University of Montana ScholarWorks at University of Montana Biological Sciences Faculty Publications Biological Sciences 1-1991 Shrimp Stocking, Salmon Collapse, and Eagle Displacement Craig N. Spencer B. Riley McClelland Jack Arthur Stanford The University of Montana, [email protected] Follow this and additional works at: https://scholarworks.umt.edu/biosci_pubs Part of the Biology Commons Let us know how access to this document benefits ou.y Recommended Citation Spencer, Craig N.; McClelland, B. Riley; and Stanford, Jack Arthur, "Shrimp Stocking, Salmon Collapse, and Eagle Displacement" (1991). Biological Sciences Faculty Publications. 292. https://scholarworks.umt.edu/biosci_pubs/292 This Article is brought to you for free and open access by the Biological Sciences at ScholarWorks at University of Montana. It has been accepted for inclusion in Biological Sciences Faculty Publications by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. Shrimp Stocking, Salmon Collapse, and Eagle Displacement Cascading interactions in the food web of a large aquaticecosystem CraigN. Spencer,B. RileyMcClelland, and Jack A. Stanford tocking of nonnative fish and 1986). In 1949, the shrimp were fish-food organisms has long introduced experimentally into been used by fishery managers Benefitsproduced by Kootenay Lake, British Columbia, to enhance production of freshwater introductionshave with the intention of enhancing rain- fisheries. Indeed, more than 25% of bow trout. However, they were the freshwater fish caught by anglers come at considerable largely responsible for a dramatic in- in the continental United States is cost crease in the growth rate and size of from nonnative stocks (Moyle et al. kokanee salmon (Oncorhynchus 1986). I nerka; Northcote 1972, 1973). After Recent studies, however, report this initial introduction, opossum negative effects of such introductions, tions have been described as the shrimp were stocked into more than especially on the native species Frankensteineffect by Moyle et al. 100 lakes in the northwestern United (Moyle 1986, Schoenherr 1981, Tay- (1986). Althoughthe most serious,or States and Canada, primarily to stim- lor et al. 1984). For example, intro- at least most widely reported,effects ulate production of kokanee (Lasenby duced rainbow trout (Oncorhynchus appear to be on native fish popula- et al. 1986, Martinez and Bergersen mykiss) have displaced native west- tions, other organisms also may be 1989, Northcote in press). slope cutthroat trout (Salmo clarki affectedby introducedspecies. Between 1968 and 1975, opossum lewisi) in many Rocky Mountain In this article,we documentpartic- shrimp were introduced into three streams (Allendorf and Leary 1988), ularly widespreadchanges cascading lakes in the upper portion of the and introduced brown trout (Salmo throughthe food web of the Flathead Flathead catchment of northwest trutta) have largely replaced native River-Lakeecosystem (Figure 1) after Montana (Figure 2) by the Montana brook trout (Salvelinusfontinalis) in the introduction of the opossum Department of Fish, Wildlife, and various streams in the Midwest shrimp(Mysis relicta). Owing to pre- Parks. The shrimp drifted down- (Moyle et al. 1986). In the Southwest, dation by the shrimp, copepod and stream, and in 1981 they appeared in native Gila topminnows (Poeciliopsis cladoceranzooplankton populations Flathead Lake, one of the largest nat- occidentalis) have been widely re- declineddramatically, contributing to ural freshwater lakes in the western placed by introduced mosquitofish the collapseof an importantplanktiv- United States (surface area 482 km2, (Gambusia affinis; Schoenherr 1981, orous fish population. Loss of this mean depth 52 m). Shortly thereafter, Stanford and Ward 1986). The result- formerlyabundant forage fish caused they began to alter the existing food ing negative, long-range, or broad- displacementof birds and mammals web. scale consequences of such introduc- that had fed on them in an upstream tributary within Glacier National Impacton zooplankton Park. and Craig N. Spenceris a researchassistant phytoplankton professorand Jack A. Stanfordis director Shrimpintroduction Opossum shrimp are voracious pred- and BiermanProfessor of Ecology at the ators of zooplankton (Figure 1); FlatheadLake BiologicalStation, Univer- are 1-2- are but clado- of MT 59860. B. Opossum shrimp small, copepods consumed, sity Montana, Polson, cold-water crusta- cerans are because Riley McClellandis a researchscientist at centimeter-long, preferred prey they GlacierNational Park,West Glacier,MT ceans that carry their young in a swim more slowly and are easier to 59936 and professor in the School of brood pouch, hence their name. They capture (Cooper and Goldman 1980, Forestry, University of Montana, Mis- are native to a limited number of Grossnickle 1982, Nero and Sprules soula, MT 59812. ? 1991 AmericanIn- large, deep lakes in North America 1986). After the appearance of opos- stitute of BiologicalSciences. and coastal Sweden (Lasenbyet al. sum shrimp in Flathead Lake, clado- 14 BioScienceVol. 41 No. 1 This content downloaded from 150.131.192.151 on Wed, 30 Oct 2013 18:54:27 PM All use subject to JSTOR Terms and Conditions , n phytoplankton McDonald Creek {f~ ~Dj~ ~ copepod cladoceran kopkanee salmon opossum shrimp Flathead Lake Figure1. The food web of the FlatheadRiver-Lake ecosystem, emphasizing those componentsaffected by the introductionof opossumshrimp. Eagles and bearsfeed on spawningkokanee, lake trouteat kokaneeand shrimp,kokanee and shrimpeat zooplankton(copepods and cladocerans), and zooplankton feed on phytoplankton.The organisms are not drawn to scale;they range in sizefrom several micrometers (phytoplankton) to meters(grizzly bears). cerans declined dramatically, with and 1989, two cladocerans,D. lon- increased nutrient loading.1 Thus it mean annualabundances reduced sig- giremis and L. kindtii, reappearedin appearsthat the phytoplanktoncom- nificantly (p < 0.0001, Student's t FlatheadLake for the first time since munity in FlatheadLake is regulated test) from 2.8 to 0.35 organisms/I 1985, albeit at low densities. largelyby "bottom-up"controls (nu- (Figure3a, also Beattie and Clancey Hrbaceket al. (1961), Spencerand trients), whereas top-down controls, in press). Two of the four principal King (1984), and Carpenter et al. involving changes in the upper cladoceranspecies (Daphnia longire- (1985) have suggestedthat predator- trophic levels, may play a largerrole mis and Leptodora kindtii) disap- induced alterations to the herbivo- in regulatingphytoplankton produc- peared from lake samples, and the rous zooplankton community can tion in eutrophiclakes. other two (Daphnia thorata and cause "top-down" effects potentially Bosmina longirostris) persisted, but regulating the abundance of phyto- Impact on fish at greatly reduced densities. Cope- plankton (free-floatingalgae) and the pods also declined significantly(p < appearanceof algal blooms in lakes. Although top-down effects extending 0.0001; Figure3b). Althoughdeclines For example, increased densities of to the phytoplanktonare not appar- in cladoceranshave been well docu- planktivorousfish have been linkedto ent in Flathead Lake, the effects of mented after opossum shrimp intro- decreasedzooplankton densities and predationby opossum shrimpon the ductionsin other lakes (Morganet al. greatly increased phytoplankton zooplanktonare widespreadand have 1978, Richardset al. 1975, Rieman abundancein nutrient-rich(eutrophic) carried over to upper trophic level and Falter 1981), similar declines in lakes. Mean annual phytoplankton organisms, including the kokanee copepods have not been reported, productionhas increasedslightly since salmon. Kokanee (landlocked sock- suggestinga higher level of predation 1978 in FlatheadLake, a nutrient-poor eye salmon) were introduced into by the shrimpon zooplanktonin Flat- (oligotrophic)lake (Stanfordand Ellis FlatheadLake in 1916, and thereafter head Lake. 1988). they replaced the native westslope After peaking in 1986, opossum However, recent experimentsindi- cutthroattrout as the dominantsport shrimp densities declined for three cate that increased phytoplankton fish (Tables 1 and 2). successiveyears (Figure4). The zoo- production after the appearance of Kokanee normally lived 3-4 years plankton community appears to be opossumshrimp is not due to declines respondingto this reductionin opos- in zooplankton grazing pressure in 'C. N. Spencer,J. A. Stanford,and B. K. Ellis, sum shrimpabundance. During 1988 FlatheadLake, but ratheris relatedto 1990, unpublisheddata. January1991 15 This content downloaded from 150.131.192.151 on Wed, 30 Oct 2013 18:54:27 PM All use subject to JSTOR Terms and Conditions (Fraley et al. 1986). From 1979 to 1985, the peak annual number of kokanee spawners varied between 26,000 and 118,000 (Figure4). By 1985, the density of opossum shrimp in FlatheadLake had increasedto 49 organisms/m2,illustrating the rapid growth of this invading species. Within two years, the kokaneepopu- lation was noticeably reduced. In 1987, only 330 kokanee migrated into McDonald Creek, and only 50 spawnedin 1989. The salmondemise also was reflectedin the annualcatch by angler fishermen,which often ex- ceeded 100,000 kokanee through 1985 (Beattie et al. 1988). As the shrimp population grew, angler har- vest declined precipitouslyto fewer than 6000 in 1987 (Beattie et al. 1988), followed by no reported catchesin 1988 and 1989.