A Northwoods Almanac for September 24 – October 7, 2010
Freshwater Jellyfish
On 9/10, I received a call from David Fellin who lives on Statenaker Lake, a 220-acre, crystal-clear lake just south of Lac du Flambeau. David reported seeing tens of thousands – “clouds” – of quarter-sized jellyfish hovering in the water column from several inches deep to as far down as he could see. He caught several, and described them as translucent with a cross shape on their back. David noted that he had lived on Statenaker Lake for 13 years and had never seen these before.
I recalled that the last time I received reports of freshwater jellyfish in our area was in September of 1989 when they appeared by the thousands on Sumach Lake and Oxbow Lake. At that time, I was very surprised to learn they even existed, and I initially wondered at the veracity of the reports.
Today I know better.
Fortunately, a good deal of information on these organisms has surfaced since 1989. It’s thought that the jellyfish are one of two species of Craspedacusta native to China, both of which (C. sowerbii and C. sinensis) live in the Yangtze River, the world’s third longest river. Freshwater jellyfish were unknown outside of China until 1880 when the jellyfish were found swimming in a large, water-lily tank at the Royal Botanic Gardens in Regent’s Park, then just outside London, England. Four years later in 1884, immature jellyfish polyps were found in a stream in Pennsylvania.
The jellyfish are speculated to have traveled from London to Pennsylvania as polyps or cysts attached to sediments, water plants, or fishes. They most likely arrived in the U.S. because during the Victorian Era various garden clubs and aquarium societies avidly imported the world’s exotic plants and fishes for display, not having a clue about the dangers of introducing exotic species.
The jellyfish eventually found their way to Wisconsin and were first reported from a farm pond near Baraboo in Sauk County where wood ducks are speculated to have carried them to the pond. By October 2006, jellyfish had been reported from 40
water basins in Wisconsin, including 37 natural lakes, 2 dugout ponds, and 1 creek.
These waters vary in size from tiny ponds to immense lakes like the 9,842-acre Lake Mendota and the 236-foot-deep Big Green Lake. As of 2006 (the latest data I can find), the jellyfish had been found in Oneida County in Clear Lake, Killarney Lake, Lone Stone Lake, Pine Lake, and Squash Lake. In Vilas County, they’ve been observed in Lac Vieux Desert, Little Bass Lake, Oxbow and Upper Oxbow Lake, Razorback Lake, Snipe Lake, Sumach Lake, and White Sand Lake. And in Forest County, only Arbutus Lake, Franklin Lake, and Mole Lake have been known to support them.
I find it interesting that September 1989 was the end of a major drought in the Northwoods, and September 2010 also marks the end of a major drought. The literature on the jellyfish’s natural history in Wisconsin says to look for jellyfish during dry and hot summers, and while this summer didn’t qualify as dry, the last eight years certainly were. When lake shallows warm rapidly during spring, the jellyfish emerge in mid-June and swim toward the water surface. Because the jellyfish are initially tiny and transparent, hardly anyone notices them. Only when they are full-grown and densely “bloomed” near the water surface do they draw attention. And that typically doesn’t happen in Wisconsin until early August to mid-September.
The jellyfish are restricted to a narrow band of water temperature between 65 to 75 degrees. If the water surface of a lake becomes warmer than 75°, as some of our lakes do during hot summer afternoons, the jellyfish congregate in deeper water where they can find their preferred temperature range.
The jellyfish feed on zooplankton. They use their tentacles to sting these critters and capture even larger prey, such as water mites and insect midge larvae. Only rarely do they stun newly hatched fish fry.
The impact of this widespread jellyfish is unclear. Their preference for large, predatory zooplankton could influence relative zooplankton species structure, but apparently no one knows if this is an issue. Only crayfish are considered an important predator of the jellyfish, though David Fellin on Statenaker Lake thought that fish were feeding on them.
These tiny freshwater jellyfish are not considered dangerous to humans. The mature jellyfish live a few weeks, release eggs, and die.
Snapping Turtles Hatching Out
On 9/11, Cathi Koch on Squirrel Lake reported watching 15 baby snapping turtles, all about the size of a half-dollar, scuttle down her drive on their way to the lake. She noted that they still had sand on them, and that this was the first time in her 20 years of living there that she had seen this.
Snappers lay anywhere from 10 to 80 eggs, which take from 60 to 90 days to hatch depending on the weather, cooler weather extending the time required for hatching. The location of the nest also influences the hatching period – a south-facing slope is certainly a warmer site than a north-facing one. Turtles were laying their eggs in early June this year, so these turtles may have taken close to 90 days to hatch.
Hawk Watching at Hawk Ridge
Last weekend, Mary and I led a group to Hawk Ridge and then up along the North Shore of Lake Superior. We were only able to spend three hours on Friday, 9/17, looking for hawks on the Ridge and unfortunately had to leave not long after the wind shifted and the hawks were starting to come through in greater numbers. Just before we left, we were able to see several massive kettles of hawks high above Lake Superior, each kettle loosely numbering 70 to 100 hawks.
The term “kettles” of hawks likely derives from the appearance of birds circling tightly in a thermal updraft like something boiling in a cauldron. “Thermals” are columns of rising warm air that occur as the sun heats the earth. Areas of dark rock heat more quickly than green forest, so thermals form earlier and more strongly over rocky ridges. The vertical gusts within a thermal can reach 10 miles an hour or more, and may rise as high as 6,000 feet.
Soaring hawks can minimize their energy loss in migration by riding the thermals up, then setting their wings and gliding as far as they can go before having to catch another thermal up, gliding out again, and so forth. The glide ratio for a broad-winged hawk is 11:1, meaning that it can glide forward 11 feet for every one foot that it sinks. So, if a hawk can ride a thermal up 500 feet, it can glide over 5,000 feet, or about 1 mile, before it hits the ground. One study of climb rates for hawks riding thermals showed a varying climbing rate from 200 feet per hour on poorer soaring days to 650 feet per hour on good soaring days. Thermals are usually strongest in early to mid-afternoon, and then decline by late afternoon as the earth cools.
The count for Friday 9/17, was 3,295, of which 2,872 were broad-winged hawks. On Saturday, 9/18, the winds held true all day out of the west-northwest, and 6,387 hawks passed over the ridge, of which 5,699 were broadwings. Sunday’s count added another 4,623 raptors.
Duluth is a hotspot for watching hawks, because most raptors are reluctant to cross large bodies of water like Lake Superior. The thermals die out almost immediately over the cooler water, so the birds naturally veer southwest along the lakeshore. They concentrate over the bluffs that loom over Duluth where they can be easily seen from the overlook at Hawk Ridge. On days with northwest winds, hundred to thousands of birds can be seen migrating past the Ridge, while southerly or easterly breezes seldom produce large flights of raptors. The migration begins in mid-August with American kestrels, sharp-shinned hawks and broad-winged hawks, and continues into December with the last of the red-tailed and rough-legged hawks, northern goshawks and eagles. Peak migration at Hawk Ridge occurs in mid-September. Raptor counts at Hawk Ridge began in the 1950's, though standardized counts didn’t begin until 1972. The results of the count are one of the two or three highest in North America. The Natural Resources Research Institute, the University of Minnesota- Duluth, and Hawk Ridge initiated a 3-year study in 2008 to examine the size of the migration, important flight lines, and important stopover habitats. Fall migration counts of raptors and passerines at Hawk Ridge are estimated to be in the hundreds of thousands to millions, indicating a massive movement that has been previously underestimated. These recent studies have begun to show the true magnitude of the fall migration and the importance of the North Shore region as a major migration corridor. This information is showing that migrating birds, especially non-raptors (mainly songbirds) are concentrating within one mile of the Lake Superior coastline, and that the non-raptor migration is much larger than previously known. For instance on 9/16, the non-raptor count was 6,927, of which 6,485 were blue Jays (including some 3,200 in 45 minutes alone).
Celestial Events
September 26 marks the first time since March 18 that the night is longer than the day. By October 5, we’ll be down to 11 ½ hours of daylight.
For planet-watching in October, look at dusk for Mars very low in the southwest and Jupiter in the southeast. At dawn, Saturn is very low in the southeast late in the month. Both Mars and Venus sink out of view in the first week of October.
