Estimating Smelt Drift through Brassua Dam into the Moose River
Fisheries biologists keep a watchful eye on smelt populations in Moosehead and other lakes throughout Maine.
Rainbow smelt are the most important forage item for landlocked salmon and lake trout, two of the most important coldwater gamefish in Maine. Smelt populations are notorious for natural fluctuations which can cause serious growth problems for these gamefish species which typically inhabit the larger, more popular lakes in Maine. As fisheries managers, we have struggled to develop methods to predict and evaluate smelt abundance and the causes for sudden declines in abundance. On Moosehead Lake, Maine’s largest lake, there are several studies are underway to address some of these issues.
In most lakes in Maine, the smelt population is produced entirely within that lake. However, we have a unique situation on Moosehead Lake. Brassua Dam sits 3 miles upstream of Moosehead Lake on the Moose River. This 30-foot dam on the outlet of the 9,000 acre impoundment is a hydro-generating facility with the intake near the base of the dam. Rainbow smelt are present in Brassua Lake and are often found in the discharge creating considerable smelt drift into the river and down to Moosehead Lake at certain times of the year.
We operated drift nets for an entire year to capture a subsample of the discharge from Brassua Dam to estimate the total number and biomass of smelt passing downstream to Moosehead Lake. A series of six metal stakes were placed on each side of the river. Two 4’x4’x6’ drift nets were attached to stakes. Stake selection was based on the flow. The nets were set for a 24 hour period on 1 day per week throughout the year, except when flows were too high to work in safely and on two occasions when we operated the nets for an entire week to examine the range of catch.
Catch was moderately related to discharge from the dam. We saw a peak in May likely associated with post-spawning movement and an increase in flow. There was another peak in January that was likely associated with an increase in flow. Overall we estimated 355,397 smelt passed downstream into the Moose River from Brassua Lake. This would not include smelt that were too small to be seen or caught in our 3/8” mesh drift nets. It appears that young-of-the-year smelt became vulnerable to netting in mid-July.
It was interesting to follow the length frequency of smelt captured during the sampling. Most notable, smelt in the 50mm range were present throughout the year, suggesting a wide range in growth rates for young-of-the-year smelt or perhaps some delayed spawning.
This work will continue for at least another year as we try to determine the potential range of smelt drift into Moosehead Lake.
Submitted by: Tim Obrey, Regional Fisheries Biologist- Moosehead Lake Region