Every day, in fishing grounds around the world, a silent drama plays out beneath the ocean's surface.
As trawl nets sweep through the water, countless fish encounter these formidable structures—some are captured, while others manage to escape through the mesh openings. For decades, fishermen and scientists alike assumed that these lucky escapees simply swam away to continue their lives unharmed.
Starting in the early 2000s, a revolutionary body of research began to challenge this assumption, revealing a complex story of survival, injury, and physiological stress that occurs after escape.
The work of pioneering scientists like Davis and Olla has illuminated this hidden aspect of fishing impacts, transforming our understanding of what really happens to fish that encounter fishing gear—knowledge that is crucial for developing more sustainable fishing practices and effective fisheries management policies.
How fish navigate and escape from trawl nets
The physical toll of escape attempts on fish
Quantifying post-escape mortality across species
When we think about fishing impacts, our attention naturally turns to the fish that are caught and landed. But what about the ones that get away? For regulatory measures like minimum mesh sizes in nets to be effective, the fish that escape through these meshes must survive to reproduce. If they die anyway from their injuries, the conservation benefit is lost. This concern launched an entirely new field of study: escape survival research.
Scientists faced a formidable challenge: how to reliably assess survival of fish after they've escaped trawl nets under commercial fishing conditions.
Early approaches seemed straightforward—collect escaped fish in cages, monitor them, and see how many survive. However, these methods contained hidden flaws that either overestimated or underestimated true mortality rates 1 .
Cages placed behind trawls created artificial swimming conditions that exhausted smaller fish, while the reduced water flow around covered codends made escape easier than in real fishing conditions.
These methodological problems meant that for years, the scientific community lacked accurate data on what happened to escapees—until more sophisticated approaches emerged.
To address the shortcomings of early methods, researchers developed innovative remote-controlled cage systems that could collect fish without the biases of previous approaches. The system developed by Lehtonen, Tschernij, and Suuronen represents a typical advanced methodology used in modern survival studies 1 .
During the trawl tow, both gates of the collection unit remain open, allowing any fish escaping from the codend to swim freely through the cage into the open sea.
At a pre-set time, a timer activates the closure of the rear gate, beginning the collection period.
After an appropriate sampling time, a second timer triggers the closure of the front gate, confining sampled escapees inside the collection unit.
A third timer activates the separation of the collection unit, which then drops to the sea bed containing the captive fish for monitoring.
This ingenious system allows researchers to control sampling duration precisely and keep it substantially shorter than in previous experiments, meaning the sampling process doesn't depend on tow duration and avoids exhausting the fish.
Once collected, the real work of survival assessment begins. Fish are transported to holding tanks or—in more advanced studies—monitored in cages at depth to avoid decompression issues.
Research revealed that survival rates vary dramatically between species, reflecting their different physiologies and vulnerabilities to capture stress. The visualization below illustrates typical short-term survival patterns across species based on studies in the southern North Sea "eurocutter" beam trawl fishery 3 .
| Species | Common Name | Survival Rate | Visual Indicator |
|---|---|---|---|
| Sole | Common sole | 66% |
|
| Plaice | European plaice | 58% |
|
| Whiting | Whiting | 22% |
|
| Cod | Atlantic cod | 20% |
|
| Common starfish | Common starfish | >90% |
|
| Edible crab | Edible crab | 84% |
|
Perhaps the most significant finding across multiple studies is the strong correlation between visible injuries and subsequent mortality. Researchers developed injury scoring systems that reliably predict survival outcomes, offering a potential shortcut for survival assessment without extended holding periods.
Disrupted osmoregulation, increased infection risk
Mortality Risk: Moderate to High
Impaired respiration, reduced oxygen uptake
Mortality Risk: High
Broken protective barrier, potential for infection
Mortality Risk: Moderate
Impaired swimming, reduced foraging and predator avoidance
Mortality Risk: Moderate
Survival rates are profoundly influenced by both environmental conditions and fishing practices. The following factors play crucial roles in determining whether an escaped fish will survive:
Longer tows = lower survival due to extended exhaustion
Higher temperatures = lower survival due to increased metabolic rate
Smaller fish = lower survival due to less swimming endurance
Modern escape survival research relies on specialized equipment and methodologies designed to collect and monitor fish with minimal additional stress.
| Tool/Equipment | Function | Research Importance |
|---|---|---|
| Trawl Covers | Collect fish escaping from codend | Essential for obtaining escapees for study; must be designed to minimize flow disruption |
| Remote-Controlled Cages | Capture escapees at depth without surfacing | Avoids decompression injury; allows sampling at any point during tow |
| Timed Release Mechanisms | Open/close collection devices at predetermined times | Enables precise control over sampling duration; critical for standardized methodology |
| Holding Tanks/Cages | Maintain captured fish for observation | Allows extended survival monitoring; must mimic natural conditions |
| Injury Scoring Systems | Standardized assessment of physical damage | Provides predictors for survival probability; enables cost-effective assessment |
| Environmental Sensors | Measure temperature, salinity, depth, oxygen | Documents conditions affecting survival; essential for interpreting results |
The progression from simple cage systems to advanced remote-operated collection devices represents the ongoing methodological evolution in this field, driven by the recognition that the methodology itself must not contribute significantly to the mortality being measured 1 .
The research into fish escape survival has transformed our understanding of fishing impacts, revealing that the story continues long after a fish escapes a net.
We now know that survival depends on a complex interplay of species-specific vulnerabilities, fishing practices, environmental conditions, and gear design. This knowledge has profound implications for how we manage fisheries and measure fishing impacts.
As this field of science advances, it brings us closer to a more comprehensive understanding of our marine ecosystems—recognizing that sustainable fishing requires considering not just what we catch, but what we affect in the process.