Largemouth Bass Micropterus salmoides, Florida Bass Micropterus floridanus, and their intergrade are socially and economically valuable sport fish. In the southeastern United States, it is customary for fisheries personnel to age black bass Micropterus species using sagittal otoliths, which requires killing the fish. Presently, fisheries managers and black bass anglers show reluctance to sacrifice large individuals. Development of a nonlethal ageing technique would not only allay concerns of sacrificing large black bass, but it could offer a pathway for new research, management, and conservation. We excised dorsal spines III–V from Largemouth Bass in Florida varying from 30 to 57 cm total length to evaluate the effects of the procedure on survival over 35 d. No mortalities were observed for fish with excised dorsal spines, and experiment-wide survival was 0.94 (0.87–1.00; 95% confidence interval). No significant differences in survival, weight change, or incidence of external injuries were observed between control and excised fish. The areas of spine excision healed with no visible infection or inflammation at the conclusion of the experiment. Therefore, dorsal spine removal offers managers a nonlethal option for collecting ageing structures of adult Largemouth Bass in Florida, including large individuals, and this result likely extends to other Micropterus species as well.
Black bass Micropterus species represent the most recreationally targeted freshwater fish genus across the United States (USDOI, USFWS, USDOC, and USCB 2016), and their fisheries are socially and economically important (Chen et al. 2003; Dutterer et al. 2014; Kerns et al. 2015; Taylor et al. 2019). Substantial taxonomic uncertainty surrounds Largemouth Bass Micropterus salmoides, Florida Bass Micropterus floridanus, and their intergrade M. salmoides × M. floridanus, hereafter referred to as LMB, as scientists have recognized Florida Bass as a subspecies of Largemouth Bass or elevated it as a distinct species at different times. The two species are so morphologically similar that genetic testing is required to distinguish between them (Barthel et al. 2010; Taylor et al. 2019). Across much of their native range, fisheries biologists manage LMB as sport fish, with some harvest permitted. However, many LMB fisheries, particularly in Florida, have low exploitation rates and high release rates, especially for trophy LMB (≥ 3.63 kg; Meyers et al. 2008; Dotson et al. 2013; Kerns et al. 2015; A. C. Dutterer, Florida Fish and Wildlife Conservation Commission [FWCC], unpublished data). This creates a situation wherein take, whether for recreational or for scientific purposes, is biologically sustainable but socially discouraged.
In the southeastern United States, fisheries personnel frequently age LMB by interpreting annuli deposited in their sagittal otoliths (Hoyer et al. 1985; Crawford et al. 1989; Maceina et al. 2007), but their extraction is lethal (Quist et al. 2012). Lethal sampling presents a public relations problem for fisheries managers who increasingly promote catch-and-release trophy LMB fisheries but also need to acquire vital demographic statistics (Dutterer et al. 2014). Thus, some fisheries managers in North America have also grown reluctant to kill larger individuals for age samples (Crawford et al. 1996; Klein et al. 2017). Although this reduces the number of large LMB that are sacrificed, failure to include these fish in age samples can bias estimates of growth, mortality, or maximum age. Ageing nonlethal structures may alleviate concerns over killing large LMB and present new age data collection opportunities.
Among the numerous fin structures found on LMB that might allow nonlethal ageing, dorsal spines are easy to remove and process, and researchers have documented promising levels of ageing accuracy and precision (Lindelien 2018; S. Lindelien, FWCC, unpublished data). However, only four other studies have assessed the accuracy of ages derived from LMB dorsal spines by comparing them to known-age fish or ages derived from otoliths, and most (besides Zhu 2015) indicated that dorsal spines were not accurate ageing structures for LMB (Maraldo and MacCrimmon 1979; Klein et al. 2017; Blackwell et al. 2019). Therefore, widespread use of dorsal spines to age LMB requires continued improvements in accuracy and precision.
Although many fisheries scientists assume that removing fin rays and spines is nonlethal and have widely accepted the practice (Quist et al. 2012), we are unaware of any published studies evaluating LMB survival after this process. To rectify the assumption that excision causes no harm, we evaluated the effects of LMB dorsal spine removal before recommending ageing via dorsal spines for use in studies in which researchers desire fish survival. We hypothesized that the excision of dorsal spines would be nonlethal and unharmful for all sizes of LMB. Our objectives were to 1) evaluate if excising dorsal spines affects LMB survival, 2) examine whether survival following dorsal spine excision was associated with LMB length, and 3) assess whether dorsal spine excision results in sublethal effects, specifically by evaluating change in weight and the occurrence of external injuries.
We targeted a broad length distribution of adult LMB (30–57 cm total length [TL]) to test the effect of size on survival. Our source was the population at Rodman Reservoir, in Putnam and Marion counties, Florida, and we collected 36 LMB (based on genetic testing by Barthel et al. , this population showed introgression of M. salmoides and M. floridanus alleles) using daytime boat electrofishing on February 27, 2019. To achieve a uniform distribution of LMB lengths, we divided the target length distribution into six 44-mm bins and retained the first six fish encountered in each bin. During collection, we held the LMB in livewells no longer than 45 min. We measured (TL; mm), weighed (g), and implanted the LMB with a Biomark APT12 FDX-B passive-integrated-transponder (PIT) tag posterior to the pelvic girdle using an MK-25 implant gun (Biomark Inc., Boise, ID), then we transferred the fish to a 1,514-L hauling box containing 0.5% salt solution under aeration to reduce stress during transport and relocation.
We transported the LMB to experimental tanks at FWCC's Freshwater Fisheries Research office in Gainesville, Florida, approximately 100 km from the collection site. We tempered the LMB for 1 h by pumping water from a designated onsite reservoir tank (19.1°C, same as experimental tanks) into the hauling box (20.5°C) until the water temperature equilibrated. We used a randomization function in Microsoft Excel version 1902 (Microsoft Corporation, Redmond, WA) to assign each LMB to one of six identical 4,543-L (filled to 3,785 L), round outdoor tanks, with the constraint that in no tank could the total fish mass exceed 9.2 kg. This was a conservative constraint to ensure that our aeration system provided adequate dissolved oxygen (DO), especially if we needed to lower tank volumes to keep the LMB from jumping out. If we exceeded the mass constraint, we repeated the randomization until we met the mass-per-tank constraints. Ultimately, we stocked each of the six tanks with six LMB, and, as a result of the uniform length distribution of our fish and the randomized tank assignments, all tanks had similar length distributions and total LMB mass. We sought this setup to minimize tank effects, facilitating replication at the level of the individual LMB. The experimental tanks were aerated and initially contained a 0.5% salt solution. After 24 h, we began adding well water in a flow-through system (∼ 3.8 L/min). This slowly dissipated the salt solution over 2–3 d and we continued this process for the remainder of the experiment. To provide cover, each tank contained 1.5 m2 of floating plants (Eichhornia crassipes and Pistia stratiotes) and one 1.0-m-high × 1.1-m-diameter Safe Haven artificial fish attractor (MossBack Fish Habitat, Springdale, AR). We placed HOBO U26-001 DO data loggers in three randomly selected tanks to record both DO and temperature, and we deployed HOBO 64K Pendant® temperature/alarm waterproof data loggers in the remaining three tanks to record temperature only (Onset Computer Corp., Bourne, MA). The loggers collected data every 30 min for the duration of our trial (Tables S1 and S2, Supplemental Material). We also measured water quality (i.e., pH, DO, specific conductance, and temperature) daily with a YSI Professional Plus Multiparameter Instrument (YSI Inc., Yellow Springs, OH) to gather instant data to ensure the tanks were functioning properly.
All LMB were acclimated in their tanks for 1 wk before we excised dorsal spines, on March 6, 2019. One tank at a time, we recovered LMB with a HT-2000 battery backpack electrofisher (Halltech Aquatic Research Inc., Guelph, Ontario, Canada) and a dip net, and temporarily held the LMB in a 76-L livewell before processing. We scanned each fish for its PIT tag ID with a Biomark HPR Lite reading device (Biomark Inc.), measured it, and placed it in a weighing pan to simulate typical workup during LMB field sampling. For each tank, we used a randomization function in Microsoft Excel to select, based on PIT tag ID, three of the six LMB (n = 18) that would have dorsal spines excised. We recorded notes about each LMB's condition on the day of dorsal spine excision, specifically noting the presence of external injuries existing before excision (Table S3, Supplemental Material). These included abrasions, lesions, inflammation, and fin erosion (Turnbull et al. 1996), all of which we classified as sublethal effects. We removed dorsal spines III–V using a pair of Craftsman® 118-mm diagonal cutting pliers (Craftsman, Towson, MD) and a pair of surgical scissors to cut the tissues between spines. We clipped spines to a level flush with the LMB's back. We standardized time out of water to 60 s for all fish regardless of treatment.
We monitored the six tanks daily following dorsal spine excision. We removed dead LMB promptly, and recorded the date of death and any externally visible injuries. We intermittently fed the LMB throughout the study but took a conservative approach during feeding to avoid elevated concentrations of ammonia in the tank water as a by-product of digestion. Prey included crayfish Procambarus spp., tadpoles Lithobates spp., juvenile Bluegill Lepomis macrochirus, and Golden Shiner Notemigonus crysoleucas. We did not weigh the mass of prey, collectively or per tank, but we evenly apportioned prey by count among tanks during each feeding. After 35 d, we removed all LMB from their tanks, and we identified (by PIT ID), measured (TL), and weighed (g) them. We inspected each LMB again for presence and location of any external injuries, especially on or near the dorsal spines, and noted its final condition (Table S3, Supplemental Material). We compared all data before and after the experiment to evaluate any sublethal effects caused from dorsal spine removal.
We used a Kaplan–Meier model (Kaplan and Meier 1958) to estimate overall mortality of LMB for the duration of the study. We used a log-rank test (Harrington and Fleming 1982) to determine whether the survival functions differed significantly between LMB that had dorsal spines excised and those that did not. To assess whether TL affected LMB survival, we fitted a Cox proportional hazard model (Cox 1972; Cox and Oakes 1984). We used paired t-tests to assess differences in pre- and poststudy weight (g) among all LMB and a Welch's t-test to evaluate differences in weight loss between control and excised groups. To assess other sublethal effects, we condensed our concluding notes of LMB condition into one of two outcomes for each fish: 1) no external injuries observed or 2) one or more external injuries observed. We used Fisher's exact test to determine whether the occurrence of observed sublethal injuries was dependent on treatment group. We conducted all statistical tests using R version 3.5.2 and compared to α = 0.05 when determining significance; we conducted survival analyses in the survival package (Therneau and Grambsch 2000; Therneau 2015) and the remaining analyses in the stats package (R Core Team 2018).
Temperature and oxygen profiles demonstrated that all six experimental tanks were functioning at similar temperature and oxygen conditions throughout 40 operating days (Figures S2 and S3, Supplemental Material). A consistent difference in tank 3 DO by about 10% was likely due to a difference in air pressure or algal growth on the tank walls. The mean temperature over 40 d (i.e., February 28, 2019–April 8, 2019) among all six tanks was 20.2°C. Mean daily tank temperature fluctuation (daily maximum − daily minimum) was 2.4°C. Mean DO among three tanks was 9.4 mg/L, minimum recorded DO was 6.1 mg/L and maximum was 11.9 mg/L. Mean daily tank DO fluctuation was 1.5 mg/L. All tanks maintained biologically favorable conditions for finfish in Florida during the experiment.
We observed no mortalities among LMB with excised dorsal spines over 35 d; therefore, survival was 1.00 (Table S3, Supplemental Material). Two LMB in the control group died; thus, survival in the control group was 0.89 (0.76–1.00; 95% confidence interval [CI]). A tank mate likely consumed one control fish (308 mm TL, the smallest LMB included in our study). A second control LMB died on March 20, 2019, after developing a large lesion on its dorsolateral surface. Experiment-wide survival was 0.94 (0.87–1.00; 95% CI). We observed no significant difference in survival between the control and excised LMB (P = 0.15). Survival was not associated with LMB total length (Z = −0.92, P = 0.36).
In all LMB, mass decreased significantly during the study (paired t-test, t-stat = 7.91, df = 34, P < 0.001). Mean weight loss among all LMB was 105 g (78–132; 95% CI) or 7.8% of initial weight, but there was no significant difference in weight loss between excised and control groups (Welch's t-test, t-stat = 0.38, df = 33, P = 0.71; Figure S1, Supplemental Material). Excision of dorsal spines III–V caused no other apparent sublethal effects for LMB. The areas of excision healed, and no infection or inflammation were visible at the excision sites at the end of the study. Some external injuries were present across treatment groups, though differences in the occurrence of those injuries between treatment groups were not significant (P = 0.69).
This is the only study we are aware of that evaluates survival and condition of LMB following dorsal spine removal. Our results provide evidence that removal of dorsal spines is nonlethal and unharmful to adult LMB. We observed neither mortality of LMB following dorsal spine removal nor any sign of infection or inflammation at the excision sites within the 35-d study period. Sublethal injuries, presumably caused from capture, handling, or a fish contacting a hard object, were present on 6 of 36 LMB from both treatment and control groups prior to and after the experiment. It appears that fish handling and cannibalism were more influential to the fish's condition and survival than the removal of dorsal spines III–V. Thus, we recommend removal of dorsal spines as a nonlethal method for adult LMB, and we expect nonlethality of dorsal spine removal to extend to other Micropterus spp. too.
Presently, anglers voluntarily release 80−90% of their LMB catches in Florida (A. C. Dutterer, FWCC, unpublished data). Some fisheries managers have also grown reluctant to sacrifice large LMB (e.g., ≥ 50 cm TL) for ageing (Crawford et al. 1996; Klein et al. 2017); therefore, biologists return many big fish to the water with only length, weight, and girth information recorded. Pressure to conserve large LMB stems from the perception that they are a required component of success in LMB fisheries (Dotson et al. 2013), especially in Florida, where trophy bass (≥ 3.63 kg [8 lb]) research, management, and conservation have been given high priority in the state's Black Bass Management Plan (FWCC 2011; Dutterer et al. 2014). To realign harvest regulations with these priorities, the FWCC and managers elsewhere have shifted harvest towards smaller, younger LMB while protecting large and old LMB (Myers and Allen 2005; Taylor et al. 2019). Routine field sampling typically does not target these individuals or the full range of areas they likely use; therefore, large LMB are encountered less frequently (Dotson et al. 2013; Dutterer et al. 2014; Hall et al. 2019). Use of nonlethal ageing structures would allow biologists to opportunistically collect age data from large LMB upon encountering them and may allow managers to leverage the effort of anglers in collecting ages of large LMB (Hall et al. 2019). This could extend to tournaments or citizen-science programs, like Florida's TrophyCatch (Dutterer et al. 2014; Zhu 2015).
Previous work has demonstrated high survival following removal of Channel Catfish Ictalurus punctatus pectoral spines (Stevenson and Day 1987; Michaletz 2005), acipenserid pectoral rays (Collins and Smith 1996; Parsons et al. 2003; Nguyen et al. 2016), and Bull Trout Salvelinus confluentus pelvic rays (Zymonas and McMahon 2006). Contrastingly, clipping pelvic, pectoral, and anal fins or removal of whole fins decreased juvenile Micropterus spp. survival across multimonth time spans, perhaps due to increased predation that could result from difficulty swimming (Ricker 1949; Coble 1971). However, dorsal spines are median fins and serve different functions in locomotion than the paired fins above (Standen and Lauder 2005). Most like our study, Metcalf and Swearer (2005) and Hobbs et al. (2014) clipped a dorsal spine of Blue Throat Wrasse Notolabrus tetricus and Coral Trout Plectropomus leopardus, respectively and held them in tanks for 4 wk. The areas of excision displayed tissue regrowth, and removal of dorsal spines caused no mortality (Metcalf and Swearer 2005; Hobbs et al. 2014). The best understanding of the effects of spine removal on long-term survival of LMB might be attained through multiyear tagging or observational studies in the wild. We removed dorsal spines III–V, which provided more than enough bony material for ageing (Lindelien 2018). Future applications of dorsal spine ageing in LMB may involve opting to remove only one spine, which would likely further diminish any unobserved negative effects of dorsal spine removal.
The only two mortalities we documented were control fish (308 and 465 mm TL). The larger LMB died 15 d into our study. We attributed its death to infection in a large lesion on its right dorsolateral surface. The fish was healthy at the outset of our study with no initial injuries recorded. Although we inspected tanks daily, we did not notice the disappearance of the smaller LMB until the end of the study. We retrieved its PIT tag from the bottom of the tank, likely egested by one of two larger fish (573 and 568 mm TL). Furthermore, the 573-mm LMB lost only 2.1% of initial weight during the study, versus the 201-g mean weight loss or 8.0% of initial weight for all 52–57-cm LMB in the experiment, suggesting that it consumed more prey than others of similar size. Cannibalism of larval and juvenile stages of cultured finfish is common (Baras and Jobling 2002), but our results show that it can also occur among cohabiting adult LMB; therefore, researchers should carefully consider the maximum and minimum sizes of cohabiting fish when designing and implementing studies of captive fish to minimize cannibalism among research subjects.
All LMB lost weight over the course of our study. Weight loss was equitable between excised and control groups; so, dorsal spine removal had no effect on weight. We did not focus feeding around a weight-maintenance diet; nonetheless, our experimental LMB were fed on 18 of 42 d of captivity. If we had given them more food, the LMB may have maintained or gained weight, but that would have increased the risk of reaching unhealthy levels of ammonia in the tank water. Additionally, weight loss could have been related to the stresses of being enclosed or handled (Jennings et al. 2012) or could have been seasonal weight loss associated with spawning (Harvey and Campbell 1989). Reproduction in LMB is multifaceted and can be easily off-put by factors like confinement and a different feeding schedule (Smith 1976; Rideout and Tomkiewicz 2011; Shaw and Allen 2014). We saw no evidence of spawning in the tanks, but based on water temperatures and timing of collection, our LMB would have been spawning-ready in the wild. Perhaps a shift in surroundings, prey, or prey availability caused the bass to behave differently and females to skip spawning (Rideout and Tomkiewicz 2011; Shaw and Allen 2014), leading to less gonadal weight and thus overall weight loss.
We timed our experiment to coincide with directed sampling for LMB and peak recreational LMB catch in Florida, which occur in early spring (Crawford et al. 1996; Dutterer et al. 2014; Bonvechio 2017). We did not regulate our tank temperatures, but allowed for daily fluctuation with ambient outdoor temperature. Thus, high, low, and mean tank temperatures in our study were consonant with water temperatures that wild LMB would experience at many Florida waterbodies in spring. We thought it would allow the best inference into LMB survival if we incorporated dorsal spine collection and ageing into fishery-dependent or -independent efforts. Our results of high survival following dorsal spine removal may not extend to the warmer conditions of summer and fall. These seasonal differences could increase the potential for infection in the areas of excision or any injuries that the fish may have, thus decreasing the survival of LMB (Cooke and Suski 2005). Given our results, we recommend sampling and excising dorsal spines during the spring to mitigate any compounding negative effects of higher water temperatures on LMB survival.
In advancing the collective knowledge of ageing techniques for LMB, our study demonstrates that removal of dorsal spines is a nonlethal method. If use and accuracy of dorsal spine ageing continue to advance, application of the method will likely expand within LMB management and research. Use of dorsal spines for ageing would benefit fishery scientists and could further engage anglers in citizen-science initiatives. Obtaining ages of some of the oldest and largest LMB in Florida would improve our understanding of the population dynamics and environmental processes that produce these valuable fish. Because most LMB anglers voluntarily release their fish, nonlethal ageing also aligns with the social ideology incorporated in modern bass fishing and managers could use it for all sizes of LMB. Therefore, fisheries managers could use nonlethal sampling of dorsal spines in a variety of fishery-dependent applications for Micropterus species and perhaps other species in which catch-and-release is expected and celebrated.
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Table S1. Temperature data collected every 30 min from February 28, 2019, to April 8, 2019, in six 4,543-L round, outdoor tanks during a 35-d acute survival experiment following excision of dorsal spines III–V on Florida Bass Micropterus floridanus.
Table S2. Dissolved oxygen data collected every 30 min from February 28, 2019, to April 8, 2019, in three 4,543-L round, outdoor tanks during a 35-d acute survival experiment following excision of dorsal spines III–V on Florida Bass Micropterus floridanus.
Table S3. Data collected over 35 d for 36 Florida Bass Micropterus floridanus during an acute survival experiment in Florida. Passive integrated transponder tag information (PIT_ID), initial and final weight (WT; g), initial and final total length (TL; mm), treatment (excised dorsal spines or control fish with no dorsal spines excised), tank number, notes recorded at the beginning and conclusion of the experiment, if the fish survived or not, and if sublethal effects were present or not are included.
Figure S1. Box-and-whisker plots of Largemouth Bass Micropterus salmoides, Florida Bass Micropterus floridanus, and their intergrade M. salmoides × M. floridanus weights recorded at the beginning and end of a 35-d survival experiment for control fish with no dorsal spines excised and fish with dorsal spines III–V excised. Plots show mean (black dot), interquartile range (i.e., 0.25–0.75 quantiles; the box), median (black bar within the box), and whiskers (upper 97.5 percentile; lower 2.5 percentile).
Found at DOI: https://doi.org/10.3996/JFWM-20-033.S2 (10.55 MB TIFF).
Figure S2. Temperature profiles measured for 40 d in six 4,543-L round, outdoor tanks. Black vertical line represents March 6, 2019, when dorsal spines III–V were excised from 18 captive Largemouth Bass Micropterus salmoides, Florida Bass Micropterus floridanus, and their intergrade M. salmoides × M. floridanus during a 35-d acute survival experiment.
Found at DOI: https://doi.org/10.3996/JFWM-20-033.S3 (602 KB TIF).
Figure S3. Dissolved oxygen (DO) profiles measured over 40 d in three 4,543-L round, outdoor tanks. Black vertical line represents March 6, 2019, when dorsal spines III–V were excised from 18 captive Largemouth Bass Micropterus salmoides, Florida Bass Micropterus floridanus, and their intergrade M. salmoides × M. floridanus during a 35-d acute survival experiment. Drop in DO on March 2, 2019, occurred due to an interruption in the aeration system power supply.
Found at DOI: https://doi.org/10.3996/JFWM-20-033.S4 (592 KB TIF).
Reference S1.[USDOI, USFWS, USDOC, and USCB] U.S. Department of the Interior, U.S. Fish and Wildlife Service, U.S. Department of Commerce, and U.S. Census Bureau. 2016. National survey of fishing, hunting and wildlife-associated recreation.
Found at DOI: https://doi.org/10.3996/JFWM-20-033.S5 (22.28 MB PDF).
We thank the Sport Fish Restoration Program for providing funding via grant number FL F-F18AF01208 (F-175-R-8). We thank J. Dotson and E. Nagid for their insight and support. T. Tuten and S. Hooley helped us collect fish. T. Lange and M. Garnett provided insight about fish tempering, care, and tank setup. We thank the U.S. Geological Survey for their methodological assistance. B. Crowder, K. Bonvechio, and M. Quist provided editorial guidance which improved the organization and clarity of this manuscript. We thank the anonymous editors and reviewers for their time and constructive feedback. The mention or use of products does not constitute endorsement by the Florida Fish and Wildlife Conservation Commission.
Any use of trade, product, website, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views of the U.S. Fish and Wildlife Service.
Citation: Lindelien S, Dutterer AC, Schueller P, Anderson CC. 2021. Evidence dorsal spine removal is nonlethal and unharmful for largemouth bass in Florida. Journal of Fish and Wildlife Management 12(1):190–196; e1944-687X. https://doi.org/10.3996/JFWM-20-033