Blue Shark life and death in the Azores
Sharks are distributed in our oceans according to many factors. Many of their lives' main events take place at discrete locations. They often pup, nurse and mate in different areas, for example because sexes and sizes tend to be segregated. We still don't know how individual sharks move around, but their rapidly declining population and prime function as top predators makes the discovery important.
We noted the new hammerhead species discovered by studying shark populations more closely in - New Hammerhead in the South Atlantic. The worldwide distribution of Prionace glauca makes them frequent members of the by-catch in pelagic longline fisheries. That means they are locally abundant, as they have a faster growth and can produce many more pups (up to 82) than other pelagic sharks.
In the North Atlantic, the species has a single population, made up of at least one pupping area just west of the Straits of Gibraltar and a juvenile male area in the western Atlantic, where mating areas could also be located. The Azores, with their central location also seem to have pupping and mating areas, so the islands were chosen as an ideal location. As well as investigating long-term migratory patterns to see how movement changed throughout the life cycle, a study of connectivity between the different areas and verification of their locations was carried out.
The results showed some heavy travelling. One female swam 28,139km in 952 days. She was a large juvenile, and it was these females who showed the greatest migratory patterns. In winter, the ladies stayed far south, between 31 and 45 degrees N, while in summer they reached the 59 degree N level of the Bay of Biscay and the Hatton Bank. They returned to the Azores, presumably every year.
The vast oceanic areas covered actually included the southern hemisphere for one tagged individual. Continental shelves were avoided, even though our old data on any shark tends to be from fishermen in these areas. We have a lot to learn! Each individual varied in his or her choice of location with a surprising lack of sexual segregation. Only he large females showed a bias to explore further and to different, probably warmer, areas than the males. There was no obvious link to the pupping areas near Europe and Africa, so the authors conclude that a central pupping area is self-sufficient as a nursery.
The young sharks hang out in the Azores for 2 years and then begin their inveterate wandering. The high mortality in the first year is due to their small size, but they grow quickly. The existence of longline fisheries close to these important Azores areas should obviously be discouraged in the future as many sharks, and many other species use the climate and topography of the Azores. Fishing will continue, but the existence of a Marine Protected Area, alongside some no-fish zones would prevent disaster for sharks, loggerhead turtles and many others.
Six authors, headed by Frederic Vandeperre of the University of the Azores produced this intriguing and forward-looking paper in PLOS ONE yesterday as - Blue Sharks across Their Life History.