Purse Seine Guidebook
While not the most common bycatch in tuna purse seine fishing, sharks and rays are the most vulnerable to its effects. Several aspects of their biology make them highly susceptible to overfishing, including:
- Slow growth rates
- Late maturation
- Long pregnancies
- Low fertility
- Long life spans
It is not true that sharks and, to a lesser extent, rays are hardy and able to withstand rough handling or prolonged exposure and still survive when returned to the sea. In fact, initial studies suggest both sharks and rays have a survivability rate of only 50%, even if they appear healthy upon release. The low survival rate is most probably due to severe stress and/or injury sustained during the fishing and handling process.
This high mortality rate of bycaught sharks and rays is likely influenced by several biological characteristics. Unlike other fish, these animals lack a hard bony skeleton to protect their internal organs. When the animals are out of water, the tissues that hold their organs in place can tear, and the force of gravity can cause organs to become crushed or damaged. This same connective tissue also supports the spinal cord and vertebrae, making sharks and rays especially susceptible to irreversible injury when handled by the head or tail.
Additionally, a shark’s head contains numerous sensitive and fragile organs used for detecting preys. If these are damaged during handling, then the shark – once released— may be unable to locate prey and could starve.
Shark anatomy
