Are global fish stocks on the brink of collapse?

Environment | Oceans

By George Blake, Kingfisher Writer

Published December 21st, 2021

Global fish stocks are in a far worse state than previously thought, with roughly half being overexploited and 10% on the brink of collapse. However, these harrowing figures cannot truly capture the state of global fisheries; as uncertainty, inadequate data, and ineffectual management means we have a limited understanding of wide-scale stock collapse risk.

The Minderoo Foundation’s Global Fishing Index is a comprehensive report detailing the state of marine fisheries around the world, covering around 1,500 fisheries across 142 coastal states. Unsurprisingly, the report makes for grim reading; of the stocks assessed, 49% are overexploited and 8% are on the brink of collapse. While these figures are concerning, they do not provide a full and robust picture of global fish stocks, as estimating the sustainability of fisheries is plagued by huge uncertainty.

Roughly half of global fish stocks are overexploited, meaning the rate at which fish are removed exceeds the rate at which they are replenished. | Jean Wimmerlin / Flickr

Most fisheries are managed using the principle of maximum sustainable yield (MSY), which put simply, is the volume of biomass that can be removed from fish stock without leading to population collapse. The idea is that among an unexploited population, the biomass of a given stock would increase until the theoretical carrying capacity (where that reproduction rate equals that of the mortality) of the environment is reached.

Based on ecological population dynamics, the growth rate of this population is assumed to be greatest at intermediate biomass, where population size is roughly half that of the unexploited population potential. Therefore, fishing mortality (MSY) should be set to ensure biomass remains at around half the unexploited potential, as if fishing mortality is less than the MSY, the stock will become dominated by larger and older fish, making the fishery less productive.

But, if fishing mortality depletes their entire biomass to below 50% of the unexploited potential, the stock is overexploited, which if unaddressed may lead to population collapse.

‘Maximum sustainable yield is the volume of biomass that can be removed from fish stock without leading to population collapse.’

While MSY has long been viewed as the ‘holy grail’ of fisheries management, it is characterised by fundamental limitations. In reality, however, it is almost impossible to determine an exact MSY, since estimating the theoretical unexploited biomass of a population is highly challenging.

Stocks have generally been exploited for many years. Stock dynamics (growth, reproduction and natural mortality) are highly complex and heavily dependent on fluctuating environmental factors and ecosystem dynamics. The marine environment is also not conducive to accurate estimates of current biomass, due to challenges in counting, sizing, and ageing fish from a population of millions. All of which influences the resilience and reproductive capacity of a given population, meaning the MSY will be hugely variable through time and between species.

The above uncertainties make applying universal thresholds at which a stock is overexploited reductive, yet it remains commonplace within large-scale assessments. For example, the headline figure of 49% of stocks being overexploited, given in Minderoo’s Global Fishing Index, is based on a biomass threshold of 40%. Were this assumption to shift to a threshold of 50%, the percentage of stocks overexploited would rise to 62%.

Maximum sustainable yield aims to ensure maximum fishery productivity without leading to a collapse in population numbers. | Our World in Data

Similar issues plague discussions around fishery collapse. All stocks should remain within safe biological limits (SBLs), which refers to the lower limit of biomass at which production of recruits (juveniles) may be compromised (very high collapse risk).

But these SBLs will also constantly be in flux, with approximate thresholds varying significantly. The SBL threshold of 8%, used within the Minderoo Global Fishing Index, translates to 10% of stocks being at risk of collapse, while using a more cautious value of 17% doubles the number of stocks at risk of collapse to 20%.

‘Using a more cautious [safe biological limit] value of 17% doubles the number of stocks at risk of collapse to 20%.’

However, most troubling is that over half (52%) of the global catch since 1990 has been taken from stocks that lack sufficient data to estimate stock abundance. Without this critical data, fishery managers are simply operating in the dark, and we have no idea whether half of all fish caught on Earth is done so in a sustainable manner.

Assuming that the stocks lacking sufficient data are likely to be characterised by worse management outcomes than those where more robust data is available, the figures quoted in the Minderoo Global Fishing Index are likely to be significant underestimations. Although, while uncertainties plague the ideas of maximum sustainable yield and safe biological limits, one thing is crystal clear, there are definitely fewer fish in the sea.

Less than half of global fisheries have robust management relevant data availability. Meaning we can make no reasonable decision regarding their sustainability. | NOAA / Unsplash

The metric that most obviously demonstrates this decline is Catch Per Unit Effort, which describes the volume of fish caught divided by fishing effort (hours trawling, boat size, area covered, etc). Since 1950, fishing effort has increased ten-fold, yet yields are half of that seen in 1950.

Given this evidence of continued stock decline, measures are urgently needed to safeguard global fish stocks. Various proposals have been suggested, the most popular of which is to continue with ‘conventional’ fisheries management (MSY), but with a focus on taking a precautionary approach.

Under such an approach, maintaining stock biomass at or above 50% should be recognised as a minimum limit, not a target. With an aim to instead follow the maximum consistent yield, which stipulates that the largest long-term yield should not reduce biomass below a certain precautionary level (often two-thirds of unexploited biomass).

‘Since 1950, fishing effort has increased ten-fold, yet yields are half of that seen in 1950.’

While this is fine in principle, the issue is that scientific advice is frequently not heeded by regulators. From 2001 to 2015, quotas within the EU’s Common Fisheries Policy (under lobbying pressure from the fishing industry) were on average set 20% higher than the advised MSY.

An alternative approach, and one gaining increasing momentum, is the idea of creating integrated networks of Marine Protected Areas (MPAs). Within MPAs, fishing activity is banned or heavily restricted (often limited to artisanal fishers), allowing ecosystems to recover and populations to grow. Ecological recovery within an MPA builds overall ecosystem resilience and will precipitate a spillover of adults, eggs and larvae, that will increase yields from fisheries surrounding the MPA.

Currently, 71 nations have signed up to the Global Ocean Alliance, which hopes to protect 30% of the global ocean by 2030 (currently it stands at 7.7%). However, such a network would only result in tangible conservation and fisheries benefits if effectively managed, with current evidence suggesting this is unlikely. Globally, less than half of implemented MPAs are fully or highly protective, while in areas such as the coral triangle, only around 1% of MPAs are considered effective.

Fishing effort must be reduced to allow depleted stocks to recover. | Brian Yurasits / Unsplash

No one measure will be sufficient alone. A cross-disciplinary approach that considers conventional management, MPAs, enhanced data collection, and increased community-based management is required.

While some argue the lack of wide scale fisheries collapse demonstrates the current rhetoric is simply ‘alarmism’, this reductionist viewpoint undermines the urgent action required, as continuing with the status quo will not lead to favourable outcomes.

The ocean is not some inexhaustible resource, unless the global fisheries management fundamentally changes, we will continue our journey towards wide scale fisheries collapse, the timing may be uncertain, but the destination is known.

Featured Image: Linda Robert | Unsplash

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