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Wednesday, January 2, 2013

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.