The Secret Migrations
Unraveling the Mysteries of Southeastern Australia's Diadromous Fishes
Introduction
Beneath the surface of southeastern Australia's rivers and coastal waters unfolds one of nature's most extraordinary migratory journeys—a world where fish defy boundaries between fresh and saltwater in a relentless pursuit of survival. These are diadromous fishes, remarkable species whose lives are divided between oceanic and freshwater environments in a delicate dance that has evolved over millennia.
Did You Know?
Many diadromous species that were once plentiful are now generally rare or appear to be extinct in some catchments, their decline going largely unnoticed amid Australia's broader biodiversity challenges 2 .
What Makes a Fish Diadromous? The Three Pathways of Migration
Diadromy represents a specialized form of migration where fish move between marine and freshwater environments during predictable phases of their life cycle. Scientists recognize three distinct types of diadromy:
Anadromy
Born in Freshwater, Grow at Sea
Anadromous species are born in freshwater, migrate to the ocean to feed and grow, and return to freshwater to reproduce.
Example: Short-finned eel (Anguilla australis)
Catadromy
Born at Sea, Grow in Freshwater
Catadromous species are born in marine environments and migrate to freshwater, where they spend most of their lives before returning to the ocean to reproduce.
Example: Congolli (Pseudaphritis urvillii) 2
Major Diadromous Fish Species of Southeastern Australia
| Species | Type of Diadromy | Conservation Status | Key Habitats |
|---|---|---|---|
| Common galaxias (Galaxias maculatus) | Amphidromous | Secure | Coastal rivers, estuaries |
| Short-finned eel (Anguilla australis) | Catadromous | Declining | Rivers, lakes, wetlands |
| Australian bass (Percalates novemaculeata) | Catadromous | Vulnerable | Coastal rivers, estuaries |
| Cox's gudgeon (Gobiomorphus coxii) | Amphidromous | Secure | Coastal streams |
| Freshwater herring (Potamalosa richmondia) | Amphidromous | Declining | Rivers, estuaries |
| Freshwater mullet (Trachystoma petardi) | Amphidromous | Vulnerable | Coastal rivers |
The Evolutionary Puzzle: Why Make the Journey?
The evolution of diadromy represents a fascinating adaptation that has puzzled scientists for decades. Why would species develop such complex life cycles requiring physiological adaptations to survive in both freshwater and marine environments?
The Productivity Hypothesis
This theory, proposed by Gross et al., suggests that differences in ecological productivity between marine and freshwater biomes drive the evolution of diadromy 6 . According to this theory:
- Oceans have higher productivity than freshwaters in temperate regions
- Freshwaters have higher productivity than oceans in tropical regions
This would predict that anadromous species should evolve in temperate areas, while catadromous species should evolve in tropical areas.
Challenges to the Hypothesis
A phylogenetic study of Clupeiformes challenged this hypothesis, finding that different modes of diadromy did not have predictable ancestry based on latitude 6 . Instead, the research suggested that:
- Predation pressure influences diadromy evolution
- Competition plays a significant role
- Geological history may be at least as important as productivity
Distribution Patterns in Southeastern Australia
In southeastern Australia, the pattern of diadromy shows interesting latitudinal variations 2 :
This distribution suggests that multiple factors—including historical biogeography, competition, and habitat availability—have shaped the evolution of diadromy in the region.
A Study in Otoliths: Decoding Fish Migration Through Ear Stones
One of the most powerful tools for understanding diadromous fish migrations has been the study of otoliths—small calcium carbonate structures found in the inner ear of fish.
Micrograph showing growth rings in a fish otolith
The Experiment: Tracing Migration Through Microchemistry
A groundbreaking study by Miles (2007) demonstrated the power of otolith microchemistry for understanding the migratory histories of diadromous fishes in southeastern Australia 2 .
Methodology
- Laboratory validation: Fish were held in various salinities to establish that otolith strontium-to-calcium (Sr:Ca) ratios reliably reflected ambient salinity levels.
- Field collection: Wild fish were collected from the Shoalhaven and Clyde Rivers using various sampling methods.
- Microchemical analysis: Using advanced techniques including laser ablation inductively coupled plasma mass spectrometry.
Research Findings
| Species | Otolith Sr:Ca Patterns | Interpreted Migration Pattern | Longevity |
|---|---|---|---|
| Freshwater mullet (Myxus petardi) | Distinct variations | Amphidromous | ~5 years |
| Striped gudgeon (Gobiomorphus australis) | Distinct variations | Diadromous | Not specified |
| Freshwater herring (Potamalosa richmondia) | Less obvious patterns | Complex movements | ~10 years |
| Cox's gudgeon (Gobiomorphus coxii) | Less obvious patterns | Complex movements | Not specified |
| Bullrout (Notesthes robusta) | Less obvious patterns | Complex movements | >5 years |
Against the Current: Threats and Conservation Challenges
Diadromous fishes face numerous threats in southeastern Australia, many stemming from human modification of river systems and coastal environments.
Habitat Fragmentation
The construction of dams, weirs, and other river structures has created significant barriers to fish migration throughout southeastern Australia. As of 2023, the Nepean River alone had at least 278 structures affecting fish passage 7 .
Fishway Passage Efficiency
| Species | Passage Efficiency | Factors Affecting Passage |
|---|---|---|
| Cox's gudgeon | Variable | Water velocity at entrance and exit |
| Freshwater mullet | Lower in entry than exit traps | Entrance trap design, water velocity |
| Sea mullet | Lower in entry than exit traps | Entrance trap design, water velocity |
Climate Change
Australian seafood production, including diadromous fisheries, is increasingly exposed to environmental extremes driven by a changing climate 1 .
Climate Impacts Include:
- Altered river flow regimes
- Changing water temperatures
- Sea-level rise impacting estuarine habitats
- Changes in ocean currents affecting larval dispersal
Additional Threats
Pollution
Agricultural and urban runoff
Habitat Degradation
Loss of riparian vegetation
Water Extraction
Reducing flow volumes
Invasive Species
Competition and predation
The Scientist's Toolkit: How Researchers Study Diadromous Fishes
Understanding the hidden lives of diadromous fishes requires specialized tools and methods. Here are some of the key approaches used by researchers:
Otolith Microchemistry
Analyzing the chemical composition of fish ear stones to reconstruct migratory histories and environmental experiences.
Environmental DNA (eDNA)
Collecting water samples and analyzing them for trace DNA left behind by fish species 5 .
Fishway Monitoring
Specialized traps and monitoring systems installed at fishways to assess species passage efficiency 7 .
Genetic Analysis
Advanced genetic techniques to identify distinct populations and understand gene flow between regions 9 .
Acoustic Telemetry
Tracking movements in water bodies to map migratory routes and identify barriers.
Stable Isotope Analysis
Determining trophic relationships and habitat use by identifying energy sources and migratory connectivity.
Conclusion: The Future of Southeastern Australia's Diadromous Fishes
The diadromous fishes of southeastern Australia represent a remarkable evolutionary adaptation to the region's variable environments, but they face an uncertain future. While challenges are significant, growing scientific understanding and conservation efforts offer hope for these extraordinary migrants.
Conservation Approaches
- Collaborative management bringing together scientists, managers, policymakers, and local communities 1 8
- Connecting knowledge and cultural perspectives of Western science and First Nations people 1
- Developing species-specific strategies based on research findings
- Protecting all stages of migratory journeys across ecosystem boundaries
Future Research Directions
- Long-term monitoring of population trends
- Studying impacts of climate change on migration patterns
- Improving fishway design for different species
- Investigating genetic connectivity between populations
- Exploring adaptive management approaches