Posted by John Filmalter and Laurent Dagorn
11 June 2011
There are approximately 60 species of sharks that are classified as being pelagic, to varying degrees. Some spend large portions of their time closely associated to the continental shelves or island slopes but at times can be found in the open ocean, while other are truly oceanic, spending extensive periods in the open ocean, and often covering immense distances in search of food and/or mates.
Generally these truly oceanic species consist of sharks from the lamnoid and carcharhinid families. These include species such as the great white, Carcharodon carcharias, mako Isurus spp., salmon, Lamna ditropis, porbeagle, L. nasus, threashers, Allopiidae, blue, Prionace glauca, silky, Carcharhinus falciformis, and oceanic whitetip C. longimanus, sharks. Certain species are captured far more regularly than others, with some, such as the blue shark, forming the primary target of certain directed longline fisheries. Of these species the three Carcharhinids, the blue, the silky and the oceanic whitetip sharks, form the largest biomass amongst the pelagic cartilaginous species. As such it is these three species that experience the highest exploitation rates and fishery-induced mortality. Although these three shark species often share overlapping habitats in all major oceans their ecology and biology can differ considerably.
Blue sharks have the fastest growth rates of the three species, with females and males reaching first maturity after 5-7 and 4-6 years respectively. In areas where the biology of this species has been well studied, differences in size at maturity have been observed between ocean basins. For example in the north Pacific Ocean both males and females mature at about 200 cm TL (Total Length), while in the western Atlantic Ocean size at maturity is about 220 cm TL for both sexes. Once mature, females produce an average of 30 offspring and it is speculated that parturition may occur on an annual basis.
Silky sharks, while being less productive than blue sharks, have also been found to show varying sizes at maturity in different areas, however data on this species is far more sparse. Females in the Atlantic Ocean mature between 232 – 250 cm TL, while those in the Pacific appear to mature at smaller sizes (180 – 218 cm TL). In the Gulf of Mexico, silky sharks are believed to mature after 6-10 years (males) and 7-12 years (females). Females typically give birth to 6-12 young every second year once mature.
Studies on the biology of the oceanic whitetip shark have been especially limited and are often based on small sample sizes. One of the few studies that has examined the biology of this species determined that in the north Pacific, female oceanic whitetip sharks reach maturity between 175-189 cm TL while males mature between 168 -196 cm TL. This equates to an estimated age at maturity of between 4 and 5 years. Females give birth to an average of 6 juveniles, however the frequency of parturition is not well known. Interestingly, studies comparing the productivity of various pelagic shark species have rated the oceanic whitetip as relatively productive, placing it just below the blue shark and above the silky shark.
It is surprising that, in the available literature, the oceanic whitetip is considered to be the most common oceanic shark species in the world’s tropical waters. This was certainly the case 30 or 40 years ago, however, comparative studies between historic and current catch rates in certain areas of its distribution have revealed a potential decrease in catch rates of between 70 and 99%. While decreases have been reported for all three species none have been as severe as those of the oceanic whitetip, suggesting possible flaws in its estimated productivity and vulnerability to rapid depletion.
Fisheries data for sharks are also generally limited. Historically, shark catches were simply not recorded in many countries, or they were lumped into a general sharks category, without keeping track of species or sizes. On top of that, shark finning, the practice of retaining only the fins and discarding the carcass at sea, has created a huge challenge to maintaining reliable statistics.
Tuna Regional Fisheries Management Organisations (RFMOs) were created (IATTC being the first in 1949) to manage fisheries in each ocean in order to control the exploitation of tuna stocks and ensure the sustainability of these fisheries. Under the general movement towards the ecosystem approach to fisheries management, Tuna RFMOs have recently begun to adopt conservation measures for some pelagic shark species. However, the lack of critical knowledge on various key biological and ecological parameters, as well as on fisheries data, for these species limit the effectiveness of these measures and hence the efforts of the RFMOs. Whether sharks are taken as targeted catch or incidental catch in tuna fisheries, we believe that RFMO members should devote more resources to studying their biology and collecting accurate catch statistics by species.