Sharks

Shortfin mako: the world’s fastest shark is speeding towards extinction

The shortfin mako shark is found almost worldwide and is currently listed as “Endangered”. They are apex predators, and it is thought that their removal from the ocean could cause ecosystem collapse, and it is essential to establish a stable, worldwide population. Overfishing is the main threat to makos, either as a target species or bycatch.

Introduction

The shortfin mako shark (Isurus oxyrinchus) is a pelagic, cartilaginous fish with a wide distribution range that covers most oceans and undergoes migrations that can be as large as 5,300km in just under 1.5 years (Barreto, de Farias, Andrade, Santana, & Lessa, 2016; Kohler, Turner, Hoey, Natanson, & Briggs, 2002). There is high demand for mako shark meat, and it is a prize game species in recreational fishing worldwide (Barreto et al., 2016).

As of 2019, I. oxyrinchus is classified as “Endangered” on the International Union for the Conservation of Nature’s (IUCN) Red List (Rigby et al., n.d.). Rigby et al. (n.d.) concluded that shortfin mako’s population trend is decreasing; there is an estimated decline everywhere except in the South Pacific and an overall estimated average reduction of 46.6% over 72-75 years. There is an estimated decline in biomass and abundance of 99.9% since the early 1800s, the main reason being overfishing (Ferretti et al., 2008).

Morphology

Shortfin mako circulatory systems utilise a heat-exchanging technique that raises their internal temperature above that of the surrounding environment (Carey, Teal, & Kanwisher, 1981; Kohler et al., 2002). They have streamlined bodies, and aerobic muscles centred closer to their rear, which aids in thunniform swimming and increases power (Donley, Sepulveda, Konstantinidis, Gemballa, & Shadwick, 2004; Wegner, Sepulveda, Olson, Hyndman, & Graham, 2010). Emery and Szczepanski (1986) concluded that the gill area of I. oxyrinchus is 2-3 times larger than other pelagic shark species, which could aid in the mako’s speed, agility, and ability to swim long distances. Shortfin mako habitat extends globally in tropical and temperate oceans and they can be found inshore in coastal areas or at least 500m down in oceanic zones (Kohler et al., 2002).

Life history

Sexual dimorphism is prevalent in I. oxyrinchus,with females often occurring larger than males, with males reaching a maximum size of 2.6m and females reaching a maximum size of 3.4m (Barreto et al., 2016; Cema & Lincandeo, 2009; Chan, 2001; Doño, Montealegre-Quijano, Domingo, & Kinas, 2014; Hsu, 2001; Natanson et al., 2006; Semba, Nakana, & Aoki, 2009). Shortfin mako sharks are oophagous and ovoviviparous (Kohler et al., 2002), and a study by Mollet et al. (2000) predicts a gestation period of 15-18 months, although a study by Duffy and Francis (2001) puts makos in New Zealand waters at a 21-month gestation period. They have a 3-year reproductive cycle (Mollet & Cailliet, 2002). Bishop, Francis, Duffy, and Montgomery (2006) concluded that New Zealand shortfin mako births are concentrated in spring and gave a theoretical birth date of 1 October, with the average length of the shark at birth to be 61cm. Francis and Duffy’s (2005) study on sexual maturity of New Zealand shortfin makos concluded that maturity occurs between 7-9 years for males and 19-21 years for females (Bishop et al., 2006). Bishop et al. (2006) found evidence of New Zealand shortfin makos living to 29 years, although this number is probably higher because there is a lower chance of catching older sharks which make up a small percentage of the overall population. The same study found the sharks grow quickly within their first year after birth; this growth rate rapidly reduces in the subsequent years to steadier growth. Late maturity, moderately long longevity, the estimated low natural mortality rate, and low annual fecundity causes low productivity in the species (Bishop et al., 2006; Mollet et al., 2000).

Diet

Stillwell and Kohler (1982) analysed the stomach contents of shortfin mako sharks and found evidence of bony fish, swordfish, and cephalopods. As shark body length increased, so did the average volume of food, indicating that as makos grow larger, they may switch to larger prey items (Kohler et al., 2002).

Mortality

Sharks have evolved for 400 million years (Donley et al., 2004), leading species such as I. oxyrinchus to be apex predators at the top of their food chain; they have no natural predators which results in low natural mortality. There is evidence of parasitic activity in shortfin makos, although it could not be determined if there was a negative effect on the shark (Borucinska & Hege, 1999). One example of anthropogenically influenced mortality is seen in 1966 when a longfin mako (Isurus paucus) died from a fishing hook retained in its flesh (Adams, Borucinska, Maillett, Whitburn, & Sander, 2015).

Ecology

Sharks are keystone species and have a strong effect on multiple ecosystems due to their predatory role and wide dispersal range (Feretti, Worm, Britten, Heithaus, & Lotze, 2010). I. oxyrinchus is considered to be a large shark with “strong, top-down forces” (Feretti et al., 2010, p1055). Therefore, their removal from an ecosystem is highly likely to drastically alter communities, induce trophic cascades, release mesopredators such as smaller sharks and rays, and consequently cause a decline in commercial fish stocks (Fig. 1).

Figure 1 “Documented ecosystem effects of fishing large sharks. Depicted are trophic (solid arrows) and behavioral (dotted arrows) interactions between humans, large and mesopredator elasmobranchs and their prey species. Block arrows represent overall population trends of the various functional groups. Regions in which particular interactions have been documented (see text) are indicated by letters (A, Australia; C, Caribbean; E, Europe; G, Gulf of Mexico; M, Mediterranean Sea; N, North American East Coast; P, Central Pacific; S, South Africa; W, North American West Coast).” (Feretti et al. 2010)

Threats

There is a high commercial demand for shortfin mako shark meat, and the species is exploited globally (Barreto et al., 2016). Peru is one of the biggest shark-fishing nations, and I. oxyrinchus is one of the top 2 most caught species with a rarely enforced catch size limit (Gonzalez-Pestana, Kouri, & Velez-Zuazo, 2016; Fischer, Erikstein, D’Offay, Guggisberg, & Barone, 2012). Brazilian fleets in the western and central South Atlantic exploit immature shortfin makos, specifically females (Barreto, 2016); it was rare for them to catch individuals greater than 2m. On a global scale, it is both a target and bycatch species in commercial and small-scale fisheries, including longline, gillnet, purse seine, trammel net, and trawls (Camhi, Pikitch, & Babcock, 2008; Rigby et al., 2019). There is likely underreporting of catch, and commercial post-release mortality from longlines alone is reported at 30-33% (Campana, Joyce, Fowler, & Showell, 2016; Rigby et al., 2019). Shortfin mako fins made up 1.2% of shark fin imported to Hong Kong in 2014 (Fields et al., 2017), and their skin, jaws, and liver oil are also used (Compagno, 2001). In New Zealand’s EEZ, I. oxyrinchus is a common bycatch species on tuna longlines and less commonly on pelagic longlines, trawls, and set nets (Bagley et al., 2000; Francis, 1998; Francis, Griggs, Baird, Murray, & Dean, 2000; Martinsohn & Muller, 1992). Since 1993, commercial catch averaged 60 tonnes annually, but observer reporting estimated 100-200 tonnes per year from the longline tuna fishery alone (Francis, 1998; Francis et al. 2000); the discrepancy between these numbers could be related to lack of accurate recording. From 1988-2015, the New Zealand tuna longline fishery total catch was comprised of 11.1% target species (southern bluefin tuna, bigeye tuna, and swordfish) and 88.9% bycatch (including albacore tuna, lancetfish, porbeagle shark, deepwater dogfish, dealfish, mako shark, moonfish, escolar, sunfish, and butterfly tuna) according to a report by Fisheries New Zealand [FNZ] (2018). Since October 2014, shark finning is illegal in New Zealand, yet over half of all makos caught by charter vessels in the 2014-15 tuna longline fishery year were kept for their flesh (FNZ, 2018). The low productivity of mako sharks causes them to be vulnerable to overfishing and makes it hard for the species to replace lost individuals. CITES (2019) reported that shortfin mako is in danger of population collapse due to the overfishing of most of the juveniles since the 1980s; this means that sharks dying of old age now will not be replaced with mature individuals leaving a 10-20-year gap.

The mining of precious metals increases the presence of toxic contaminants in the environment, especially in coastal environments, specifically mercury (Maz-Courrau et al., 2011). Sharks bioaccumulate mercury through their tissues and organs from the environment and through the food they eat; mercury travels up the trophic web and magnifies at each level, eventually accumulating in shark muscular tissue (Maz-Courrau et al., 2011). Maz-Courrau et al. (2011) found that 33% of shortfin mako sharks they studied off the Baja California coast had levels of mercury higher than was estimated to be fit for human consumption by Mexican Law and Watling et al. (1981) found mercury concentrations in mako sharks off the coast of Australia to be almost twice that. As Maz-Corrau et al. (2011) only sampled juveniles, it is likely that as sharks grow and eat more, more mercury is accumulated into their tissue over time. When people eat this contaminated tissue, it bioaccumulates up the food chain again and causes adverse health effects in humans.

Current/Proposed Management Actions

As of 2019, I. oxyrinchus officially met criteria to be listed on the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Appendix II, which “includes species not necessarily threatened with extinction, but in which trade must be controlled in order to avoid utilisation incompatible with their survival” (CITES, 2019, p. 2).

Pressure from overfishing appears to be the most significant contributor to shortfin mako shark decline globally. Due to the widespread distribution of I. oxyrinchus and migratory nature, conservation effort and management is needed at both a local and international scale (Adams, Flores, Flores, Aarestrup, & Svendsen, 2016; Corrigan et al., 2018). Because of their “Endangered” status on the with a decreasing population trend, it is essential to increase their population trend and bring them to the next highest, more stable level on the Red List, “Vulnerable”, and take measures to ensure they do not go the other direction and become classified as “Critically Endangered”. The most effective way this can be achieved is through strict management of fisheries and the implementation of Marine Protected Areas (MPA) in habitats that are suitable for shortfin makos by means of international treaties (Birkmanis, Partridge, Simmons, Heupel, & Sequeira, 2020; Dulvy et al., 2008; Rigby et al., 2019).

Community Education & Awareness

Films, documentaries, and TV series are a great way of moving an audience and opening their eyes to what is going on in the world around them in an entertaining way. Saving Jaws by Ocean Ramsey, Shark Week by The Discovery Channel, and BBC’s Blue Planet, narrated by David Attenborough, are a few.

For the booklovers, don’t worry, we got you. Ocean Ramsey’s What You Should Know About Sharks: Shark Language, social behavior, human inter-actions, and life saving information educates readers on shark behaviours, how to swim with them safely, and debunks common shark myths. Sylvia A. Earle’s book The World is Blue: How Our Fate and the Ocean’s are One shows readers how every one of us literally breathes the ocean, and her other book, Sea Change: A Message of the Oceans, has been compared to Rachel Carson’s Silent Spring. For those who do not enjoy reading, Christian Vizl’s Silent Kingdom is a photography-based book that shows the raw beauty, power, and elegance of marine creatures, such as sharks, in stunning black and white photos.

NGO’s also play an essential role in community education and awareness. In New Zealand, groups such as Auckland Whale & Dolphin Safari and Whale Watch Kaikōura take their customers to visit sea life up close. SEA LIFE Kelly Tarlton’s Aquarium educates both adults and children alike and cares for sick and dying turtles.

Monitoring & Research

There is enough research on I. oxyrinchus populations to know something has to change, but what lacks is monitoring the factors contributing to their dismal fate. As of 2019, CITES began to monitor the commercial catch and trade of shortfin mako but agreed that it might not be enough (CITES, 2019). In 2013, MPI released a National Plan of Action for the Conservation and Management of Sharks (NPOA). In the plan, they stated one of their objectives was to “systematically review management categories and protection status to ensure they are appropriate to the status of individual shark species” (MPI, 2013, p. 19) among others, yet there is a lack of reporting available for public access to back this up. They also had intentions of issuing a revised NPOA in 2018, which seems not to exist altogether.

Bycatch records need to be scrutinised and fisheries must be forced to show accurate record-keeping. Data collected from accurate bycatch records of I. oxyrinchus can then be accumulated around the globe to highlight the fisheries that are exploiting shortfin makos.


References

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1 comment on “Shortfin mako: the world’s fastest shark is speeding towards extinction

  1. Pingback: Shortfin mako: the world’s fastest shark is speeding towards extinction — The Ocean: Explained | huggers.ca

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