Sharks and Rays
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, and
- long life spans.
Contrary to the belief that sharks (and to a lesser extent rays) are hardy and that they can sustain rough handling or extensive exposure and still survive when returned to the sea, initial studies suggest a survivability rate of only 50 percent, even if they appear healthy upon release, most probably due to severe stress and/or injury sustained during the fishing and handling process.
This high mortality rate is surely influenced by a number of biological “weaknesses” in sharks and rays. Unlike other fish, these animals do not have a hard skeleton of bone to protect their internal organs. When out of water, the tissue that holds organs in place can tear and the weight of gravity can result in crushed or damaged organs. This same connective tissue holds the spinal cord and vertebrae in place, and for this reason, animals handled from the head or tail can suffer irreversible damage as a result. A shark’s head also holds a number of sensitive and fragile organs used to detect prey, and if handling damages these, then the shark – once released—could be unable to locate prey and starve.