Do Jaw Deformities Adversely Affect Largemouth Bass?

Largemouth Bass Micropterus nigricans anglers have mainly adopted catch-and-release practices with limited direct fishing mortality. However, catch-and-release angling could cause delayed mortality and nonlethal effects that could potentially affect population dynamics. For instance, hooking can result in jaw deformities, with unknown subsequent effects on vulnerability to angling, condition, growth, and mortality. Our objectives were to assess the prevalence of Largemouth Bass jaw deformities and test for differences in angling vulnerability, growth, condition, size structure, and mortality of fish with and without jaw deformities. Largemouth Bass were captured by electrofishing and by anglers at 41 tournaments at Brushy Creek Lake, Iowa, between April and August 2015. Jaw deformities were observed in 7.0% of fish caught at tournaments and 3.3% caught while electrofishing (5.8% overall). Angling recapture rates of tagged fish with (78.9%) and without (76.8%) jaw deformities were similar. Condition, growth, size structure, and mortality of fish with and without jaw deformities were also similar. Our results indicate jaw deformities have little effect on Largemouth Bass angling vulnerability, condition, growth, or survival; however, we still recommend careful handling of fish to minimize injuries that may occur during angling.

Catch-and-release angling has become a popular practice since the 1970s (Quinn 1996) and has become commonplace for certain fisheries, including Largemouth Bass Micropterus nigricans, where release rates rose from 27% in the early 1980s to 99% by 2006 (Myers et al. 2008; Schramm and Gilliland 2015; Sass and Shaw 2020). In addition to increased release rates of recreationally angled bass, most competitive angling events held in the United States now require the use of catch-and-release practices (Noble 2002; Kerr and Kamke 2003; Driscoll et al. 2012). Although catch-and-release angling has increased exponentially and direct harvest fishing mortality has declined, delayed sources of mortality and sublethal angling effects on reproductive success, movement rates, and physiological parameters remain a concern for fish management (e.g., Cooke et al. 2000; Thompson et al. 2008; Cline et al. 2012; Kerns et al. 2016).

Largemouth Bass catch-and-release mortality can occur and may represent the largest source of delayed angling mortality for populations (Meals and Miranda 1994; Thompson et al. 2008; Sylvia et al. 2021b; Maahs et al. 2021). However, catch-and-release angling may have a number of other lethal and sublethal effects. For instance, all angled fish receive a hooking injury and the prevalence of hook wounds in a population can increase with catch-and-release angling pressure (Cooke et al. 2000; Suski et al. 2003; Thompson et al. 2008; Fernholz et al. 2018). Fish care and handling practices (e.g., fish landing devices, tournament cull tags) can also result in damage to fish mouths and result in higher rates of broken jaws for some fishes, potentially resulting in deformities (Danylchuk et al. 2008; Milliken et al. 2009; Skaggs et al. 2017; Chong et al. 2021). Some fish mouth wounds can heal and are no longer detectable (Fernholz et al. 2018). Other wounds are more severe and can result in split dentary, separation of the tongue from the floor of the mouth and left and right maxilla breaks. Because of their severity, these wounds may heal improperly, resulting in jaw deformities (Figure 1).

Jaw deformities may have important physical and physiological consequences that could alter populations. Jaw deformities can decrease oxygen recovery after physical exertion and swimming performance in Atlantic Salmon Salmo salar (Lijalad and Powell 2009). Additionally, Largemouth Bass use a combination of ram and suction feeding to capture prey (Grubich and Wainwright 1997; Sass and Motta 2002), and alterations to their jaws may reduce foraging efficiency, potentially resulting in lower prey consumption and increased energetic expenditures. The inability of fish to capture prey can lead to long-term effects including reduced condition and growth, increased vulnerability to parasites and predators, and higher mortality (Chipeniuk 1997; Miyashita et al. 2000; Meka and Margraf 2007; Cobcroft and Battaglene 2009). Further, more aggressive black bass Micropterus spp. within a population may be more likely to receive jaw deformities resulting in increased capture rates of those individuals (Philipp et al. 2009; Sutter et al. 2012). Despite the potential implications of jaw deformities on Largemouth Bass populations, we are aware of no studies that have evaluated their prevalence in populations or their effects on population characteristics and dynamics.

Our objectives were to document the prevalence of jaw deformities in a Largemouth Bass population that experienced extensive angling pressure and to test for differences in 1) the relative angling vulnerability (i.e., angler captures), and 2) growth, condition, size structure, and mortality of fish with and without jaw deformities. We hypothesized Largemouth Bass with jaw deformities could be more aggressive in the population and thus recaptured by anglers at a higher rate than those without jaw deformities, but both jaw types would be captured at similar rates with electrofishing. We also hypothesized fish with jaw deformities would have lower condition, slower growth, smaller size structure, and higher mortality than those without jaw deformities.

We collected Largemouth Bass from Brushy Creek Lake, Iowa, USA, between April and August 2015. Brushy Creek Lake is a 280-ha impoundment and supports a popular Largemouth Bass fishery within Iowa. Forty-one competitive angling tournaments, resulting in 6,232 angler-hours (22 h/ha), were held during the sampling period. All Largemouth Bass weighed in at tournaments were placed in a tank with a continuous flow of oxygen following the tournament weigh-in. We also conducted sampling using day and night boat electrofishing (pulsed DC 300 V, 8 amps) during the first weeks of every month to sample all of the accessible shoreline. We spent 30.61 h electrofishing and captured 774 Largemouth Bass (25 bass/electrofishing-hour) across the 4-mo study period. We collected length (mm), weight (g), scales, and dorsal spines, and tagged all fish ≥330 mm with an individually numbered metal Monel jaw tag and visually examined each for jaw deformities before releasing it. We considered fish to have jaw deformities if either or both the right and left maxilla were disfigured, the bottom dentary was split, or any piece of the jaw had healed missing parts of bone (Figure 1). We did not consider recent, small hook wounds (i.e., holes in parts of the soft tissue of the mouth or head) to be jaw deformities.

All analyses included Largemouth Bass captured by both electrofishing and tournament angling events. Recapture data could also contain reported recreational angler captures. We estimated age for all fish captured with jaw deformities (n = 87), whereas we estimated age in an additional 87 fish with normal jaws that were randomly selected within the same time period that jaw deformity fish were captured. We mounted dorsal spines in epoxy and then cross-sectioned (1 mm width) them at least three times using an IsoMet low speed saw. Two independent readers aged cross-sectioned spines under a dissecting microscope. If two readers could not assign a consensus age for a dorsal spine section (6% of fish), a third reader provided an age estimate. We used scales for estimating age for fish <330 mm (total length [TL]) that did not receive a tag. Scales can provide accurate age estimates for bass up to age 7+ y (Maraldo and MacCrimmon 1979) and Largemouth Bass <330 mm were all estimated at age 3 y or younger. Biases in the ages of dorsal spines may exist for bass >7 y, but we achieved high reader agreement (>80% after initial readings) and believe they provided a relative comparison of von Bertalanffy growth parameters between individuals with and without jaw deformities in our study.

We calculated the proportion of Largemouth Bass with jaw deformities (number of fish with jaw deformities/number of fish captured) within the population separately for fish collected during tournaments and electrofishing. We also calculated the number of recaptures with and without jaw deformities for tournaments and electrofishing as an index of angling vulnerability (Table S1, Supplemental Material). We treated the recapture rate of electrofishing as a control because we assumed that no differences would exist in recapture rates of fish captured by electrofishing. We used a Kolmogorov–Smirnov test (Chakravarti et al. 1967) to test for differences in recapture frequency between jaw types by gear. We assessed condition using relative weight W_r = (W/W_s) where W_r is the relative condition, W is weight of the individual, and W_s is the standard weight for its length predicted by a length–weight regression (W_s = aL^b; Neumann et al. 2012; Table S2, Supplemental Material). We compared condition between jaw types using a two-tailed student's t-test. All statistical tests used a significance level of α = 0.05.

We tested differences in length at age among Largemouth Bass with and without jaw deformities using von Bertalanffy models,

where L_∞ is the asymptotic length, and K is the Brody growth coefficient (Neumann et al. 2012) fit to length at age data for fish with and without jaw deformities (Table S3, Supplemental Material). We compared von Bertalanffy growth models using 95% profile likelihood confidence intervals (Haddon 2021). We also conducted a test for differences in growth between fish with and without jaw deformities as grams of growth per gram of fish per day (G/G/D) using mark–recapture data (Table S4, Supplemental Material). We established three categories for growth comparisons: 1) initially captured with a jaw deformity (n = 22), 2) initially captured without a jaw deformity and recaptured with a jaw deformity (n = 21), and 3) captured and recaptured without a jaw deformity (n = 21). We calculated growth (somatic growth differences) as where G_t is grams of growth per day, W_i is the initial weight, and W_c is recaptured weight. We used only Largemouth Bass at large ≥14 d between tagging and recapture periods (n = 43 of 87 with jaw deformities) because of depressed feeding initially following release of an angling event (Siepker et al. 2006); however, refractory periods are variable for black bass (Cline et al. 2012; Sass et al. 2018). We compared growth (G/G/D) as a function of length (covariate) and jaw type (deformity or nondeformity) using analysis of covariance (ANCOVA). Finally, we compared size structure between groups by creating length-frequency histograms for jaw and gear types and a two-sided Kolmogorov–Smirnov test (Chakravarti et al. 1967) tested for differences in size structure of fish with and without jaw deformities for each gear type (Table S5, Supplemental Material).

We estimated survival of Largemouth Bass with and without a jaw deformity using a Cormack–Jolly–Seber mark–recapture model (Lebreton et al. 1992) in Program Mark (White and Burham 1999; Table S6, Supplemental Material). We collected monthly recapture data from 87 Largemouth Bass with jaw deformities and 87 with non–jaw deformities captured between April and September 2015 with recapture histories beginning in April of 2015 and lasting until April of 2017 (see Sylvia et al. [2021a]; Sylvia and Weber [2022] for additional sampling information during this time). We analyzed one model that included a group effect of jaw deformities for detection probability and survival and considered differences in survival to be significant if 95% confidence intervals did not overlap between the two groups.

Observations of jaw deformities were low and recapture rates did not differ between Largemouth Bass with and without jaw deformities by gear type. A total of 1,429 fish were captured during 2015: 1,011 were captured by tournament anglers and 418 were captured with electrofishing. Jaw deformities were observed on 71 of 1,011 (7%) caught at tournaments and 16 of 481 (3.3%) caught with electrofishing (5.8% overall). Tournament anglers captured individual Largemouth Bass between one and four times, whereas electrofishing captured individuals between one and five times (Figure 2). Of the jaw deformity fish captured by anglers, 67.0% were captured once, 28.0% were captured twice, 4.0% were captured three times, and 1.0% were captured four times. Non–jaw deformity fish captured by anglers only once made up 94.0% of the population, whereas 6.0% were captured twice, and 1.0% were captured three times. Jaw deformity Largemouth Bass captured by electrofishing were caught a maximum of two times: 85.0% were caught once, and 15.0% were caught twice (Figure 2). In contrast, 81.0% of fish without jaw deformities were captured once, 16.0% twice, and 3.0% were captured three times with electrofishing. The number of fish recaptured with and without jaw deformities with angling (D_5,5′ = 0.2, P = 0.99) and electrofishing (D_5,5′ = 0.4, P = 0.82) were similar.

We found no differences in condition, length at age, and somatic growth of Largemouth Bass with and without jaw deformities. Average relative weight for fish with jaw deformities was 105 (±2 SE), whereas the average relative weight for fish without jaw deformities was 107 (±2 SE). Relative weight decreased with increasing length for both jaw types but did not differ between individuals with and without jaw deformities (T₁₇₁ = −1.38, P = 0.17; Figure 3). Jaw deformity (K = 0.58, 0.55–0.60; L_∞ = 432 mm, 427–437 mm) and non–jaw deformity (K = 0.58, 0.55–0.60; L_∞ = 432 mm, 427–437 mm) fish had similar von Bertalanffy growth curve parameters (mean, ±95% CI), where the profile likelihood confidence intervals for K and L_∞ for jaw deformity and non–jaw deformity model parameters overlapped with the estimate (Figure 4). Largemouth Bass with normal jaws appeared to have greater G/G/D growth at longer initial lengths than did those initially having jaw deformities and recaptured with jaw deformities (Figure 5); however, G/G/D did not differ across lengths (ANCOVA: F_1,60 = 0.435, P = 0.51) or jaw types (F_2,60 = 0.61, P = 0.54).

We found that Largemouth Bass with and without jaw deformities were captured across a large length distribution. A few individuals <330 mm were captured with jaw deformities, the shortest being 220 mm. The majority (97%) of fish captured with a jaw deformity were >380 mm with some fish as long as 520 mm captured with jaw deformities. Size structure of fish captured with either angling (D_50,50′ = 0.06, P = 0.76) or electrofishing (D_50,50′ = 0.16, P = 0.11) was similar for fish with and without jaw deformities (Figure 6).

Mortality rates did not differ between Largemouth Bass with and without jaw deformities. We observed the minimum age of fish captured with both jaw types was 3 y, but maximum age differed between jaw deformity and non–jaw deformity groups. Fish with a jaw deformity reached a maximum of 8 y, whereas those without a jaw deformity reached a maximum of 10 y. Monthly mortality for non–jaw deformity fish was 79.3% (95% CI = 72.1–85.1), whereas monthly mortality for those with jaw deformities was 82.8% (95% CI = 75.9–88.1). Consequently, mortality did not differ between jaw types because 95% confidence intervals overlapped.

Our results suggest jaw deformities have little effect on angling vulnerability, condition, growth, or survival of Largemouth Bass. Although Largemouth Bass in Brushy Creek Lake often display hook wounds in proportion to catch-and-release angling pressure (Fernholz et al. 2018) that may have developed into a jaw deformity, these injuries do not appear to be adversely affecting individuals or the population. One challenge to assessing the effects of jaw deformities is that we do not know with certainty what caused the injury and when the jaw deformity was obtained. Catching fish via rod and reel and angler fish-care practices can cause tissue damage to the maxilla, but we do not know whether jaw deformities were solely caused by angling. Other potential sources of injury that could cause this damage are unknown; however, the source of jaw wounds was not relevant for this study where we assessed how jaw wounds, regardless of their source, could potentially affect populations. Additionally, survivorship bias may account for our findings of a lack of influence of jaw deformities in fish populations. Severe hooking wounds may have resulted in immediate mortality before fish could develop jaw deformities. Thus, our results apply only to fish that have survived initial injury and where initial wounds actually developed into a deformity versus those with a hook wound or injury. This research evaluates long-term influences of fish that have survived initial injury; we believe these data are useful for building an understanding of how permanent jaw deformities affect a highly angled fish population.

Although we know of no other study that has tested for population-level effects of jaw deformities on wild Largemouth Bass populations, past hooking injuries (missing maxillary, inverted maxillary, and scarring to the dentary) in a catch-and-release Rainbow Trout Oncorhynchus mykiss fishery constituted 29% of captured fish (Meka 2004). Jaw deformities in aquaculture can result in impaired respiration (Lijalad and Powell 2009), decreased or total inability of fish to capture and ingest food (Pittman et al. 1990; Sadler et al. 2001), reductions in growth rates, and decreased survival rates (Miyashita et al. 2000; Cobcroft and Battaglene 2009). Thus, testing for effects of jaw deformities on a wild Largemouth Bass population is important to understanding potential effects of catch and release and for population conservation. Recent studies have found differences in the level of Largemouth Bass aggressiveness within a population, where individuals that are more aggressive may be more likely to be captured by anglers (Philipp et al. 2009). We hypothesized aggressive Largemouth Bass (those captured multiple times) would be more likely to obtain a jaw deformity (Sutter et al. 2012) and be recaptured again by anglers at higher rates compared with individuals without jaw deformities. However, our hypothesis was not supported, because both jaw types were recaptured by both gear types (electrofishing and angling) with similar capture frequencies. High fishing pressure may have resulted in both aggressive and nonaggressive individuals being captured at similar rates (Cooke et al. 2007) because improvements in fishing technology can increase capture rates regardless of aggressiveness (Feiner et al. 2020). Angler skill can also play an important role in capture rates, especially on Brushy Creek Lake given the large amount of tournament angling pressure (Beardmore et al. 2011; Sylvia et al. 2021a; Maahs et al., in press). Aggressiveness and nonaggressiveness may not play a significant role in natural systems and Largemouth Bass capture rates may be regulated by other factors including learned behaviors and recruitment into the population (Clark 1983; Hessenauer et al. 2016; Wegener et al. 2018). The extent of jaw deformities and suction feeding behavior in Largemouth Bass led us to hypothesize that jaw deformities would result in reduced condition and growth and higher mortality. For instance, lower jaw deformities were common among farmed Atlantic Salmon and condition of these individuals was lower than individuals with normal jaws (Sutterlin et al. 1987; Quigley 1995). Yet, our hypothesis was not supported, because no differences in condition were observed between individuals with and without jaw deformities. Largemouth Bass consume a diversity of prey types (Anderson 1984; Hodgson and Kitchell 1987; García-Berthou 2002) and predator capture success can be influenced by prey type, habitat complexity, and turbidity (Savino and Stein 1982, 1989; Shoup and Wahl 2009). An abundance of woody structure in Brushy Creek Lake may have resulted in a more varied diet compared with a less woody system (Ahrenstorff et al. 2009). Additionally, nearly all fish were in good condition (W_r > 90) and an abundance of prey may have provided plenty of foraging opportunities. Fish can also compensate for jaw deformities by using different foraging and feeding strategies. For instance, Largemouth Bass can use different cranial muscle movements to achieve similar buccal pressures (Grubich and Wainwright 1997) and primarily use ram feeding on fish prey (Sass and Motta 2002). Thus, if fish with jaw deformities can alter feeding methods and habits, population characteristics and dynamics would not be adversely affected.

Fish condition is a valuable metric of short-term prey availability that has potential consequences for fish growth, where fish with poor condition often experience slower growth rates (Neumann et al. 2012). However, we did not detect an effect of jaw deformities on growth or size structure of Largemouth Bass that survived the initial injury. Growth parameters (von Bertalanffy, G/G/D) and size structure were similar between fish with and without jaw deformities. Previous studies suggested deep hooking and catch and release hook wounds may influence growth and condition (Aalbers et al. 2004; Meka and Margraf 2007), but Largemouth Bass with jaw deformities in this study differed from hook wounds in previous studies because most deformities occurred outside of the mouth. Wound severity may be a greater influence on these parameters than is a simple presence or absence of a deformity or bass may simply overcome deformities given their ability to modulate jaw function for feeding conditions (Sass and Motta 2002) and adapt to food availability within a system.

Had condition or growth been negatively affected by jaw deformities, it may have resulted in reduced survival rates. However, with no differences in growth or condition, we also found no effects of jaw deformities on survival. Largemouth Bass are particularly resilient to catch-and-release angling practices and there may be little influence of catch-and-release mortality on populations (Sylvia et al. 2021a; Sylvia and Weber 2022; Maahs et al., in press). Increased catch-and release angling may lead to failed fishery management objectives, including decreased population size structure, increased density dependence, and increased competition with other species (Miranda and Bettoli 2007; Hansen et al. 2015; Sass et al. 2018; Sylvia et al. 2021b). Understanding the role jaw deformities in populations is important, but it may be inconsequential in systems with high mortality. Increased mortality rates of fish with jaw deformities may even benefit fisheries where size-structure is affected by a density-dependent growth. Consequently, we conclude that jaw deformities do not influence angler captures or Largemouth Bass condition, growth, or survival despite being common in the Brushy Creek Lake fishery.

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Table S1. Data collected on tournament- and electrofishing-captured Largemouth Bass Micropterus nigricans at Brushy Creek Lake, Iowa, from April and August 2015 to assess the effects of jaw deformities on population characteristics. Numbers of bass recaptured with and without jaw deformities were calculated for bass tournaments and electrofishing as an index of angling vulnerability. Table includes the percentage of total captured bass recaptured 1, 2, 3, and 4 times for jaw deformity and non–jaw deformity fish captured by angling and electrofishing.

Available: https://doi.org/10.3996/JFWM-21-096.S1 (15 KB DOCX)

Table S2. Data collected on tournament- and electrofishing-captured Largemouth Bass Micropterus nigricans at Brushy Creek Lake, Iowa, from April and August 2015 to assess the effects of jaw deformities on population characteristics. Bass condition was assessed using relative weight W_r = (W/W_s). Table includes length (mm) and the calculated relative weight (W_r) between jaw deformity and non–jaw deformity bass.

Available: https://doi.org/10.3996/JFWM-21-096.S2 (21 KB DOCX)

Table S3. Data collected on tournament- and electrofishing-captured Largemouth Bass Micropterus nigricans at Brushy Creek Lake, Iowa, from April and August 2015 to assess the effects of jaw deformities on population characteristics. von Bertalanffy growth curve data for jaw deformity and non–jaw deformity bass was used to assess growth differences between the two groups. Table includes age of fish, mean length (mm), and von Bertalanffy growth estimate (mm).

Available: https://doi.org/10.3996/JFWM-21-096.S3 (15 KB DOCX)

Table S4. Data collected on tournament- and electrofishing-captured Largemouth Bass Micropterus nigricans at Brushy Creek Lake, Iowa, from April and August 2015 to assess the effects of jaw deformities on population characteristics. Differences in growth between bass with and without jaw deformities was compared as grams of growth per gram of fish per day (G/G/D) using mark–recapture data. Table includes G/G/D and initial length for Largemouth Bass initially captured with a jaw deformity, captured and recaptured without a jaw deformity, and initially captured without a jaw deformity and recaptured with a jaw deformity.

Available: https://doi.org/10.3996/JFWM-21-096.S4 (17 KB DOCX)

Table S5. Data collected on tournament- and electrofishing-captured Largemouth Bass Micropterus nigricans at Brushy Creek Lake, Iowa, from April and August 2015 to assess the effects of jaw deformities on population characteristics. Size structure was compared between groups (fish with and without jaw deformities for each gear type) by creating length-frequency histograms for both jaw and gear types. Table includes the length, number, and percentage of Largemouth Bass within each of the jaw and angling type groups.

Available: https://doi.org/10.3996/JFWM-21-096.S5 (22 KB DOCX)

Table S6. Data collected on tournament-captured Largemouth Bass Micropterus nigricans at Brushy Creek Lake, Iowa, from April and August 2015 to assess the effects of jaw deformities on population characteristics. Survival of Largemouth Bass with and without a jaw deformity was estimated using a Cormack–Jolly–Seber mark–recapture model where mortality was estimated using group effect of jaw type on survival and recapture probability [S(g) p(g)]. 95% confidence interval that did not overlap indicated significant differences between groups. Table includes the recapture input file used in the analysis.

Available: https://doi.org/10.3996/JFWM-21-096.S6 (22 KB DOCX)

We thank Savanah Fernholz for assistance in the field and numerous bass tournament organizations in Iowa for participating with this project. We would also like to thank the journal reviewers and Associate Editor for taking the time and effort to review the manuscript. We appreciate the valuable comments and suggestions, which helped to improve the quality of the manuscript. Funding for this project was provided by an Iowa State University Presidential Wildlife Initiative grant.

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