FEATURE ARTICLE - New indicators could help manage global (ecosystem) overfishing

Our latest feature explores potential international standard for tracking the status of global fisheries ecosystems.
Published: 6 September 2019

​​​Jason Link, NOAA Fisheries Senior Scientist for Ecosystems. Link has been a member and past chair of numerous ICES working groups, study groups, workshops, and symposia related to marine fisheries and ecosystems.

Real limits to production

Primary production forms the basis for all other biological production in an ecosystem. Phytoplankton power entire food webs, ultimately setting the limits on all productivity in a given part of the ocean. Sometimes we forget that when dealing with living marine resources. 

This ultimate limit set by primary producers is particularly true for marine fisheries production. And it holds promise for helping us manage marine fisheries.

In a paper published in Science Advances, Reg Watson and I suggest that scientists and resource managers really do need to focus on whole ecosystems rather than solely on individual populations. Population-by-population fishery management is more common around the world, but this new approach could help avoid the damages of widespread and sequential stock overfishing and the insecurity that it brings to fishing economies and communities.

Marine ecosystems experience overfishing

In simple terms, to successfully manage fisheries in an ecosystem, the rate of removal for all fishes combined must be equal to or less than the rate of renewal for all those fish. And knowing the ecosystem-level primary production can inform what the rate of that total fishery removal should be.

Using global databases and adopting a large marine ecosystem focus, we think we have discovered a way to determine when ecosystem overfishing is occurring. We propose three indices, each based on widely available catch and satellite data, to link fisheries landings to primary production and energy transfer up the marine food web. Specific thresholds developed for each index make it possible to determine if ecosystem overfishing is occurring. We functionally define ecosystem overfishing as occurring when the total catch of all fish is declining, the total catch rate or fishing effort required to get that catch is also declining, and the total landings relative to the production in that ecosystem exceed suitable limits. This definition of ecosystem overfishing is analogous to overfishing of single stocks, but more comprehensive for a large region of the ocean. Although several people have been working on the topic, this is the first time that an ecosystem has had overfishing clearly defined, described, and quantitatively demarcated.

Detecting overfishing at an ecosystem level will help to avoid many of the impacts we have seen when managing fished species on a population-by-population basis, and holds promise for detecting major shifts in ecosystem and fisheries productivity much more quickly. In several ecosystems, for example in the North Sea which is quite familiar to the ICES community, declines in these indices, indicative of ecosystem overfishing, was occurring about 5–10 years earlier than what was pieced together by looking at sequential collapses in individual populations of cods, herrings and related taxa. This could have avoided undue loss of value and avoided shifting the catches in that ecosystem to one now dominated by smaller fishes and invertebrates.

​Whole ecosystem indices

The first index used is the total catch per unit area, or how much fish a given patch of ocean can produce. The second is the ratio of total catches to total primary productivity, or how much fish can ultimately arise from the base of the food web, capitalizing on primary production limits. The third index is the ratio of total catch to chlorophyll, another proxy measure for marine primary production.

Notably, we develop thresholds for these indices. Proposed thresholds for each index are based on the known limits of the productivity of any given part of the ocean. Using these limits, local or regional context should be considered when deciding what management actions to take to address ecosystem overfishing. If ecosystem overfishing is detected, some form of lowering overall fishing rate would likely be recommended. But what that would look like and how it would be done really depends on the ecosystem and any policies already in place there. We don't want to be too prescriptive for a particular geography or region, given the jurisdictions or regulatory conditions that could already exist. But we also don't want to ignore that there are real limits to fishery production in a given marine ecosystem, and exceeding them has real consequences.

The thresholds clearly show when an entire ecosystem is being fished beyond what it is able to produce, basically noting that for any given patch of the ocean there is only so much fished species that can be produced. These thresholds are theoretically developed with ample supporting empirical evidence. Despite their novelty, the data needed to estimate these indicators is already available or relatively easy to acquire. Having international standards would make those decisions about overfishing much easier to determine and would help to emphasize sustainable fisheries.

We named the indices in honor of the late marine biologist John Ryther and NOAA Fisheries scientists Michael Fogarty and Kevin Friedland. All have worked extensively on integrating ecosystem and fishery dynamics for better resource management. Like so much of the work done in the ICES community, we appreciate and recognize the valuable contributions of those who have laid down a legacy for us to follow.

Tropics, temperate regions face most ecosystem overfishing

We know that climate change is shifting many fish populations toward the poles, yet the fishing fleets and associated industries are not shifting with them. That already has had serious economic and cultural impacts. We are able to follow these shifts over time and how they can exacerbate or even contribute to ecosystem overfishing.

Fisheries are an important part of the global economy. In addition to trade and jobs, fish provide the primary source of protein to more than 35 percent of the world's population, and 50 percent of the people in the least developed countries, according to FAO reports. Regions where the greatest amount of ecosystem overfishing occurs are also where impacts can be the greatest.

When we looked at 64 large marine ecosystems around the world, approximately 50% are experiencing ecosystem overfishing. We found those in the tropics, especially in Southeast Asia, have the highest proportion of ecosystem overfishing. Temperate regions also have a high level of ecosystem overfishing, with limited capability to absorb shifting fishing pressure from the tropics as species move toward the poles. This has ramifications for many European marine seas. It also has implications for marine fish trade to supply European seafood demands.

Even if tropically-oriented fleets were able to shift latitudes and cross claims for marine exclusive economic zones, it remains unclear if temperate regions could absorb shifts from the tropics, given that many temperate regions are also experiencing ecosystem overfishing and catches there have been flat for more than 30 years. It seems likely that fisheries of the global ocean are at or near carrying capacity, even accommodating a dynamic ocean environment. Thus the utility of monitoring these ecosystems for overfishing is heightened.

Ecosystem overfishing does not occur in every marine ecosystem, but it is widespread. The fact that nearly half the world's large marine ecosystems are experiencing ecosystem overfishing is not a trivial observation.

Potential international standard

The three indices proposed represent a potential international standard for tracking the status of global fisheries ecosystems.

They are easy to estimate and interpret, are based on widely repeated and available data, and are a practical way to identify when an ecosystem would be experiencing overfishing based on well understood and well-accepted primary production and food web limitations. Adopting these would eliminate a lot of the debate about whether or not ecosystem overfishing is happening and instead focus attention on solutions. But until we can define and identify what ecosystem overfishing is, we cannot begin to address it.

One of ICES science priorities is impacts of human activities. This includes work that explores how pressures on the marine environment act, independently and collectively, to modify the variety, quantity and distribution of marine life and the structure, function and dynamics of food webs and marine ecosystems (including cumulative pressures and their cumulative impacts).  ​

Read all ICES science priorities in our Science plan.

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​Schematic of population and ecosystem overfishing: ​
(A) Schematic of population overfishing. While a population is experiencing overfishing, the abundance and biomass (here as the number of fish icons) and fish size declines over time, along with many other facets related to population and fleet dynamics. (B) Schematic of EOF. Analogous to population overfishing, EOF is the result of continued fishing pressure on multiple populations, leading to sequential depletion across populations in an ecosystem over time. Click to enlarge.

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FEATURE ARTICLE - New indicators could help manage global (ecosystem) overfishing

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