Where Have All the Blue Sharks Gone?

Blue Shark (c)KipEvans

While Blue Sharks are some of the most extraordinary, personable swimmers of the ocean as well as one of the most abundant pelagic shark species in the world, little is still known about the biology or population health of this beautiful shark. This is primarily due to a disregard by fisheries management and general indifference to the high amount of bycatch and the emergent threat of shark finning.

Blue sharks, known also by the scientific name: Prionace glauca, are distinguished by their darker blue backside [dorsal] with a sharp demarcation of a white underside [ventral], and a considerably slim build, as seen in their pectoral fins as well. P. glauca also characteristically have conically shaped snouts and large eyes without spiracles, used to circulate oxygenated blood directly to important organs including the eyes and brain using a separate blood vessel. These graceful marine creatures exist in typically temperate or tropical pelagic oceanic environments, within temperatures of 44.6℉ – 77℉ [7℃ – 25℃] and depths of 0-1,148 feet [350 m]; possibly making them one of the most wide-ranging shark species on the planet. The diet of P. glauca mainly consists of small, bony fishes including herring or sardines, invertebrates including squid, cuttlefish and pelagic octopi, bottom dwelling fish, smaller sharks and even seabirds at the water’s surface at times. Typically actively feeding at night, they also feed throughout a twenty-four hour period and practice aggregating to feed on groups of prey. 

The reproductive methods and practices of P. glauca, set it apart from other shark species. Being one of the few placentally viviparous shark species, their young develop within a uterus and they give birth to live pups. After maturation [males: 5-7 years of age, females: 4-6 years of age], members of both sexes will convene annually to mate; it is unclear, however, if this consistently occurs among all members. During courtship, males will bite or mark the female repeatedly. This has actually led to the females developing skin three times as thick as their male counterparts. An incredible feat of evolution has occurred within females of the species; they can store the sperm of males, delaying fertilization until an optimal time is reached. The gestation period of this species is typically 9-12 months. This range is due to females being able to delay the birth of their offspring until ideal conditions are met. Litter sizes average around 30-35 pups, but can be as large as 135; normally they are born at around fourteen inches to twenty inches in length, and reside in their nursing grounds for two years before beginning migration. 

Another unique aspect of P. glauca is their developed migratory patterns [including transatlantic migration] and methods, known as being considerably complex and lengthy for a number of reasons. Those reasons being that both sexes of shark tend to spatially segregate throughout most of the year; females migrate to new areas after mating to give birth to offspring, and blue sharks of different ages and sexes typically are not found residing in the same areas, as tracking data has shown. A study performed in Canada, regarding using archival satellite pop-up tags on a number of blue sharks, noted that after entering the Gulf Stream all sharks initiated in diel vertical migration, representing a trade off between the function of food gathering and avoiding predators. This study showed a greater mean night depth for the sharks [72 m] and a lower daytime depth [412 m], providing a thermoregulatory advantage to feeding in cooler waters. There was no indication that this diving of the sharks resulted in a modification to their net migratory pathway, but rather, that the Gulf Stream area serves as a key feeding group for large pelagic predators. 

As seen with multiple shark species, apex predators maintain a critical ecological role within their ecosystem. Apex predators including P. glauca regulate and control populations of various organisms they prey on, generally allowing for greater biodiversity through lessening competition between prey species with lower numbers of individuals. Without the presence of an apex predator, there is an increase in competition and pressure of other existing species within the same habitat while possibly affecting food supply. Prey species themselves can bring their demise with greater population size, including an increase in disease, destroying their own food sources, and eventually depleting in numbers or going extinct. The blue shark is also vulnerable to various natural predators at more juvenile stages in their life cycle, these predators include killer whales [Orcinus orca], the shortfin mako shark [Isurus oxyrinchus] and the white shark [Carcharodon carcharias]. A variety of parasitic copepods affect blue sharks as well including species: Pandarus satyrus, Kroeyerina elongata, Echthrogaleus coleoptratus, Kroyeria carchariaeglauci, Phyllothyreus cornutus, attaching to pectoral fins, their nose, body surfaces, and gills, potentially leading to sight impairments and alterations in gill structure. The protection, conservation, and knowledge of shark species, including P. glauca, is essential to the ecological health of numerous ecosystems and our planet.

The detrimental threats facing P. glauca currently and throughout time, are greatly due to the influence of humans. Blue sharks are most threatened by the threat of intentional harvest [shark finning], sport fishing [shark derbies], and bycatch effects of fisheries. One study estimated that the fins from as many as 20 million blue sharks were in the shark fin trade in Hong Kong, and was the most common large shark fin to be sold for shark fin soup.

Sport fishing is also a major threat to this species. Studies performed in Canada by the Shark Research Lab, have focused on examining and measuring blue sharks caught at shark derbies to determine the practice’s impact on the health and number of  blue shark populations. Analyzing the year to year trends in catch rates, size compositions, maturity at size and mortality rates in derbies and commercial bycatch has indicated that blue shark abundance has declined and mortality has increased over the past decade, with derbies maintaining a minor component of overall mortality compared to that of bycatch seen in tuna and swordfish fishers. Another study performed by Shark Research Lab  to determine the mortality rate of blue sharks caught through bycatch [commercial fishers or longliners] by using archival satellite pop-up tags on a percentage of the population to assess water temperature and depths, providing evidence of death, and eventually releasing stored data back to the lab for analyzing. Results found that the overall blue shark bycatch mortality in pelagic longline fishery was thirty-five percent, with an estimated mortality rate for blue sharks released alive was nineteen percent.. Thirty-three percent of considerably injured blue sharks subsequently died, with ninety-five percent of mortality occurring within eleven days following release, indicative of death caused by trauma. In the North Atlantic, blue shark populations are estimated to have declined over sixty percent from 1986 to 2000, with the global status being evaluated with an estimate of 10.7 million sharks being killed every year [2003]. Shark Research [Campana] Lab in Canada is placing emphasis on increased tagging measures to quantify post release mortality from recreational shark fishing, population numbers and health, and migratory patterns. 

Conservation measures, efforts and practices implemented for the protection of P. glauca have existed and evolved over the last few decades. The blue shark has been categorized as Near Threatened by the IUCN [International Union for Conservation of Nature], potentially being moved to a substantially worse category with current available data and population trends. Making it critically important for re-evaluation efforts and reliable gathering of important data or information regarding the blue shark species [the current method of estimating shark/marine organisms (CPUE- catch per unit effort) makes the process difficult with numerous problems including amounts of harvesting not being reported to organizations receiving the data, and intense migratory patterns] to gain more certainty; improved conservation efforts rely on this. Various countries are establishing “shark sanctuaries”, areas completely banning the fishing of sharks, with others at least banning shark finning, reducing the overall catch of sharks. Some countries have created Marine Protected Areas, helping to protect the crucial habitat of marine species, with an increased commitment of governments and scientists towards reliable tracking of fishing practices and establishing increasingly sustainable fisheries worldwide. However, these MPAs are generally not large enough to protect highly migratory species like blue sharks. International action through the ICCAT will essentially be necessary in regulating blue shark population mortality and addressing the numerous threats to this species as described previously.

Caught as bycatch in the west coast driftnet fishery for swordfish, this once abundant shark is disappearing from the high seas and our coastal ocean off California. A proposal to list this species under CITES Appendix II at CoP 16 in 2013 failed to advance for consideration, blocked mainly by large fishing nations like Spain, China and the USA. Increasing education and availability of reliable information and data of shark species such as P. glauca, including their crucial ecological importance, the level of threat and exploitation toward them, and the potential of a brighter future for these irreplaceable organisms and the entire planet through manageable efforts. I believe that this is a universal responsibility that will be accomplished with our commitment. Like the curiosity, beauty and functional behavior of the beautiful blue shark; the potential of what committed individuals can accomplish is limitless. 


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