The impact of drought on the distribution of fish communities in the Mullaroo-Creek – Lindsay River complex

"June 2009".

Project Number: Impacts of drought on distribution of fish communities in the Mullaroo Creek - Lindsay River Complex - M/BUS/255.

MDFRC item.

This study was initiated and designed in response to a protracted period of low flows predicted to occur in the lower River Murray region as a result of extreme drought, low inflow conditions and imposed water management strategies across the Murray-Darling catchment during 2007. Previous research has demonstrated that the Lindsay Island anabranch complex in north-western Victoria supports robust populations of native and exotic fish species. The Mullaroo Creek, an anabranch of the River Murray that dissects Lindsay Island is of particular importance to management of Murray cod in the lower Murray region as it supports what is considered to be a robust self sustaining population that is important to sustainability of Murray cod at the broader regional level. During the predicted ‘Low Flow’ period, the River Murray at Lock 7 ceased to flow for a total of 166 days and average daily flow decreased by 87% as compared to the preceding five year or 'Base Flow' period. Despite prevailing low flows and protracted periods of zero flow in the Murray River at Lock 7 during 2007 and 2008, flow into the Mullaroo Creek, with its off-take 2 km upstream from Lock 7, did not cease to flow. During the Low Flow period, average daily flow in the Mullaroo Creek was reduced by 16.5% compared to the average daily flow recorded during the Base Flow period. There was an opportunity to combine five years of existing fish assemblage data (2002 – 2006) compiled by the MDFRC for the Lindsay Island anabranch complex at Base Flows, with two years of data gathered during the protracted period of Low Flow (2007 - 2008), to enable examination of changes in the spatial distribution, assemblage structure, abundance and population size structure of native and exotic fish species in response to the influence of the protracted Low Flow period. Distributing the analysis of fish sampling effort across Base Flow and Low Flow periods revealed that there was no significant change to the composition of the large bodied fish assemblage (all species combined) relative to flow conditions across Lindsay Island during this study. However, there were significant changes regarding the abundance of particular species at particular reaches. The relative abundance of Murray cod in the Upper Mullaroo Creek increased during the Low Flow period as compared to the Base Flow period. The most important consideration here is that the relative abundance of Murray cod in the Mullaroo Creek did not decrease during the Low Flow period and there are several potential explanations to explain this observation. Unlike Murray cod, the relative abundance of golden perch decreased during the Low Flow period, potentially result of a lack of observable recruitment across the study reaches since 2005. In the context of decision making regarding the alteration of flow regimes and therefore the flow habitats of fish at TLM Icon sites, consideration should be given to the findings of this project. Examination of the length-frequency distribution of Murray cod in the Mullaroo Creek highlights the significant impact result of recreational harvest upon the population structure of Murray cod in the Mullaroo Creek. We consider recreational harvest to be the single most eminent threat to the status of Murray cod in the Mullaroo Creek and as a consequence, the broader region. We recommend that anglers be denied access to the Mullaroo Creek, from its off-take on the Murray River, to its confluence with the Lindsay River via the creation of a fisheries conservation zone, consistent with provisions existing within the Victorian Fisheries Act (1995).