Similar to how the age of a tree is determined by the rings in the trunk, certain bony structures on a fish can be cross-sectioned and examined for bands, or annuli, that are laid down each year. The ages of fish are often determined by the annuli on otoliths (inner ear bones), dorsal or pectoral fin spines.
The second dorsal fin spine was previously used to identify the ages of dogfish, but often with imprecise results due to normal calcification and damage to the structure. So instead, Bubley is using vertebrae to determine their age, a technique typically used on other Elasmobranchs. Two individuals examine vertebrae cross-sections that have been processed to reduce the calcification and stained to bring out the annuli more clearly.
"The chance of the two readers getting the same age estimate is vastly improved with the vertebrae than from the dorsal spines," Bubley says.
The preliminary results show that the dogfish age at maturity typically between 10-15 years has not changed dramatically over the last couple decades. However, the growth rate may be slowing down, so that smaller sized fish are reaching maturity at around the same age as is typical for that species in the Atlantic.
"I think this trend was initially set into motion by the fishing pressure on them in the 1990s," Bubley says. "The largest sized dogfish were harvested by fishermen, and so the smaller ones remained behind to reproduce. They passed their genes on to the next generation, causing size selection to occur where smaller fish are able to reproduce."
The average number of offspring produced by a female dogfish has decreased from 10 two decades ago to four today, according to Tsang. Changes
|Contact: Rebecca Zeiber|
University of New Hampshire