How Do Spawning Benches Work?

Spawning Bench or half-log. (C) Eric Engbretson

In northern states, fish managers sometimes use spawning benches to promote the spawning of smallmouth bass.  While largemouth bass, rock bass, and sometimes bluegill also use spawning benches, it is the spawning of smallmouth bass that inspired the design. 

 

In a healthy lake, smallmouth bass build spawning nests against rocks, sunken trees, or large pieces of wood in about four to ten feet of water.  Next to these structures, the male excavates a shallow, circular crater in the lake bottom.  This system provides good natural protection to eggs and fry. 

 

But in lakes without coarse woody habitat, large rocks or similar objects, smallmouth bass may be forced to construct their nests out in the open.  When spawning and is over and the female has deposited her eggs in the nest, the male diligently guards the eggs and later the fry from predatory fish and crayfish.  When the nests are out in the open without natural protection, the male must guard up to 360 degrees of the crater he has dug.  This is exhausting and dangerous, since his back is always turned away from part of the nest.  Far fewer eggs incubate, and far fewer fry survive their first few weeks when fish have to use nests that lack the natural shield of a habitat's woody elements. 

 

Fish managers have studied the hard work put in by bass and have noted the decreased recruitment of young fish.  The managers came up with an idea for a simple structure they hoped would meet the needs of nesting fish and make it easier for eggs and fry to survive.  The idea for spawning benches was born. 

 

A spawning bench consists of a four to six foot piece of log sawed lengthwise in half and attached to concrete or cinder blocks on each end.  Spawning benches are therefore sometimes called half-logs.  Once placed on the suitable substrate, the spawning bench provides overhead cover from birds of prey.  The concrete blocks on each end protect the nest from raiders on two sides. 

Smallmouth bass guarding nest built adjacent to one of the concrete blocks of a spawning bench. (c) Eric Engbretson

It was a sound design and one that smallmouth bass readily used, but not exactly as intended.  It turned out that smallmouth bass weren't concerned about overhead cover. The benches usually sat in water deep enough to preclude threats from above by ospreys and other birds of prey.  While nests are occasionally built between the two concrete blocks as the designers intended, smallmouth bass usually construct nests next to one side or the other, thus allowing the male to guard the nest from only three sides.  The key element seems to be the concrete block itself and not so much the half log.  In fact, if the spawning bench falls on its side, it still provides excellent protection. 

 

Spawning benches are poor substitutes for the naturally occurring woody cover that fish prefer.  But in lakes devoid of suitable wood, rocks, or trees, spawning benches provide a superb means of helping smallmouth bass defend their nests and allowing more of their offspring to survive.

Where Have All the Perch Gone?

There's an interesting new story that just appeared in Minnesota Outdoor News.  Researchers with the Minnesota Department of Natural Resources say that in 900 lakes they've surveyed in that state, numbers of yellow perch are down significantly.

Over the years, I've noticed the same thing while diving and photographing fish in Wisconsin lakes... there just aren't as many perch as there used to be.  The news report says in part that DNR researchers are looking at not only what might be holding back yellow perch, but also potentially dragging them down. 

"A new study looking at yellow perch is being led by DNR research biologists  Jeff Reed, of Glenwood, and Bethany Bethke, of Duluth. Related research conducted by Bethke during the past couple of years pointed to a likely decline in perch numbers; in more than 900 lakes studied, perch numbers had dropped on average, from about 10 fish per gill net to around five fish per gill net during DNR surveys.
“When you see something going in half over a 25-year period, it’s something you want to look at,” Reed said.

Examination of netting data yields some clues regarding why perch might be declining in some places. High water levels have created fertile breeding grounds for northern pike, and a corresponding increase in the pike population could be putting a bigger dent in the perch population, Reed said. Also, invasives – such as zebra and quagga mussels, and spiny water fleas, where they exist – could be reducing the foods available – like zooplankton – for very small perch.

One other factor could be in play, too. Longer growing seasons in lakes are helping species like bluegills, which compete with yellow perch.
  
Bethke adds yet another item to the list of possible culprits for perch decline: the loss of near-shore habitat, which could be lakeshore property owners removing woody debris or bulrush."

To read the entire article, go to this link.

Lake on The Brink-The Unexpected Consequences of Treating Eurasian Watermilfoil

In 2012, Greg Matzke, a fisheries biologist for the Wisconsin Department of Natural Resources, made a startling discovery on Florence County’s Lake Ellwood. During a comprehensive fish survey which included spring, summer and fall netting and electrofishing surveys, Matzke discovered that all of the lake’s largemouth bass were older than 5 years of age, with approximately 91% of the largemouth bass population being at least seven years old.  The absence of younger fish indicated a recruitment failure for a number of years.  Such failures in largemouth bass recruitment over multiple years are unprecedented in the state of Wisconsin. 

“The current largemouth bass population is in serious trouble,” Matzke reported. “It appears that natural reproduction of largemouth bass has not occurred since 2007. As these older/larger fish move through the population, a significant reduction in largemouth bass abundance will take place, with the potential for the complete loss of this species of fish unless the current situation changes.”

Matzke next began looking at the lake’s panfish population.  What he found was stunning. Overall, the lake’s panfish abundance had fallen an estimated 75% in just the last 10 years, with bluegill and rock bass abundance down an estimated 65% and 89% respectively, showing that these populations also appear to be collapsing. Intense sampling throughout 2012 found only a single black crappie under six years of age, showing another alarming recruitment failure in several consecutive years. When Matzke analyzed the ages of Lake Ellwood’s northern pike population, the results were even more disappointing: There were no pike under the age of eight!  

Matzke stared at the data he had collected. His department had never seen a mystery like the absolute and complete recruitment failures of native northern pike, black crappie, and largemouth bass (along with significant reductions in recruitment of other panfish populations). He shared his findings with other fisheries professionals across the state and they all said the same thing.  They had never seen a collapse like this in their careers. Matzke and his team scrambled to collect more data and tried to find a cause that could have brought the fish to the brink of extirpation in Lake Ellwood. Surveys from 2002 had shown normal abundance, size structure, growth, and recruitment in all of these species. What had happened in the last ten years that was preventing fish from successfully reproducing? 

The only thriving species of game fish in the lake were smallmouth bass. Their abundance and size structure had grown in the last decade and recruitment was high.  This suggested that the problem was targeting specific species of fish. Because Lake Ellwood’s smallmouth bass were doing so well while the other species were collapsing, the focus turned to the lake’s historically sparse but important aquatic plant community. All the species showing recruitment failures are highly dependent on aquatic vegetation for spawning as well as cover and food for their young. Matzke observed that smallmouth bass seem to be different. “The fact that this species was not affected by the reduction in plant life,” he said, “is not a major surprise since as a species smallmouth bass are less dependent on aquatic vegetation.”

 

The Smoking Gun

Eurasian Water Milfoil was discovered in Lake Ellwood in 2002. Herbicide treatments began in 2003 and increased every year. By 2007 recruitment of northern pike, largemouth bass and black crappie had come to an end. “When I started to analyze the data it was strikingly obvious to me that there are some problems associated with the herbicide”, said Matzke. When he graphed the fish abundance (by year class) over the last decade and overlaid it with a graph showing yearly herbicide treatments, he found what he believed was a critical connection.  Fish numbers fell as the amount of herbicide increased.  Interestingly, in the year following a relatively low application of herbicide, young bluegill (and black crappie to a much smaller degree) began to appear again, but their numbers are still very low and they will likely disappear before they reach age 2.

 
“We still wonder which stage of reproduction has failed in these species”, says Matzke. “Aquatic vegetation plays a major role in spawning site selection and in the survival of eggs and fry. Plants are also the source of primary production providing food and habitat for young fish and prey items, including invertebrates and minnows. It seems likely that one or all of these important phases of reproduction are dwindling in Lake Ellwood.”

On April 17, 2013 Matzke met with the Lake Ellwood Association to reveal his data and conclusions. He told the group, “The main cause for failed northern pike, largemouth bass and black crappie recruitment (along with the massive reduction in panfish abundance) appears to be the loss of aquatic vegetation.” The 2-4-D herbicide used on Eurasian watermilfoil had been successful in reducing the abundance of this invasive species significantly. Conversely, other native plants were also harmed by years of chemical treatment. Matzke said he has no reason to believe the chemicals have directly caused a failure in reproduction of any species of fish in Lake Ellwood. However, Matzke does believe that the chemicals have indirectly caused recruitment failure by eliminating too many of the aquatic plants young fish need in order to survive.  Matzke has called for a change in the way the Lake Ellwood Association has been managing the lakes aquatic plants.  He recommended that further chemical treatments for milfoil be stopped.

“First and foremost,” says Matzke, “we need to promote and strengthen aquatic vegetation in Lake Ellwood.” He stresses the role of aquatic vegetation in spawning and concludes that the loss of vegetation (including the invasive milfoil) has almost certainly wiped out a great deal of forage for young fish.

It seems that milfoil treatments controlled the invasive plant but also jeopardized the health of the lakes fishery. Today the lake contains a dwindling and rapidly aging population of largemouth bass, black crappie, northern pike, and bluegill. Matzke hopes the plants will come back in time for the remaining old fish to produce at least one year class before they die. If that doesn’t happen, many fish populations will likely be extirpated from Lake Ellwood. New fish can be stocked, of course, but the lake would lose the unique genetic lineage of the fish that have lived there for thousands of years.

 

The Future

Could chemical herbicide treatments for Eurasian watermilfoil be reducing fish recruitment in other lakes? None of the other lakes that have been receiving chemical treatments have had their fish populations surveyed this intensely. Large scale recruitment problems due to loss of important plant cover could be taking place throughout the region where the invasive plant is now being fought. There is no way to know if this is happening, and frankly, up until now, there has been no reason to find out.  Fisheries experts around the state are only now learning of Matzke’s findings on Lake Ellwood. In the future, they will likely start paying more attention to fish recruitment on lakes treated for Eurasian watermilfoil which would allow the Department of Natural Resources to determine whether this crisis is an isolated instance or a more widespread problem. 

In the meantime, it’s a race against time for Lake Ellwood’s native fish. The question remains: Will the plants come back in time to save these fish populations?

Read this story in In-Fisherman Magazine here.

(Note: An updated, follow-up article (Back From The Brink-How Lake Ellwood, Once Doomed Is Being Rescued), from May 2014 can be viewed by clicking here.)

 

How Do Spawning Benches Work?

Spawning Bench or half-log. (C) Eric Engbretson



In northern states, fish managers sometimes use spawning benches to promote the spawning of smallmouth bass.  While largemouth bass, rock bass, and sometimes bluegill also use spawning benches, it is the spawning of smallmouth bass that inspired the design. 

 

In a healthy lake, smallmouth bass build spawning nests against rocks, sunken trees, or large pieces of wood in about four to ten feet of water.  Next to these structures, the male excavates a shallow, circular crater in the lake bottom.  This system provides good natural protection to eggs and fry. 

 

But in lakes without coarse woody habitat, large rocks or similar objects, smallmouth bass may be forced to construct their nests out in the open.  When spawning and is over and the female has deposited her eggs in the nest, the male diligently guards the eggs and later the fry from predatory fish and crayfish.  When the nests are out in the open without natural protection, the male must guard up to 360 degrees of the crater he has dug.  This is exhausting and dangerous, since his back is always turned away from part of the nest.  Far fewer eggs incubate, and far fewer fry survive their first few weeks when fish have to use nests that lack the natural shield of a habitat's woody elements. 

 

Fish managers have studied the hard work put in by bass and have noted the decreased recruitment of young fish.  The managers came up with an idea for a simple structure they hoped would meet the needs of nesting fish and make it easier for eggs and fry to survive.  The idea for spawning benches was born. 

 

A spawning bench consists of a four to six foot piece of log sawed lengthwise in half and attached to concrete or cinder blocks on each end.  Spawning benches are therefore sometimes called half-logs.  Once placed on the suitable substrate, the spawning bench provides overhead cover from birds of prey.  The concrete blocks on each end protect the nest from raiders on two sides.

 

Smallmouth bass guarding nest built adjacent to one of the concrete blocks of a spawning bench. (c) Eric Engbretson

 

It was a sound design and one that smallmouth bass readily used, but not exactly as intended.  It turned out that smallmouth bass weren't concerned about overhead cover. The benches usually sat in water deep enough to preclude threats from above by ospreys and other birds of prey.  While nests are occasionally built between the two concrete blocks as the designers intended, smallmouth bass usually construct nests next to one side or the other, thus allowing the male to guard the nest from only three sides.  The key element seems to be the concrete block itself and not so much the half log.  In fact, if the spawning bench falls on its side, it still provides excellent protection. 

 

Spawning benches are poor substitutes for the naturally occurring woody cover that fish prefer.  But in lakes devoid of suitable wood, rocks, or trees, spawning benches provide a superb means of helping smallmouth bass defend their nests and allowing more of their offspring to survive.