LDWF Fishery Biologists Develop New Survey To Effectively Set And Evaluate Largemouth Bass Harvest Regulations
story by JOE WEST, LDWF Fisheries Biologist
Bass fishing in Louisiana is big business. Nearly 55 percent of Louisiana’s freshwater anglers target largemouth bass and their passion is unmatched.
This is certainly no revelation to the folks with the Louisiana Department of Wildlife and Fisheries (LDWF) responsible for the monitoring and management of Louisiana’s largemouth bass populations and the fisheries they support.
To determine the effectiveness of waterbody-specific largemouth bass harvest regulations, fishery resource managers need to understand the current population and fishery in terms of the vital rate functions that govern the population, such as growth, mortality and recruitment rates, and the characteristics of the anglers utilizing that resource, including harvest and release rates.
Fishery biologists conduct surveys on inland waterbodies across the state to provide that information. These surveys were historically conducted on as many waterbodies as possible in a given year but sometimes provided an incomplete picture of the population/fishery in question due to limited sample size.
In an effort to increase sample size and improve the information necessary to effectively set and evaluate largemouth bass harvest regulations, LDWF biologists began a new project in 2010 surveying waterbodies with important largemouth bass fisheries more intensively over a three-year sampling period. The overall goal of this project was to develop a statewide database of largemouth bass population and fishery characteristics to inform future LDWF fishery management decisions.
Office of Fisheries biologists of the Research and Assessment and Inland Fisheries Sections collaborated on this project. Inland Fisheries staff provided field collections from the multi-year survey period for each waterbody assessed. Research and Assessment Section staff provided the population and fishery analyses and developed the final assessment reports. As of December 2017, LDWF biologists have completed the multi-year largemouth bass surveys on 23 waterbodies and are currently conducting surveys on another four.
The data gathering process begins with the collection of largemouth bass from LDWF electrofishing surveys conducted over a continuous three-year sampling period. These fishery-independent surveys sample each largemouth bass population directly allowing insights into the dynamics controlling each population.
Samples are collected each year during the same season and under similar environmental conditions. The overall goal of this survey is the collection of a minimum of 500 largemouth bass per waterbody over the multi-year sampling period to represent the current age structure of the largemouth bass population in question.
Of these fish, 10 per inch group per year are sacrificed for age and growth analyses where biologists remove the ear bones, or otoliths, and record the length and sex of each fish. Ear bones are then sent to the LDWF Age and Growth Lab located at the Baton Rouge headquarters for processing. Similar to trees, fish are aged by counting the number of rings on the otolith. After all largemouth bass have been aged, age-length-keys are developed to assign ages to largemouth bass that weren’t sacrificed.
Largemouth bass growth and mortality rates are important components of implementing successful harvest regulations as they determine how quickly fish reach certain sizes and how rapidly fish disappear from the population. Mortality rates consist of two parts: Population losses due to natural causes such as disease and predation, or natural mortality, and losses due to angler harvest and release, or fishing mortality. Also important is the rate of recruitment, or reproduction, in each largemouth bass population and the degree of variation observed in recruitment from year to year.
Growth rates of each largemouth bass population are estimated by fitting growth curves to the age and length samples collected from the multi-year electrofishing survey. The growth curves are then used to estimate the amount of time required to reach specific sizes.
These rates vary considerably from population to population. As an example, it takes 1.6 years for Poverty Point Reservoir largemouth bass to grow to 12 inches, the fastest rate in the state. In contrast, it takes 2.5 years for Calcasieu River largemouth bass to grow to 12 inches, the slowest rate.
Variation in growth rates from waterbody to waterbody is affected by a number of factors, including forage availability, submerged aquatic vegetation coverage and water quality.
Mortality rates of each largemouth bass population are estimated by fitting catch curves to the age-specific catch per hour from the multi-year electrofishing surveys. The decline observed from younger to older ages in the samples indicates the level of mortality in the population.
Populations with a steeper decline have higher mortality rates than populations with a more gradual decline. These rates also vary noticeably from population to population. As an example, the total mortality rate for Poverty point Reservoir largemouth bass is 34 percent per year, the lowest rate in Louisiana. At this rate, if you start with 100 largemouth bass at 1 year old, only 19 fish remain alive by the age of 5.
In contrast, the Toledo Bend Reservoir has the highest total mortality rate, 64 percent per year. At this rate, if you start with 100 largemouth bass at 1 year old, only two fish remain alive by the age of 5.
Recruitment variability of each largemouth bass population is estimated from the changes observed in each year’s electrofishing catch per hour of 1 year old largemouth bass. Populations with small differences in annual catch per hour of 1-year-old largemouth bass have more stable recruitment than populations with large differences.
Those populations with stable recruitment from year to year are better candidates for certain harvest regulations than populations that exhibit boom or bust recruitment cycles.
Recruitment varies considerably from population to population. Of all the waterbodies included in this project, the Caney Creek Reservoir largemouth bass population exhibited the least variation in recruitment and the Atchafalaya Basin population exhibited the largest.
In addition to the multi-year electrofishing survey, a LDWF creel survey was conducted a single year on each waterbody included in this project to characterize each fishery. These fishery-dependent surveys interview anglers utilizing the fisheries directly and provide estimates of harvest and release rates allowing insights into angler behavior.
In recent decades, voluntary catch-and-release has become popular with largemouth bass anglers. When anglers are hesitant to harvest largemouth bass of legal size, potential benefits of certain length limit restrictions may not be realized.
The degree of voluntary largemouth bass catch-and-release varies considerably across the state. The Cross Lake fishery releases 96 percent of all legal sized largemouth bass caught, the highest rate in the state. In contrast, the Toledo Bend Reservoir and Iatt Lake fisheries release nearly 40 percent of all legal sized largemouth bass caught, the lowest rates.
INFORMED FISHERY MANAGEMENT
The overall goal of this project was to develop a statewide database of largemouth bass population and fishery characteristics to inform future LDWF fishery management decisions. Using this database, LDWF fishery resource managers determine which harvest regulations are best suited to individual waterbodies.
Minimum length limits are typically recommended for populations characterized by low rates of recruitment and natural mortality, moderate to fast growth rates and high fishing mortality. Protected slot limits, on the other hand, are typically recommended for populations characterized by high recruitment and low growth rates.
Fishery resource managers also identify candidate waterbodies from this database with potential for trophy largemouth bass production and can examine management tradeoffs of trophy production versus harvest-oriented fisheries. This allows resource managers to make informed decisions regarding waterbody-specific regulations while also accounting for the preferences of constituents utilizing these fisheries.
BASS STOCK ASSESSMENT CASE STUDIES
TOLEDO BEND RESERVOIR
Toledo Bend Reservoir currently has a 14-inch minimum length limit harvest regulation. This regulation was implemented in 1991 to standardize the regulations of Louisiana and Texas. Results of this study indicates that the Toledo Bend Reservoir largemouth bass population currently has the highest level of mortality in the state, 64 percent, and is influenced more by fishing mortality than by natural mortality.
Toledo Bend largemouth bass also exhibit a moderate growth rate and stable recruitment levels when compared to the other populations included in this project. Creel survey results suggest that Toledo Bend anglers catch largemouth bass at a much higher rate than other fisheries included in this project and harvest a much larger percentage than they release, nearly 60 percent of legal-sized largemouth bass are harvested.
Given these dynamics, the existing 14-inch minimum length limit appears beneficial by protecting largemouth bass from harvest until they can reproduce at least once while also supporting one of the top fisheries in the nation.
The Chicot Lake largemouth bass fishery was managed from 1991 to 2014 with a 14 to 17 inch protected slot limit harvest regulation. In April 2014, this regulation was rescinded and replaced with statewide harvest regulations (10 fish per day creel and no length limit).
Results of this study indicates that the Chicot Lake largemouth bass population has one of the lowest levels of mortality in the state, 35 percent, and is influenced more by natural mortality than by fishing mortality.
Chicot Lake largemouth bass also exhibit a slow to moderate growth rate and larger variability in recruitment levels when compared to the other populations included in this project. Creel survey results suggest that Chicot Lake anglers harvest a much smaller percentage of largemouth bass than they release, less than 20 percent of legal-sized largemouth bass are harvested.
Given these dynamics, the earlier 14 to 17 inch protected slot limit was determined ineffective and rescinded. Protected slot limits are intended to increase growth rates of largemouth bass within the slot and increase the numbers of largemouth bass above the slot by utilizing angler harvest of fish below the slot as a management tool. Because Chicot Lake anglers harvest so few legal size largemouth bass caught the effectiveness of the protected slot limit regulation as a management tool is limited.