Overfishing & Climate Change

What is overfishing?

Overfishing occurs when too many fish are extracted from a fishery at a rate and intensity high enough to cause over-depletion of stock, leading to reduction in reproductively mature individuals able to breed at sufficient levels to replenish the population, to an extent the fishery cannot sustain continued extraction over the long-term.

Increased numbers of commercial fishing fleets with the ability to maximize catches using new technologies to locate, extract and process target species efficiently; enabled by policies, laws and subsidies that have historically favoured industrial fishing; and using destructive, non-selective and unsustainable methods of extraction, contribute to overfishing.

Fisheries operating within a nation’s Exclusive Economic Zone (EEZ) are generally managed well. Fisheries operating on the high seas, beyond 200 nautical miles (outside the EEZ), are notoriously difficult, if not impossible, to govern, as is illegal, unreported and unregulated (IUU) fishing. Overfishing is more widespread in large-scale, industrial fisheries than in small-scale, subsistence fisheries supporting local economies.

Current global sustainability trends

The Food and Agriculture Organization of the United Nations (FAO UN) has tracked global trends on the sustainability status of selected fish stocks since 1974 and reports their findings every two years in The State of World Fisheries and Aquaculture (SOFIA) publications.

Aggregated data of assessed fish stocks between 1974 and 2013 indicated 10.5% of fisheries were under fished, 58.1% were fully fished (maximally sustainably fished)* and 31.4% were over fished. Therefore, 68.6% of fisheries were at biologically sustainable levels, while 31.4% were at biologically unsustainable levels (FAO 2016).

Aggregated data of assessed fish stocks between 1974 and 2017 indicated 6.2% of fisheries were under fished, 59.6% were fully fished (maximally sustainably fished)* and 34.2% were over fished. Therefore, 65.8% of fisheries were at biologically sustainable levels, while 34.2% were at biologically unsustainable levels (FAO 2020).

Assessed fish stocks at biologically sustainable levels have decreased from 90% in 1974, to 68.6% in 2013 (FAO 2016) and to 65.8% in 2017 (FAO 2020). Two-thirds of fisheries are being managed at sustainable levels.

Assessed fish stocks at biologically unsustainable levels have increased from 10% in 1974, to 31.4% in 2013 (FAO 2016) and to 34.2% in 2017 (FAO 2020). One-third of fisheries are not being managed at sustainable levels.

*The Australian Marine Conservation Society (AMCS) definition of ‘fully fished’ or maximally sustainably fished – “when fishing pressure is at the maximum or targeted limit (typically Maximum Sustainable Yield) of what can be sustained before overfishing will likely occur.” (italics mine). Over 50% of assessed fish stocks are ‘fully fished’.

Isn’t there ‘plenty more fish in the sea’?

In 2009, during a TED talk, oceanographer Sylvia Earle told her audience when she first began exploring the ocean some fifty years ago, neither she nor her contemporaries ever thought extracting large amounts of marine life could damage the ocean. The ocean was believed to be “a sea of Eden”, but humanity is now “facing paradise lost”.

There may once have been ‘plenty more fish in the sea’, but overfishing and increased consumer demand for seafood has depleted some wild fisheries almost to the point of collapse, placing immense pressure on remaining stocks. Sustainable fisheries management must become the norm, or there may one day be no more fish in the sea.

The ‘2048’ date controversy

That there may one day be no more fish in the sea was an idea universally embraced by the mainstream media and all but accepted by the public – that global fisheries would collapse by 2048 – but it was a view based on the results of a study that was subsequently retracted.

The original study, published in the journal Science, used aggregated catch data to predict that if current fishing rates were to continue unabated, global fisheries would likely collapse by 2048 (Worm et al. 2006). The study, as with all scientific research, made assumptions to support the potential validity of its hypothesis however, after extensive peer review, its conclusion was rejected by the fisheries science community.

Three years later, the lead author retracted the original study and published new research (Worm et al. 2009), claiming some species would be affected, and there was hope global fisheries could be rebuilt if management tools and strategies were successfully implemented.

A global assessment five years later projected a worse-case scenario of 10-20% of fish stocks in 2050 at sustainable levels, with most stocks being potentially sustainable if successful management strategies were replicated in other regions (Costello et al. 2016).

Fishing further down the marine food chain

As overfishing continues to exploit stocks, larger species become depleted and fisheries return consistently lower yields, commercial fleets are exploring at greater depths and fishing further down the marine food chain. We are often told we must stop eating the species we are over-consuming (carnivorous fish), and eat fish lower on the food chain (herbivorous and omnivorous fish).

On face value, this is great advice and we may think we are taking pressure off larger species higher on the food chain. But what happens if we all started eating the smaller fish, where stocks may currently be within sustainably managed levels? Over time, won’t their stocks become over-exploited too, unless we collectively rein in our seafood consumption?

Carnivorous marine fish species rely on smaller fish as their primary food source but must compete with our increasing appetite and demand for seafood. What kind of impact would we have on their prey availability if we all started eating the smaller fish?

Sardines are an important forage fish, but sardine populations may be vulnerable to overfishing. Research shows sardine levels were six times lower than normal when fishing rates were high (Essington et al. 2015).

The Atlantic Cod fishery

The collapse of the Gadus morhua (Atlantic Cod) industry in Newfoundland, Canada in 1992 saw a 95% decline in the population. In 2013, after a 20-year moratorium on fishing, Atlantic Cod are showing signs of recovery, though it is unclear if the fishery will recover to historical levels. The collapse and subsequent slow recovery of Atlantic Cod over decades highlights the potential long-term impacts of overfishing.

Overfishing and climate change

Overfishing exacerbates the impacts of climate change, including rising sea temperatures, ocean acidification, deoxygenation and changes to marine currents, resulting in altered marine ecology and ecosystem structure, habitat and species loss, and decreased resilience of some marine organisms to withstand the effects of global warming.

The relationship between overfishing and climate change is far more complex than can be discussed in depth here, but current research shows how fisheries are being impacted from climate change and that overfishing is contributing to climate change:

  • Overfishing depletes fish stocks and releases stored carbon into the atmosphere (Mariani et al. 2020)
  • Removing top predators from marine ecosystems may lead to increased carbon dioxide production in the ocean (Spiers et al. 2016)
  • Range expansions, population declines and re-distributions of some fish stocks
  • Spatial and temporal changes in fish body size driven by temperature (Audzijonyte et al. 2020)
  • Changes to seasonal biological events – corals spawning out of sync with environmental cues which impacts reproduction (Shlesinger & Loya 2019)
  • Increases in maximum sustainable yield (MSY) for temperate and polar region fisheries and decreases in MSY for tropical region fisheries (Cheng et al. 2010)

A literature review found the combination of overfishing and climate change would impact fish stocks and marine ecosystems and that ending overfishing would make those fish stocks and marine ecosystems more resilient to climate change (Sumaila & Tai 2020).

© 2016 – 2021 Seafood Free September


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