Ecology of the Alligator Gar, Atractosteusspatula, in the Vicente G1;Jerreroreservoir, Tamaulipas, Mexico

THE SOUTHWESTERNNATURALIST 46(2):151-157 JUNE 2001

ECOLOGY OF THE ALLIGATOR GAR, ATRACTOSTEUSSPATULA, IN THE VICENTE G1;JERRERORESERVOIR, TAMAULIPAS, MEXICO

.-., FRANCISCO J. GARCiA DE LEON, LEONARDO GoNzALEZ-GARCiA, JOSE M. HERRERA-CAsTILLO, KIRK O. WINEMILLER,* AND ALFONSO BANDA-VALDES

Laboratorio de Biologia lntegrativa, lnstituto Tecnol6gicode Ciudad Victoria, Boulevard Emilio Fortes Gil1301, Ciudad Victoria, CP 87010, Tamaulipas, Mexico (F]GL, LGG,]MHC) Department of Wildlife and FisheriesSciences, Texas A&M University, CollegeStation, TX 77843-2258 (KO~ Direcci6n Generalde Pescadel Gobiernodel Estado de Tamaulipas, Ciudad Victoria, Tamaulipas, Mexico (ABV) * Correspondent:[email protected]

ABsTRACf-We provide the first ecological account of the alligator gar, Atractosteusspatula, in the Vicente Guerrero Reservoir, Tamaulipas, Mexico. During March to September, 1998, the local fishery cooperative captured more than 23,000 kg of alligator gar from the reservoir. A random sample of their catch was dominated by males, which were significantly smaller than females. Males and females had similar weight-length relationships. Relative testicular weight varied little seasonally, but relative ovarian weight showed a strong seasonal pattern that indicated peak spawning activity during July and August. Body condition of both sexes also varied in a pattern consistent with late summer spawning. Fishing for alligator gar virtually ceased from October to February, when nonreproductive individuals were presumed to move offshore to deeper water. Alligator gar fed primarily on largemouth bass, Micropterus salmoides,and less frequently on other fishes. The gillnet fishery for alligator gar in the reservoir appears to be based primarily on individuals that move into shallow, shoreline areas to spawn. Males probably remain in these habitats longer than females. The dominance of largemouth bass in the alligator gar diet reflects this centrarchid's high occurrence in littoral habitats of the reservoir, and the diet of alligator gar inhabiting deeper habitats of the reservoir could be different.

RESUMEN-Son aportados por primera vez algunos aspectos ecologicos del catan, Atractosteus spatula, en la represa de la Presa Vicente Guerrero, Tamaulipas, Mexico. El trabajo fue desarrol- lado durante el periodo de marzo-septiembre de 1998, en el cuallas cooperativas pesqueras locales capturaron mas de 23,000 kg de catan de la represa. La captura estuvo dominada por machos, los cuales fueron mas pequeiios que las hembras. Ambos sexos registraron una relacion peso- longitud similar. El peso relativo de los ovarios mostro un fuerte patron estacional no asi el peso relativo de los testiculos, los cuales exhibieron poca variacion estacional. En ambos sexos el coe- ficiente de condicion vario mensualmente, indicando un rico de actividad de desove durante julio-agosto. La pesca de catan disminuyo virtualmente de octubre a febrero, cuando el catan no se reproduce y se presume que migra de la zona litoral hasta los sitios mas profundas de la represa. El catan se alimento principalmente de la lobina negra, Micropterus salmoides,y en menor porcen- taje de otros peces. La pesca de catan con redes agelleras en esta represa, se confina a individuos que se desplanza las zonas litorales someras en donde ocurre el desove. Los machos probablemente permanecen mas tiempo en estos habitats que las hembras. La dominancia de la lobina negra en la dieta del catan refleja la alta incidencia del centrarquido en habitats litorales de la represa, siendo la dieta del catan de aguas profundas de la represa probablemente diferente.

The alligator gar, Atradostcus spatula, is wide- (Suttkus, 1963; Alvarez del Villar, 1970; Wiley, ly distributed in rivers and lakes of the Missis- 1976; Lee and Wiley, 1980; Page and Burr, sippi River drainage from the lower Missouri 1991). In spite of its ancient phylogenetic his- and Ohio rivers to the Gulf Coast from the tory, comparatively little ecological research Econfina River, Florida to Veracruz, Mexico has been conducted on the gar family, Lepi- 152 TheSouthwestern Naturalist vol. 46, no. 2 sosteidae. In part, lack of interest in gars stems length (TL), interorbital distance, total weight, and from their perceived lack of value as a fishery weights of consumable meat and gonads. Fishermen resource in North America, and the belief that frequently clean their catch in the field, and the lat- these predators reduce populations of more ter measurements were made for calculation of con- d . bl . (S h . 1992) I version equations to aid future research. Length was eSIra e specIes carnecc la, . n con- . . th 11' . h. hI ~ 1 d fi d measured wIth a metrIc tape and fish measuring trast e a Igator gar IS Ig Y v",ue as a 00 . . : . . board to the nearest 1 cm, or WIth calipers to the ~sh III northeastern ,MeXIC?, where th.e spe~Ies nearest 1 mm; weights were measured with a spring IS known as the catano AllIgator gar IS an Im- balance to the nearest 1 g, or for larger quantities portant food fish consumed by at least 100 with a DETECTO series MCS dial balance to the families in the municipality of Padilla, Tamau- nearest 28 g. Gonads and digestive tracts were pre- lipas (Porras-Escamilla, 1998). Until now, no served in 10% formalin and transported to the lab- studies have been conducted on Mexican pop- oratory for examination. ulations of the alligator gar. Here we report. r:ata Analysi~To ~xamine varia~on in body con- findings from a 7-month study of population dltlon, the relationshIp between weIght and 10gloTL tr t d U. l ' ty d d . t f was estimated by using linear regression. Differences s uc ure, repro uc ve seasona I , an Ie 0.. .. th 11' . th V . G R m the weIght-length relationshIp of males and fe- e a. Igator. gar III e .,Icente uerrero. es- males were examIned. by means of analysIs. of co- erVOir III central TamaulIpas, MexIco. variance. Size at sexual maturity of each sex and reproductive activity were estimated based on macro- MATERIA1BAND METHODs-Study Area- The study scopic examination of gonads and calculations of was conducted in the Vicente Guerrero Reservoir, the gonadosomatic index (GSI): 100 X (gonad which is located within the Gulf coastal plain be- weight/body weight). Gonads were classified as 3 tween 23°45' and 24°05'N and 98°40' and 98°57'W, stages (Rossemblum et al., 1987): immature (cate- ca. 54 km northeast of Ciudad Victoria, Tamaulipas. gories I-IV), mature (V-VI), and spent. Sex was not The reservoir was constructed in 1971 by the Secre- determined in 25 cases in which gonads were im- taria de Recursos Hidraulicos to provide irrigation mature. water for 42,000 ha of cropland, as well as recrea- Following Lagler (1956), diet was evaluated using tion. The reservoir has a maximum depth of 62 m the descriptors of prey frequency of occurrence and a capacity of 5.28 X 109 m3. Its main tributaries (%F), number (%N), and volume (% V). In addi- are the Purificacion, Pilon, Grande, and Corona riv- tion, George and Hadley's (1979) index of relative ers. The region receives an average annual rainfall importance (Rl) was calculated for each prey item: of 7(J0 to 800 mm, with heaviest precipitation during summer.A verageannu al temperature IS. 24 to 26°C Rl. = (%F' + %N. + %V;)100 , , n (Secretaria de Programacion y Presupuesto, 1983). L (%1'; + %N; + % Vi) Sampling-Monthly samples of alligator gar from ;~I Vicente Guerrero Reservoir were obtained from h . th b f . . were n IS e num er 0 prey categorIes. In George commercIal catches from the Villa de Padilla Fishery d H dl ' d . h d . . an a ey s stu y, welg t was use m paceI 0 f vo-I Production Cooperative Society during 2 periods ume. (March through September 1998 and April through July 1999). Fishing effort by the cooperative was 17 . days (March), 18 days (April), 22 days (May), 16 days RESULTS-The number of alligator gar that (June), 11 days (July), 18 days (August), and 17 days we obtained each month was a function ofvar- (September), with nearly equal effort expended iation in our survey effort and the ability of each day. We surveyed their tQtal catch 1 to 4 days fishermen to catch fish with gillnets (the latter per month during 1998 for a total of 15 survey days. is a function of fish density and movement in Our survey effort during 1999 was lower, and was littoral habitats). Expressed on a per-survey-

v . . . ~.-~~ ...... 01/)01/)01/)01/)01/)01/)01/)01/)01/)01/)01/)0 i'-i'-~~O)O)OO~~C\lC\lC')C')~~I/)I/)COCOi'-i'-~ ~~~~~~~~~~~~~~~~~ Total Length (cm) FiG. l-Size frequency distribution by sex of alligator gar from Vicente Guerrero Reservoir, Tamaulipas, Mexico, 1998 to 1999 (n = 163). move offshore to deeper regions of the reser- + 27.0. We converted data for total biomass voir. During 1998, biomass of alligator gar per (kg) of alligator gar meat harvested each survey day by month was: 48.3 kg (March), month by the cooperative to the total weight 143.9 kg (April), 97.9 kg (May), 158.1 kg (kg) of whole fish harvested using the follow- (June), 72.9 kg (July), 167.0 kg (August), and ing conversion equation obtained by regres- 256.0 kg (September). sion analysis of meat (lacking skin and viscera) With r2 = 0.96, the regressionequation de- versus whole body for 121 gar: W;otalbody = scribing the relationship between total length 0.268 + 1.863Wmeat(r = 0.975, P < 0.0001,n and interorbit distancewas TL = 10.668 (ill) = 121). The total monthly harvest of gar by the cooperative was: 2,015 kg (March), 3,130 kg (April), 7,315 kg (May), 3,157 kg (June), 1,072 kg (July), 1,294 kg (August), and 5,513 kg (September). Total length of alligator gar ranged from 97 to 180 cm. Our samples were dominated by males (male = 115, female = 25, immature = 25), and males were significantly smaller than females (t-tes~o-tailed'P < 0.0001, Fig. 1). On average, females weighed 5.4 kg more than males. The minimum size of maturation for males was 95 to 100 cm TL, and for females was 125 to 130 cm TL. The weight-length re- -.- lationships for males and females were not sig- 1.9 2.0 2.1 2.2 2.3 nificantly different (ANCOVA, P > 0.05, Fig. Log(Total Length in mm) 2), with a common regression equation: lOglOW = 3.176(logloL) - 2.540, where Wis expressed FIG. 2-Regressions of weight (log body weight in in g, and Las cm TL (r" = 0.94). g) versus total length (log TL in cm) for male (n = 115) and female (n = 25) alligator gar from Vicente During March to September 1998, mean GSI Guerrero Reservoir, Tamaulipas, Mexico, 1998 to of males showed low temporal variation (1.53 1999. Regressionequation for femalesis 10g(W) = to 2.91), but mean GSI of females significantly 3.330 10g(L) - 2.863; for males 10g(W) = 2.951 increased from 2.8 in March to 11.7 in April. 10g(L) - 2.077. Mean GSI of females was relatively high 154 The SouthwesternNaturalist vol. 46, no. 2

14 3 5 ~ l ;~~~~;~o female 12 male I' \. 3 1 '"0' 10 II" .1 \,\,\, f "..,...0-.'.1

8 ' . " ~00: I . " W ,I (!) I 6 I "I 4 1,1/: , '" f 2 f-- f ' 1~~~ ~~ f '~'" 5 27 23 8 8 6 0 Mar Apr May Jun Jul Aug Sep Month FIG. 3-Monthly variation in mean gonadosomatic index (bars = ::tl SD) for male (n = 77) and female (n = 14) alligator gar from Vicente Guerrero Reservoir,Tamaulipas, Mexico, 1998. throughout the summer and declined during quency, numerical abundance, % volume, in- September (Fig. 3). The percentage of mature dex of relative importance), largemouth bass gonads increased from a minimum of 25 dur- (Micropterus salmoides,local name is lobina ne- ing March to a maximum of 87 during July gra) was the predominant prey in stomachs (Fig. 4). No spent gonads were detected dur- (Table 1). Though of less importance, shads ing March, and low percentage of spent go- (Dorosomacepedianum and D, petenense),cichlids nads was observed during May to July and Sep- (' Cichlasoma' cyanoguttatum), and catfish (Ari- tember (no gonad data were obtained for idae) also were consumed, April). Thirty-seven percent of gar gonads were spent during August, thus indicating the peak DISCussIoN-Alligator gar captured from lit- spa'wning period for this species in Vincente toral habitats of Vicente Guerrero Reservoir Guerrero Reservoir. were mainly adults with either mature or spent Fishes were the only prey identified from al- gonads. Because alligator gar are extremely dif- ligator gar stomachs. By all measures (% fre- ficult to capture during October to February,

n=6 30 26 9 9 11 100

80 (/) "C to c: 0 (!) 60 ~0 Spent "6 ~ Mature - . Immature ~ 40 !:! OJ a. 20

0 Mar May Jun Jul Aug Sep Month

FIG. 4-Monthly percentages of immature, mature, and spent gonads (male and female) in alligator gar from Vicente Guerrero Reservoir, Tamaulipas, Mexico, 1998 (n = 91). June 2001 Garcia de Leon et al.-Ecology of Atractosteusspatula 155

TABLEI-Frequency of occurrence,numerical percentage,volumetric percentage,and index of relative importance for 4 prey taxa taken from stomachsof 60 alligator gar from Vincent Guerrero Reservoir (n examined = 138,n empty= 78).

Relative Prey taxon % Frequency % Number % Volume importance Microp~s salmoides 51.5 50.0 78.0 59.8 'Cichlasoma'cyanoguttatum 15.7 12.8 8.4 12.3 Dorosomaspp. 12.9 20.9 6.4 13.4 Ariidae 1.4 1.2 0.9 1.2 Unidentified fish 18.5 15.1 6.3 13.3

we presume that these individuals move from to estimate the size of whole fish based on mea- limnetic habitats of the reservoir to littoral surements of heads found in the field or meat habitats for spawning. We found little pub- delivered to markets. For example, 7 kg of lished information on spawning behavior of meat corresponds to a fish weighing approxi- the alligator gar. May and Echelle (1968) col- mately 12.8 kg and measuring 145 cm TL. Giv- lected tw~ juvenile alligator gar in an arm of en that our best estimates of stocks currently Lake Texoma, Oklahoma-Texas, the first doc- come from the commercial catch, such con- umentation of reproduction by this species in versions will be necessary to obtain adequate a reservoir. Suttkus (1963) reported that Lepi- descriptions of alligator gar stocks. sosteusspecies spawn adhesive eggs in shallow Monthly changes in the gonadosomatic in- areas with 1 to 4 males joining a single female, dex and percentages of mature and spent go- and all departing soon afterward. A recent nads indicated a summer spawning period with study of the spotted gar (Lepisosteusoculatus) a peak during August. By comparison, the revealed seasonal movement into seasonally in- spawning period of alligator gar in Louisiana undated littoral regions of the Atchafalaya Riv- is from April to June (Suttkus, 1963). Based on er swamps in Louisiana (Snedden et al., 1999). documentation of early life stages,Echelle and The fact that male alligator gar greatly out- Riggs (1972) estimated A. spatula in Lake Tex- numbered females in the commercial catch oma, Oklahoma-Texas, spawn from January to (ratio females to males = 1:7.6) suggests that September. Evidence of spawning (spent go- males remain in the littoral zone longer than nads) was observed in Vicente Guerrero Res- females. By remaining within a spawning hab- ervoir only during June to September, so that itat, these males might be able to fertilize the alligator gar captured from March through clutches of several females in succession. May might have been individuals that moved Female alligator gar are larger than males, a into littoral habitats in search of potential sites pattern also observed for longnose gar, Lepisos- for spawning. Since at lower latitudes the teus osseus,from reservoirs in Missouri (Netsch lengthening of photoperiod during spring oc- and Witt, 1962) and Kansas (Klaassen and curs earlier, and annual temperature variation Morgan, 1974). The overall size range of Vi- is lower, it would be reasonable to assume that, cente Guerrero Reservoir alligator gar was within a species, spring spawning dates would larger than that obtained by Suttkus (1963) for be advanced and duration of the reproductive a sample of 21 alligator gar in Lake Ponchar- period would be longer (Hubbs, 1985). Al- train, Louisiana (41 to 147 cm TL). Slope of though the longer reproductive period (rela- the weight-length relationship for Vicente tive to data summarized in Suttkus, 1963) of Guerrero Reservoir alligator gar indicated pos- the lower-latitude population of alligator gar in itive allometric growth. Vincente Guerrero Reservoir follows this as- Given the high correlations we obtained for sumption, initiation of spawning was actually relationships between weight of harvested later in central Mexico (24~) than in either meat and total body weight, between body Louisiana (30 to 33°N) or Lake Texoma (Lat. weight and TL, and between TL and interor- 34~), a topic that deserves additional study. bital distance, future investigators will be able Largemouth bass, the most important food 156 TheSouthwestern Naturalist vol. 46, no. 2 and sportfish in the reservoir, is the primary permit (No. 041099-213-03) was provided by SE- prey of alligator gar from littoral zones of the MARNAP. reservoir in summer. Because we do not have diet data for alligator gar from other regions LITERATURE CITED of the reservoir and for other periods of the - year, it currently is not possible to estimate ALVAREZDEL VILLAR, J. 1970. Peces Mexicanos overall consumption of largemouth bass by al- (c~a~es). Secreraria de Industria y Comercio, I. C tfi h . d. MeXICO. Igator gar. a s were a millor letary com- d II .al .d .fi d fr I ECHELLE, A. A., AND C. D. RiGGS. 1972. Aspects of the ponent, an a maten we I enn e om a - I I... h . f (Lepis ) . ear y I.e Istory 0 gars osteus In Lak e T~ex- ligator gar stomachs were ariids. Normallyes- oma. Transactions of the American Fisheries So- tuarine inhabitants, ariid catfish have in- ciety 101:106-112. creased in abundance within the reservoir, and GEORGE,E. L., AND W. F. HADLEY.1979. Food and ariids now outnumber freshwater ictalurid cat- habitat partitioning between rock bass (Amblopli- fishes in this system (F. J. Garcia de Leon, pers. tes rupestris) and smallmouth bass (Micropterus da- obser.). Most alligator gar stomachs were emp- lomieui) young of the year. Transactions of the ty, a common finding in studies of large pisci- American Fisheries Society 108:253-261. vores. Additionally, high frequency of empty HUBBS,C. 1985. Darter reproductive seasons.Copeia h b bl . fl d b . b. 1985:56-68. stomac s pro a y was ill uence y Immo 1- I . . .11 £ 12 h S all KlAASSEN, E. H., AND L. MORGAN. 19 74. Age and aznon ill gl nets or up to . m er prey wth fl . . gro 0 ongnose gar In Tut tl e Cree kR eserVOIr,. would have digested faster, which could have Kansas. Transactions of the American Fisheries biased our dietary findings toward larger prey. Society 102:402-405. Other studies have found alligator gar feeding LAGLER,K. F. 1956. Freshwater fishery biology, Sec- on gamefish, mullet (Mugil cephalus) , crabs, ond ed. William M. Brown Publishers, Dubuque, and water birds (Suttkus, 1963). Iowa. Currently, the alligator gar population of LEE, D. S., ANDE. O. WILEY.1980. Atractosteusspatula Vincente Guerrero Reservoir does not appear (~cepede), Alligator gar. In: L~e, D. S., C. R. to be highly vulnerable to overfishing because GIlbert, C. H. Hocutt, R. E. JenkIns, D. E. McAl- commercial fishing is confined to 7 ~onths of lister,.andJ. R. Stauffer,Jr., editors. Atlas of North h d tak tl I Th I £ AmerIcan freshwater fishes. North Carolina State t e year an es mos y ma es. e arge e- M f N I H . Ral . useum 0 atura Istory, eig.h Pg. 47. mal.es, which are critical !or maintaining re- MAy, E. B., ANDA. A. ECHELLE.1968. Young-of-year crultment, were captured ill much lower num- alligator gar in Lake Texoma, Oklahoma. Copeia bers in this study. If one assumes a 1:1 sex ratio, 1968:629-630. most females are avoiding capture by the com- NETSCH,N. F., ANDA. W. WITT. 1962. Contributions mercial fishery on an annual basis. Because the to the life history of the longnose gar (Lepisosteus species is long-lived, a relatively low level of osseus)in Missouri. Transactions of the American successful recruitment should be able to main- Fisheries Society 91:251-262. tain a relatively stable population in the reser- PAGE,L. M., AND B. M. BURR.199~. A field guide t?

. A th th h d . f th n .. t freshwater fishes: North AmerIca north of MexI- VOIr. n e 0 er an ,I e sex ra 0 IS na - . . all k d d d . . b I co. Houghton MIfflIn Company, Boston, Massa- ur y s ewe towar omillanon y ma es, our c huse.tts catch data would justify c~ncern if escape of PORRAS-ESCAMILLA,P. M. 1998. Catalogo de especies spawners from the fishery was low. Clearly, de interes comercial en la produccion pesquera more field data are needed to serve as the de Tamaulipas. Memorias de Experiencia Profe- foundation for a basic model of alligator gar sional. Disertacion para Licenciatura en Biologia. population dynamics in this system. Unpublished dissertation, Instituto Tecnologico de Ciudad Victoria, Ciudad Victoria, Taumalipas. This project was funded by grant #970406019 ROSSEMBLUM,M.,J. PUDNEY,AND I. CALLARD.1987.Go- from the Sistema Regional Alfonso Reyes (SIRRE- nadal morphology, enzyme histochemistry, and YES) of the Consejo Nacional de Ciencia y Tecnol- plasma esteroid levels during the annual repro- ogia, Mexico. Our thanks go to the members of Villa ductive cycle of male and female brown bullhead de Padilla Fishery Production Cooperative Society catfish, Ictalurus nebulosusLeSueur. Journal of for logistic support in the field; also to J. H. Rodri- Fish Biology 31:325-341. guez C. for helping us with collection of data in the SCARNECCHIA,D. L. 1992. A reappraisal of gars and field. The Instituto Tecnologico de Cd. Victoria pro- bowfins in fishery management. Fisheries 17:6- vided support for lab work. A scientific collecting 12. June 2001 Garciade Leon et al.-Ecology of Atmctosteusspatula 157

SECRETARIADE PROGRAMACIONY PRESUPUESTO. 1983. H. B., and W. C. Schroeder, editors. Fishes of the Sintesis geografica del Estado de Tamaulipas. western North Atlantic. Memoirs of the Sears Coordinacion General de los Servicios Nacion- Foundation for Marine Research I, Part 3. New ales de Estadistica, Geografia e Informatica. In- Haven, Connecticut. pp. 61-88. stituto Nacional de Estadistica Geografia e Infor- WILEY.E. 0.1976. The phylogeny and biogeography matica. of fossil and recent gars (Actinopterygii: Lepisos- SNEDDEN,G. A., W. E. KELSO.AND D. A. RUTHERFORD. teidae). Miscellaneous Publications of the Muse- 1999. Diel and seasonal patterns of spotted gar urn of Natural History of the University of Kansas movement and habitat use in the lower Atchafa- 64:1-111. laya River Basin, Lousisiana. Transactions of the American Fisheries Society 128:144-154. Submitted 7 October1999. Accepted5 June 2000. SUTfKUS.R. D. 1963. Order Lepisostei. In: Bigelow, AssociateEditor was David R. Edds.