Sunday, July 12, 2009

Trout in the St. Joseph River Valley of Michigan: Species Diversity and Annual Variation

In a previous post, I examined 20th century distribution of trout in streams within the Michigan portion of the St. Joseph River. Here, I continue my assessment of trout populations by looking at species diversity and annual variation. The information presented here is derived from the creel-census data provided by Wesley and Duffy (1999).

In streams (including McCoy Creek) where trout were known to be present during the period 1928-1965, anglers could universally expect to encounter (at least on occasion) up to three different species of trout, not always in the same year but over a period of years. Of the 41 streams with trout, 58 percent (24) yielded three species, 17 percent (7) two species, and 42 percent (10) one species. Details follow:

Number of Trout Species by Stream (1 – 2 – 3)
  • Headwaters: 1 – 0 – 0 (n=1)
  • Upper: 0 – 0 – 1 (n=1)
  • Middle: 2 – 2 – 5 (n=9)
  • Lower: 6 – 2 – 8 (n=16)
  • Mouth: 1 – 3 – 10 (n=14)
  • TOTAL: 10 – 7 – 24 (n=41)
  • There was a strong correlation between the number of trout species caught in a stream and the number of creel censuses conducted. For example, in streams with one species detected the median number of creel censuses conducted was 1 (range 1 – 4), in streams with two species it was 2.5 (range 1 – 5), and in streams with three species it was 7 (range 2 – 19).

    I next examined yearly variability in species diversity by taking a closer look at the 24 streams in which three species of trout were detected. Trout were detected in 78 percent (149) of 190 yearly creel censuses. In the 149 census-years in which trout were reported as being caught, there was a 30 percent probability of encountering one species, a 42 percent probability of two species, and a 28 percent probability of three species. Details follow:

    Yearly Variation in Number of Trout Species Caught (1 – 2 – 3)
  • Upper: 1 – 1 – 1 (n=11)
  • Middle: 8 – 14 – 7 (n=37)
  • Lower: 24 – 30 – 15 (n=80)
  • Mouth: 12 – 17 – 9 (n=62)
  • TOTAL: 45 – 62 – 42 (n=149)
  • PERCENT: 30 – 42 – 28
  • There was variability in both the number of species and the specific species caught each year, perhaps because of variability in stocking patterns of different species in each of the 24 streams.

    The creel-census data show that trout were encountered less frequently by anglers in McCoy Creek (57 percent) than at other trout streams in the St. Joseph River drainage (78 percent), and average species diversity was lover in years when present (1.5 versus 2.0).

    Reference:

    Wesley, J. K., and Joan E. Duffy. 1999b. St. Joseph River assessment. Appendix 2 (.pdf). Miscellaneous historical creel data. Michigan Department of Natural Resources Fisheries Division Fisheries Report 24: 117-227. URL: http://www.michigandnr.com/PUBLICATIONS/PDFS/ifr/ifrlibra/special/reports/sr24/sr24App2.pdf

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