Surveys Growth and Mortality of Invasive Flathead Catfish in the Tidal James River, Virginia Corbin D. Hilling,* Aaron J. Bunch, Jason A. Emmel, Joseph D. Schmitt, Donald J. Orth C.D. Hilling, D.J. Orth Department of Fish and Wildlife Conservation, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 A.J. Bunch Virginia Department of Game and Inland Fisheries, Charles City, Virginia 23030 J.A. Emmel SOLitude Lake Management, Charlottesville, Virginia 22901 J.D. Schmitt US Geological Survey, Lake Erie Biological Station, Sandusky, Ohio 44870 Abstract Invasive species are a major threat to biodiversity of native fishes in North America. In Atlantic coastal rivers of the United States, large catfishes introduced from the Gulf of Mexico drainages have become established and contributed to native species declines. Flathead Catfish Pylodictis olivaris were introduced to the Chesapeake Bay drainage in the 1960s and 1970s in the James and Potomac river systems in the eastern United States. Diet studies have found James River Flathead Catfish function as apex predators and are known to consume at-risk Alosa spp. To limit further range expansion and impacts to native species, resource management agencies need information on population characteristics to support population assessments and management plan development. Thus, we examined temporal trends in growth rates and estimated total instantaneous mortality for tidal James River Flathead Catfish collected by Virginia Department of Game and Inland Fisheries from 1997 to 2015. Parameters of the von Bertalanffy growth model with length-at-age observations pooled across sampling years were estimated as L ‘ ¼ 1,059 mm, k ¼ 0.231/y, and t 0 ¼ 0.55 y. Flathead Catfish growth differed among sampling years, especially for the years 2007 and 2014, which had the largest sample sizes. However, there were no obvious temporal trends in growth trajectories. James River Flathead Catfish tend to grow much faster than most populations used in development of the relative growth index, but the species is known to grow faster in its nonnative range. Consequently, scientists and managers should use caution when applying growth indices if native and nonnative populations are not expressly considered in development of the index. We estimated total instantaneous mortality as Z ¼ 0.50 and mean natural mortality from six estimators as M ¼ 0.30. A lack of older individuals in the population means that mortality rates may be overestimated as a result of gear selectivity or ongoing maturation of the population. These data provide information to support future work examining the species in the James River and development of population models to evaluate management strategies and management plans. Keywords: Chesapeake Bay; Pylodictis olivaris; relative growth index; weight; von Bertalanffy Received: May 2, 2019; Accepted: July 20, 2019; Published Online Early: July 2019; Published: December 2019 Citation: Hilling CD, Bunch AJ, Emmel JA, Schmitt JD, Orth DJ. 2019. Growth and mortality of invasive Flathead Catfish in the tidal James River, Virginia. Journal of Fish and Wildlife Management 10(2):641–652; e1944-687X. https://doi.org/ 10.3996/052019-JFWM-033 Copyright: All material appearing in the Journal of Fish and Wildlife Management is in the public domain and may be reproduced or copied without permission unless specifically noted with the copyright symbol &. Citation of the source, as given above, is requested. Journal of Fish and Wildlife Management | www.fwspubs.org December 2019 | Volume 10 | Issue 2 | 641 Downloaded from http://meridian.allenpress.com/jfwm/article-pdf/10/2/641/2462054/052019-jfwm-033.pdf by guest on 12 June 2020
Transcript
Surveys
Growth and Mortality of Invasive Flathead Catfish in theTidal James River, VirginiaCorbin D. Hilling,* Aaron J. Bunch, Jason A. Emmel, Joseph D. Schmitt, Donald J. Orth
C.D. Hilling, D.J. OrthDepartment of Fish and Wildlife Conservation, Virginia Polytechnic Institute and State University, Blacksburg, Virginia24061
A.J. BunchVirginia Department of Game and Inland Fisheries, Charles City, Virginia 23030
J.A. EmmelSOLitude Lake Management, Charlottesville, Virginia 22901
Invasive species are a major threat to biodiversity of native fishes in North America. In Atlantic coastal rivers of theUnited States, large catfishes introduced from the Gulf of Mexico drainages have become established and contributedto native species declines. Flathead Catfish Pylodictis olivaris were introduced to the Chesapeake Bay drainage in the1960s and 1970s in the James and Potomac river systems in the eastern United States. Diet studies have found JamesRiver Flathead Catfish function as apex predators and are known to consume at-risk Alosa spp. To limit further rangeexpansion and impacts to native species, resource management agencies need information on populationcharacteristics to support population assessments and management plan development. Thus, we examined temporaltrends in growth rates and estimated total instantaneous mortality for tidal James River Flathead Catfish collected byVirginia Department of Game and Inland Fisheries from 1997 to 2015. Parameters of the von Bertalanffy growth modelwith length-at-age observations pooled across sampling years were estimated as L‘¼ 1,059 mm, k¼ 0.231/y, and t0¼0.55 y. Flathead Catfish growth differed among sampling years, especially for the years 2007 and 2014, which had thelargest sample sizes. However, there were no obvious temporal trends in growth trajectories. James River FlatheadCatfish tend to grow much faster than most populations used in development of the relative growth index, but thespecies is known to grow faster in its nonnative range. Consequently, scientists and managers should use cautionwhen applying growth indices if native and nonnative populations are not expressly considered in development of theindex. We estimated total instantaneous mortality as Z¼ 0.50 and mean natural mortality from six estimators as M¼0.30. A lack of older individuals in the population means that mortality rates may be overestimated as a result of gearselectivity or ongoing maturation of the population. These data provide information to support future work examiningthe species in the James River and development of population models to evaluate management strategies andmanagement plans.
Received: May 2, 2019; Accepted: July 20, 2019; Published Online Early: July 2019; Published: December 2019
Citation: Hilling CD, Bunch AJ, Emmel JA, Schmitt JD, Orth DJ. 2019. Growth and mortality of invasive Flathead Catfishin the tidal James River, Virginia. Journal of Fish and Wildlife Management 10(2):641–652; e1944-687X. https://doi.org/10.3996/052019-JFWM-033
Copyright: All material appearing in the Journal of Fish and Wildlife Management is in the public domain and may bereproduced or copied without permission unless specifically noted with the copyright symbol &. Citation of thesource, as given above, is requested.
Journal of Fish and Wildlife Management | www.fwspubs.org December 2019 | Volume 10 | Issue 2 | 641
The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views of theU.S. Fish and Wildlife Service.
Invasive species are considered one of the greatestthreats to native fishes (Cambray 2003; Dudgeon et al.2006). Fishes have been moved outside their nativeranges for various reasons, including aquaculture,aquaria, biological control, forage enhancement, bait,and establishment of new fisheries (Padilla and Williams2004). Although species are often moved to providehuman benefits, unexpected consequences can result(Moyle et al. 1986). Nonnative species are consideredinvasive if they exacerbate economic or biological losses,and invaders can facilitate native fish declines throughcompetition, habitat manipulation, hybridization, para-sitism, and predation (Gozlan et al. 2010; Vilizzi et al.2014). Consequently, invasive species can be importantpredictors of regional native fish declines (Light andMarchetti 2007; Hermoso et al. 2011).
In many Atlantic coastal rivers of the United States,large-bodied catfishes have been introduced via sanc-tioned stockings, holding pond escapes, illegal introduc-tions by the public, or unintentional introductionthrough contaminated stockings of similar species(Burkhead et al. 1980; Brown et al. 2005; Bonvechio etal. 2011; Schloesser et al. 2011). Blue Catfish Ictalurusfurcatus and Flathead Catfish Pylodictis olivaris have beentransported to the Atlantic slope for their food qualityand popularity as sportfish (Kwak et al. 2011). Althoughthese species are native to North America, their naturaldistributions are largely restricted to many drainages ofthe Gulf of Mexico (Jackson 1999; Boschung and Mayden2004), but Flathead Catfish’s native range may alsoinclude the Great Lakes (Stauffer et al. 2016). Thesespecies exhibit relatively flexible life-history strategiesand habitat requirements; therefore, they have becomeestablished in many river systems and reservoirs alongthe Atlantic coast. The presence of these large nonnativecatfishes has generated concern for native speciesconservation. Nonnative Flathead Catfish populationsare particularly concerning because of their piscivorousfeeding habits (Jolley and Irwin 2003; Pine et al. 2005).Consequently, Flathead Catfish may have a greaterimpact per capita on native fishes than Blue Catfish(Schmitt et al. 2017, 2019a, 2019b). Further, FlatheadCatfish has a wide nonnative range on the Atlantic slopewith established populations from Georgia to theDelaware River basin (Brown et al. 2005; Kaeser et al.2011). Published works have implicated Flathead Catfishin native fish declines (Thomas 1993; Dobbins et al.2012), and modeling exercises have predicted up to a50% decline in native fish biomass in other Atlantic rivers(Pine et al. 2007).
Flathead Catfish entered the Chesapeake Bay water-shed when the species was introduced into the Potomacand James river systems in the 1960s and 1970s (Jenkinsand Burkhead 1994). The Chesapeake Bay is of greateconomic and cultural importance, but many importantspecies, especially diadromous fishes, have declined as aresult of habitat alteration, dams, and overfishing (Houde2006; McBride 2006). The presence of Flathead Catfishmay present an additional challenge for these species.Schmitt et al. (2017) reported that Flathead Catfishconsumed at-risk Alosa spp. during their spring spawn-ing migrations in the James River. Further, James RiverFlathead Catfish have been characterized as apexpredators based on trophic level estimation (Schmitt etal. 2019a). The species has also recently expanded itsrange into the Pamunkey River, possibly with the help ofanglers (Schmitt et al. 2019a). The Chesapeake BayProgram Sustainable Fisheries Goal ImplementationTeam Executive Committee created the Invasive CatfishTask Force in 2012 to develop and recommendmanagement options to respond to invasive catfishrange expansions in the Chesapeake Bay (Invasive CatfishTask Force 2014). However, no region-wide managementschemes have been adopted. Currently, Flathead Catfishharvest is unrestricted with no creel or size limits belowthe fall line of Virginia coastal rivers, but angling groupsare interested in preservation of trophy catfishingopportunities. Consequently, conflicting preferences ofstakeholder groups and biological uncertainty havemade management of Chesapeake Bay catfishes chal-lenging. Management plans are needed to effectivelyreduce Flathead Catfish population sizes and limitcolonization of new areas; however, no documentedestimates exist for many life-history characteristics in theJames River to guide management strategy evaluations.Further, we lack information on how these populationcharacteristics have changed over time.
Information on population rate functions (growth,mortality, and recruitment) helps managers monitorpopulations and generate management strategies (Kernsand Lombardi-Carlson 2017). Growth information isimportant to understand stock production (Dutil andBrander 2003) and is incorporated in many stockassessment models (Francis 2016; Lorenzen 2016).Further, coupling growth and mortality information canreduce the likelihood of growth overfishing (Haddon2011). The goal of this study was to improve ourunderstanding of invasive Flathead Catfish growth andmortality in the tidal freshwater portions of the JamesRiver based on data collected over a 19-y period. Wewere interested in estimating growth to determinewhether differences exist over temporal scales, as wellas average mortality rates for the period. Although
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recruitment is of interest, data suitable to evaluaterecruitment of this population are unavailable. Thesedata will provide useful inputs for simulation models toevaluate fishing levels needed to reduce populationsizes. In addition, this study helps us understand howgrowth should be considered in those models over time.
Study Site
The James River flows east across Virginia from theAppalachian Mountains to the Coastal Plain where itconverges with the Chesapeake Bay near Norfolk,Virginia. The James River is the longest river entirely inVirginia and its watershed makes up .25% of the landarea of the Commonwealth (Jenkins and Burkhead 1994).At Richmond, Virginia, the James changes from a rocky,high-gradient river to a tidal river system. The tidal JamesRiver forms a subestuary of the Chesapeake Bay andtransitions from freshwater to brackish water movingtoward its mouth. Elevated nutrient loads make theJames River a highly productive river, despite recentdeclines in point-source nitrogen and phosphorus(Bukaveckas and Isenberg 2013). The subestuary sup-ports many recreational and commercial fisheries,including blue crab Callinectes sapidus, Striped BassMorone saxatilis, and Largemouth Bass Micropterussalmoides. The James River has a long history ofnonnative catfish introductions, beginning with theintroduction of Channel Catfish Ictalurus punctatus inthe late 1800s, followed by Blue and Flathead Catfish inthe 1960s and 1970s (Jenkins and Burkhead 1994).Burkhead et al. (1980) reported Flathead Catfish enteredthe James River in 1965 when a holding pond wasflooded at the Hog Island Wildlife Management Areareleasing approximately 50 individuals. The James Rivernow supports a Flathead Catfish population (Schmitt etal. 2017, 2019a), in addition to an established and denseBlue Catfish population (Bunch et al. 2018; Fabrizio et al.2018; Hilling et al. 2018; Schmitt et al. 2019b). FlatheadCatfish largely inhabit areas upstream of Hopewell,Virginia, whereas the range of Blue Catfish extendsfurther downstream into more saline waters with higheroverall densities in the system. The current study focusedon the tidal-freshwater sections of the James River whereelectrofishing surveys have monitored populations ofnative (e.g., White Catfish Ameiurus catus) and nonnativecatfishes.
Methods
Data collectionWe examined data from Virginia Department of Game
and Inland Fisheries (VDGIF) electrofishing surveystargeting Flathead Catfish. Flathead Catfish–specificsurveys occurred within the known species distributionin the tidal James River (fall line at Richmond, Virginia toHopewell, Virginia) at fixed stations periodically from1997 to 2015 to provide hard parts for ageing. Personnelfrom VDGIF sampled Flathead Catfish during or near themonth of June; a time when Flathead Catfish seem to be
particularly susceptible to capture by electrofishing inthe system (RS Greenlee, VDGIF, personal communica-tion). Personnel collected fish using a boat-mountedSmith-Root 9.0 GPP electrofishing unit, low-frequency(15 Hz) pulsed direct current, and a weighted 7-minsolated dropper cable with a distal 1-m section ofexposed braided stainless cable acting as the anode(Greenlee and Lim 2011). Fish were netted by a crewoperating two chase boats to increase capture efficiency(Daugherty and Sutton 2005). Scientists from VDGIFmeasured length (mm), weight (g), and extracted lapillarotoliths from all collected fish.
Age estimationScientists processed lapillar otoliths collected during
Flathead Catfish sampling according to the proceduresof Buckmeier et al. (2002), with a notable modification tothe slide-mounting technique. Scientists mounted oto-liths parallel to the slide, dorsal side down, with theposterior half encased in Crystalbonde 509 adhesive.They left the anterior end overhanging the longest edgeof the slide. Once the adhesive was fully cured, they usedthe slide edge as a guide while grinding the otolith(using wetted 600 grit, then 1,200 grit, waterproofsandpaper on a Buehler Metaserv 2000 grinder–polisher)until the nucleus and annuli were visible in thetransverse plane (Greenlee and Lim 2011).
An experienced technician read otoliths, and a seniorbiologist conducted secondary reads on a subsample of30% of the fish collected for each year. Biologistsperformed secondary reads using a nonrandom, strati-fied method that helped identify common patterns inmisreading ageing structures, such as reader drift(Campana 2001). The first reader stratified individual fishbased on sequential order of reading; and the secondaryreader examined fish from the beginning, middle, andend of the sequence. If 150 fish were aged in a givenyear, a secondary reading would occur on 45 otolithsfrom the first 15, middle 15, and last 15 fish examined bythe first reader. If each age class was not represented inthe second readings, then technicians selected addition-al samples for secondary readings. Disagreement amongreaders was rare, but was alleviated using the secondotolith from the same fish to reach a consensus age.
GrowthWe described growth of Flathead Catfish using the
von Bertalanffy growth model fitted to length-at-ageobservations (Data S1, Supplemental Material) by collec-tion year (1997, 2006, 2007, 2011, 2014, and 2015). Thevon Bertalanffy growth model describes length at age tas
Lt ¼ L‘ 1� e�kLðt�t0LÞh i
þ e; e~ð0;r2Þ;
where L‘ is average maximum length, kL is the Brodygrowth coefficient describing how quickly L‘ is reached,and t0L is the theoretical age when length is equal to
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zero, and errors (e) followed an additive structure (Quinnand Deriso 1999; Haddon 2011). We examined diagnosticplots and compared with those generated from use of amultiplicative error structure to determine which ap-proach was most appropriate (Ogle 2016). We fitted vonBertalanffy growth models using nonlinear least-squaresregression using a Gauss–Newton algorithm (R CoreTeam 2015). We chose starting values for nonlinearregression based on parameters of the Flathead Catfishstandard growth equation and adjusted them asnecessary when convergence issues arose (L‘ ¼ 1266.5mm, k ¼ 0.103/y, t0 ¼ 0.05 y; Jackson et al. 2008). Todetermine whether growth differed by year, we com-pared growth between years using Kimura’s likelihoodratio test (Kimura 1980; Haddon 2011). The age structureof yearly samples differed, so we truncated the data setfor comparison to ensure the age ranges examined wereconsistent. We compared growth by year via pairwiseKimura likelihood ratio tests (a ¼ 0.05) of year-specificvon Bertalanffy growth models and year-aggregatedmodels with length-at-age observations pooled betweentwo collection years.
To understand how James River Flathead Catfishgrowth in length compares with other populations, wecompared our data with published growth standardsand fitted growth curves from other studies. Jackson etal. (2008) developed a relative growth index (RGI) forFlathead Catfish, providing a standard length equationallowing for comparison with Flathead Catfish acrosstheir range. Using the standard length equation, wedetermined whether Flathead Catfish growth in theJames River was faster or slower than the ‘‘average’’population (RGI ¼ 100) and estimated the percentilerange representative of the population. Kwak et al.(2006) examined the differences in native and nonnativeFlathead Catfish growth, reporting that nonnativepopulations typically grow faster. Further, reservoirpopulations may grow faster than river populations, sowe graphically compared our temporally aggregated vonBertalanffy model with fitted von Bertalanffy curves from7 native and 11 nonnative river populations (Table 1;Data S1, Supplemental Material). We only includedpopulations from studies where ages were assignedusing otoliths to avoid any biases that may skew growthrates related to underestimation of ages using spines(Olive et al. 2011).
We also examined gravimetric growth as parametersof the weight–age relationship are used in other models,such as empirical natural mortality and consumption tobiomass ratio estimators (Djabali et al. 1993; Lorenzen1996; Palomares and Pauly 1998). We estimated gravi-metric growth using the von Bertalanffy growth modelparameterized for weight at age t as,
Wt ¼ W‘ 1� e�kwðt�t0wÞh ib
ee; e~Nð0;r2Þ;
where W‘ is the average maximum weight, kw describeshow quickly W‘ is reached, t0w is the theoretical age
when weight equals zero, b is the allometric growthparameter, and errors were assumed to follow amultiplicative structure (Hilborn and Walters 1992;Haddon 2011). Starting values were based on our bestjudgement because we did not encounter other studiesestimating the weight-at-age model for Flathead Catfishin the literature. The parameters of the weight–agerelationship are often difficult to estimate (Quinn andDeriso 1999), so we pooled data from all years becauseof small sample sizes in some years. Spurious estimatesof b led us to estimate the parameter outside of theweight-at-age model using the allometric growthequation,
W ¼ aLb;
where W is weight, L is the length, and a and b areparameters estimated using linear regression of thenatural logarithms of length and weight (Quinn andDeriso 1999).
MortalityAlthough we examined growth over time, a temporal
analysis of mortality seemed inappropriate based on anincreasing trend in maximum age over the samplingperiod. Consequently, we estimated total instantaneousmortality (Z) using a weighted catch curve based on allaged fish from 1997 to 2015 (Smith et al. 2012). The firstage of full recruitment was the age with the greatestcatch in the sample (Smith et al. 2012), whereas the lastage considered in catch curve regression was the oldestage with �5 fish (Miranda and Bettoli 2007). The catchcurve was weighted based on a preliminary catch curvefitted to determine age-class weightings and a secondary
Table 1. Flathead Catfish Pylodictis olivaris populationspresented in the graphical assessment of native and nonnativegrowth in comparison with the nonnative population in tidalJames River, Virginia, from 1997 to 2015.
Location Status Reference
Altamaha River, GA Nonnative Kaeser et al. 2011
Apalachicola River, FL Nonnative Massie et al. 2018
Cedar River, IA Native Massie et al. 2018
Coosa River, AL Native Sakaris et al. 2006
Des Moines River, IA Native Massie et al. 2018
Flint River, GA Nonnative Kaeser et al. 2011
Ichawaynochaway Creek, GA Nonnative Kaeser et al. 2011
Iowa River, IA Native Massie et al. 2018
Little Pee Dee River, SC Nonnative Bonvechio et al. 2016
Llano River, TX Native Massie et al. 2018
Lumber River, NC Nonnative Kwak et al. 2006
Neuse River, NC Nonnative Kwak et al. 2006
Northeast Cape Fear River, NC Nonnative Kwak et al. 2006
Ocmlgee River, GA Nonnative Sakaris et al. 2006
Satilla River GA Nonnative Bonvechio et al. 2016
Susqehanna River, PA Nonnative Massie et al. 2018
Tallapoosa River, AL Native Sakaris et al. 2006
Upper Mississippi River Native Steuk and Schnitzler
2011
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fitting based on those weightings to estimate Z (Ogle2016). We also estimated instantaneous natural mortality(M) based on empirical estimators developed by Pauly(1980; Table 2), Hoenig (1983), Djabali et al. (1993),Jensen (1996), Lorenzen (1996), and Cubillos et al. (1999).We based our selection of M estimators on thoseexamined by Maceina and Sammons (2016), given thesimilarities in data available. We omitted three estimatorsexamined by Maceina and Sammons (2016), includingthe Quinn and Deriso (1999) estimator, because ofseemingly arbitrary selection of the parameter describingthe proportion of fish reaching maximum age; the Chenand Watanabe (1989) estimator because of poorperformance in the evaluation by Kenchington (2014);and the Kenchington (2014) estimator because it wasderived as an estimator of Z and we were unclear how tocalculate its inputs appropriately. We estimated inputsfor life-history traits for M estimators from age andgrowth studies discussed above, and calculated meanannual temperature for the Pauly (1980) estimator fromthe nearest continuously (every 15 min) recording U.S.Geological Survey water gage (USGS streamgage02035000, U.S. Geological Survey 2019) at Cartersville,Virginia (~75 km upstream of the fall line at Richmond).Water temperature data were available beginning inOctober 2007 at the Cartersville gage; therefore, wecalculated the mean from 1 January 2008 to 31December 2015. Based on the estimated Z from catch-curve regression and the mean M from six naturalmortality estimators, we estimated fishing mortality as F¼ Z�M. We conducted all analyses in R version 3.1.3 (RCore Team 2015).
Results
GrowthWe estimated average individual growth of the
population with data pooled from all collection yearsusing the von Bertalanffy growth model as L‘¼ 1,059 (SE¼ 29) mm, kL¼ 0.231 (SE¼ 0.016)/y, and t0L¼ 0.55 (SE¼0.093) y. When estimating growth by collection year,there were no obvious trends over time in growthparameters (Table 3) or trajectories (Figure 1). There was
a nonsignificant increasing trend (simple linear regres-sion, F¼ 3.437; df¼ 1,4; P¼ 0.137) in maximum observedage by sampling year (Table 3). Diagnostic plotsgenerally supported the use of an additive errorstructure although some heteroscedasticity was evidentfor 2014 and 2015 (Data S1, Supplemental Material).
The youngest fish among all sample years were age-2,whereas all samples had fish as old as 7 y. Consequently,the age range 2–7 y was used for growth comparisons.Of the 15 pairwise comparisons we examined for growthmodels, 10 were significantly different and 5 were not(Table 4). Statistically similar growth years were notalways nearest to each other temporally. Often, therewere significantly different years of growth betweenyears with similar growth. The years with the two largestsample sizes (2007 and 2014) were significantly differentfrom all other years we compared.
James River Flathead Catfish scored consistently highon the RGI. Of the mean RGI values by year, none werebelow 100 (Table 5). However, RGI of a single age-1 fishcollected in 1997 was estimated as 74. Many mean RGIvalues were above the 95th percentile. All age classeswere above the 50th percentile when multiple fish werecollected. Examination of fitted growth curves alsosupported fast growth in the James River (Figure 2).
We modeled gravimetric growth for 584 FlatheadCatfish collected from 1997 to 2015 ranging in age from1 to 15 y. We first estimated the parameters of theallometric growth equation as a¼ 1.97 3 10�6 (SE¼1.653 10�7) and b¼ 3.281 (SE¼ 0.013). Using b as a constant,we estimated parameters of the weight-parameterizedvon Bertalanffy growth model as W‘ ¼ 20,670 (SE ¼3,324) g, kw ¼ 0.198 (SE ¼ 0.019)/y, t0w ¼�0.026 (SE ¼0.073) y. The weight-parameterized von Bertalanffymodel appears to fit data well, although older fish (age.7 y) were relatively uncommon in the sample (Figure3).
MortalityExamination of the age structure of the entire sample
revealed age-3 fish were most abundant and served asthe starting point for catch curve regression. Afterexcluding age classes with fewer than five fish, age-10fish were the oldest included in mortality estimation. Weestimated total instantaneous mortality as Z¼ 0.50 (95%
Table 2. Empirical natural mortality (M) estimators andestimates from life-history information pooled from the period1997 to 2015 for tidal James River, Virginia, Flathead CatfishPylodictis olivaris. Tmax is the maximum observed age, W‘ isaverage maximum weight, k is the Brody growth coefficient, t0
is the theoretical time when size is zero from growth modeling,and s¼mean annual water temperature (estimated as 16.28C).
Source Equation M
Pauly 1980 M ¼ 0:9849 3 L�0:279‘ k0:6543s0:4634 0.37
Hoenig 1983 M ¼ 4:31T�1:01max 0.26
Djabali et al. 1993 M ¼ 0:8598W�0:0302‘ k0:5280 0.33
Jensen 1996 M ¼ 1:5k 0.35
Lorenzen 1996 M ¼ 3:00W�0:288‘ 0.17
Cubillos et al. 1999 M ¼ 4:31½t0 � lnð0:05Þk�1��1:01 0.31
Table 3. Sample sizes (n), maximum observed age, andparameter estimates for von Bertalanffy growth models fit tolength-at-age observations for Flathead Catfish Pylodictisolivaris from the tidal James River, Virginia, by year. Standarderrors for parameter estimates provided in parentheses.
Year n Max. age L‘ kL t0L
1997 55 7 1,100 (124) 0.27 (0.07) 0.64 (0.18)
2006 90 8 1,091 (80) 0.24 (0.04) 0.28 (0.17)
2007 130 9 966 (58) 0.29 (0.05) �0.22 (0.22)
2011 102 11 1,079 (58) 0.22 (0.03) 0.16 (0.22)
2014 141 15 1,035 (33) 0.28 (0.02) 0.91 (0.13)
2015 66 9 952 (60) 0.28 (0.05) 0.21 (0.19)
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CI ¼ 0.32–0.68). Consequently, annual survival wasestimated as S ¼ 0.61 (95% CI ¼ 0.51–0.73). Naturalmortality estimates varied from 0.17 to 0.37 (Table 2)with a mean of 0.30. Fishing mortality was subsequentlycalculated as F ¼ 0.20.
Discussion
Although Blue Catfish receive a great deal of attentionin the Chesapeake Bay region as a result of theirabundance and spatial range, Flathead Catfish likelyhave also generated concern for at-risk fishes. JamesRiver Flathead Catfish are known to consume at-riskAlosa spp. as they migrate to and from their spawninggrounds (Schmitt et al. 2017, 2019a, 2019b), posing athreat to Alosa spp. conservation. Consequently, man-aging Flathead Catfish is imperative to limit a source ofadditional mortality to these at-risk fishes. Prior to thisstudy, synthesized information on James River FlatheadCatfish was limited to diet information collectedopportunistically during a study focused on Blue Catfish(Schmitt et al. 2017, 2019a) and von Bertalanffyparameter estimates derived from a Flathead Catfishgrowth meta-analysis (Massie et al. 2018). The current
study provides critical information on population char-acteristics for use in population assessments to under-stand fishing levels necessary to reduce Flathead Catfishabundance and potentially reduce predation mortalityon at-risk species. Further, growth and mortality param-eters are components of consumption estimators (e.g.,Palomares and Pauly 1998) that can be coupled withbiomass and diet information to estimate total con-sumption of prey species.
Flathead Catfish in the James River appear to growfaster in terms of length than most populationsevaluated in published indices, but may exhibit nearerto average growth when compared with other nonnativepopulations. Several studies have examined the growthof nonnative and native Flathead Catfish, with findingsindicating that nonnative populations generally growfaster (Kwak et al. 2006; Sakaris et al. 2006; Rypel 2014).However, Massie et al. (2018) found no difference in L‘
Table 4. Matrix of likelihood ratio test results from pairwisecomparisons of von Bertalanffy growth models fit to length-at-age data collected from 1997 to 2015 for tidal James RiverFlathead Catfish Pylodictis olivaris in Virginia. The upperquadrant provides F-values, whereas the lower quadrantprovides P-values where significance was assessed at a ¼0.05. Significant differences denoted by *.
Year 1997 2006 2007 2011 2014 2015
1997 — 4.383 26.869 6.490 25.849 8.076
2006 0.223 — 26.575 2.954 36.929 6.953
2007 ,0.001* ,0.001* — 35.364 81.787 19.299
2011 0.090 0.399 ,0.001* — 24.063 2.048
2014 ,0.001* ,0.001* ,0.001* ,0.001* — 24.752
2015 0.044* 0.073 ,0.001* 0.563 ,0.001* —
Figure 1. Total length-at-age observations for Flathead Catfish Pylodictis olivaris in the tidal James River, Virginia (black circles) withfitted von Bertalanffy growth models (red lines) by year from 1997 to 2015.
Table 5. Mean relative growth index (RGI) values by age andsampling year (standard deviation in parentheses) for tidalJames River Flathead Catfish Pylodictis olivaris collected byVirginia Department of Game and Inland Fisheries from 1997 to2015. Percentile range for RGI values shown as shades of red.Empty boxes indicate no fish were collected.
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and k between introduced and native populations. Wefound that James River Flathead Catfish grew faster thanrange-wide standards, but studies used in the develop-ment of the RGI for Flathead Catfish featured mostlynative populations (Jackson et al. 2008). Interpreting theresults from growth standards should be done withcaution if there is reason to believe native and nonnativepopulations for a given species exhibit differences ingrowth and were not expressly considered in develop-ment of the index.
Despite annual differences in length-based growthrates, we did not find any discernable temporal trends ingrowth for James River Flathead Catfish. Invasionecology would lead us to predict that growth woulddecline over time as nonnative populations becomeestablished, densities increase, and resources becomeless abundant. Studies on nonnative fishes have reportedfaster growth at the invasion front (or recently intro-duced areas) than for fishes in previously establishedhabitats (Bøhn et al. 2004; Feiner et al. 2012; Gutowskyand Fox 2012; Azour et al. 2015; Kornis et al. 2017).However, growth rates during establishment may notalways follow expected patterns (Masson et al. 2016,2018). Similarly, Massie et al. (2018) reported that growthparameters were not related to time since introductionfor 13 nonnative Flathead Catfish populations, but theauthors suggested they lacked populations early enoughin the invasion process to observe differences. This mayalso influence the current study where Flathead Catfishwere likely introduced 32 y before the first age andgrowth samples were collected in 1997. Regardless of alack of simple trends in growth, the current studysupports heterogeneity among years, which could berelated to annual differences in climatic variables such assunshine fraction, wind speed, evapotranspiration rates,and flooding (Kwak et al. 2006; Schramm and Eggleton2006; Jones and Noltie 2007; Rypel 2011; Massie et al.2018). Sample sizes may have been too small in someyears to observe statistical differences in growth—wefound the 2 y with the largest sample sizes weresignificantly different from all other years we compared.Further, small sample sizes may have caused impreciseestimation of growth parameters and maximum age
(Kritzer et al. 2001). Despite small sample sizes, signifi-cant differences in growth by year suggest that futurepopulation assessments should consider temporalgrowth heterogeneity when size-based Flathead Catfishecology is of interest.
James River Flathead Catfish are subject to highermortality rates than many other native and nonnativepopulations. Published studies on native populationsreported Z-estimates ranging from 0.15 to 0.33 (Sakariset al. 2006; Makinster and Paukert 2008; Marshall et al.2009). Kwak et al. (2006) estimated Z ranged from 0.17 to0.22 for three nonnative populations in North Carolina,which was similar to estimates for the nonnativepopulation in Ocmulgee River, Georgia (Z¼ 0.23; Sakariset al. 2006). The Little Pee Dee River in South Carolinahad slightly higher Z estimates at 0.36 (Bonvechio et al.2016). Our estimates of instantaneous total mortality (Z¼0.50) were most similar to an introduced Flathead Catfishpopulation that underwent removals to reduce abun-dances where Z ranged from 0.46 to 0.74 (Sakaris et al.2006; Bonvechio et al. 2011, 2016). Our estimates of Mappear to be higher than published values—M for anative population in Lake Wilson, Alabama, averaged0.13 from five empirical estimators (Marshall et al. 2009).Further, Marshall et al. (2009) estimated much lowerfishing mortality (F¼ 0.06) than the present study. JamesRiver Flathead Catfish may experience high mortalityrates as a result of commercial and recreational harvest(including testing of commercial electrofishing; Trice andBalazik 2015) or environmental factors characteristic ofdynamic estuarine systems.
In addition, mortality rates in the current study maybe overestimated as a result of a truncated agestructure. This could be due to continued maturationof the population as it approaches stabilization (Sakariset al. 2006). Further, the lack of older fish in the samplemay be related to size-based selectivity of low-frequency electrofishing for Flathead Catfish based onanecdotal evidence summarized by Bodine et al. (2013).Future studies may consider the addition of trotlinesampling to target large individuals (Bodine et al. 2013)to determine whether large, older individuals weremissed with low-frequency electrofishing. In addition,Flathead Catfish exist above the fall line in the James
Figure 2. Fitted von Bertalanffy growth curves for 7 native and11 nonnative Flathead Catfish Pylodictis olivaris river popula-tions. The fitted curve for the tidal James River, Virginia, wasestimated from the current study with length-at-age observa-tions pooled over six sampling years from 1997 to 2015.
Figure 3. Weight-at-age observations for Flathead CatfishPylodictis olivaris in the tidal James River, Virginia (black circles)with a fitted weight parameterized von Bertalanffy growthmodel (red line) with observations pooled from 1997 to 2015.
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River and currently we lack information on how fishabove and below the fall line are related. FlatheadCatfish is a highly mobile species that makes longseasonal migrations (Vokoun and Rabeni 2005). It ispossible that fish could migrate into or out of tidalportions of the river, violating assumptions of negligiblelosses and additions due to movement. If managed forfisheries, overestimates of mortality may mean thatFlathead Catfish are less productive and less resilient tofishing than analyses imply (Harry 2018). However, ifmanaged as an invasive species, overestimates ofmortality rates could lead to ineffective strategies toreduce population sizes if actual mortality rates arelower than believed or desired. Consequently, assess-ments evaluating management strategies should in-clude sensitivity analyses to evaluate the influence ofpotential biases in mortality estimation.
Flathead Catfish present a threat to native species inthe James River, but information is limited. Publishedstudies cover topics of diet and distribution using datathat were opportunistically collected. Future workdirected toward Flathead Catfish behavior and ecologywould help managers understand the species within theJames River and beyond. Further, Flathead Catfish areexploited as part of the James River commercial catfishfishery, but available harvest data lacked details onspecies identity until recently, when Blue Catfish weresegregated (Virginia Marine Resources Commission,unpublished data). Many scientists and the public havecalled for increased harvest to reduce invasive catfishabundances. Flathead Catfish removal efforts in theSatilla River, Georgia, have resulted in lower biomass pereffort in electrofishing runs, as well as truncated sizestructures (Bonvechio et al. 2011). Therefore, it may beprudent to require species-level reporting for catfishharvest moving forward. Information on Flathead Catfishharvest would help fishery analysts estimate populationsizes and evaluate responses to harvest.
Invasive catfish management in the Chesapeake Bayfeatures numerous conflicts among user groups, be-tween user groups and management agencies, andbetween management agencies. Using growth andmortality estimates presented in the current study asinputs in population assessments, consumption estima-tors, and simulation studies may help managementagencies see what the possibilities and uncertainties arefor management of Flathead Catfish in the ChesapeakeBay region. This may facilitate more discussions withstakeholders and begin the process of defining objec-tives for management to promote cooperation andsatisfaction among diverse stakeholder groups anddevelopment of management plans.
Supplemental Material
Please note: The Journal of Fish and Wildlife Managementis not responsible for the content or functionality of anysupplemental material. Queries should be directed to thecorresponding author for the article.
Data S1. Data presented and analyzed in the current
study organized in three sheets (Fish, Growth Parameters,and Diagnostic Plots). Fish: Biological data for FlatheadCatfish Pylodictis olivaris from the tidal James River,Virginia from 1997 to 2015. Biological data presented arefrom Flathead Catfish–specific low-frequency electrofish-ing surveys, targeting areas known to support thespecies. The spreadsheet provides total length (mm),weight (g), otolith ages (Age_O, years), and the year inwhich the individual was collected. Growth Parameters:Growth parameters from the von Bertalanffy growthmodels (Linf¼ L‘) used in graphical assessment of nativeand nonnative growth. Location refers to the riversystem from which otoliths were sampled from FlatheadCatfish. Min.age and Max.age are the youngest andoldest ages presented in the referenced paper, respec-tively. Estimates of t0 from Bonvechio et al. (2016) wereprinted in error (T. F. Bonvechio, Georgia Department ofNatural Resources, personal communication) and havebeen corrected in the Growth Parameters sheet. Diag-nostic Plots: Diagnostic plots to assess assumptions ofhomoscedasticity and normality from von Bertalanffygrowth model fitting with additive and multiplicativeerrors by year fish were collected.
Found at DOI: https://doi.org/10.3996/052019-JFWM-033.S1 (198 KB XLSX).
Acknowledgments
C.D. Hilling was supported by a Virginia Sea GrantGraduate Research Fellowship. D.J. Orth was supportedin part by the U.S. Department of Agriculture throughthe National Institute of Food and Agriculture Programand Virginia Tech University. Data collection and analyseswere supported by the Virginia Department of Game andInland Fisheries through a Federal Aid in Sport FishRestoration Grant from the U.S. Fish and Wildlife Service.We are indebted to Catherine Lim and others from theVDGIF Age and Growth Lab who processed and agedotoliths. We thank K.E. Keretz, three anonymous review-ers, and the Associate Editor for comments thatimproved a previous version of this manuscript.
Any use of trade, product, website, or firm names is fordescriptive purposes only and does not imply endorse-ment by the U.S. Government.
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