Michigan Department
of Natural Resources
Fisheries Research Report No. 2045, 1997
EVALUATION OF ALTERNATE
METHODS FOR ESTIMATING
NUMBERS OF OUTMIGRATING STEELHEAD SMOLTS
Tammy
J. Newcomb
and
Thomas G. Coon
Abstract.-The
annual juvenile (smolt) migration to the lake by Great Lakes salmonids
provides an opportunity for fishery managers to estimate the abundance
of the smolting cohort. This information is valuable for modeling recruitment
potential to the fishery and for estimates of the returning broodstock.
The purpose of this study was to compare several techniques for estimating
the number of steelhead Oncorhynchus mykiss smolts emigrating from
the Betsie River in northwestern Michigan. We also compared the production
of steelhead smolts from this river with other rivers in the state. In
May and June, 1993-1996, we monitored emigrating smolts by use of visual
observations, time-lapse videography and mark-recapture to estimate the
number of steelhead smolts migrating past a lamprey weir located 18 km
upstream of the river mouth. In 1993 and 1994, two observers counted smolts
passing over the weir for 20 minutes out of each hour from 21:00 to 05:00
hours. In 1995 and 1996, black and white video cameras and time lapse
videocassette recorders continuously monitored the passing smolts through
the night. We reviewed each videotape and counts of the number of smolts
passing over the weir from 21:00 to 05:00 for each night. We also collected
20 minute samples from the videotapes following the same schedules as
was used for visual observations. A subsample of videotapes were reviewed
a second time to measure variation of the counts. In all four years, a
constriction weir was constructed every 5th night during the smolt run
to capture emigrating fish and to quantify species composition and the
origin of steelhead (hatchery or wild). Steelhead smolts comprised 30-61%
of all the salmonid juveniles sampled. Other species present were brown
trout Salmo trutta, coho salmon Oncorhynchus kisutch, and
chinook salmon Oncorhynchus tshawytscha. Wild steelhead comprised
12-52% of the steelhead smolts. Mark-recapture estimates and 95% confidence
intervals for hatchery and wild steelhead smolts ranged from 13,837 (12,583-15,215)
in 1985 to 56,661 (46,036-69,703) in 1993 and were 2-9 times greater than
the estimates from direct observation and videography. The results from
time-lapse videography and visual observation were similar and gave the
most reliable estimates for steelhead smolts produced from the watershed.
Estimates of steelhead smolt numbers from observation methods ranged from
2,198 (± 512) in 1996 to 9,645 (± 1,111) in 1994. The Betsie
River produces fewer wild steelhead smolts (12-22/ha) than other streams
studied in Michigan.