Developing Non-lethal Aging Methods for Largemouth Bass in Florida


Largemouth bass in tank

Florida is known as the “Bass Fishing Capital of the World,” where black bass species are the number one recreationally targeted freshwater fishes. For many people in Florida, bass fisheries are important for numerous social and economic reasons; yearly, these fisheries account for $1.25 billion in economic activity. In Florida, bass are managed as game fish, with some harvest legally permitted. Although harvest is permitted and biologically sustainable in moderation, most bass anglers have adopted a strong catch-and-release ethic that places high value on putting bass back alive. FWC’s management and conservation of bass relies on maintaining up-to-date data on growth and population dynamics across Florida. To do this, FWC biologists must be able to reliably age individual fish. Largemouth bass are typically aged by interpreting annular rings deposited in sagitta otoliths but removing them requires the fish to be killed for extraction. Other bony parts of a fish’s anatomy can also form yearly growth rings, and some of them, such as fin rays and spines, may be removed without causing lasting harm to the fish. However, these alternative aging structures often are difficult to interpret and provide less reliable ages than otoliths in largemouth bass.

In 2018, University of Florida scientists uncovered promising results about aging largemouth bass in Florida via fin rays and spines. After substantial practice interpreting the banding patterns in cross-sections of the bass fin structures, dorsal spine ages were identified as the most precise and comparable to otolith ages.

These encouraging results and the high social value placed on live release of bass create an opportunity for FWC biologists to continue developing and validating non-lethal aging techniques, which could lead to novel future investigations. For example, FWC partners with bass anglers through a trophy bass conservation program—TrophyCatch, tournaments and tagging studies which largely, if not explicitly, involve catch and live-release of bass. Incorporating a non-lethal aging method in these activities would increase the breadth and application of data collected by anglers and scientists; thus, it would be a new and valuable tool for largemouth bass research, management and conservation.

To continue the assessment of non-lethal largemouth bass aging methods in 2019, FWC biologists formed three main goals: (1) measure the survival of largemouth bass following dorsal spine excision, (2) compare the accuracy and precision of ages derived from largemouth bass dorsal spines among six waterbodies in Florida with different growth rates, and (3) disseminate information about use of dorsal spines in largemouth bass aging through a series of publications and internal workshops for FWC fisheries biologists.

To tackle the first objective, Fish and Wildlife Research Institute and Division of Freshwater Fisheries Management biologists collected 36 wild-largemouth bass from Rodman Reservoir, clipped dorsal spines III–V, and held the bass in six outdoor tanks to assess their survival. The bass (12–22 inches in length) were observed daily, fed (e.g., crayfish), and water quality was monitored.

At the conclusion of the acute survival tank experiment, biologists demonstrated that dorsal spine removal was non-lethal over a 35-day period. No mortalities were observed for largemouth bass with excised dorsal spines, and experiment-wide survival was 94%. Removal of dorsal spines did not result in any apparent, lasting negative effects for the largemouth bass.

  • 1,200-gallon tanks housing 36 largemouth bass

    FWC designed and constructed aeration system, and the six 1,200-gallon tanks housing 36 largemouth bass for the acute survival experiment.

  • biologist removing largemouth bass dorsal spines III-V with a pair of cutting pliers

    An FWC biologist removes largemouth bass dorsal spines III-V with a pair of cutting pliers.

  • View from above an experimental tank where largemouth bass are housed

    View from above an experimental tank where five visible largemouth bass are housed during the 35-day acute survival study. Fish attractor and microhabitat are seen in the background.

  • microscopic image of an ear stone cross section

    Dorsal spine (top) and sagitta otolith (bottom) sections from the same 7-year-old individual. To estimate ages, sections are viewed under a microscope using two different light sources. Clear (i.e., translucent) bands are counted in dorsal spines and opaque bands are counted in otoliths to determine the age of the Largemouth Bass. Distinct enumerated bands are indicated by light and dark points.

  • microscopic image of an ear stone cross section

    Ambiguous banding pattern on a dorsal spine section (top). Difficult double-banding patterns where two bands are joined occurs frequently when aging dorsal spines. Straightforward banding pattern on a sagitta otolith section (bottom). Both sections are taken from the same Largemouth Bass (age 4). Light and dark points represent enumerated bands.

The second objective aims to determine if aging largemouth bass populations with dorsal spines has potential broad application across Florida. A suite of factors, like lake productivity and temperature regimes that vary across the state, can influence bass growth and life-history. It is not known how variation in these factors might affect researchers’ ability to age largemouth bass using dorsal spines; therefore, multiple largemouth bass populations were tested. Gainesville Freshwater Fisheries biologists partnered with multiple Freshwater Fisheries Research (FFR) and DFFM field offices to collect dorsal spines in addition to otoliths that were already being garnered for largemouth bass aging studies in 2019. The targeted waterbodies included: Lake Griffin, Fellsmere Reservoir, Stick Marsh/Farm 13 Reservoir, L-67A Canal, Escambia River marsh and Apalachicola River.

Field collections occurred during spring and fall of 2019. Laboratory processing, which consisted of cutting thin cross-sections of each structure and mounting them onto microscope slides, was also completed by fall 2019. Aging of the samples is still ongoing, but preliminary results suggest that banding patterns in dorsal spines can be challenging to interpret and appear to vary among largemouth bass populations. Waterbody specific training and preparation may be required to age dorsal spines from other largemouth bass populations with varying growth patterns. Between-reader and structure analyses remain in-process. FWC biologists are drafting two manuscripts covering this research for peer-reviewed publication and are preparing information to present at workshops where members of other FWC offices can learn and benefit from the techniques.

As the role of anglers in natural resource stewardship and conservation in Florida increases, development of non-lethal aging techniques for largemouth bass opens new doors for FWC and stakeholders to work together through citizen-science and cooperative research. These tools could help guide management actions which ultimately meet bass conservation goals. Once the project results move from preliminary to final, we can determine if dorsal spines will form the connection between collecting viable information about Florida’s largest and perhaps oldest largemouth bass.