There are several hypotheses to explain why marine mammals become entangled in fishing nets. Here are some of them:
- Marine mammals that live in the open ocean do not
understand the concept of a large barrier such as a
gill net. They don't recognize nets as a danger, or
they make mistakes while trying to navigate around
- The animals become entangled while investigating
nets as strange, new objects.
- Marine mammals (especially seals and sea lions) learn to associate nets with a "free lunch." They are attracted to the fish caught in the nets and become entangled themselves while eating the fish.
We also want to investigate when marine mammals use echolocation (biological sonar) and when they use their sense of sight. It may be possible that:
- Dolphins and porpoises do not see the nets with
their eyes because the water is murky (clouded) and
shallow where the nets are used, or they do not see
the nets at night.
- Dolphins and porpoises are able to "see"
gill nets with their echolocation. However, they panic
when they run into nets at times when they are not
using their echolocation.
- Marine mammals would see the nets if they were designed to be easily detected by the animals' biological sonar.
There are many questions that we hope our study will try to answer. These kinds of questions are very difficult to answer in the wild because conditions cannot be easily controlled. That is why our research is being done with captive animals at Sea World. In this laboratory setting, we can control things like how many animals are present during experiments, how deep the water is, how big the net fragments are, how close the animals can get to the nets, and which species we want to see interact with the nets.
We can also use a variety of research techniques with captive animals that are difficult or impossible to use in the wild. We can use both under- and above-water video cameras to observe an animal's behavior. We can use hydrophones (underwater microphones) mounted in the pools to record the calls and echolocation of the dolphins and porpoises. We can see whether the animals become used to the pingers by controlling how often, and for how long, we let different species hear the sounds.