Extinction in Sharks

I found a research article regarding extinction in sharks, skates, rays, and chimaeras in different oceanic habitats.  Sharks, skates, rays, and chimaeras make up chondrichthyans.  They are compared amongst different habitats, which include: continental shelves, the deep sea, and the open ocean.  Overall, the chondrichthyans that dwell in the deep water have an overall higher rate of maturity and longevity.  Extinction risk was highly associated with phylogeny and reproduction.

Fig 1: Shark

The traits that are often associated with susceptibility to extinction are: low productivity, smaller litter sizes, slower growth rates, slower sexual maturity, and long "interbirth" intervals.  As stated earlier, the different habitats are: the continental shelf, the deep sea, and the open ocean.  The continental shelf is the zone of the ocean that goes from the shoreline to 200 meters deep, the open ocean goes deeper than the continental shelf, and the deep sea goes all the way to the largest depth of the sea.  

In comparison to fishes that live within the shallow-water, deep-water fishes have a slower growth rate, sexually mature later, live longer, and have smaller metabolic rates.  All of these factors cause them to have longer turnover times, which implies that the populations will be less productive.  Predation increases with an increase in metabolism and accelerated turnover rates.  Predation is moderate in both shallow habitats of continental shelves as well as the deep sea.  

The first step of this research paper is to figure out whether deep-water chondrichthyans have a longer turnover time than shallow-water chondrichthyans.  The second step of the paper is to show the consequence of the habitat and life-history traits on extinction.  

In order to complete the experiment, scientists chose 105 different species.  The took data on maximum body size, size at maturity, longevity, age at maturity, growth completion rate, litter size, the interbirith interval, reproduction mode, and habitat.  

Overall, the study concluded many vital aspects of the chondrichthyan habitat.  Firstly, the results from this study showed that the extinction of chondricththyans is indeed linked to habitat.  The chondrichthyans that occupy the deep water have enlarged turnover times.  Chondrichthyans that occupy the shallow and oceanic shelf have a lower extinction risk than those in the deep water.

It was also concluded that reproduction played a role in extinction.  It was found that non-matrophic females who reproduce at higher rates have less of a chance in becoming extinct.  The study also concludes that the body size of the fish has little to do with extinction rate.  If extinction were to occur in these fish, it would happen in fish that are extremely large or small.  The study concluded that the two most susceptible species of fish extinction were: Lamniformes and Squaliformes.  The article says that the solution of the problem is, "Minimizing fish mortality in deep-water habitats already exploited and preventing new deep-water ecosystems to be exploited are necessary to avoid the extinction of these species." (Garcia)

Source: Garcia, Veronica B., Lucifora, Luis O., Myers Ransom A. (2008). The importance of habitat and life history to extinction risk in sharks, skates, rays, and chimaeras. The Royal Society, 275(1630).  Retrieved from: http://rspb.royalsocietypublishing.org/content/275/1630/83#ref-49