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Mansfield Creek (2008)

Fisheries Survey Summary

Mansfield Creek fish samplingIn July 2008, the DEC Fisheries Unit along with angler volunteers sampled the wild trout population in Mansfield Creek. This stream was last sampled in 2003. The lower 2.5 miles of the stream are stocked each spring with yearling and two year old brown trout while the upper six miles are managed as a wild trout fishery for brown trout and rainbow trout. Wild rainbow trout were introduced to Mansfield Creek by DEC in 1995-1997. They have been successfully reproducing since then. The stream is managed with a year-round fishing season with the time period from 10/16 to 3/31 restricted to catch and release fishing with artificial lures. This regulation took effect in October, 2006.

Six sites were electrofished in 2008, five of which were at the same locations as sampling in 1999 and 2003. The stream is characterized by having an unstable, gravel/cobble substrate with good riparian vegetation of shrubs and trees in most areas. The watershed mainly consists of agricultural and forested land uses with scattered residential development. The stream has extensive Public Fishing Rights (PFR) access.

Mansfield Creek wild rainbow troutWild trout abundance varied greatly in our sampling; the highest numbers were found in the three upper sites and low abundances were found at the lower three sites in the stocked section. This was expected as summer water temperatures often are above optimum limits for wild trout at the lower two sites. The average number of wild brown trout per mile for the upper fives sites (also done in 1999 and 2003) was 180/mile, while for wild rainbow trout it was 291/mile. This represents a substantial drop in numbers for wild brown trout from the last sampling (416/Mile in 2003) and an increase for numbers of wild rainbow trout (190/mile in 2003) (Table 1). In each year of sampling since 1995 (1997, 1998, 1999, 2003 and 2008), rainbow trout abundances have increased while brown trout abundance has varied greatly. A corresponding large decrease in the biomass of wild brown trout was seen compared with the 2003 sampling. The estimated biomass of wild brown trout in 2003 was the highest ever found at 67 lbs/acre, but had fallen to only 18 lbs/acre this year. The biomass of rainbow trout (12 lbs/acre) was similar to that found in 2003 (15 lbs/acre) (Table 1). Lower than normal numbers of yearling and two year old brown trout likely explain the drop in abundance and biomass of wild brown trout in Mansfield Creek in 2008. Trout habitat at all the shocking sites had changed substantially, but not decreased, in the five years since the prior sampling due to flooding and unstable substrates.

The largest brown trout captured in this year's survey included a 17" fish in the lower section and a 16" fish in the upper section. A 12.5" rainbow trout was captured in the upper section. At the two sites done in the stocked section, only three hatchery yearling trout were captured, indicating either a very high creel rate, or more likely poor survival due to high water temperatures.

Table 1. Estimated abundance (Number/mile) and biomass (Pounds/acre) of yearling and older wild brown trout and rainbow trout in Mansfield Creek from 1992 to 2008.
Year Brown Trout Rainbow Trout
Number / mile Pounds / acre Number / mile Pounds / acre
1992 363 15 --- ---
1995 217 30 --- ---
1997 288 41 54 3
1998 449 51 22 3
1999 252 34 104 6
2003 416 67 190 15
2008 180 18 291 12

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