Fishing Mortality and Habitat Loss Affect Largemouth Bass Fishery in the Potomac River (Maryland)

Largemouth Bass Micropterus salmoides offer sport fish angling opportunities throughout most of the continental United States (Claussen 2015). The density and distribution of Largemouth Bass affect angling success (Matthias et al. 2014). Conservation strategies for Largemouth Bass currently aim to reduce negative impacts due to angling (Gilliland and Schramm 2009) and long-term changes in habitat condition (Allen et al. 2002). Reducing negative impacts from long-term changes in habitat conditions may not improve a fishery (Allen et al. 2002), particularly when those projects do not target the whole watershed (Cowx et al. 2004). This makes it necessary to manage angler behavior by regulating harvest or possession, regulating fishing gear, and providing catch-and-release guidelines. However, because Largemouth Bass fisheries are often catch-and-release in practice, evidence that managing angler behavior results in better conservation of Largemouth Bass populations is largely lacking.

Largemouth Bass population size declines with reduced recruitment, defined here as birthed fish that later sexually mature. Factors that may periodically reduce recruitment include a shorter growing season because of late-season nest building (Post et al. 1998), poor growth during summer because of dense or absent submerged aquatic vegetation (SAV; Miranda and Pugh 1997; Maceina and Slipke 2004; Love 2011, 2015), poor growth during summer leading to overwinter mortality (Miranda and Hubbard 1994; Post et al. 1998), antagonistic interactions with invasive species such as Northern Snakehead Channa argus (Love et al. 2015), and possibly nest failure because of angling (Gwinn and Allen 2010). Of these, the density and distribution of SAV is among the most well studied for influence on survival, growth, and distribution of Largemouth Bass (Miranda and Pugh 1997; Allen et al. 2002; Paukert and Willis 2004). Intermediate SAV cover (10–25%) correlates with higher numbers of overwintered juvenile Largemouth Bass (Miranda and Pugh 1997) and possibly greater recruitment. Body growth benefits from drier, early summers (Maceina and Bettoli 1998; Rypel 2009). Because of the overwhelming influence of habitat conditions on recruitment, recruitment does not depend on the number of reproducing adults (Allen et al. 2011) or the number of stocked fish (Buynak et al. 1999).

Declines in the density and distribution of adult Largemouth Bass reduce population size. A reduction in adults is caused by increases in annual mortality, which can limit angling success in a fishery (Pauly et al. 2002; Driscoll et al. 2007). Annual mortality that includes natural and fishing mortality can range between 31% and 84% (Allen et al. 2002). Natural mortality is usually a small fraction of annual mortality (e.g., average of 20% for unexploited populations in Hudson River drainage; Maceina and Sammons 2016), but can be greater, with some estimates near 50% (Beamesderfer and North 1995).

To help reduce some aspects of fishing mortality, the Bass Anglers Sportsman Society (BASS) circulates guidelines for catch-and-release fishing for black bass (Gilliland and Schramm 2009). Catch-and-release fishing for Largemouth Bass lowers annual mortality (Seidensticker 1975; Allen et al. 2008), but some aspects of fishing still cause delayed death of adults. Mortality of adults after their release can influence a fishery when catch is substantially greater than harvest and exploitation is 15% or more (Allen et al. 2004). Population growth by approximately 10 to 30% per year offsets losses due to high levels of mortality (Hayes et al. 1995). Generally low levels of exploitation or high population growth may account for the paucity of studies that indicate catch-and-release guidelines are needed to prevent declines of Largemouth Bass fisheries.

We hypothesized that a decline in abundance of Largemouth Bass in the Potomac River resulted from a period of greater fishing mortality associated with an increase in the number of anglers and catch rates and followed by habitat loss that limited recruitment and recovery of the population. We used a combination of fishery-independent (e.g., fish surveys) and fishery-dependent (e.g., reported catch) data to examine trends in catch of juveniles and adults. These results were examined in light of annual differences in mortality, distribution of SAV, and fishing effort. Fishing effort was estimated with annually reported catch rates by tournament directors and the number of anglers participating in tournaments. Because tournament angling is a small fraction of the fishery, our estimates of fishing effort do not realistically reflect total fishing effort in the fishery for any year. For that reason, we did not definitively parse fishing mortality into components of harvest or catch-and-release mortality. Instead, we examined the relationships of fishing mortality and SAV to population growth rates to determine when management actions may become necessary to protect the fishery.

The Potomac River is the second largest drainage in the Chesapeake Bay watershed (Figure 1) and is approximately 652 km long and drains approximately 38,000 km² from West Virginia, Maryland, Virginia, and the District of Columbia. Beginning in 2012, widespread reports to Maryland Department of Natural Resources (MD DNR) suggested that the abundance of Largemouth Bass was declining in the Potomac River (Drake 2014). The Largemouth Bass fishery in the Potomac River was excluded from the top 100 bass fisheries in the United States in 2014 and remains absent in 2016 (BASS 2016). Until that time the tidal freshwater portion of the Potomac River was the most consistently fished Largemouth Bass fishery in Maryland (MD DNR 2015a).

The SAV in the Potomac River was annually monitored in two ways: 1) aerially using light detection and ranging and 2) visually during boat electrofishing surveys conducted in the fall. Aerial images were obtained by Virginia Institute of Marine Science (Orth et al. 2014). Fish surveys were conducted by MD DNR at randomly chosen sites (see below). Dominant species of SAV included hydrilla Hydrilla verticillata, Eurasian milfoil Myriophyllum spicatum, and water celery Vallisnaria americana. Submerged aquatic vegetation estimates were not available for 2011 because neither the MD DNR Tidal Bass Survey nor Virginia Institute of Marine Science aerial survey was conducted because of the occurrence of tropical storms.

Angler effort was measured as the number of anglers participating in Potomac River tournaments staged in Maryland since 1989 (Table S1). The total number of anglers participating in the Largemouth Bass fishery in the Potomac River was not measured. The number of tournament anglers was a measurable index of annual participation in the fishery and was monitored using a registration system by Smallwood State Park, the most popular Maryland location for tournaments on the Potomac River. Beginning in 2004, the World Wide Web was also searched weekly by using search engines to identify Potomac River tournaments, their dates, the number of anglers participating, and the catch. Tournaments were also attended each weekend by MD DNR until 2011 (March–November). After 2011 the MD DNR began requiring tournament directors to report participation and catch data. Data were reported through 2015.

Catch was reported as the number of Largemouth Bass weighed per angler for the tournament. Weighed catch per angler-hour (CPAH) was determined as the total number of fish brought to the scale and weighed by anglers divided by effort for the tournament. Tournament effort was the number of anglers in the tournament multiplied by the hours fished by the anglers during the tournament day. Average CPAH was calculated for tournaments that allowed a five-fish creel limit per angler per day and a minimum total length of 305 mm for Largemouth Bass. Data were excluded for tournaments held during the spawning season (March 1–June 15) because the minimum total length (TL) to possess Largemouth Bass differs from the rest of the year. The CPAH was averaged among tournaments for each year and a standard deviation (SD) in CPAH for each year was computed. The CPAH did not include all black bass caught by an angler throughout the day but still provided an index of fish weighed over time. The average catch weighed per angler-hour was not used to infer trends in population size. Anglers likely varied in experience (Heermann et al. 2013) and were not likely distributed randomly (Matthias et al. 2014), both of which confound repeatability of methods from year to year and complicate inference of annual trends in catch as related to population size. Additionally, angler catches may be invariant with fish density (Post et al. 2002) and be independent of relative abundance or population size.

The following indices were calculated from MD DNR autumn electrofishing surveys (September–October; 1999–2015): proportional size distribution (PSD) for 381 mm or longer fish (PSD₃₈₁), catch per electrofishing hour (CPH) for all Largemouth Bass caught, CPH for juveniles, CPH for age 1+, the proportion of high-quality sites with juveniles, and annual mortality (MD DNR 2015b; Table S2). The daytime survey targeted shorelines using boat electrofishing (Smith-Root, 5.0 generator-powered pulsator or 9.0 generator-powered pulsator; 340 to 680 V) at a subset of all possible shoreline sites in tidal freshwater of Maryland's Potomac River. Settings were adjusted by MD DNR biologists to ensure that fish electrotaxis was achieved before each site was sampled. Power output was not standardized among habitats (Miranda 2005), but an oscilloscope was used in 2014 and 2015 to determine that power output using settings by staff were sufficient for generating power that was necessary to elicit electrotaxis (Bonar et al. 2009).

Sites were randomly chosen each year from a stratified design and the pool of sites within a stratum did not change over time. We used two strata: 1) moderate- to high-quality habitat sites; and 2) poor-quality habitat sites. Habitat quality for Largemouth Bass was defined with habitat suitability criteria (Markham et al. 2002; Love 2011) that included availability of SAV and submerged structure, water chemistry, and salinity. Proportionately more moderate- to high-quality sites (67%) were chosen than poor-quality sites with uniformly poor, homogenous habitat.

The number of sampled sites annually ranged between 31 and 55 between 1999 and 2015. When stunned from electrofishing, Largemouth Bass were removed from the water and placed within a well-oxygenated live well for recovery. Each fish was measured (mm) and weighed (g), then released after the site was fully sampled. The PSD was calculated as the proportion of fish that were 381 mm or longer in an annual survey divided by the total number of fish that were greater than 200 mm in the survey (Bonar et al. 2009). Age of each fish was assigned using an age at length key (ALK; Fridrikson 1934) that was generated using Largemouth Bass collected from tidal rivers of the Chesapeake Bay watershed (spring through fall, 2005–2015; N = 412). An ALK is a series of length ranges that each include measured age probabilities for each length range. The ALK is used when conducting stock assessments to estimate the fraction of ages represented in total catch (Goodyear 1997). Many methods for assigning age probabilities to length ranges have been used, including length frequencies (AGEKEY; Isermann and Knight 2005) and probability theory (Salthaug 2003). These methods can produce consistent results for predicting ages for fish of unknown age (DeVries and Frie 1996; Bettoli and Miranda 2001; AGEKEY, Isermann and Knight 2005).

We used length ranges that reflected differences in growth among ages to create the ALK. Fish grew up to 200 mm TL within their first year, 50 mm per year until age 4, and then 25 mm per year thereafter (Fewlass 1996). Length ranges for the ALK included: 200 mm or less (n = 3); 201–250 mm (n = 9); 251–300 mm (n = 9); 301–350 mm (n = 40); 351–375 mm (n = 35); 376–475 mm (n = 261); 476–500 m (n = 33); and 500 mm or greater (n = 23). Because of high overlap in length at age between 376 mm and 475 mm, fish within that length range were combined to generate a single age probability distribution. Age of a fish was determined using otoliths (Buckmeier and Howells 2003; Maceina et al. 2007). Otoliths were dried, cracked along the transverse plane, lightly sanded with wet sandpaper, and examined for annulus number with a dissecting scope equipped with an AcuView digital camera.

In most cases the observed distribution of ages within the length range was used to predict unknown ages for a fish. A nonparametric probability distribution of measured ages was assigned for each length range (Gerritsen et al. 2006), but only when TL was less than or equal to 475 mm. Because of smaller sample sizes and greater diversity of ages for fish greater than 475 mm, predicting age from a nonparametric distribution of otolith ages was not practical. The accuracy of an ALK to estimate ages for fish in a population depends on the number of fish aged (n) within the length range (Isermann and Knight 2005; Coggins et al. 2013). To improve age prediction for fish greater than 475 mm in total length, we used Poisson distributions with central tendency based on mode age within the length range for lengths greater than 475 mm.

The estimate of age from an ALK may be wrong because of density-dependent growth and interannual differences in density or habitat conditions. Growth rates were not correlated with habitat conditions, which varied over time and among rivers of the Chesapeake Bay (Love 2011). Growth rate estimates using scales were similar among 12 riverine populations in the Chesapeake Bay watershed (1987–1993; analysis of covariance, F_12,91 = 1.07, P = 0.40; unpublished analysis by J.W.L. using data from Fewlass 1996). Growth rates for Largemouth Bass in Maryland were also similar over time from 1949–1959 (68 mm/y; mean length of fish aged from scales; Elser 1962) to 2001–2005 (60–80 mm/y; length difference between marked period and recaptured period; MD DNR 2015b) and to 2015 (67 mm/y; computed from growth coefficient of von Bertalanffy growth model; MD DNR 2015a). We tested the accuracy of the key for the 412 individuals aged with otoliths. Most (68%) were correctly predicted to age within 1 y of true age and at least 95% of fish were predicted to within 3 y. The parameter of interest here, instantaneous rate of mortality (Z, see below), did not statistically differ when fish were aged with the ALK as compared with that when fish were aged with otoliths (Z_otolith = −0.49, SE = 0.05, 95% CI = 0.10; Z_ALK = −0.42). Therefore we considered the ALK a useful tool for predicting unknown ages for fish with a known TL.

A CPH of all Largemouth Bass caught at a site was calculated by dividing total catch by seconds spent electrofishing, then multiplied by 3,600 s to generate a CPH. The CPH was averaged among sites sampled within a year and a SD was computed for each year. Annual trends in CPH for juveniles (≤ 200 mm TL) and age 1+ Largemouth Bass (> 200 mm TL) from high-quality sites was examined by similarly plotting average CPH for juveniles and age 1+ fish (with standard error) for each year (1999–2014). Annual differences in juvenile CPH calculated during fall can reflect annual differences in recruitment (Fuhr et al. 2002). We used a two-sided nonparametric trend analysis (Mann–Kendall test) to statistically test whether there was a monotonic trend in CPH for age 1+ Largemouth Bass. This analysis did not assume residuals were normally distributed or a linear trend. A positive or negative value of the Mann–Kendall Z statistic indicated that the data would increase or decrease with time, respectively.

Average juvenile CPH was computed with a geometric mean to reduce influence of values that could vary by an order of magnitude. The geometric means did not include sites when juveniles were not collected. To include information in our analyses regarding the absences of juvenile fish at a site, the proportion of sites with at least one juvenile was calculated for each year. Presence–absence data have been used to estimate spawning stock biomass for pelagic species (Mangel and Smith 1990). With the level of sampling effort used here, these data may not imply a decline in population size but highlight local extirpations (Strayer 1999). In the context of this paper, these data were used to evaluate a trend in the proportion of habitats used for reproduction by Largemouth Bass, which reproduces juveniles that maintain site fidelity in embayments or reservoirs (Copeland and Noble 1994; Jackson et al. 2002).

Annual mortality was calculated from total instantaneous mortality rate (Z), which was the negative slope from a nonparametric regression of age (independent variable) and the instantaneous change in number of individuals caught within consecutive age cohorts (2+ y; dependent variable); the number was transformed by the natural log before analysis. A nonparametric regression was used to minimize the influence of outliers by ranking residuals rather than observations and using a median to calculate the slope parameter, Z (Rousseeuw and Leroy, 1987). Fish younger than age 2 were excluded to help minimize the influence of variable recruitment on the estimate of Z. Highly variable recruitment in crappie (Pomoxis spp.) led to estimates of annual mortality that were within 10% of true values (Allen 1997). The Z was calculated for each year of the survey; Z estimates with 95% confidence intervals that included 0 were excluded from further analysis. Once estimated, Z was used to determine annual mortality (A) as A = 1 −e^−Z. For presentation, A was converted to a percent by multiplying A by 100. Annual mortality included both percentages of natural mortality and fishing mortality, which were considered additive (Allen et al. 2008).

Average natural mortality rate (m) was estimated as 0.34 for the Potomac River population (Love et al. 2015) following Pauly (1980) using von Bertalanffy growth parameters determined from age at length data. This value was slightly lower than that in a review by Allen et al. (2008; m = 0.49) and greater than that estimated by Maceina and Sammons (2016) for unexploited populations in Hudson River drainage (m = 0.23). Maceina and Sammons (2016) found that using Pauly (1980) and parameter estimates from a von Bertalanffy growth equation (ages ≤ 15 y) produced a m estimate that was similar to empirically derived estimates from unexploited populations of Largemouth Bass. Natural mortality rates can vary among populations and among years, and the accuracy of its estimate can be negatively affected by observation error because of sampling bias. Thus we did not assume a fixed value of m. The SD in m among years (SD = 0.07; Love et al. 2015) was used to reflect uncertainty from natural variation in m among years. To include that uncertainty in a parameter estimate for annual fishing mortality, we used Monte Carlo methods to generate parameters (Haddon 2001). Cortés (2002) similarly used published accounts of demographic traits for populations of sharks to determine variance in the parameter with Monte Carlo methods and ultimately correlate demographic parameters with population growth rates for 38 species of sharks. We used Monte Carlo methods to randomly draw m from a normal distribution 1,000 times assuming m was 0.34 and SD was 0.07. For each random draw, annual natural mortality (v) as v = 1 − e^−m and annual fishing mortality (u) as u = A − v were calculated. Averages and variances were calculated for v and u for each year.

Fishing mortality is an indirect estimate that depends on realistic estimates of m and A. A more direct measure of exploitation of the resource was also used, but it pertains only to tournament angling. Exploitation of the resource, relative to the population size, was calculated by multiplying relative catchability of bass (q) by the number of tournament anglers per year (N). Relative catchability of fish by anglers was estimated by dividing the average weighed CPAH among tournaments in a year by the relative population abundance (i.e., CPH) in a year. Therefore, q was CPAH/CPH and relative exploitation was q × N. Sinclair (1998) similarly described relative fishing mortality as a ratio of commercial catch to relative population abundance. Because tournament anglers practice live release, we considered this calculation as relative exploitation rather than relative fishing mortality. The metric was used to evaluate annual differences in relative exploitation and to qualitatively compare that trend with that in u. This measure of relative exploitation did not describe all aspects of exploitation for the population, but was expected to illustrate general annual trends.

We examined the relationships of changes in relative abundance to SAV area in tidal freshwater of the Potomac River and u. Changes in relative abundance between years were measured as lambda, or population growth rate: average CPH for all Largemouth Bass caught in year t + 1 was divided by CPH for year t. Lambda was negative when population size declined, assuming fish were equally catchable among years. Before calculating lambda, variance in CPH among years was smoothed using a moving average with a window of 3, resulting in the effect of each years' CPH being a mean of the three surrounding values and including the current value. This smoothing was done to provide a general depiction of change in average CPH over time and generate a value for 2012 that was missing because of unavailable data for 2011. Statistical analyses were conducted using Systat (version 10.0, Systat Software Inc.).

The relationships between lambda, SAV area, and u were examined using Spearman's correlation analysis (rho) and a contour plot. A contour plot is a three-dimensional relationship depicted in two-dimensional space and composed of isolines or contour lines. The contour lines depict interpolated values of the dependent variable (lambda) with changes in independent variables, SAV and u. The plot was used to determine how lambda varied with changes in SAV and u and to examine management targets for u that allow for positive population growth (lambda = 1.1) at average levels of SAV in tidal freshwater of the Potomac River. The contour plot was generated with SigmaPlot (version 11.0, Systat Software, Inc.).

Total instantaneous mortality for the population of Largemouth Bass was estimated across years at −0.66 (SE = 0.03), yielding an A of 48.1%. Annual mortality ranged between 41 and 57% for most years (ages 2–10; number of age cohorts = 6–9). Values for A between 2013 and 2015 were not included for further analysis because the estimated slope for each included 0 in its 95% confidence interval (2013: −1.38, 1.09; 2014: −1.38, 1.50; 2015: −1.58, 0.12). For years between 1999 and 2013 u ranged between 16 and 39% (average = 28%, SD = 6%) and was relatively high for 2008 and 2009 (Figure2a). Relative exploitation was high in 2007 (Figure 2b) and tournament anglers reportedly weighed more fish between 2007 and 2010 than previous years (Figure 3), both highlighting a period of popular fishing activity. Relative exploitation remained high after 2010 because q was high rather than because the number of anglers participating in the fishery was high. Relative abundance of age 1+ Largemouth Bass was greater when relative exploitation was low (< 300) than when relative exploitation was high (> 400; Figure 2c). There was a decline in relative abundance of age 1+ Largemouth Bass (Figure 4a; Z = −3.4, P = 0.001) and relative abundance was consistently low since 2010. This decline may be related to a loss of fish larger than 381 mm TL. The PSD₃₈₁ from MD DNR fall surveys declined in 2008 and 2009, indicating a loss of fish larger than 381 mm between 2008 and 2009 relative to stock size (Figure 4b). The decline in PSD₃₈₁ may also result from an increase in catch of fish that range between stock size (200 mm) and 381 mm, but catch of age 1+ fish was not noticeably greater during those years (Figure 4).

The relative abundance of juveniles was consistently low after 2011 compared with previous years (Figure 5a). The proportion of surveyed sites occupied by juveniles began declining before 2011 (Figure 5b). The PSD₃₈₁ increased from 2012 to 2015 and catch of age 1+ was relatively low, suggesting a decline in the number of fish that were 200 to 381 mm. Shortly after 2011 the area of SAV in tidal freshwater portions of the Potomac River sharply declined by 57% (Figure 6). In 2014 and 2015, there was increased area of SAV and increased catch of juveniles and the proportion of sites with juveniles relative to 2013 (Figure 6), as well as a decrease in PSD₃₈₁, suggesting improved recruitment.

The area of SAV and fishing mortality influenced population growth rates, though independently were not highly correlated with population growth rate. The area of SAV was positively correlated with population growth rate (rho = 0.35), whereas fishing mortality was negatively correlated with population growth rate (rho = −0.46). When fishing mortality exceeded 35%, population growth was negative (Figure 7). The impact of fishing mortality on population growth depended on the area of SAV. Despite the level of fishing mortality, population growth tended to decline when the area of SAV was less than 1,500 ha. As the area of SAV increased above 2,000 ha, population growth rates were stable (lambda = 1.0) or positive (lambda > 1.0) at higher levels of fishing mortality. At exceptionally high levels of SAV cover (> 2750 ha), population growth rates were negative, suggesting that a moderate level of SAV cover is beneficial for population growth. To maintain a population growth rate of 10% per year (lambda = 1.1) with an approximate average area of SAV across the study period of 2,000 ha, u was 28%.

Relative abundance of Largemouth Bass declined after a period of relatively high levels of fishing mortality and exploitation in the Potomac River. At average SAV cover we estimated that fishing mortality should not exceed 28% for a sustainable fishery in the Potomac River. This estimate is given with caution because population growth rates were not highly correlated with fishing mortality or SAV. Moreover, a target for fishing mortality may be a moving target when unexpected habitat disturbances reduce population growth. Because the quality of a Largemouth Bass fishery may decline when fishing mortality or exploitation is high and population growth is low (Gwinn and Allen 2010), fishery management may benefit by conservatively managing when using reference points for exploitation.

Our data indicated that a greater fraction of the population was caught and released in the late 2000s, possibly resulting in greater exploitation of the resource. The number of fish caught, weighed, and released by tournament anglers (per angler hour) was greater in the late 2000s than in the 1990s or early 2000s. Similarly, data from access intercept surveys also suggest that total catch per hour by nontournament anglers was greater in the late 2000s than in previous time periods. In 1990 Potomac River catch rates of Largemouth Bass reported by recreational anglers were 0.28 bass/angler hour (Fewlass 1991), which is lower than that estimated in 2009 during a similarly designed creel survey (0.85 bass/angler hour; unpublished data, U.S. Fish and Wildlife Service, Table S3). This nearly threefold increase suggests an increase in number of Largemouth Bass caught by recreational anglers. Approximately 10% of Largemouth Bass caught and released in a year may die (Allen et al. 2002; Bartholomew and Bohnsack 2005; Love et al. 2015). Catch-and-release mortality may be caused by hooking injuries (Cooke et al. 2003; Wilde and Pope 2008) and hastened because of handling time and air exposure (Siepker et al. 2007). In addition to catch-and-release mortality, fishing mortality was due to annual differences in the level of harvest. Approximately 6% of anglers reportedly harvested 1 bass/angler day in 1990 (Fewlass 1991), whereas 6.7% of anglers reportedly harvested 3 bass/angler day in 2009 (unpublished data, U.S. Fish and Wildlife Service, Table S3). Although only a small fraction of the population may be harvested from the Potomac River (< 20% of population size; Fewlass 1991), an increase in harvest rate per angler indicated that controlling harvest may continue to play a role in fishery management for Largemouth Bass. Finally, fishing mortality was due to tournament angling. The retention, weighing, and release of bass can increase mortality risks (Schramm et al. 2006; Ostrand et al. 2011; Kerns et al. 2012). Additionally, adult Largemouth Bass can be removed from areas when transported many miles from distant streams or waterways (Wilde 2003), leading to a redistribution of adults because of short-term stockpiling at release sites (< 7 days; Brown et al. 2015). The proportion of fish weighed during a tournament is small relative to population size. In 2009, multiplying the estimated number of tournament anglers with an average catch rate of 3.3 bass/d (MD DNR 2015a) yielded approximately 12,000 bass weighed and released, which was only 7% of the population size estimated in 1990 for the Potomac River (Fewlass 1991). Even if harvest and catch-and-release mortality affect a small proportion of the population, future offspring may be negatively affected if the most fit individuals are caught by anglers (Sutter et al. 2012) and those fish later die or fail to reproduce.

The decline in relative abundance of age 1+ fish was followed by a decline in the relative abundance of juveniles after 2011. Reproduction and recruitment likely depended on habitat conditions rather than spawning stock size (Post et al. 1998; Allen et al. 2011; Shaw and Allen 2016). Nest failure owed to some forms of angling occurred less often when refugia was present (Siepker et al. 2009). Such refugia may include SAV. Here, we suggest that the loss of SAV was a major factor associated with the decline in recruitment. The density of recruits for Florida bass Micropterus floridanus was lower in habitats with about 30% cover of SAV than about 65% cover in lentic environments (Shaw and Allen 2016). In the Potomac River we found a positive relationship between the estimated number of juvenile Largemouth Bass during fall surveys (1999–2009) and area of SAV from 0 to 25 ha (R² = 0.32, P < 0.0001; unpublished data, J.W.L.) for 39 identified spawning coves in the Potomac River (Love 2015). In addition to a loss of SAV, average density of SAV between 2010 and 2014 increased slightly to between 3.3 and 3.5 (average = 3.4) and was greater than between 2002 and 2008 (range 2.7–3.2; average = 2.9) on a 4.0 scale. Dense SAV species in the Potomac River, such as invasive hydrilla (Orth et al. 2010), may yield fewer benefits on growth and survival of young Largemouth Bass than those afforded by SAV species with naturally low or intermediate densities (Maceina and Slipke 2004; but see Stahr and Shoup 2016).

There are other habitat conditions that may have influenced reproduction and recruitment. Environmental pollution of endocrine disruptors may have negatively affected reproduction and contribution of gametes by some males in the population (Iwanowicz et al. 2009; Yonkos et al. 2014). Invasive species such as Northern Snakeheads may pose risks that negatively affected recruitment (Love et al. 2015). Severe overwinter water temperatures may have caused the death of some less fat offspring (Post et al. 1998). Finally, a history of intense fishing pressure during the spawning season may have led to evolution of less aggressive males defending nests (Sutter et al. 2012; Philipp et al. 2015) and to stressed adults (Siepker et al. 2007) with a lower quality of gametes, as observed for Rainbow Trout Oncorhyncus mykiss (Campbell et al. 1992). Empirical evidence is lacking to support any of these influences on Largemouth Bass fisheries of Maryland.

Population growth and abundance were likely affected by other factors than those measured here. Abundance may decline if natural mortality increased because of increased levels of predation or starvation (Post et al. 1998; Fullerton et al. 2000), greater levels of overwinter mortality for juveniles (Hurst 2007), and disease (Schramm et al. 2006; Neal et al. 2009). Prey resources may have declined in the Potomac River, but neither body condition nor somatic growth rate of Largemouth Bass has declined (MD DNR 2015a). Body condition may decline from normal levels near 1.0 to approximately 0.82 when disease is prevalent (Overton et al. 2003). Body condition for Largemouth Bass from the Potomac River has not declined (on average) for fish greater than 150 mm (relative weight average = 1.01, SD = 0.02; unpublished data, J.W.L.). Therefore, body condition data indicate that neither starvation nor disease is commonplace among adults. Both prey resources and fish health may be negatively affected by pollution, but eutrophication and pollution have lessened on the river (potomacreportcard.org; Potomac Conservancy 2016) and signs of disease from pathogens and occurrence of Largemouth Bass virus (Grizzle et al. 2002) are at low levels (unpublished data, J.W.L.; http://dnr2.maryland.gov/fisheries/Pages/pathogen-map.aspx). Additionally, no outbreak of disease from Largemouth Bass virus has ever been reported for the population in the tidal Potomac River. Although changes in the aforementioned factors may have influenced natural mortality without being observed or measured, evidence that an increase in natural mortality explains apparent changes in relative abundance of adult bass is lacking.

Abundance of sport fish in recreational fisheries can increase as sources of mortality become less intense (Allen et al. 2008). Such changes in abundance may go unnoticed when angler catches are more influenced by environmental factors (Heermann et al. 2013) than fish density (Post et al. 2002). Angler catch rates reported during this study were high throughout a period when fishery-independent surveys indicated a decline in relative abundance, which could be explained if adults were more concentrated when SAV (i.e., fish-aggregating structure) was more limited in availability. In our study we provide justification that management actions were necessary to protect the fishery. Limiting harvest is unlikely to be a sole solution to increase abundance because of an increasing trend to catch and release bass (Allen et al. 2008). Other actions that improve post-release survival could include requiring catch-and-release guidelines (e.g., Cooke and Suski 2005; Gilliland and Schramm 2009). More specifically, these actions entail: 1) clarified, consistent language; 2) established mechanisms to share guidelines; 3) creation of an incentive program; and 4) improved efficiency in information exchange between stakeholders and agency staff. Language clarified by writing with anglers will facilitate understanding of the language. Existing guidelines can differ among states and their implementation varies (Pelletier et al. 2007). Mechanisms to share guidelines include websites and social media, e-mail, video media, and State Fishing Guides. Maryland Department of Natural Resources developed infrastructure to deliver electronic information to black bass anglers who buy licenses and tournament directors who obtain permits (Schramm et al. 1991). States such as Florida have progressively adopted a waiver system that creates an incentive for tournament anglers to adopt certain catch-and-release guidelines by allowing them an exemption to possess a creel limit of fish longer than the maximum size allowed. Other incentives could include certificates and participation in stakeholder-agency management committees. To ensure guidelines are used and are effective, a partnership of key stakeholders and agency staff facilitates an exchange of information. Brewer (2002) further noted that such coalitions may stimulate broader interest in species conservation. Maryland Department of Natural Resources recently created a technical advisory group with key personnel from the fishing industry and the agency. Development of these coalitions germinates thought that can produce enforceable action for a culture of anglers who share the interests of conservation with agency staff but often feel disenfranchised in management decisions.

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Table S1. Fishery-dependent data for Largemouth Bass Micropterus salmoides tournaments on the Potomac River (1999–2015). These data included tournament chapter, date, number of fish caught, number of anglers, hours fished in a day, and a calculated catch per angler hour. Found at DOI: http://dx.doi.org/10.3996/022016-JFWM-015.S1 (157 KB XLSX).

Table S2. Fishery-independent data for Largemouth Bass Micropterus salmoides population in the Potomac River (1999–2015) as collected during the Maryland Department of Natural Resources' Tidal Bass Survey. These data included site name, date of sampling, duration of electrofishing (s), length of the shoreline site (m), and number of Largemouth Bass caught. The latter was further partitioned by the number of juveniles (≤ 200 mm) and the number of adults (> 200 mm). The catch per hour (CPH) was computed for the total number of fish, the number of juveniles, and number of adults. Found at DOI: http://dx.doi.org/10.3996/022016-JFWM-015.S2 (96 KB XLSX).

Table S3. Creel survey data collected for Largemouth Bass Micropterus salmoides caught by anglers during a 2009 angler-intercept survey conducted at five access points in tidal freshwater areas of Maryland's Potomac River by U.S. Fish and Wildlife Service. Access points included Fort Washington (FW), Marshall Hall (MH), Slavens Ramp (SR), Sweden Point (SP), and Friendship Landing (FL). 41 KB Found at DOI: http://dx.doi.org/10.3996/022016-JFWM-015.S3 (41 KB XLSX).

Reference S1. [BASS] Bass Anglers Sportsman Society. 2016. Toledo Bend retains top spot on Bassmaster's 100 Best Bass Lakes list. Bass Anglers Sportsman Society, Birmingham, Alabama. Found at DOI: http://dx.doi.org/10.3996/022016-JFWM-015.S4 (351 KB PDF).

Reference S2. Drake J. 2014. Bass are in trouble in the Potomac. The Enterprise, B-4. Found at DOI: http://dx.doi.org/10.3996/022016-JFWM-015.S5 (27.342 MB PDF).

Reference S3. Fewlass F. 1991. Study V: investigations of largemouth bass populations inhabiting Maryland's tidal waters. In Davis R, editor. Annapolis, Maryland: Statewide Fisheries Survey and Management, Federal Aid in Sport Fish Restoration, Project F-29-R. Found at DOI: http://dx.doi.org/10.3996/022016-JFWM-015.S6 (5152 KB PDF).

Reference S4. Fewlass F. 1996. Study V: investigations of largemouth bass populations inhabiting Maryland's tidal waters. Annapolis, Maryland: Final Report, F-48-R Maryland Department of Natural Resources Freshwater Fisheries Division. Found at DOI: http://dx.doi.org/10.3996/022016-JFWM-015.S7 (3469 KB PDF).

Reference S5. Gilliland G, Schramm HL Jr. 2009. Keeping bass alive: a guidebook for tournament anglers and organizers. Lake Buena Vista, Florida: Bass Anglers Sportsman Society. Found at DOI: http://dx.doi.org/10.3996/022016-JFWM-015.S8. Also found at https://www.bassmaster.com/sites/default/files/bassmaster2011/imce/KeepingBassAlive_guidebook%20comp.pdf (231 KB PDF).

Reference S6. [MD DNR] Maryland Department of Natural Resources. 2015a. Survey and management of Maryland's fishery resources, annual (2015) performance report. Annapolis, Maryland: MD DNR, Federal Aid in Sport Fish Restoration, Project F-48-R-25. Found at DOI: : http://dx.doi.org/10.3996/022016-JFWM-015.S9 (8689 KB PDF).

Reference S7. [MD DNR] Maryland Department of Natural Resources. 2015b. Fishery management plan for largemouth bass Micropterus salmoides in Maryland Tidewater. Annapolis, Maryland. Found at DOI: http://dx.doi.org/10.3996/022016-JFWM-015.S10 (1400 KB PDF).

Reference S8. Elser HJ. 1962. Growth rates of Maryland freshwater fish. Annapolis, Maryland: Reference Number 62-13, Natural Resources Division, University of Maryland. Found at DOI: http://dx.doi.org/10.3996/022016-JFWM-015.S11 (2930 KB PDF).

We thank the biologists who assisted in data collection for this project, specifically Tim Groves, and Ross Williams. We also thank numerous tournament directors and anglers who have contributed meaningful data toward the study of the Potomac River fishery for Largemouth Bass. This manuscript was considerably improved by thoughtful comments made by anonymous reviewers. This work supports actions of the Maryland Department of Natural Resources' Tidal Bass Fishery Management Plan and was partially funded by the Sport Fish Restoration Act and a cooperative agreement with U.S. Fish and Wildlife Service (Agreement #16 U.S.C. 777). 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.

Any use of trade, product, website, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.