La Grange bass ate silver carp under 4 inches in summer 2015

STEP 1 · Stomach counts

Silver carp ranked behind sunfish and shad

Western Illinois University crews electrofished the La Grange Pool from June through November 2015. They worked backwater, side channel and tributary shorelines where they expected largemouth bass and juvenile silver carp. They kept only silver carp shorter than 14.8 inches (375 mm). They examined diets from 389 bass of 2.8 to 22.8 inches (70 to 578 mm).

Of those 389 bass, 220 had empty stomachs or diets with no fish. The prey comparison uses the 169 bass with identifiable fish, and empty stomachs were left out of that tally. Thirty of the 169 contained silver carp. That share is the 18 percent reported for fish diets. Those 30 bass are 7.7 percent of all 389 bass examined.

Sunfishes occurred in 24 percent of the 169 fish diets. Shads occurred in 21 percent, and the discussion treats that share as gizzard shad. Silver carp ranked third at 18 percent. Sunfish and shad turned up in more of these fish diets than silver carp did. Our read is that young silver carp were one summer forage item among several in these backwaters and did not lead the diet.

Measurable silver carp parts were in 20 of the 30 bass that contained silver carp. Every bass with silver carp came from a backwater. Diet bass came from backwaters at 98.5 percent, side channels at 1.0 percent and tributaries at 0.5 percent.

Silver and bighead carp have identical chewing pads. The authors counted every pad as silver carp after no bighead carp turned up in 2015 in their sampling, Illinois River Biological Station sampling or genetic checks.

STEP 2 · Measured meals

Consumed carp stayed between 1.3 and 4.0 inches

Digestion had gone too far to measure any consumed silver carp whole. Lengths came from chewing pads on 18 carp and from pharyngeal teeth on the other six. The estimates ran from 1.3 to 4.0 inches (34 to 101 mm). Those carp came from bass of 3.7 to 10.3 inches (94 to 262 mm).

On average a consumed carp was about 42 percent of the bass that held it, and the spread was 29 to 61 percent. Larger bass in this set held larger carp, and bass length accounted for about 61 percent of the variation in estimated carp length (r squared 0.61).

STEP 3 · Quiet months

Fall diets in this sample held no silver carp

Silver carp in these diets came from June through August 2015. The September through November diets contained none. Missing carp in this set of stomachs does not set the predation rate at zero. The paper does not say how many of the 389 diets came from September through November, so the strength of that absence cannot be judged.

Size sampling took 577 silver carp from 1 June through 9 November 2015. The 305 summer carp had a median of 5.4 inches (137 mm). The 272 fall carp had a median of 6.1 inches (156 mm). The fall sample included no carp from 1.6 to 3.1 inches (40 to 80 mm).

The authors tie the missing 40 to 80 mm class to bass of 3.9 to 7.9 inches (100 to 200 mm) and call it an optimal size near 40 percent of bass length. They also note that carp of 3.1 to 5.5 inches (80 to 140 mm) in the fall would still have suited bass of 7.9 to 13.8 inches (200 to 350 mm), and they say those bass were abundant. They allow that habitat or behavior may have changed and reduced encounters. The authors conclude that size selection likely played a role. Our read is that the fall gap fits a missing small class without ruling out a change in where the fish were.

In the combined Western Illinois University and Illinois River Biological Station size samples, June through August bass had a median of 5.0 inches (127 mm) and fall bass had a median of 10.4 inches (264 mm). Those medians pool both crews and are not the diet bass alone. Station bass were not opened for diets. The authors warn that bass may not respond the same way in a year when silver carp are scarce. They cite a Long Term Resource Monitoring daytime electrofishing catch of 25.16 silver carp per 15 minutes on this pool in 2014 and 2.12 in 2015.

STEP 4 · Longer bass

Predictions run past the measured meals

The fitted line gives a 7.9 inch bass (200 mm) a mean carp of 3.1 inches (79 mm). The same line gives an 11.8 inch bass (300 mm) a mean carp of 4.2 inches (107 mm) and a 15.0 inch bass (380 mm) a mean carp of 5.1 inches (129 mm). Measured carp came from bass no longer than 10.3 inches, so those two larger examples sit outside the meals. The 4.2 and 5.1 inch means also sit past the longest recovered carp at 4.0 inches.

The authors saw silver carp of 3.1 to 3.9 inches (80 to 100 mm) at the end of the first growing season and 4.3 to 8.7 inches (110 to 220 mm) after the first year. They say those age-1 carp would be prey for bass of 11.8 to 24.8 inches (300 to 630 mm) in this pool. In combined Long Term Resource Monitoring and Western Illinois University electrofishing on the pool in 2015, 92.3 percent of largemouth bass were under 11.8 inches (300 mm). The Long Term Resource Monitoring surveys had no bass over 20.1 inches (510 mm). The Western Illinois University diet sample included bass up to 22.8 inches (578 mm).

The authors judge that boosting largemouth bass is unlikely to substantially reduce silver carp in the Illinois River or in other waters where the two species live together. The study did not stock bass or measure whether carp numbers changed.

STEP 5 · Size check

No eaten carp topped 61 percent of the bass

Our proposed check is an observation to test. It was not part of the study. When you land a bass in a summer backwater that also holds young silver carp, compare its length with the small carp you can see in the same water. In this 2015 sample the measured pairs were carp of 1.3 to 4.0 inches inside bass of 3.7 to 10.3 inches, and those carp ran 29 to 61 percent of the bass that held them. That look can show whether both sizes are present where you fish. It cannot show that one bass ate a carp or confirm the length line or show that bass are reducing carp.

Our read of the control question would change if diets from your river showed bass regularly eating carp past their first year. The authors put those age-1 carp at 4.3 inches (110 mm) and longer in this pool. Where bass over 11.8 inches are common, a diet check for carp past their first year would be worth doing. Bass size alone would not show control. A carp survey that came in lower after bass numbers rose would be a reason to test control further. That lower survey would not by itself show that bass caused the drop.

The evidence covers the La Grange Pool in 2015 and comes almost entirely from backwaters. Stomach contents show carp that were consumed. The paper does not separate live capture from scavenging, so stomach contents show consumption, not confirmed kills. The counts cannot show how many carp bass removed from the pool. Main-channel bass were outside the diet sample. A size look on another river or in another year is a separate observation this paper cannot settle.