The Hidden Killers
Unraveling Why Bycatch Doesn't Survive
The Unseen Toll of Fishing
Bycatch—marine species caught while targeting other fish—drives declines in sea turtles, seabirds, sharks, and marine mammals. While capture is devastating, post-release mortality often remains invisible. Even animals discarded alive may die later from injuries, stress, or exhaustion. Understanding why requires examining physiological trauma, fishing methods, and environmental factors 1 8 .
Bycatch Fast Facts
- Over 40% of global catch is bycatch
- 300,000+ cetaceans die annually as bycatch
- Some shark populations have declined by 90% due to bycatch
A sea turtle entangled in fishing gear - one of millions affected annually
How Fishing Turns Deadly: Key Mortality Factors
Physical Injury & Barotrauma
Gear-inflicted wounds: Gillnets suffocate entangled animals, while longline hooks cause internal bleeding. Trawls crush species in heavy nets.
Pressure changes: Deep-dwelling fish hauled rapidly to the surface experience "barotrauma"—swim bladders rupture, eyes bulge, organs protrude from mouths. Few survive release 8 .
Physiological Stress
Exhaustion: Prolonged fights in nets deplete oxygen reserves. Sharks enter metabolic acidosis, their blood turning toxic.
Thermal shock: Surface temperature extremes kill species adapted to stable depths.
Handling & Exposure
Air exposure: Sea turtles drown when trapped underwater but suffer cardiac stress when hauled on deck. Just 5 minutes of air exposure triples mortality in some reef fish 6 .
Predation: Discarded animals become easy prey.
Mortality Pathways in Major Gear Types
| Gear Type | Key Mortality Factors | High-Risk Species |
|---|---|---|
| Gillnets | Entanglement, suffocation, seabird diving | Seabirds, turtles, small cetaceans |
| Longlines | Hook ingestion, internal bleeding | Sharks, albatrosses, turtles |
| Trawls | Crushing, barotrauma, compaction | Starfish, juvenile fish, rays |
| Purse seines | Net compression, "rollover" release | Redfish, jacks, forage fish |
Spotlight Study: Decoding Death in the Gulf Menhaden Fishery
The Experiment
A landmark 2024 study quantified bycatch mortality in Louisiana's menhaden fishery. Researchers observed 418 net sets (3.2% of total) using cameras and holding tanks to track survival 6 .
Methodology
Observed pathways:
- Rollover bycatch: Animals released from nets after pumping (e.g., large redfish).
- Chute bycatch: Fish separated via grate systems during pumping.
- Retained bycatch: Non-target species stored in holds (100% mortality).
Survival assays: Tagged released fish and held subsamples in seawater tanks for 48h.
Results & Analysis
- 22,000+ breeding redfish killed annually despite Louisiana's ban on harvesting them recreationally.
- 98% mortality in "chute bycatch" due to scale loss, gill damage, and pressure shock.
- Forage fish devastation: 81 million croaker and 25 million sand seatrout discarded dead annually, destabilizing food webs 6 .
| Pathway | Example Species | Mortality Rate |
|---|---|---|
| Rollover release | Large redfish | 17% |
| Chute separation | Black drum | 98% |
| Retained in hold | Croaker | 100% |
The Vessel Factor: How Skipper Skill Alters Survival
Australian studies reveal individual vessel operators dramatically sway bycatch rates. In 15 fisheries examined, "high-target, low-bycatch" vessels proved:
- Proactive mitigation: Adjusting set times to avoid seabird feeding, using deeper hooks.
- Gentler handling: Wet towels protect turtle shells; dehookers reduce bleeding in sharks.
- Real-time adaptation: Moving after rare species interactions 1 .
The Scientist's Toolkit: Technologies Tracking Mortality
Electronic Monitoring
Video surveillance of hauling. Monitors handling practices in trawls 2 .
Genetic Stock ID
Links bycatch to breeding populations. Found 61% of albatross bycatch from one colony 7 .
Accelerometer Tags
Tracks post-release movement & mortality. Revealed 80% of discarded sharks die within 12h.
BRDs
Exclusion devices (e.g., turtle TEDs). Cut sea turtle deaths by 99% in trawls 8 .
Rethinking Solutions: From Gear to Incentives
Timing Matters: The Gillnet Revolution
Newfoundland herring fishers tested 12-hour vs. 24-hour net soaks. Night-only sets reduced seabird bycatch to near zero while maintaining herring catch. Why? Herring feed nocturnally; seabirds dive by day 9 .
The Payment Paradox
A 2025 Indonesian trial paid fishers to release endangered hammerheads. Live releases rose 71% for wedgefish, but some vessels increased hammerhead catches to earn more. Solutions:
- Capped payments per vessel
- Gear swaps (e.g., circle hooks) to avoid catches altogether 4 .
Toward a Mortality-Free Future
Reducing bycatch deaths demands more than new nets. It requires:
- Vessel-specific management leveraging skilled fishers 1
- Quiet technologies like infrared cameras to non-invasively assess stress
- Handling protocols minimizing air exposure and injury.
As genetic tools reveal which populations are most vulnerable 7 , and AI predicts high-risk zones, the goal shifts from counting corpses to saving survivors.
Infographic showing a fish's path from capture to release/death, with mortality "hotspots" labeled