Trans. Am. Fish. Soc., 107(3): 500-504, 1978© Copyright by the American Fisheries Society, 1978

Predator-Prey Interactions of Fishes Under theInfluence of Ammonia'

D. M. WOLTERING, J. L. HEDTKE 2 , AND L. J. WEBERSOak Creek Laboratory of Biology, Department of Fisheries and Wildlife

Oregon State University, Corvallis, Oregon 97331

ABSTRACT

Food consumption and growth rates of a predator, the largemouth bass (Micropterus salmoides),and the behavior of both the predator and its prey, the mosquitofish (Gambusia affinis), weresensitive indices of sublethal effects of ammonia on predator-prey interactions. Prey consumptionand growth rates of bass in control tests increased with increasing prey densities. Ammonia con-centrations of 0.63 and 0.86 mg/liter NH 3 substantially decreased prey consumption and growth ofbass in test tanks stocked with densities of 30, 60, and 120 mosquitofish. At a given ammoniaconcentration, there were greater decreases in prey consumption and growth of bass at higher preydensities. This can be attributed in part to the bass being more sensitive than mosquitofish toammonia and to the harassment of the predator by the prey which occurred at high ammoniaconcentrations and high prey densities.

Changes in predator-prey interactionsmay be sensitive and meaningful indicatorsof sublethal toxicity. Sublethal concentra-tions of a toxicant may change the interac-tions of coexisting and dependent speciesand be as detrimental to an organism asacute exposures to the same toxicant. Good-year (1972) found that exposure of mosqui-tofish to a sublethal level of ionizing radia-tion caused abnormal behavior resulting inincreased mortality from predation. In asimilar test situation, the ability of mosqui-tofish to avoid predation by bass was im-paired after the former had been exposed tosublethal concentrations of mercury (Kaniaand O'Hara 1974).

The consumption of prey by a predator isin part dependent upon prey density. Thechange in the number of prey consumed byan individual predator as a response tochanges in prey density is termed the func-tional response (Solomon 1949). Consump-tion of prey should increase as a function ofprey density up to a maximum where thepredator becomes satiated or runs out oftime in which to eat more prey. Forms ofthe functional response have been empiri-cally determined in studies involving either

' Oregon Agricultural Experiment Station TechnicalPaper No. 4400.

Present address: Department of Pharmacology,University of Oregon Health Sciences Center, Schoolof Dentistry, Portland, Oregon 97201.

3 Present address: Oregon State University MarineScience Center, Newport, Oregon 97365.

one or two prey species and predation byinvertebrates (Holling 1965; Murdoch 1969),fish (Ivlev 1961), and small mammals (Holl-ing 1959).

The objective of this study was to look atchanges in predator-prey interactions oflargemouth bass (predator) and mosquito-fish (prey) when both were exposed to sub-lethal concentrations of ammonia, a com-mon pollutant. Direct measurements weremade of food consumption and growth of thepredator over a wide range of prey densities.

METHODSJuvenile largemouth bass were hatchery

raised, and mosquitofish were collected atleast 1 month before each experiment froma wild population which was subject to pre-dation by fish including largemouth bass.Prior to the experiments bass were fed mos-quitofish and Oregon Moist Pellet; mosqui-tofish were fed dry Purina Trout Chow. Testbass were between 9.7 and 11.3 cm in totallength. Mosquitofish averaged 2.5 cm totallength and could be readily consumed bythe bass.

Tests were carried out in large glass tanks(122 cm long x 46 cm wide x 41 cm high),each covered on three sides with white plas-tic to eliminate visual contact of fish be-tween adjacent tanks. Water depth was 28cm above 5 cm of pea gravel substrate giv-ing a utilized water volume of 157 liters.Total flow into each tank was 400 ml/mincausing a 99% replacement in 30 hours

500

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WOLTERING ET AL.-PREDATOR-PREY INTERACTIONS

501

TABLE 1.-Temperature, pH, and concentrations of dissolved oxygen, total ammonia (NH 3 + NH4 +), and un-ionized ammonia (NH3) for each of the interaction experiments. Values are means (SD) of all test tanks for 10days, except for dissolved oxygen which are means of all test tanks for 1 day.

Experiment

2 3 4 5

Temperature (C) 24.6 (0.4) 24.5 (0.8) 23.7 (0.3) 24.7 (0.3) 24.9 (0.3)pH 7.66 (0.05) 7.64 (0.06) 7.47 (0.11) 7.45 (0.16) 7.62 (0.10)Dissolved oxygen (mg/liter) 7.4 (0.1) 6.0 (1.3) 6.3 (1.2) 6.5 (1.0)Total ammonia

(mg/liter NH 3 + NH4*) =-0.07 4.85 (0.69) 20.46 (0.92) 45.07 (1.99) 37.42 (1.96)Un-ionized ammonia

(mg/liter NH3) ---0.002 0.13 (0.03) 0.34 (0.09) 0.63 (0.07) 0.86 (0.15)

(Sprague 1973). Mosquitofish could find cov-er above two fiberglass screen shelves (20cm x 46 cm x 1.3 cm thick), one at eachend of the tank. The shelves made contactwith the sides of the tank and angled fromthe water surface to a depth of 2 cm. It wasdetermined that this refuge could providefunctional cover for about ten mosquitofishat a time.

The dilution apparatus was similar to thatdescribed by Chadwick et al. (1972). Heatedwell water, thermostatically maintained at25 C and aerated to prevent gas supersatur-ation, was mixed with a concentrated stocksolution of ammonium chloride and deliv-ered to eight of ten test tanks. For each 10-day experiment the concentration of toxi-cant delivered was the same for these eighttanks. The remaining two test tanks servedas controls and received heated well waterwithout toxicant. One 15-watt daylight flu-orescent tube was suspended above eachtank; photoperiod was 16 hours of light and8 hours of darkness.

A test range of ammonia concentrationswas selected following 10-day acute bioas-says conducted according to Sprague (1973).The lowest lethal concentration of ammoniawas 1.3 mg/liter NH 3 . Exposure to 3.3 mg/liter NH 3 killed all the largemouth bass butnone of the mosquitofish. Based on thesestudies, bass and mosquitofish were ex-posed to ammonia concentrations of 0.13,0.34, 0.63, and 0.86 mg/liter NH 3 in preda-tor-prey experiments.

Five 10-day experiments were carriedout. In all tests there were duplicate tanksat prey densities of 15, 30, 60, and 120 pertank. The initial experiment was run with-out toxicant to establish baseline responses

of both predator and prey (control). Toxicantwas introduced in the four subsequent ex-periments. Two additional control tanks atselected prey densities were included ineach of these experiments to detect anychange in baseline responses which mighthave occurred during the course of thestudy.

Bass and mosquitofish were deprived offood for 24 hours prior to the start of anexperiment. At that time the fish were an-esthetized with tricaine methanesulfonate(MS222); initial weight and length for eachbass, and the total weight for all of the mos-quitofish to be stocked in each tank wererecorded. Mosquitofish were stocked in ap-propriate tanks approximately 18 hours af-ter being anesthetized and 2 hours beforebass were stocked. One bass was randomlyassigned to each tank 4 to 5 hours afterbeing anesthetized and their introductioninto the tanks was designated as time zerofor the experiment. Mosquitofish were fedon days 3, 6, and 9 by floating small amountsof the dry trout chow over the shelves ineach tank. Initial prey densities were main-tained in each tank throughout each exper-iment by restocking mosquitofish at 24-hourintervals. Restocked prey were acclimatedto actual test conditions including aquari-um, predator, and toxicant.

At the end of a test the surviving mos-quitofish in each tank were counted, mea-sured, weighed, and dried in an oven at 70C for 4 days. The mosquitofish showed noappreciable growth over 10 days in any ofthe tests; they averaged 0.145 g wet weightand 0.033 g dry weight. Bass were left in thetanks an additional 24 hours to allow timefor clearing of the gastrointestinal tract; fi-

Control A (small bass)A 0.13 mg/I NH3

Control B (large bass)0.34 mg/I NH30.63 mg/I NH30.86 mg/I NH3

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Control B (large boss)0.34 mg/I NH30.63 mg/I NH30.86 rng/I NH3

–120

502 TRANS. AM. FISH. SOC., VOL. 107, NO. 3, 1978

15 30 60 120Prey Density (per tank )

FIGURE 1.—Ter -day prey consumption rates of juvenilelargemouth bass as functions of prey (mosquitofish Idensity during exposure to 0 (control), 0.13, 0.34,0.63, or 0.86 mg/liter of NH3. Smaller bass initiallyweighed 3.26 ± 0.30 g dry weight (upper curves);larger bass initially weighed 4.45 ± 0.41 g dry.Curves were fitted by inspection.

nal lengths and wet and dry weights werethen obtained for each bass. The initial dryweight to wet weight (g) relationship forlargemouth bass was dry = 0.3789 (wet) —1.1714; r = 0.96. Final dry weight to wetweight relationships indicated water weightgains of up to 9% in 10 days for bass ex-posed to ammonium chloride. Therefore,calculations of food consumption rates of in-dividual bass (mg mosquitofish consumedper g mean weight of bass per day) and theiraverage relative growth rates (mg weightchange of bass per g mean weight of bassper day) were based on dry weight values ofboth bass and mosquitofish (Warren 1971).

Water temperature, pH. and total am-monia in each tank were monitored daily.Dissolved oxygen was determined once ineach tank during each of the five experi-

5 30 60 120Prey Density (per tank)

FIGURE 2.—Ten-day growth rates of juvenile large-mouth bass as functions of prey (mosquitofish ) densityduring exposure to one of four concentrations of NH3or with no ammonium chloride added (controls). Dryweights of smaller bass (upper curves) were 3.26 ±0.30 g, and of larger bass were 4.45 ± 0.41 g. Curveswere fitted by inspection.

ments. Total ammonia was assayed with aspecific ion electrode (Orion Research In-corporated 1970). Un-ionized ammonia(NH 3) is primarily responsible for the tox-icity of ammonia solutions to fish (Downingand Merkens 1955; Lloyd and Herbert1960). From assayed total ammonia levels,temperature, and pH in the test tanks, theconcentrations of un-ionized ammonia werecalculated (EIFAC 1970) and are listed inTable 1.

RESULTS

Initial sizes of all test fish were kept asuniform as possible within and between ex-periments. Growth of largemouth bass inthe holding tanks, however, resulted in a25-30% initial weight difference in the fishused for the first two versus the last three

WOLTERING ET AL.-PREDATOR-PREY INTERACTIONS 503

experiments. Because of these size differ-ences, responses of the exposed fish in thesecond experiment (0.13 mg/liter NH 3) arecompared to small control bass (Control A),and exposed fish in the third, fourth, andfifth experiments (0.34, 0.63, and 0.86 mg/liter NH 3) are compared to large controlbass (Control B).

Control bass from all five experimentsconsumed more prey as prey density in-creased; the large bass ate more than smallbass at intermediate prey densities (Fig. 1).At a prey density of 120, bass consumed upto 10% of body weight, i.e., 30 mosquitofishor more per day. There was a correspondingincrease in the growth rates of control basswith increased prey density (Fig. 2). Smallbass grew at higher rates than large bass.Since their consumption rates were lower,this indicates that small bass converted foodto growth more efficiently than large bass.

Consumption and growth rates of bassexposed to ammonia concentrations of 0.13and 0.34 mg/liter NH 3 were similar to therates seen in controls A and B, respectively.There was a general decrease in both con-sumption and growth rates at the higher am-monia concentrations; these decreases weregreater with increasing prey density.

The consumption and growth data can bebetter understood in light of certain behav-ioral observations on both predator andprey. As the concentration of ammonia in-creased the bass showed signs of distress(increased respiratory rate and coughingfrequency, and loss of equilibrium) and adecrease in predatory activity (lower fre-quency of attacks on prey and a general lossof aggressiveness). In the control tanks andat low ammonia concentrations, the usualresponse of the mosquitofish to an activebass was to avoid the predator and move tothe water surface and onto the shelves. Atlower prey densities a higher proportion ofthe mosquitofish took refuge from the bass.At the two highest concentrations of am-monia predatory activity of the bass oftenceased and the mosquitofish began chasingand nipping the bass, especially when theprey were at high densities. In six tankswith high ammonia concentrations and highprey densities the bass died within 10 days;

continuous harassment by the mosquitofishwas observed in all cases and was very like-ly a contributing factor.

DISCUSSION

The results of this study support the hy-pothesis that predator-prey interactions, asmeasured by the consumption and growthof a predator, are sensitive to sublethal con-centrations of ammonia. In the controls andat low NH3 concentrations, prey consump-tion and growth of largemouth bass in-creased as a function of prey density. Con-sumption and growth decreased withincreasing concentrations of ammonia incombination with increasing prey density(i.e., the greater the density, the greater theNH 3 effect).

If direct ammonia toxicity were the onlyfactor altering the baseline consumptionand growth rates, these responses wouldeither increase or remain unchanged withincreasing prey density for bass at a givenammonia concentration. However, at 0.63and 0.86 mg/liter NH 3 most of the consump-tion and growth rates decreased as preydensity increased. The inverse of an ex-pected functional response, with the highestconsumption at the lowest prey density, wasseen at the highest ammonia concentration.There was an apparent additional stress onthe bass brought about by the aggressivebehavior of the mosquitofish. Harassmentwas probably in response to a reduction inpredatory activity of the bass initiated by acombination of stresses from ammonia andthe number of mosquitofish encountered.

With increasing concentrations of am-monia fewer mosquitofish were needed toelicit the aggressive behavior. Noticeabledecreases in predatory activity and the oc-currence of predator harassment were firstobserved at prey density 120 for 0.34 mg/liter NH 3 tests, 60 for 0.63 mg/liter NH 3, and30 for 0.86 mg/liter NH 3 . In preliminary ex-periments, involving a single bass and 400mosquitofish with no ammonia added, basswere continuously harassed and did not livefor more than a few days. At the higher am-monia concentrations deviations of bass'consumption and growth in duplicate testtanks (e.g., at 0.63 mg/liter NH 3 and prey

504 TRANS. AM. FISH. SOC., VOL. 107, NO. 3, 1978

density 60) can he related to onset and du-ration of mosquitofish harassment, and ifand when the bass died.

Experiments at other laboratories havebeen designed to study changes in predator-prey interactions resulting from exposure ofthe prey to various stressors. Increased vul-nerability of prey to predation by fish hasbeen demonstrated for sublethal levels ofradiation (Goodyear 1972), thermal stress(Coutant et al. 1974), mercury (Kania andO'Hara 1974), and pesticide (Tagatz 1976).In the present study both predator and preywere simultaneously and continuously ex-posed to the toxicant for the duration of theexperiment. The largemouth bass showed agreater sensitivity to ammonia than the mos-quitofish both in acute lethal tests and inbehavioral parameters when exposed tosublethal concentrations. Ammonia, at con-centrations not directly lethal to either spe-cies, altered the survival of both through acombined concentration and density-depen-dent interaction; it was detrimental to thebass (lethal at the extreme) and as a resultof decreased predation was beneficial to themosquitofish.

ACKNOWLEDGMENTS

Research was supported by SupplementNo. 127 to the Memorandum of Understand-ing between Oregon State University andthe Pacific Northwest Forest and Range Ex-periment Station, USDA Forest Service;and by US Public Health Service TrainingGrant No. 5T01 GM01192.

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