SPICER, Minn. — Tullibee numbers continue to decline in Green and Koronis Lakes.
The cold water fish are an important forage fish for the walleye that attract anglers to these lakes.
Tullibees, also known as cisco, are considered canaries in the coal mine in terms of what they may be telling us about climate change. Peter Jacobsen, a researcher with the Minnesota Department of Natural Resources, and Brad Carlson, with the DNR fisheries staff in Spicer, co-authored a paper a few years ago documenting the decline of tullibees in the two lakes.
Gill nets set in the lakes this autumn produced the fewest numbers of tullibees ever. Fisheries staff reported an average of only 0.5 per gillnet this fall in Green Lake, and they did little better in Koronis, according to Dave Coahran, fisheries supervisor in Spicer.
That compares to the late 1990’s, when it wasn’t unusual to collect as many as 100 tullibees in a net. Tullibee numbers have been declining since. In 2012, the declining numbers led the DNR to close the fall netting season on them.
Green and Kornis Lakes are at the very southern edge of the tullibee’s range in Minnesota. These silver-colored fish require cold water and oxygen levels above 2 parts per million.
Green and Koronis Lakes have seen die-offs of tullibee during long, hot stretches in late July and August.
Coahran said the continued decline in tullibees remains under study. Along with losses due to warmer summers, it’s possible that the shorter and milder winters also associated with a changing climate are also adversely affecting the fish.
They spawn in November and early December, and their eggs lie on the bottom of the lakes until March and April before hatching. It’s possible the shorter winter prompts the eggs to hatch before their food sources are available.
Coahran noted that other changes in Green Lake may now be adversely affecting the tullibees as well. The arrival of zebra mussels in the lake has greatly decreased the amount of food available for zooplankton, an important source of food for smaller, forage fish.
There has also been a significant decline in the perch numbers in the lake. With fewer perch as forage, there is likely increased pressure by predators such as walleye and northern pike on the remaining tullibees.
The clear waters of the zebra mussel infested lake and declining perch numbers may be adversely affecting walleye numbers too. Not too many years ago, the fisheries staff would collect nine to 10 walleyes in gill nets. Now, a stocking program is aimed at trying to keep it at six per net, said Coahran.