Auburn study proves that fishing pressure affects bass populations

dolski-bass — Even though a high percentage of anglers practice catch-and-release, low mortality coupled with high capture rates year after year can reduce the abundance of big bass. (Photo courtesy Brian Dolski)

I’ve heard it again and again: fishing pressure is increasing. Based on what I see on the lakes I fish, I fully agree.

Whether the number of anglers is increasing or staying the same, a lot of guys and gals are on the water catching bass. A recent Auburn University study by researchers Jeff Buckingham and Dr. Matt Catalano assessed the capture rate: the proportion of the largemouth bass population greater than 15 inches — the legal minimum length — caught by anglers in Lake Guntersville.

The study design

A variable-reward tagging study was used to estimate bass capture rates during 2014 and 2015. The concept is simple: the number of angler-recaptured tagged bass divided by the number of tagged bass at large equals the proportion of the population caught. Recaptures were totaled separately for harvested bass, caught-and-released (C&R) bass, and bass caught in tournaments.

While the math for calculating capture rate is straightforward, obtaining accurate numbers can be difficult.

First, tagging can cause mortality; this reduces the number of tagged bass at large and, without appropriate adjustment to calculations, leads to underestimation of capture rate. To estimate tagging mortality, the Auburn researchers monitored the survival of bass in small research ponds tagged with the same tag as applied to Guntersville bass. All but one of 68 tagged bass were recovered when the ponds were drained after one month, a mortality rate of 1.5%.

Second, bass can shed tags, and failure to account for tag loss can also lead to underestimation of capture rate. To estimate tag loss, the researchers monitored tag retention of bass stocked into research ponds tagged with the same tag applied to Guntersville bass. Tag loss was zero after one month and only 3.7 percent after one year.

One more variable — tag reporting rate — must also be known to accurately estimate capture rate. Numerous past studies to estimate bass capture rate have found that, unfortunately, many anglers failed to report tagged fish, even when cash rewards were offered.

Offer a reward

The standard approach to estimate tag reporting rate is to tag a few fish with high-reward tags and assume that all these tags are reported and the capture rate of bass tagged with high-reward tags is the same as bass tagged with low-reward tags. The ratio of high-reward tags returned to high-reward tags in the population is compared to the same ratio for low-reward tags to estimate the reporting rate of the far-more-numerous low-reward tags.

Other studies have established that a $100 reward per tag is sufficient to achieve 100% tag reporting. This study used $150 high-reward tags and $5 reward for the low-reward tags.

It might be hard to believe that anglers can catch more than half of the legal bass in 69,000-acre Lake Guntersville, but similar high bass catch rates have been measured in other popular bass lakes.

A total of 3,885 bass larger than 15 inches were captured by electrofishing and tagged in the spring and fall from November 2013 to February 2015; 280 of these fish were tagged with high-reward tags. Anglers caught and reported 890 bass during the two-year study.

Tag reports were tallied separately for harvest anglers, non-tournament catch-and-release anglers, and tournament anglers. The reporting rates were 49% for harvest anglers, 38% for catch-and-release anglers, and 15% for tournament anglers.

Although similar to reporting rates in some other studies, these low tag-reporting rates hinder this and other studies conducted to improve fishing. The tags were 5 inches long, readily visible and had clearly printed instructions to report tags by phone or email. Posters at all lake access points and elsewhere alerted anglers to the study and the importance of reporting tags. Yet reporting rates were less than 50%. Please be a player in efforts to improve your fishing and, at the same time, put a few bucks in your pocket by reporting all tagged fish caught.

Study results

When all the data were brought together, necessary adjustments made for tag loss, tagging mortality, and tag reporting, 56% and 50% of the legal-size bass were caught per year from 69,000-acre Lake Guntersville during the study. More than half of these fish were caught by non-tournament catch-and-release anglers, and less than 5%were caught by harvest anglers.

With such a high proportion of the population captured, mortality is a concern. For harvest anglers, mortality is 100%. Using mortality rates available from other studies, the Auburn researchers estimated catch-and-release mortality at 5% and tournament mortality at 5% to 14% depending on seasonal water temperature. Total annual fishing mortality was 10%. Of that mortality, about 55% was attributable to harvest anglers, 18% percent to catch-and-release anglers and 27% to tournament anglers.

Not surprising were the seasonal capture rates: far greater in the spring than winter. Summer capture rates were about half those of the spring, and fall capture rates about one-third of the spring rates.

Tucked away in the results was something I found interesting: based on tag returns, capture probability was not affected by age of bass. In other words, the numbers of bass may get fewer with age, but big bass are just as catchable as smaller bass.