Piked Dogfish Squalus acanthias
Sharks are not normally shoaling fish, but piked dogfish aggregate into huge groups numbering thousands of individuals, often all of one sex and size. Also known as the spurdog or spiny dogfish, this sleek, dark gray shark has two dorsal fins, each with a sharp spine in front of it. Irregular white spots decorate its sides, especially in young fish, and it has a pointed snout and large oval eyes. It was once very common and was possibly the most abundant species of shark, but it is now threatened globally as a result of overfishing. These sharks do not begin to breed until they are 10–25 years old and may live to be 70–100 years old. They grow very slowly, and the young take up to two years to develop inside the mother. Some populations migrate thousands of miles seasonally in order to avoid very cold water.