148 



Fishe 



Our Liviii/i Kcsiiunes 



— Native range 

 □ FL subspecies 

 a 



■ Hybrid subspecies 

 □ North- subspecies 



Figure. Loss of genetic variation 

 among largenioutli bass popula- 

 tions, a. Tlie native range of the 

 largemouth bass {Micivpleriis 

 salmoides) is delineated by tfie red 

 lines (MacCnnimon and Robbins 

 1975). As first described by Bailey 

 and Hubhs (1949), the Florida sub- 

 species, M.S. floridamis. was 

 restricted to peninsular Florida 

 (blue); the northern subspecies, 

 M.S. scilmoides. covered most of the 

 rest of the range of the species; and 

 there was a relatively small inter- 

 grade zone between the two result- 

 ing from some indeterminable com- 

 bination of natural hybridization 

 and human-caused mixing of 

 stocks, b. The expansion of the 

 intergrade by 1980 was described 

 by Philipp et al. (1983). Because 

 detailed ranges were not explored 

 in all states, and because this inter- 

 grade zone expansion was likely 

 caused by state stocking programs, 

 entire states are classified according 

 to whether the intergrade zone was 

 expanded, c. The current intergrade 

 zone is now even larger because of 

 the addition of more states in which 

 largemouth bass containing at least 

 some M.S. floridaiius genes are 

 being introduced either by the state 

 fish and game agencies themselves 

 or by private groups. Notice that the 

 entire southern and eastern portion 

 of the original range of the northern 

 subspecies. M.s. salmoides, is at 

 risk of being inundated with M.s. 

 floridanus genes. 



practice among fisheries management agencies 

 and the private sector. 



Largemouth Bass 



Largemouth bass (Micropteriis salmoides) 

 exemplify how introduction programs cause the 

 loss of genetic diversity. The original range of 

 the largemouth bass was restricted to parts of 

 the central and southeastern United States 

 (Figure), extending northward into some of 

 southern Ontario (MacCrimmon and Robbins 

 I97.S). Bailey and Hubbs (1949), however, 

 described two subspecies. The Florida sub- 

 species, M.S. floridanus, was formerly restricted 

 to much of peninsular Florida (Figure, a), 

 whereas the range of the northern subspecies, 

 M.S. salmoides, extended north and west of an 

 intergrade zone that included pails of South 

 Carolina, Georgia. Alabama, and northern 

 Florida, It is likely, though, that the intergrade 

 zone had already been expanded from the orig- 

 inal natural hybrid zone as a result of early fish 

 stocking programs. 



Since 1949, however, much more serious 

 stocking efforts have extended this intergrade 

 zone. A survey of largemouth bass populations 

 conducted in the late 1970"s (Philipp et al. 

 1983) revealed that the intergrade zone had 

 grown considerably larger through the deliber- 

 ate stocking efforts of the involved state agen- 

 cies (Figure, b). Additional introductions of 

 M.S. floridanus since that genetic survey have 

 now spread the genes of that subspecies across 

 the entire southern range of M.s. salmoides 

 (Figure, c). 



This introduction of the Florida largemouth 

 has compromised the genetic integrity of all the 

 populations of the northern largemouth bass 

 into which the species has been introduced 

 (populations in Texas. Oklahoma. Arkansas. 

 Louisiana, Mississippi, Tennessee, Alabama, 

 Georgia, South Carolina, North Carolina, 

 Virginia, and Maryland, at a minimum). Those 

 now-genetically mixed populations have lost 

 much of their distinctness because of the loss of 

 among-population genetic variation that accom- 

 panies this type of homogenization. Populations 

 other than those in the water bodies actually 

 stocked will be affected as well because of 

 inevitable gene flow into and between other 

 connected populations. As a result, genetic 

 integrity is now at risk for all populations of this 

 important sportfish species throughout the 

 southern and eastern portions of its native 

 range. 



In addition, because the two subspecies have 

 quite different characteristics (Cichra et al. 

 1982; Fields et aL 1987; Kleinsasser et al. 

 1990), these massive stock transfers will likely 

 result in outbreeding depression. More specifi- 



cally, the Florida subspecies exhibits signifi- 

 cantly poorer survival, growth, and reproductive 

 success in Illinois than does the northern sub- 

 species (Philipp 1991; Philipp and Whitt 1991). 

 Also, the offspring resulting from crossing the 

 two subspecies (in either direction) are less fit 

 in Illinois than are the offspring of the pure 

 northern subspecies (Philipp 1991). These 

 results extend to populations of the northern 

 subspecies across its range from Texas to 

 Minnesota (unpublished data). 



Conclusions 



The genetic integrity of largemouth bass 

 stocks, and likely of many other managed fish 

 species as well, is eroding as a result of man- 

 agement programs that inadvertently permit or 

 deliberately promote stock transfers. This caus- 

 es not only the loss of genetic variation among 

 populations, but through outbreeding depres- 

 sion it is also probably negatively affecting the 

 fitness of many native stocks involved. We need 

 to address genetic integrity when restoring 

 native populations. 



References 



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 1-19 in N. Ryman and F.M. Utter, eds. Population genet- 

 ics and fishery management. Washington Sea Grant 

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Altukhov. Y.P. and E.A. Salmenkova. 1987. Stock transfer 

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Bailey. R.M.. and C.L. Hubbs. 1949. The black basses 

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