Variable stocking effect and endemic population genetic structure in Murray cod Maccullochella peelii

Abstract

Microsatellite markers were utilized to examine the genetic structure of Murray cod Maccullochella peelii throughout its distribution in the Murray--Darling Basin (MDB) of eastern Australia, and to assess the genetic effects of over three decades of stocking hatchery-reared fingerlings. Bayesian analysis using the programme Structure indicated that the species is largely genetically panmictic throughout much of its extensive range, most probably due to the high level of connectivity between catchments. Three catchments with terminal wetlands (the Lachlan, Macquarie and Gwydir), however, contained genetically distinct populations. No stocking effects were detected in the catchments that were genetically panmictic (either because of low genetic power or lack of effects), but the genetically differentiated Gwydir and Macquarie catchment populations were clearly affected by stocking. Conversely, there was no genetic evidence for survival and reproduction of stocked fish in the Lachlan catchment. Therefore, stocking of M. peelii throughout the MDB has resulted in a range of genetic effects ranging from minimal detectable effect, to substantial change in wild population genetic structure.

Identifying environmental correlates of intraspecific genetic variation

“…We used existing genotype data for 16 microsatellite loci that were previously published in Rourke et al (2011), removing individuals with data missing for 420% of loci. Murray cod samples used here (N = 616) were sourced from 15 major river catchments across the MDB (Figure 1 and Table 1) and collected between 1994 and 2006 (87% were sampled between 2000 and 2006).…”

“…The larval duration of the Murray cod is typically o1 month, and sexual maturity is reach at ∼ 4-6 years of age (Rowland, 2004). Terminal wetlands partially isolate the Lachlan, Macquarie and Gwydir catchments, and these catchments are modestly genetically differentiated (mean pairwise F ST = 0.06; range: 0.03-0.13) (Figure 1; F ST values from Appendix VI in Rourke et al, 2011). The remaining major river catchments in the Murray Darling Basin are connected by moderate to high levels of gene flow (mean pairwise F ST = 0.01; range: 0-0.04) (Figure 1; F ST values from Appendix V1 in Rourke et al, 2011).…”

“…Although wild Murray cod are no longer fished commercially (the last fishery closed in 2003), recreational fishing is still permitted (Lintermans et al, 2005). Supplementation of wild populations using hatchery-reared stock has been used since the 1970s to support the continuation of recreational fishing, with tens of millions of fish released across the MDB (mostly as fingerlings into impoundments) (Lintermans et al, 2005;Rourke et al, 2010;Rourke et al, 2011). Limited monitoring of recruitment success of stocked individuals means that the effects of stocking on wild populations are not well understood.…”

“…Evidence of localised effects of stocking exists in some catchments. For example, a proportion of genetically nonadmixed individuals in the Warrego, Namoi and Border rivers may indicate the presence of stocked individuals (Rourke et al, 2011). However, across large areas of the MDB (thousands of river km across the southern parts of the basin and the more northern Darling and Condamine catchments), stocking has not had detectable effects on patterns of genetic diversity or population structure (Rourke et al, 2010(Rourke et al, , 2011.…”

Adaptive management in action: using chemical marking to advance fish recovery programs in the Murray–Darling Basin

“…Stock enhancement has occurred in virtually every single sub-catchment (Rourke et al 2011). Any large scale fish monitoring programs are therefore confounded by stocking because there was no mechanism to discriminate hatchery and wild fish.…”