The science of sustainable seafood, explained

What is the population status of small, pelagic forage fish worldwide?

Forage fish are the link in the food chain from planktonic primary producers to more familiar creatures higher up. They are small, pelagic fishes found in large schools or bait balls. Herring, smelt, shad, sprat, sardines, mackerel, anchovies, menhaden, and sand eels are some you may have heard of. Seemingly everything that can fit a forage fish in its mouth eats them (hence their moniker, being forage for others). All types of seabirds and marine mammals live off forage fish, as do several commercially-important fish like tuna, salmon, and cod. Forage fish themselves are commercially important: Some populations are fished for food while others support a large reduction industry, where fish are reduced to fishmeal and fish oil to feed livestock, farmed fish, and other species that humans find tastier (and pay more for).

Because of their significance in the food chain, understanding and monitoring forage fish populations is vital for conservation and ocean sustainability. Good news: Hilborn et al. 2022, a recent paper in Fish and Fisheries, reported the population status of forage fish over the last 50 years and found stability—the amount of forage fish has remained consistent over the previous five decades. Though individual populations naturally cycle between periods of high and low abundance, most forage fish are not threatened by overfishing; just one large population (European sand eel) is well above overfishing targets.

A global team of researchers assembled data on forage fish populations from the RAM Legacy database, the most complete database of worldwide stock assessments, representing 60% of the world’s forage fish catch. Lead author Ray Hilborn (and founder of this site) said, “Our paper paints a hopeful picture for the future of forage fish.” Their analysis reinforces emerging theories on forage fish ecology—that environmental factors drive population status much more than fishing pressure. 

Forage fish ecology

Forage fish congregate in highly productive areas with lots of plankton. Thanks to modern satellites, we can see where those areas are. The map below shows chlorophyll concentrations in the ocean—a measure of how much plankton is in the water. When you mouse over, you can see how areas of high productivity correlate with higher fish catches and areas of low productivity correlate with lower fish catch.

Plankton, and thus forage fish, are dependent on the number of nutrients in the water. Nutrient density is determined by various environmental and geological factors like currents, temperature, salinity, and underwater structure.

Forage fish populations boom and bust based on changes in those environmental factors (and many others we don’t fully understand), but their predators don’t. Forage fish predators evolved alongside the boom/bust cycles to prey switch—eat a wide variety of species, typically whatever is most abundant. Two recent research papers found no relationship between forage fish populations and their predators.

The paper released earlier this month shows how global forage fish populations have fluctuated over the last 50 years—or haven’t. They found that the worldwide abundance of forage fish has remained steady while individual stocks boom and bust. The researchers acknowledge a portfolio effect across monitored populations—a collective stability—like when a stock portfolio remains steady as individual stocks go up and down. This creates strange regulatory and conservation challenges. How much fishing pressure should be applied to a booming or busting population? If fishing pressure has less bearing on a fish population than previously thought, can fishermen extract more food and profit?

Historical data on forage fish populations

Commercial fishing for pelagic fish rapidly developed in the 1950s, but reliable monitoring of populations or fishing effort didn’t start until the 1960s and 70s. Without monitoring or regulation, many forage fish populations were overfished and several collapsed, notably herring stocks in the North Atlantic. The RAM Legacy Database doesn’t thoroughly capture the status of those stocks before 1970, so the authors of Hilborn et al. 2022 chose to start their analysis then.

They conglomerated data on 82 forage fish stocks and, below, presented the two most important metrics in fishery management: relative biomass and relative fishing pressure. Relative biomass is the ratio between measured biomass (B) and the target biomass (Btarget), often referred to as maximum sustainable yield (Bmsy). Relative fishing pressure is the ratio between fishing pressure (noted as U or F) and the targeted amount of fishing pressure (Utarget or Umsy). Good fishery management aims to keep the biomass and fishing pressure ratios as close to 1 as possible. An especially low biomass ratio (e.g., below 0.5) indicates an overfished stock, while an exceptionally high fishing pressure ratio (e.g., above 1.5) indicates a stock is subject to overfishing.

Figure 4 from Hilborn et al. 2022 shows relative biomass and fishing pressure on forage fish stocks from 1970-2018. You can see both ratios hover close to 1. The big error bars show how boom/bust forage fish stocks can be. You can also notice how monitoring coverage, indicated by the blue shading, has increased over time as more governments have dedicated time and money to managing their fisheries.

Figure 4 from Hilborn et al. 2022. Trends in small pelagic fish global estimates
Figure 4 from Hilborn et al. 2022. Trends in small pelagic fish global estimates from RAM legacy database of relative abundance, B/Btarg; and fishing pressure, U/Utarg, relative to MSY‐based or other target reference points from 1970–2018. Red dots show the median of all stocks assessed in that year. Boxplots show distributions of individual stocks in each year, with shading reflecting the fraction of stocks with assessments covering that year. Stocks are equally weighted.

Figure 4 looks like an expected outcome—fish the right amount (close to 1) and biomass will be the right amount (close to 1)—but digging deeper, the authors found something quite surprising: hardly any correlation between fishing pressure and biomass!

They grouped forage fish by FAO region and plotted the expected vs actual relationship between fishing pressure and biomass. In most areas, individual stocks showed hardly any correlation between fishing pressure and stock abundance. The Mediterranean-Black Sea region showed the most substantial relationship. Several stocks have been severely overfished there—it could be that depleted stocks are more susceptible to increases or decreases in fishing pressure.

Figure 8 from Hilborn et al. 2022. Bivariate mean trends in B/Btarg and U/Utarg from RAMLDB by major FAO area. The solid line going from upper left to lower right is the predicted equilibrium abundance for each level of fishing pressure. In the region above and to the right of this line, abundance is expected to decrease and is expected to increase in the region below and to the left of the line. Only regions with n > 5 stocks are included in this figure.

Aside from the Mediterranean-Black Sea region, the relationship between fishing pressure and forage fish biomass looks random across nearly every region. The authors have three hypotheses for why:

  1. Fishing pressure is low compared to natural mortality (forage fish are delicious prey)
  2. Recruitment, or the number of fish larvae that make it to adulthood, is variable and unrelated to adult biomass.
  3. Populations are subject to strong bottom-up/top-down food chain interactions.

Hypothesis 1 has a lot of merit. Lower trophic level species are more abundant, and humans extract a much lower percentage than higher trophic species. The authors state that “fishing mortality of small pelagic fish is lower in relation to predation than it is for higher trophic level species.” In other words, higher trophic level species die more often at the hands of humans than lower trophic species, which are usually eaten by other species or die naturally. In addition, hypotheses 2 and 3 would reaffirm recent discoveries in forage fish ecology—namely that they are greatly influenced by environmental factors and naturally boom or bust.

These natural fluctuations, combined with the portfolio effect and fishing being a relatively small portion of total mortality for small pelagic fish, together appear to explain the lack of relationship between fishing pressure and abundance… None of this argues that fishing pressure does not need to be regulated, but the impacts of regulation will be less evident in small pelagic fish than other functional groups.

Future of forage fish management

If forage fish are less responsive to fishery management interventions, how should we govern them? It’s a difficult question with many perspectives. Conservation groups see the population swings, uncertainty, and importance of forage fish and advocate for a more precautionary approach to fishing, while industry groups see the portfolio stability and lack of correlation to fishing pressure as an imperative to produce more food and profit.

The scientists in Hilborn et al. 2022 suggest a dynamic fishery management scheme that meets both sides in the middle. Strong, but flexible regulations would probably allow for increased catch and food production while also safeguarding busting stocks and preserving the portfolio of forage fish around the world.

Don’t forget about unassessed areas

The main issue in forage fish fisheries isn’t necessarily between conservation groups and industry (or at least regulated industry)—it’s in how many stocks are not monitored. The data presented in Hilborn et al. 2022 captured most of the world’s oceans but left out 40% of the world’s forage fish catch, most of which is caught in developing parts of Asia. Many of those countries can’t yet monitor their fisheries and are probably overfishing. The first step to arguing over fishery management is having it in the first place.

Regardless, Hilborn et al. 2022 paints an optimistic picture for the future of forage fish. Read the entire paper here, and check out our previous coverage of forage fish below.

Max Mossler

Max Mossler

Max is the managing editor at Sustainable Fisheries UW.

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