112

WETLANDS, Vol. 25, No. 1, March 2005, pp. 112–121� 2005, The Society of Wetland Scientists

THE OCCURRENCE OF NON-INDIGENOUS NILE TILAPIA, OREOCHROMISNILOTICUS (LINNAEUS) IN COASTAL MISSISSIPPI, USA: TIES TO

AQUACULTURE AND THERMAL EFFLUENT

Mark S. Peterson1, William T. Slack2, and Christa M. Woodley3

1Department of Coastal SciencesThe University of Southern Mississippi

703 East Beach DriveOcean Springs, Mississippi, USA 39564

E-mail: mark.peterson@usm.edu

2Mississippi Department of Wildlife, Fisheries and ParksMississippi Museum of Natural Science

2148 Riverside DriveJackson, Mississippi, USA 39202-1353

3Department of Wildlife, Fish and Conservation BiologyUniversity of California, DavisDavis, California, USA 95616

Abstract: We studied the distribution and abundance of Nile tilapia, Oreochromis niloticus near two aqua-culture facilities for two years in coastal wetlands in southeastern Mississippi, USA. In 280 collections, werepresented 29 families, 65 taxa, and 86,415 fishes with a variety of gear types. Oreochromis niloticus rankedsixth in abundance overall and ranked second among those stations sampled in the Pascagoula River water-shed and sixteenth among Coastal River stations. Water temperature downstream from the facility effluentswas always warmer than ambient, and at the Pascagoula River facility, it never dropped below 15.1�C overthe three years examined. Thus, normal environmental conditions, the presence of the downstream thermalrefuge, and the generally low salinity of the bayous of our region all combine to provide a quality environ-ment for continued survival of released fish. Furthermore, O. niloticus seem to spawn year-round, and fishas small as 79.9 mm TL were found to carry mature eggs, suggesting that, if they are born early in theseason, they could reproduce during their first summer of life. Further spread of O. niloticus and introductionof new species are expected as aquaculture expands. The philosophy that allows the escape or release ofnon-indigenous taxa into our present landscape, justified by the belief that species will not survive or becomeestablished, is fallible.

Key Words: aquaculture, invasion, non-indigenous, thermal refugia, tilapia, wetlands

INTRODUCTION

Land-based and offshore aquaculture activities areincreasing worldwide (Naylor et al. 2000, Stickney2002, Costa-Pierce 2003); many of these facilities in-clude members of the family Cichlidae (Beveridge andMcAndrew 2000). However, cichlids are recognizedas having the potential to alter aquatic communitiesinto which they are introduced (Courtenay 1997), inpart because they are tolerant of variable environmen-tal conditions such as temperature and salinity (Tre-wavas 1983). When environmental salinity is nearisosmoticity (�10–15 psu), tilapiine cichlids have alower energetic cost of osmoregulation (Febry andLutz 1987), have significantly reduced oxygen con-

sumption rates (Farmer and Beamish 1969), and cantolerate lower temperatures (Beamish 1970, Zale andGregory 1989, Avella et al. 1993). These physiologicaldata suggest that life in low salinity conditions com-pared to fresh water should allow for more energy tobe allocated toward growth and promote low temper-ature tolerance in tilapia species. In addition to thesephysiological adaptations, tilapiine fishes are trophicgeneralists (Moriarty and Moriarty 1973a,b, Zale andGregory 1990, Dempster et al. 1993, Traxler and Mur-phy 1995), and their reproductive biology is charac-terized by short generation time, multiple clutches, andextended breeding seasons (Naylor et al. 2000, Stick-ney 2002, Peterson et al. 2004). These adaptationsmake tilapiine fishes excellent aquaculture species.

Peterson et al., NON-INDIGENOUS NILE TILAPIA IN MISSISSIPPI 113

The invasion of non-native environments by phys-iologically tolerant tilapiine fishes has been docu-mented worldwide, coinciding with their increased useas an aquaculture species. For example, blackchin ti-lapia (Sarotherodon melanotheron Ruppell) represent-ed 90% of the biomass and occurred in over 80% ofall collections in an impounded mangrove habitat inFlorida, which was attributed to their ability to surviveconditions in the altered mangrove environment(Faunce and Paperno 1999). Crutchfield (1995) indi-cated that redbelly tilapia (Tilapia zilli Gervais) be-came the fourth most abundant species in a powerplant reservoir in North Carolina within the first threeyears after accidental introduction. Foraging by red-belly tilapia eliminated all aquatic macrophytes (sub-merged and floating) in the reservoir within a two-yearperiod, which coincided with significant declines inpopulations of native fishes.

The actual process of maintaining aquaculture spe-cies may also be detrimental to downstream aquaticsystems. The effluent from aquaculture facilities canalso alter the receiving environment through changesin water quality (water temperature, conductivity, nu-trients, sedimentation, disruptive chemical messagers,etc.; Chua et al. 1989, Kamps and Neill 1999, Banaset al. 2002) and subsequent habitat alteration. Thesealterations have been shown to support the prolifera-tion of non-indigenous species (Ross 1991, Courtneyand Williams 1992, Williams et al. 1998). Often, thereceiving environment becomes less suitable for nativespecies but more favorable for non-indigenous speciesand their proliferation (Courtney and Williams 1992,Goldburg et al. 2001, Ricchardi 2001, Myrick 2002).In a recent study of community assembly rules, Belyeaand Lancaster (1999) argued that environmental con-straints restrict species establishment and mediate in-teractions among successive recruits; changes in theseconstraints, either exogeneous or endogenous, maydrive community change.

Nile tilapia, Oreochromis niloticus Linnaeus, iswidely used and an adaptive aquaculture species (Cos-ta-Pierce 2003) that is cultured in coastal Mississippi.We have recently observed Nile tilapia in the coastalwetlands of Mississippi, but the importance of this isunclear since brackish marshes along the northern Gulfof Mexico are among the least studied regions of theU.S. in terms of introductions (Ruiz et al. 2000, Carl-ton 2001). The objectives of this project were to 1)document the establishment of the non-indigenousNile tilapia in coastal watersheds in Mississippi and 2)document the presence and extent of thermal refugiadownstream of two existing aquaculture facilities.

MATERIALS AND METHODS

We collected fish and water quality data at 20 sta-tions/areas (Figures 1 and 2) between 16 November

2000 and 18 June 2002, except during February 2001.We sampled monthly at six fixed stations that wereclosely associated with two aquaculture facilities. Ofthe six stations, two were in the Pascagoula River (PR)system (Figure 2, stations 21 and 22) and 4 were inSimmons Bayou (SB) (stations 1, 2, 3 and 4), a trib-utary of Davis Bayou (DB). In addition, fourteen gen-eral areas were sampled haphazardly within coastalwatersheds and were sampled periodically to deter-mine the extent of the tilapia invasion (Figure 1).Fixed stations were sampled with seines (3 or 15 mlong) and dipnets, while stations within the generalareas were sampled with trammel nets (31 and 61 mlong), hook and line, minnow traps, and plastic coatedwire crab traps. Trammel nets were fished as day sets,overnight sets, or strike net sets (White 1959). Vouch-ered specimens of all species were euthanized in thefield with MS-222, fixed in 10% formalin for 7–14days, washed overnight in fresh water, and preservedin 70% ethanol. Depth (m), water temperature (�C),dissolved oxygen (D.O., mg/L), salinity (psu), currentflow (m/sec), and conductivity (�S) were also mea-sured at all stations/areas, and all were geo-referencedwith a Garmin 45 GPS unit. Total length (TL, mm) ofall fishes was measured with digital calipers or a metertape. Fish � 160g blotted wet weight (g) were weighedon a digital balance (to 0.001g), whereas larger fishwere weighed on a spring balance (� 57g). Cichlidscientific names used in this paper follow Trewavas(1983) and Shafland (1996), and all specimens are ar-chived in the Mississippi Museum of Natural Science(MMNS) Ichthyology Collection in Jackson.

We also established five long-term water tempera-ture recording stations (Figure 2) with Onset Stow-Away� Tidbit� data loggers during the two years offish collections plus one additional year. Water tem-perature was recorded hourly from 24 January 2001 to23 January 2004 (1094 potential record days) at theeffluent (station 21) and 1.55 km downstream (station26) at Little River Marina on Robinson Bayou of thePR. Actual records were 1083 days for station 26 and1069 days at station 21. Within SB, water temperaturewas recorded hourly from 9 March 2001 to 22 August2002 (531 record days) at stations 100 m upstreamfrom the aquaculture effluent, 100 m downstream fromthe effluent, and 1.32 km downstream from the efflu-ent. Mean daily water temperature (24 January 2001to 23 January 2004, 927 actual days) was also obtainedfrom the USGS gauging station (02480212) located atriver kilometer 1.61 on the PR.

Descriptive statistics were generated with SPSSsoftware (SPSS, Inc., Ver. 10.1 or 11.5, Chicago, Ill).Analysis of variance (ANOVA) and Sidak pairwisecomparison tests were used to compare O. niloticusTL by sampling date for standardized collections at

114 WETLANDS, Volume 25, No. 1, 2005

Figure 1. General map of stations and areas where 280 collections were made over the course of this study in coastalMississippi. Solid circles represent sampled areas where Nile tilapia was not present; triangles represent areas where Niletilapia was present. The arrows in circles A and B refer to the location of the aquaculture facilities. See Figure 2 for moredetail on A and B.

the effluent and downstream stations. Normality andhomogeneity of variance assumptions of ANOVAwere examined, and, if violated, the data were log10-transformed prior to analysis. Results were consideredsignificant when p � 0.05.

RESULTS

Twenty-nine fish families representing 65 taxa and86,415 individuals were collected in 280 samplingevents, with O. niloticus ranking sixth in total abun-dance for all stations sampled. In the Pascagoula andEscatawpa River systems, 22 families representing 44species and 25,600 individuals were collected. In thissystem, O. niloticus ranked second in abundance. Inthe Coastal Rivers systems (Figure 1: Biloxi Back Bay,Davis Bayou, Graveline Bayou), 22 families repre-senting 45 species and 60,815 individuals were col-lected, and O. niloticus ranked sixteenth in abundance.Oreochromis niloticus was captured in three of six(50%) study areas (PR, DB, Black Creek CoolingPond) and was collected in every month except June2002.

Within the Pascagoula and Escatawpa River sys-tems, measured O. niloticus ranged from 4.53 to430.00 mm TL (n � 3,709; mean � 41.78), whereasin SB fish were 12.09 to 400.00 mm TL (n � 58; mean� 64.56). A noticeable relationship between season,abundance and TL at stations 21 and 22 was evident.When water temperatures were low in the fall, bodysize was large and vice versa (Figure 3). Mean TL wassimilar across dates at the PR effluent station, exceptthat specimens from December 2001 were significantlysmaller (ANOVA, F13,602 � 12.19, p � 0.001; Sidak, p� 0.05) (Figure 4) than all other monthly collections.This supports the persistence of O. niloticus at the ef-fluent station. However, fish lengths were significantlysmaller (ANOVA, F12,2951 � 116.31, p � 0.001; Sidak,p � 0.05) 1.55 km downstream (station 22) in latespring and summer compared to other months (Figure4). Some larger individuals were carrying embryos andeggs in their mouths, and 286 eggs and yolk sac larvaewere collected at station 22 in February 2002. We in-terpret this to mean that mature individuals moved toa downstream nursery as water temperature increased.

Table 1 lists the minimum-maximum values of the

Peterson et al., NON-INDIGENOUS NILE TILAPIA IN MISSISSIPPI 115

Figure 2. Map of specific stations in the Pascagoula River (A) and Simmons Bayou (B). Stars represent fixed monthlysampling stations, whereas open circles represent the locations of the water temperature data loggers. The arrow indicatesstations where the aquaculture facility effluents enter the system.

physical-chemical factors measured by system and sta-tion/area. Water temperature and D.O. followed a typ-ical seasonal pattern. However, maximum water ve-locity, water depth, conductivity, and salinity variedwith precipitation, effluent discharge, and tidal stage,which was generally higher or deeper in the PR systemnear the effluent station. Sampling stations/areas fur-ther away from the effluent typically had lower con-ductivity and salinity in the PR but increased down-stream in the Escatapwa River (stations 29 and 30,Figure 1, Table 1). The SB/Coastal River stations/areashad higher salinity, as they were tidally influenced andcloser to the Mississippi Sound. Water temperature atthe PR effluent station experienced less monthly var-

iability and a consistent yearly pattern over the threeyears of our study when compared to station 26 (Table1). Deviation in water temperature graphically illus-trates this trend (Figure 5), with the largest differencesbetween these two stations occurring during the coldermonths. Thermal profiles depicting water temperatureat the two stations converge during May as the rise inambient water temperature corresponds with seasonalwarming in all years. Water temperature at station 26was comparable to water temperature in the PR 17.7river kilometers downstream. A comparison of meandaily water temperature between the two stations in-dicated that daily water temperature differed by lessthan 2.5�C in 90% of the observations, and in the most

116 WETLANDS, Volume 25, No. 1, 2005

Figure 3. Plot of abundance of Oreochromis niloticus by sampling date. Dark bars represent the effluent station (21) andgray bars represent the downstream station (22). No collections were made in February 2001.

Figure 4. Plot of Oreochromis niloticus total lengths (mean � se) for the Pascagoula River effluent station (solid circle, n� 604) (station 21, A of Figure 1) and 1.55 km downstream station (open circle, n � 2964) (station 22, B of Figure 1) bycollection date. No collections were made in February 2001.

Peterson et al., NON-INDIGENOUS NILE TILAPIA IN MISSISSIPPI 117

Table 1. Description of physical-chemical data collected within each station/area presented as minimum and maximum values. D.O. �dissolved oxygen, and ‘—’ � missing data. Detailed monthly data are available in Peterson et al. 2002.

Pascagoula & Escatawpa Systems

Station/Area

Collection Date,n � dates

WaterTempera-ture (�C)

Conductivity(�S)

Salinity(psu)

D.O.(mg/L)

MaxCurrentVelocity(m/sec) Depth (m)

21222324

16 Nov. 2000 to 20 Feb. 2002, n � 1524 Jan. 2001 to 20 Feb. 2002, n � 1324 Jan. 2001 to 29 Jun. 2001, n � 54 May 2001 to 18 Jun. 2002, n � 6

20.2–29.221.5–30.219.9–28.415.4–29.5

1066–26161050–26111726–21061808–1908

0.7–1.30.5–1.30.1–1.10.1–0.9

1.9–6.12.1–6.33.0–5.23.1–10.7

0.0–4.50.0–3.30.00.0–0.5

0.1–0.650.1–3.61.0–3.00.7–4.8

252627

2 May 2001 to 31 Aug. 2001, n � 314 Dec. 2000 to 18 Jun. 2002, n � 52 and 3 Aug. 2001, n � 2

23.4–28.914.8–28.128.9–31.6

245.2–642175–3368

1319–1579

0.1–0.30.1–2.10.7–0.8

3.2–7.73.2–6.85.6–6.5

0.0–0.50.0–1.00.9–3.5

1.0–6.30.0–2.00.8–1.8

282930

27 Mar. and 25 Apr. 2002, n � 217 Jun. 2002, n � 118 Dec. 2001 and 17 Jun. 2002, n � 2

21.4–27.427.9–29.513.6–30.3

43–286621–4241540–12450

0.0–0.10.2–2.00.3–6.6

4.5–9.42.9–10.00.8–9.9

——0.0

0.3–3.00.8–4.30.3–2.4

Coastal Rivers (Simmons, Graveline, Davis and Biloxi Back Bay) System

1234

25 Jan. 2001 to 29 Nov. 2001, n � 1025 Jan. 2001 to 29 Nov. 2001, n � 1025 Jan. 2001 to 29 Nov. 2001, n � 1025 Jan. 2001 to 29 Nov. 2001, n � 10

15.1–29.717.3–27.817.3–28.011.5–27.3

1075–2962090.7–25520399–2315089.2–23800

0.6–13.50.0–11.30.0–12.10.0–11.0

1.8–9.71.4–11.51.1–11.51.4–9.3

0.0–0.00.0–3.00.0–5.50.0–2.0

0.15–1.30.15–0.80.15–0.40.15–0.8

567

25 Jan. 2001 to 28 Nov. 2001, n � 83 May 2001, n � 1

30 Jan. 2002, n � 1

10.2–32.023.2–23.619.8–22.4

2105–3178018250–193707330–17750

1.1–19.911.1–11.54.2–9.5

1.7–10.44.6–6.03.6–8.1

0.0–1.0—0.0

0.33–2.0—0.15–1.4

89

10

21 Feb. 2002, n � 128 Mar. 2002 and 3 Jun. 2002, n � 210 Jun. 2002, n � 1

18.9–19.618.8–29.228.0–28.5

17380–2084086–133

7620–14710

10.6–12.60.0–8.14.3–8.3

7.6–8.73.1–6.53.5–4.4

0.0——

0.5–1.6—0.3–0.4

Figure 5. Plot of water temperature (�C) deviation (effluent-downstream) values between the aquaculture effluent at station21 and the downstream station 26.

118 WETLANDS, Volume 25, No. 1, 2005

Table 2. Compilation of the lowest water temperature and date, plus the percent of total records when water temperature was � 10.5�C. These are based on hourly records taken from 24 January 2001 through 23 January 2004.

Drainage Station(s)Lowest

Temperature Date % � 10.5�C

Pascagoula River Effluent (21)Little River (26)

15.17.51

2 March 200225 January 2003

0 (never below 15.1�C)7.4

Simmons Bayou Upstream (4)Downstream (3)Furthest away (5)

4.586.245.96

24 January 20028 January 20023 January 2002

6.44.13.8

Pascagoula River River kilometer 1.61 9.6 4 January 2002 1.3

extreme case, the two stations differed by only 4.3�C.A similar, but less distinct, pattern was depicted forwater temperature in SB, with water temperature re-corded upstream from the aquaculture facility beinggenerally cooler (Table 1, station 4) than the effluentstation (station 3). The deviation in water temperaturebetween the stations/areas (Table 1, stations 1,2,3,5)showed little difference and is not presented here.

Water temperature was seasonally elevated at theeffluent station compared to either up- or downstreamwithin both systems (Table 2). However, it was con-sistently greater and seemed to cover a larger area atthe PR stations. Furthermore, the number of days whenwater temperature was � 10.5�C was always greaterwithin the PR effluent stations (Table 2), and watertemperature never dropped below 15.1�C over thethree years of this project. This temperature was se-lected because Jennings (1991) noted loss of equilib-rium in S. melanotheron when water temperature de-creased to 10.5�C. These data suggest a potential ther-mal refugia for tilapiine fishes may exist in RobinsonBayou of the PR system during times of the year whenambient water temperature is typically low (late fall,winter, early spring). The potential for thermal refugiaexists in SB but is limited spatially when compared tothe potential at the Robinson Bayou station.

DISCUSSION

The demand for seafood, coupled with the declineof fisheries species worldwide, has resulted in an in-crease in land-based and offshore aquaculture facili-ties. Given the continued increase in the human pop-ulation, we are likely to see further increases in aqua-cultural products for public consumption to replace orsupplement existing fisheries stocks (Naylor et al.2000). Consequently, there is an increased threat ofestablishment of non-indigenous species, as well as theecosystem impacts. Pimentel et al. (2000) estimatedthat 50,000 non-indigenous species in the U.S. alreadycause major environmental damage totaling about$137 U.S. billion annually.

Coastal wetlands in the northern Gulf of Mexico are

characterized by warm temperate to subtropical waterswith salinities of 0 to 25 psu. These environmentalconditions support an abundant and diverse ichthyo-fauna (Rakocinski et al. 1997, Ross 2001) and are alsoquality environments for non-indigenous species.Trexler et al. (2000) indicated that it is likely that thebalance between native and non-indigenous speciesdepends on local environmental conditions, whichvary with annual rainfall and minimum temperaturepatterns. Thus, spatial and temporal abiotic conditionswithin the northern Gulf of Mexico might promote thesurvival and potential expansion of non-indigenousspecies, as has been documented elsewhere (Meng etal. 1994, Ruiz et al. 2000, Matern 2001).

A number of tilapiine species are affected by lowwater temperature and high salinity conditions (Jen-nings and Williams 1992, Ross 2000), which influencesurvival, movement, and distribution. For example,Shafland and Pestrak (1982) determined that the lowerthermal limit for Oreochromis aureus Steindachner(6.2�C), Oreochromis mossambicus Peters (9.5�C), Ti-lapia mariae Boulenger (11.2�C), and S. melanotheron(10.3�C) were lowered when acclimated to 2 psu com-pared to 0 psu. Chervinski and Lahov (1976) deter-mined that O. aureus, O. niloticus, and Oreochromisvulcani Trewavas (a subspecies of O. niloticus, Tre-wavas 1983) began dying at 11�C, and all died within2 days; their lower thermal limit was reduced to 9�Cwhen acclimated to 5 psu. The environmental condi-tions documented in our study indicate that ambientwater temperature, coupled with both the thermal ref-uge downstream of the Robinson Bayou aquaculturefacility and the generally low salinity of the bayous,are conducive for continued survival and spawning ofO. niloticus and other cichlids used in aquaculture. Ac-climation to low salinity (�10–15 psu) reduces thelower water temperature limit (Beamish 1970, DeSilvaand Perera 1985, Zale and Gregory 1989, Avella et al.1993) and oxygen consumption rate (Farmer andBeamish 1969, Febry and Lutz 1987) of many tilapiinefishes. These physiological data suggest that life in lowsalinity conditions should allow for more energy to be

Peterson et al., NON-INDIGENOUS NILE TILAPIA IN MISSISSIPPI 119

allocated toward growth and promote low temperaturetolerance than in fresh water. Behrands et al. (1990)indicated that environmentally-induced selection ofcold tolerance occurs in O. niloticus across its nativerange. In contrast, S. melanotheron, exposed to rapiddecreases in water temperature showed modified socialbehavior to 12�C, with loss of equilibrium occurringat 9.6�C and death occurring at 6.9�C; 5 to 35 psu didnot influence thermal tolerance (Jennings 1991). Thepresence of a thermal gradient in Robinson Bayouprobably aids O. niloticus survival and may drive ther-mal selection for low temperature tolerant individualsover time. In fact, fish naturally select warmer com-pared to cooler water temperature areas (Baltz et al.1987), and thus, it is not unusual that non-indigenousspecies with warmer temperature requirements seekout thermal refugia (e.g., Legner and Pelsue 1977,Shafland and Pestrak 1982, Stauffer et al. 1988, Trex-ler et al. 2000).

Multiple escapes from a single source foster estab-lishment (Stauffer 1984, Myrick 2002), and followinginvasion, some populations explode quickly while oth-ers experience a long lag period between initial inva-sion and subsequent population explosion. Further-more, the establishment of non-indigenous species isgenerally more successful in disturbed systems (e.g.,thermal alteration, habitat alteration, water diversion)in contrast to relatively undisturbed habitats that oftenshow a large resistance to invasion by non-indigenousspecies (Moyle and Light 1996). Our data suggest thatthere seem to be repetitive escapes of O. niloticus todownstream, thermally-altered environments, asshown by the lack of change in monthly mean TL overtwo years at the PR effluent station (station 21) com-pared to other stations.

The advanced reproductive strategy of maternalmouthbrooding in O. niloticus also influences survival,proliferation, and movement. Our data indicate that O.niloticus are spawning all year in the PR, fish as smallas 79.9 mm TL can carry eggs, and the size at 50%maturity is 113 mm TL (Peterson et al. 2004). Thesedata suggest that, if they are born early in the season,they could reproduce in their first summer of life.McBay (1961) noted that O. niloticus between 96–118mm TL (50 d old) carried eggs and previously sug-gested that reproduction occurs within their first sum-mer of life. Nile tilapia collected in both SB/CoastalRivers and the PR stations ranged from 4.53 to 430.00mm TL (about 5–7 years old, Trewavas 1983; unpubl.data), suggesting overwintering and spawning in bothsystems. The collection of yolk sac and very smalljuvenile O. niloticus 1.55 km downstream (station 22)in February 2002 supports our assertion. Clearly, thisspecies is well-adapted for life in the coastal bayousof the northern Gulf of Mexico, and given that the

intrinsic rate of population increase plays a significantrole in determining which species survive and prolif-erate (Ruiz et al. 2000), we expect O. niloticus to con-tinue to invade.

Large-scale ecosystem changes have been driven byintroductions and subsequent interactions among na-tive and non-indigenous species. For example, Ogutu-Ohwayo (2001) indicated that Nile perch, Lates nilo-ticus Linnaeus, O. niloticus, and two other cichlidswere introduced into Lake Victoria, Africa in the1950s and early 1960s, and now, the former two spe-cies dominate the fish fauna. Lates niloticus is a pis-civore and has almost eliminated the native haplo-chromine cichlids in the lake. Oreochromis niloticusis a herbivore that now outcompetes other native andtwo other non-indigenous cichlids. Oreochromis nilo-ticus is more fecund, grows to a larger size, has a fastergrowth rate, and lives longer than the native or othernon-indigenous tilapiines of Lakes Victoria and Kyoga(Fryer and Iles 1972). Finally, nest predation by theMayan cichlid (Cichlasoma urophthalmus Gunther)and walking catfish (Clarias batrachus Linnaeus) onnative centrarchids has been observed in southernFlorida, USA (Trexler et al. 2000) and may contributeto decreases in centrarchid reproduction and changesin population dynamics.

It is paramount that we not continue to approachnon-indigenous organisms with the current naivety.The philosophy that allows the escape or release ofnon-indigenous taxa into our present landscape, justi-fied by the belief that species will not survive or be-come established, is fallible. There are too many ex-amples in the literature citing problems from seeming-ly innocent escape or release of non-indigenous spe-cies (e.g., Courtenay and Stauffer 1984, Fuller et al.1999, Courtney and Williams 2004). As noted byShelton and Smitherman (1984), ‘‘for whatever pur-pose an exotic fish is used, escape is virtually inevi-table; thus, this eventuality should be considered.’’ Of-ten, the reaction to the discovery of an introducedaquatic species is a ‘‘wait and see’’ approach. Thisdefault response is argued to be the most economicallyviable (i.e., doing nothing costs nothing). However, bythe time there is evidence that a harmful non-indige-nous species has become established, it is usually toolate to pursue effective management or achieve erad-ication. Documentation of successful and completeeradication after release and establishment, however,is limited. There are far more cases detailing failederadication efforts (Stauffer 1984, Mills et al. 1993,Courtney and Williams 2004). More research is need-ed on ontogenetic thermal and salinity requirements,growth, movement, habitat alteration, and behavioralinteractions with native species. In particular, interac-tions among non-indigenous and native substrate-

120 WETLANDS, Volume 25, No. 1, 2005

spawning species have rarely been examined. Theseimportant life-history metrics are vital as input datanecessary to predict and model the invasion of O. nil-oticus into other coastal watersheds. This is particu-larly true relative to predicted changes in coastal land-scapes due to global climate change and sea-level risescenarios (Scavia et al. 2002).

ACKNOWLEDGMENTS

Ches Vervaeke, B. Lezina, G. Waggy, J. Finley, M.Partyka, and J. Brookins provided field and laboratoryassistance. Richard and Grady Scott of Moss Point,Mississippi allowed access to their property for sam-pling. James D. Williams of the USGS Florida Carib-bean Science Center verified our tilapia. The JacksonCounty Port Authority allowed access and providingassistance with our sampling on the Black Creek Cool-ing Ponds facility. Nancy J. Brown-Peterson and M.Dugo reviewed this manuscript and made many help-ful comments. This Mississippi Department of Wild-life, Fisheries and Parks funded this project throughgrant F-129.

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Manuscript received 28 January 2004; revisions received 16 July2004 and 13 September 2004; accepted 25 October 2004.