Temporal Changes in the Growth Patterns of a Long-Lived Percichthyid from the Murray-Darling Basin; Anthropogenic or Natural Influences, or Both?

Understanding the age-and-growth of fish stocks is a cornerstone of successful fisheries management. However, collecting large numbers of individuals for ageing is most often not well received where catch-and-release practices dominate the fishery.  The result can be that these recreational fisheries are managed using outdated and potentially erroneous information. Murray cod is endemic to Australia’s largest river system; the Murray–Darling River. Murray cod is Australia’s largest freshwater fish (>one m and 100 kg) and is relatively long-lived, having being aged up to 48⁺ years. Anthropogenic influences led to dramatic a decline in the abundance of the species during the latter 20^th century.  Strict fisheries regulations and a range of remediation activities have resulted in partial recovery in some populations. To compare current with historical growth patterns from the 1970-80’s, we collected Murray cod otoliths in five river systems from across the Murray-Darling in 2010-12. There were significantly slower growth rates in all five systems compared to historical samples. We hypothesise that the differences result from a combination of anthropogenic and natural influences. We suggest that future enhancement programs not only consider stock numbers but also the antecedent and current environmental conditions and the potential carrying capacity.