found in Nett Lake during May 9-10, GPS Reference Sand Gravel Cobble Bolder/ Bedrock N 48º W 93º (Big Point) N 48º W 93º (Big Point) N 48º W 93º (Spirit Island) 4

5 Recommendations: The extensive wetland and bog conditions surrounding Nett Lake serve to trap soil within the thick vegetative mat surrounding the lake. As a result, the substrate type most common within Nett Lake is composed of sand, muck and clay. Only three small outcropping of gravel and boulders were noted during this survey. My recommendation for the long term management of Nett Lake is to continue to build on the natural strengths that the lake can offer. Attempting to enhance the recreational fishery (specifically for northern pike) would be limiting due to the shallowness of the lake (<8 feet deep) and frequent winter kill conditions. Chris Holm (Nett Lake Reservation Water Resources Program Manager) noted that fish species within Nett Lake are able to overwinter only by migrating into adjacent stream inlets during the winter months. The great strength that Nett Lake has to offer is the habitat and food resources it can provide to migratory waterfowl and songbirds. I believe that keeping the lake open for wild rice production and managing for waterfowl would be the best use of Nett Lake. This management approach would also support the tribe s most prominent and culturally important natural resource, the continued production of wild rice. I understand that the consumption of wild rice seed by waterfowl is of concern to the tribe. This specific issue will be addressed next year by our wildlife biologist within the second phase of this project. Due to the gradual contour, shallow lake depths and the apparent inability of the lake to flush out accumulations of sediment, the encroachment of emergent vegetation will continue to pose significant impacts on wild rice production. The tribe may want to identify the cause of the increased sediment loading into Nett Lake. The 2004 Tribal Landowner Incentive Grant proposal submitted by the tribe made the following two statements: Evidence suggests that wild rice decline in Nett Lake started decades past, with the advent of more sophisticated timber harvesting techniques and the decline of fur trapping and trading. Damming of the Nett River outfall has, over seventeen years, resulted in the accelerated stagnation of the basin and consolidation of rice straw and sediment within the basin. Taking steps to reduce sediment loading and restore the natural flushing actions of the lake will increase the areas for wild rice production. If Nett River dam modifications are deemed necessary to achieve this goal, I would again suggest that the tribe consider applying for a Tribal Landowner Incentive Grant and also seek other grant opportunities to help address the financial concerns associated with such an undertaking. 5