Macroinvertebrates
Understanding the ecological consequences of macroinvertebrate community-structure change
A healthy and productive fish community relies on the underlying food web to provide energetic and nutritional needs. Macroinvertebrates are central to those food webs and in the Murray River are an important food source for native fish. Any changes that lead to alterations in macroinvertebrate communities will alter the nutritional landscape available for native fish.
Background
The 35-year monitoring program of macroinvertebrates in the Murray River (the River Murray Biological Monitoring Project (RMBMP), funded by the Murray–Darling Basin Authority has shown that macroinvertebrate communities in the Murray River have changed over time and at different sites.
The aim of this component of the MMCP Collaboration is to quantify the ecological consequences of those changes. In doing so, we will generate a conceptual model that provides information to target management activities directly towards the key elements of the food web that have the greatest effect on the productivity of fish communities. This project links with activities undertaken within the Long Term Intervention Monitoring (LTIM) and Murray–Darling Basin Environmental Water Knowledge and Research (EWKR) Food Web and Fish projects.
In the first instance, we propose examining two approaches:
Energetics approach
The Murray–Darling Basin Environmental Water Knowledge and Research (EWKR) Food Web Project has recognised the need to understand the role of trophic dynamics in determining outcomes of managed hydrological regimes. Established ecological theory allows prediction of the energy distribution within different food web structures. We will use this approach to further interrogate the River Murray Biological Monitoring Program (RMBMP) long term dataset, to establish how changes in size structures within the macroinvertebrate community alters the energy distribution of Murray River food webs. The overall impact is the extent to with energy can be transferred to fish production.
Nutritional approach
Macroinvertebrates in the Murray River are an important food source for native fish and any changes that lead to alterations in macroinvertebrate communities, either abundances of all or specific organisms and/or community composition, will alter the nutritional landscape available for native fish.
As with all consumers, a ‘balanced diet’ is critical to maintaining healthy individuals and populations. Diets with balanced amounts of macronutrients are known to be important in fish communities. At the biochemical level, the aquaculture industry has identified a suite of ‘indispensable amino acid requirements (IAA)’ (also termed essential amino acids) for fish growth (Cowey 1995; Mambrini and Kaushik 1995). Understanding the overall nutritional requirements has informed the design of food sources to optimise fish growth in the aquaculture sector. While the aquaculture industry has examined nutritional needs from an industrial perspective (e.g. including growth of Murray cod ;De Silva et al. 2000), the ecological significance of changes in fish diets has not received attention.
Early work at the Centre for Freshwater Ecosystems has shown that the food sources (macroinvertebrates) can vary in the extent to which they supply a balanced diet to fish, but it remains to be shown how ecologically significant these controls are, compared to other drivers of fish populations.
Objectives
The objectives of this research theme are to:
- Draw on the long-term Murray River Monitoring dataset to quantify the extent to which changes in the size structure of the macroinvertebrate communities has led to changes in the energy distribution, and thus energy resources available for fish.
- Synthesise and summarise the relevant literature on the key aspects of fish nutrition that have greatest ecological significance in driving fish growth and reproduction. This will involve investigating the relative importance of macronutrients and micronutrients in generating healthy populations of higher order consumers in the river.
- Overall, to explore whether or not the decline in native fish can be attributed to observed shifts in the types and distributions of potential prey items and how management options can be used to optimise food resources.
Immediate objectives
- Drawing on the long-term Murry River Monitoring dataset, to quantify the extent to which changes in the size structure of the macroinvertebrate communities has lead to changes in the energy distribution, and thus energy resources available for fish.
- Synthesise and summarise the relevant literature on the key aspects of fish nutrition that have greatest ecological significance in driving fish growth and reproduction. This will involve investigating the relative importance of macronutrients and micronutrients in generating healthy populations of higher order consumers in the river.
- The overall objective is to explore whether or not the decline in native fish can be attributed to observed shifts in the types and distributions of potential prey items and how management options can be used to optimise food resources.
Medium to long-term objectives
Management implications
‘Protect and restore the ecosystem functions of water-dependent ecosystems’
The management and restoration of native fish populations has primarily targeted the maintenance of flows and habitats that promote recruitment and growth of larval and juvenile fish. The success of these actions may be limited due to a poor understanding of how changed flow regimes have modified the occurrence of appropriate food resources that are necessary to support the recruitment and growth of native fish. The MMCP Collaboration will provide water resource managers with the knowledge on how to manipulate flow regimes to support and maintain food resources that promote the growth of native fish.