Population structure, effective population size and adverse effects of stocking in the endangered Australian eastern freshwater cod, Maccullochella ikei
Nock, C.J., Ovenden, J.R, Butler, G.L., Wooden, I., Moore, A. and Baverstock, P.R., 2011. Population structure, effective population size and adverse effects of stocking in the endangered Australian eastern freshwater cod, Maccullochella ikei. Journal of Fish Biology, 78(1): 303–321.
Hatchery breeding programs have been used extensively to supplement populations of threatened fish species. The potential detrimental genetic effects have long been recognised but there is only limited empirical evidence for a loss of genetic variability in recipient populations following stocking. Genetic markers were used to examine diversity over space and time in the endangered eastern freshwater cod Maccullochella ikei.
High levels of population structure were detected in the Clarence River system, in north-eastern New South Wales. Distinct wild populations were found in the upper Nymboida River, the lower Nymboida and Mann Rivers, and in the remote western tributaries of Guy Fawkes and Sara Rivers and Washpool Creek. Estimates of effective population size for upstream populations were consistently below 50. The findings of this study suggest that the distribution of eastern freshwater cod is fragmented, and that remnant populations are small and isolated with restricted dispersal particularly among upstream sites.
Hatchery breeding programmes were used to re-establish locally extinct populations and to supplement remnant populations. Analyses of hatchery-bred fingerlings provided evidence for population mixing in the hatchery, with the majority of parental stock sourced from the upper Nymboida River. Comparison between historical bone and contemporary wild caught samples showed a significant loss of genetic variation (>20%) in the lower Mann and Nymboida Rivers since the commencement of stocking. Fragmentation may have been a causative factor; however, temporal shifts in allele frequencies suggest swamping with hatchery-produced cod has contributed to the genetic decline in the largest wild population of eastern freshwater cod.
This study suggests that the consequences of fragmentation and subsequent population mixture in the hatchery used for supportive breeding of eastern freshwater cod reduced genetic variation in captivity and in the wild. It demonstrates importance of using information on genetic variation and population structure in the management of breeding and stocking programs, particularly for threatened species.