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Sharks and the Impacts of Overfishing

Reading Time: 7 minutes

Dr Leonardo Guida, Australian Marine Conservation Society

 

Healthy populations of sharks are critical to a healthy ocean. Typically occupying the top of food webs as apex predators, they keep said food webs in balance. The impact of overfishing of shark populations can cause food webs to become unstable and potentially collapse. Recent science is also revealing the important role sharks have in regulating the behavior of other animals, such as tiger sharks preventing overgrazing of seagrass by turtles and dugongs1,2. In preventing overgrazing, tiger sharks are indirectly maintaining the health of seagrass habitat which forms important nurseries for smaller fish, and stores carbon for 1000s of years at 40x’s the speed of terrestrial forests, which in comparison, store carbon for around 100 years1.

 

There are 516 species of shark in the world3, with 182 calling Australian waters home4. Sharks are characteristically long-lived animals that mature late and give birth to few young – on average the mature at 10 years, and give birth to 4-6 pups following a reproductive cycle of 1-2 years5. Consequently, they are extremely vulnerable to fishing pressure and overfishing over sharks has resulted in some alarming statistics whereby the numbers of their oceanic species have crashed by 70% in just the last 50 years6

 

Since the dawn of industrialised fishing in the last 100 years, the impact of overfishing has driven a third of sharks and rays to extinction. Here’s a ‘fun’ statistic to consider – sharks have been on earth for around 450 million years surviving five mass extinction events, and so if we think of their existence as a 24 hour clock, overfishing has wreaked havoc in just 0.02 seconds!

 

What is overfishing and is different from being overfished?

Overfishing is depleting a population faster than it can replenish itself. When a species is overfished, this means it has declined to ‘danger zone’, a point below which you can no longer remove individuals from its population at a maximum rate over an indefinite period (in fisheries science, this ‘rate’ is called Maximum Sustainable Yield; MSY). Importantly, a population may be overfished but not experiencing overfishing.

 

Can a species experience overfishing even if it isn’t threatened?

Yes, it is entirely possible for species to be experiencing overfishing when it isn’t a threatened species. Examples include the tiger shark which is currently in decline however is assessed as ‘Near Threatened’ in Australia4,7. Conversely, the Vulnerable shortfin mako shark is a threatened species that is currently experiencing overfishing4,7. There are a total of 14 shark species in Australia that are considered overfished, including the iconic great white (Vulnerable), scalloped and great hammerheads (Endnagered), grey nurse (east coast: Critically Endangered, west coast: Vulnerable), and oceanic whitetip sharks (Critically Endangered)

Alarmingly, despite being overfished and Endangered, Australia still legally permits fishing and sale of Endangered school, and both Endangered scalloped and great hammerheads for meat and fins. 

 

The environmental impact of overfishing sharks

Measuring the environmental impact of overfishing of sharks is inherently very difficult to do in real life. That being said, there are a handful of cases including one in a ‘natural’ laboratory which reveals some interesting statistics.

The Scott Reefs & Rowley Shoals of northwestern Australia, form a unique natural laboratory where we can directly compare a relatively pristine, protected reef (Rowley) with a reef that has been fished for over 300 years (Scott). Rowley has twice as many shark species, and they’re twice as abundant and 20% longer! On Rowley where sharks are common, there was also an overall greater biodiversity, abundance and biomass of bony fishes compared to the shark-depleted Scott Reefs9.

The overfishing of dusky sharks in Western Australia’s (WA) Northern Shark Fishery (NSF) in the early 2000s resulted in a significant decline of their juvenile population over 1000km away. The NSF is a region spanning the north-west and northern waters of WA and the duskies here are predominantly mature adults who make a return-trip down to the temperate south of WA (over ~1000-4000 km) where they pup. It’s in the south where the pups spend the first few years of life before they’re mature enough to make the journey north. The temperate south is where WA’s Temperate Shark Fishery targets juvenile duskies and it was in the early 2000s that the juvenile stock was being severely depleted because of overfishing of adults in the north. Suffice to say, fishers in the south were not happy with the fishers in the north. Fortunately, since the closure of the NSF in 2010 to stop the overfishing of sharks, the statistics are looking positive and we’re seeing sandbar populations slowly recover and dusky numbers stabilise10.

 

In South Africa, overfishing is thought to be having a cascading effect on the food web. The overfishing of school (a.k.a. tope) and smoothhound sharks combined with the increased presence of shark-hunting orcas, is currently thought to be driving away white sharks for months and even years at a time11. In turn, with the whites away, there has been a greater presence of bronze whalers and seven gill sharks. Furthermore, seals are less concerned about being eaten, are reproducing more and potentially spending more time hunting, all of which are placing greater pressure on fish populations8. The take-home message here is that overfishing of sharks can result in instability of food webs and possibly lead to a collapse.

 

Unsustainable shark fishing

Sustainability is probably one of the most misconstrued and misused ‘buzzwords’ going around. When we talk about sustainable fishing we must realise that ‘sustainability’ is not just the number of a given fish, but the environmental impact that fishing has as a whole.

 

Australia’s gummy shark fisheries are a perfect example of how ‘sustainable’ is misused and is frankly, misleading. Although gummy shark in Australia are ‘sustainable’ in terms of their individual numbers, the fisheries that target them (Southern and Eastern Scalefish and Shark Fishery; and WA-managed Temperate Shark Fishery) are unsustainable. Fishing for gummy shark has resulted in some damning statistics, driving the Endangered school shark and the Endangered Australian sea lion towards extinction. As we noted earlier, Endangered school sharks are overfished (possibly even experiencing overfishing12) and are at an estimated ~10-15% of their original numbers12. The incidental catch of Australian sea lions has resulted in their numbers declining by 60% in the past 40 years13.

 

How to stop overfishing?

The most fundamental component to any modern-day, sustainable fishery is independent observation of fishing activity, whether by human observers or cameras onboard vessels. Independent observation allows the accurate collection of data enabling us to get a better understanding of what species is being caught, where it’s being caught and in what numbers. Importantly, it’s not always necessary or possible to cover 100% of fishing activity and so the levels of observation must be a scientifically-determined, statistically-robust representation of all fishing activity in both time and space.

In the case of threatened shark species which are overfished or experiencing overfishing, the detailed data and statistics collected via independent observation and fishers’ logbooks, enable mortality limits for a given species to be put in place which, when reached, can then result in fishing activity to be appropriately slowed or stopped altogether. This will then promote species recovery as soon as possible because, short of stopping fishing altogether, bycatch is essentially inevitable. Species that can trigger the closure of a fishery are known as ‘choke’ species, like the choke on a throttle that determines the speed of an engine. The school shark – in theory – is an example of this in that it is impossible to not catch them when you’re targeting gummy sharks, and so should a scientifically determined mortality limit be reached (which enables species to recover back to healthy numbers) the fishery could be closed to prevent further deaths. In reality, this unfortunately does not happen in gummy shark fisheries but it’s one strategy to help the Endangered school shark recover its numbers.

Complementary to mortality or catch limits, there are multiple strategies to promote threatened species recoveries. Perhaps one of the most fundamental is protecting critical habitats like aggregation sites or breeding grounds (sometimes they’re one and the same) to limit interactions with fisheries in the first place and enable species to boost their numbers over time. The Critically Endangered grey nurse shark on the east of Australia is a classic example, where many of their known aggregation sites throughout New South Wales and Queensland are protected from all or most types of fishing. Consequently, they appear to be showing tentative signs of recovery of ~4-5% per year, however significantly more protection is required to definitely recover the species14.

 

Take home messages

Sharks are facing their biggest threat in their entire existence, us. Ironically, our very existence depends on theirs given that healthy oceans need healthy populations of sharks.

Overfishing and being overfished are different concepts. Overfishing is the continued decline of a species irrespective of its extinction risk. Being overfished is once a species has fallen below its population danger-zone and fishing for the species has to stop. It’s possible for a species to experience overfishing and not be overfished – i.e. its numbers are declining but haven’t reached a critical level… yet.

We must understand that sustainability is more than just the number of a given fish, it is the fishing impact on the environment as a whole which determines sustainability. There is no point calling one species sustainable because of its individual numbers when its harvest comes at the cost of other threatened species – where does one then draw the line?

Independent observation fishing activity and the protection of critical habitats are fundamental and complementary strategies to stop the overfishing of threatened species and enable their recovery back to healthy numbers.

We are their biggest threat, and their only hope.

 

References

  1. Heithaus, M. R. et al. Seagrasses in the age of sea turtle conservation and shark overfishing. Front. Mar. Sci. 1, (2014).
  2. Nowicki, R. J., Thomson, J. A., Fourqurean, J. W., Wirsing, A. J. & Heithaus, M. R. Loss of predation risk from apex predators can exacerbate marine tropicalization caused by extreme climatic events. J. Anim. Ecol. 90, 2041–2052 (2021).
  3. Last, P. R. & Stevens, J. D. Sharks and Rays of Australia. (CSIRO Publishing, 2009).
  4. Kyne, P. M., Heupel, M. R., White, W. T. & Simpfendorfer, C. A. The Action Plan for Australian Sharks and Rays 2021. (2021).
  5. Heupel, M. R., Kyne, P. M., White, W. T. & Simpfendorfer, C. A. Shark Action Plan Policy Report. http://epubs.aims.gov.au//handle/11068/15377 (2019).
  6. Pacoureau, N. et al. Half a century of global decline in oceanic sharks and rays. Nature 589, 567–571 (2021).
  7. Simpfendorfer, C. A., Chin, A., Rigby, C. L., Sherman, S. & White, W. T. Shark futures: a report card for Australia’s sharks and rays. (Centre for Sustainable Tropical Fisheries and Aquaculture, 2019).
  8. Hammerschlag, N. et al. Loss of an apex predator in the wild induces physiological and behavioural changes in prey. Biol. Lett. 18, 20210476.
  9. Barley, S. C., Meekan, M. G. & Meeuwig, J. J. Species diversity, abundance, biomass, size and trophic structure of fish on coral reefs in relation to shark abundance. Mar. Ecol. Prog. Ser. 565, 163–179 (2017).
  10. Braccini, M., Molony, B. & Blay, N. Patterns in abundance and size of sharks in northwestern Australia: cause for optimism. ICES J. Mar. Sci. 77, 72–82 (2020).
  11. Towner, A. et al. Fear at the top: killer whale predation drives white shark absence at South Africa’s largest aggregation site. Afr. J. Mar. Sci. 44, 139–152 (2022).
  12. Thomson, R., Bravington, M., Feutry, P., Gunasekera, R. & Grewe, P. Estimating the abundance of School Shark in Australia using close kin genetic methods. https://www.frdc.com.au/project/2014-024 (2020).
  13. Goldsworthy, S. D. et al. Assessment of the status and trends in abundance of a coastal pinniped, the Australian sea lion Neophoca cinerea. Endanger. Species Res. 44, 421–437 (2021).
  14. 14. Rayns, N. Review of Recover Planning for Threatened Sharks: Status, Analysis & Future Directions. (Self published, 2019).
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