Fish faeces and ocean life

Abstract

Better knowledge of marine carbon cycles, their effect on global warming, and the important role of fish in both, can convince politics to establish more sustainable fishery policies.

Fishing stirs primordial instincts in nations that far exceed its contribution to wealth or even nutrition; sometimes, disputes over fishing rights even erupted into armed conflict as in the “cod wars” between the UK and Iceland over access to Atlantic cod fisheries near Iceland (Fig 1). Just recently, the UK and France have been embroiled in an increasingly bitter dispute over access to the former’s territorial waters by the latter’s fishermen. This has seriously affected the relations between these two countries, despite the fact that fishing accounts for a mere 0.1% of the UK’s GDP and even less for France at just 0.06%.

Figure 1.

Atlantic cod (Gadus morhua). Illustrations de Ichtyologie ou histoire naturelle générale et particulière des Poissons Bloch, Marcus Elieser, J. F. Hennig, Plumier, Krüger, Pater, Schmidt, Ludwig, Bodenehr, Moritz 1795‐1797. Wikimedia.

… a growing understanding of the impact of fishing on marine ecosystems has led to intensifying efforts to implement and enforce strategies that will preserve fish stocks …

Behind these headlines, a growing understanding of the impact of fishing on marine ecosystems has led to intensifying efforts to implement and enforce strategies that will preserve fish stocks and protect delicately balanced marine ecosystems. While it is difficult to persuade governments to impose sufficiently strict quotas amid a field where emotions run high and communities are determined to maintain long established fishery traditions, there has been some success establishing Marine Protected Areas (MPAs) similar in concept to national parks where all life is protected. However, convincing governments of their vital contribution to marine ecosystems, especially coastal waters, requires comprehensive data about the impact of human activities across the ecosystem as a whole.

The biochemical role of fish excretions

One significant step in that direction was a 2021 paper that demonstrated that fish—or rather their excretions sinking down to deeper waters where they feed the ecosystem near the bottom of the seas—play an even bigger role in the oceanic ecosystem than had been thought before. The study by a US team also provided evidence that, by the 1990s, the total biomass of fish in the range 10 g to 100 kg had been reduced by half as a result of human activities, along with associated carbon cycling rates (Bianchi et al, ²⁰²¹).

The starting point was that biomass and biogeochemical role of fish in the ocean were known to be ecologically important but with no clear idea of the extent, given the lack of quantitative data. The authors first developed a model to infer the biomass, metabolic rate and biogeochemical importance of fish targeted by fisheries, as well as spatial distribution in the oceans, and their historical change based on historical records of fish catch and stock assessments. They then calibrated and refined their model with contemporary data so as to be able to predict the biomass and biogeochemical impact of fish stocks.

“There are two main impacts we would like to have on the scientific community,” commented Daniele Bianchi, lead author and an oceanographer specializing in biogeochemistry at the University of California, Los Angeles. “The first is renewing the attention of the importance of marine fish on ocean biogeochemistry, specifically the cycles of carbon, oxygen and nutrients. While these cycles are mostly driven by microbes in the ocean, there’s a growing recognition that animals shape them in ways we are only now beginning to unravel. The second is to point out that such effects may have significantly changed over time following the inception of industrial fisheries, overlapping with ongoing climatic and environmental change. In general, we argue that marine animals such as fish should be included as essential elements of Earth system models used to understand and project Earth’s cycles.”

In general, we argue that marine animals such as fish should be included as essential elements of Earth system models used to understand and project Earth’s cycles.

Ocean carbon fluxes

That point has gained much resonance in the light of climate change impact on the ocean’s carbon cycle, for although the amount of carbon stored in marine biota at around 3 Gigatons is only around 0.5% of that stored in terrestrial vegetation at 610 Gigatons, the flux, that is the amount of carbon exchanged, is almost equal at around 50 Gigatons each (Schlesinger & Bernhardt, 2013). The major contributors are plants and algae by fixing atmospheric CO₂ during photosynthesis and converting it into organic compounds which are then distributed laterally and downwards through ecosystems. This includes phosphorus and nitrogen which exhibit cycles of their own closely connected to the carbon cycle, as well as micronutrients. Some of the organic carbon eventually sinks to the deep ocean, where it is recycled back into inorganic carbon.

As Bianchi pointed out, there have been ongoing efforts to better represent marine animals in Earth system models through the Fisheries and marine ecosystem Model Intercomparison Project (Fish‐MIP), an international collaboration to address both human and natural aspects of ocean ecosystems. This is drawing together diverse modellers to assess impacts, provide projections for international policy formulation, and contribute towards improvement in prediction. The output of FishMIP simulations is then made publicly available through the online ISIpedia platform to provide information about possible impacts of climate change for decision makers (https://www.isipedia.org/). “The FishMIP project has already generated important projections of future abundance of marine animals in a changing ocean,” Bianchi noted. “However, the feedback of this change on marine chemistry and the ecosystem is only now beginning to be studied with the same tools. I would hope our paper points to potentials directions and approaches to do so.”

Indeed, other scientists in the field have acknowledged the seminal role of their paper. “Basically, just having decent numbers on these things will open up the door for a lot more research and a lot more interest in these consumer‐derived processes,” said Jacob Allgeier, a specialist in tropical coastal ecosystems at the University of Michigan, USA. “There have been other studies that have done similar things, but this is certainly the most comprehensive yet. Hopefully this will spurn on folks to dig in a bit more and actually develop some more concrete estimates of these processes.” The findings were also regarded as timely by Boris Worm from the Ocean Frontiers Institute at Dalhousie University, Halifax, Canada, and a prominent specialist in fisheries. “Yes, I think that this is a topic that is coming under closer scrutiny, especially in the context of climate change and the effects of fishing and the ocean carbon cycle,” he said. “I don’t think the results were that surprising, but it is still important to see some of these processes quantified.”

One obvious question is whether more robust data on the importance of fish in the global marine ecosystem would convince politicians to adopt more sustainable fisheries policies.

Allgeier does have one caveat, noting that the research had not taken account of the interaction between fish excretions and ocean microbes while on the way down. “I interpret the carbon export estimates very very cautiously because they don't even begin to calculate how much of that carbon from poop is respired back by microbes, which I would think would be significant,” he explained. Bianchi commented that his team was just beginning to developing consistent unified models for assessing accurately the global biomass and biogeochemical cycling rates of fish. Catering for interactions between microbes and faecal matter as it descends would be an important step.

A case for sustainable fisheries

One obvious question is whether more robust data on the importance of fish in the global marine ecosystem would convince politicians to adopt more sustainable fisheries policies. There is not much optimism though. “To be totally honest, I don't think these numbers will affect commercial fishing at all and the direct effect of these numbers on policy are not going to be substantial,” Allgeier said. “I do think that these numbers provide another reason we should be thinking about ecosystem‐based management and more sustainable fishing practices, and I think these numbers add important and tangible numbers to that argument. But will policy makers pay any attention to this? I personally don't think so. There have been a lot more damning numbers reported in the past with little to no effect on policy.”

Allgeier is optimistic though that sustainable fisheries are achievable in the longer term and that meanwhile research can provide other benefits. “I study nutrient and energy dynamics in coastal tropical ecosystems with an overarching goal of helping to promote sustainable fisheries and ecosystems that provide useful services, for example carbon sequestration in seagrass,” said Allgeier. Indeed, there has been growing focus on the role of seagrass and other plants such as mangroves that inhabit the coasts. Seagrasses in particular are especially valued for their potential contribution to atmospheric carbon reduction because, although they account for less than 0.2% of the world's oceans, they sequester approximately 10% of the carbon buried in ocean sediment annually (Fig 2). Per hectare, seagrasses can store up to twice as much carbon as terrestrial forests (https://themeaningofwater.com/2021/05/26/largest‐seagrass‐restoration‐project‐in‐england‐the‐blue‐carbon‐initiative).

Figure 2.

Seagrass meadow in the Florida Keys National Marine Sanctuary. Wikimedia / US National Oceanic and Atmospheric Administration.

The bigger focus though is on the whole ocean and that is where the recent work on fish will help to assess the effect of human activities and climate change on populations of all species, according to Bianchi. “The main effect of human activities is obviously depletion of animal abundance, and the cascading effects on other species, what we generally refer to as trophic cascades,” he explained. “Fishery management tends to focus on effects on the abundance of targeted species, and only now are we beginning to consider whole‐ecosystem impacts, a much more complex undertaking. Implications for seawater chemistry and carbon cycle are yet to be carefully quantified and considered for management.”

Marine protected areas

Given the pessimistic prospects for immediate changes to fishing policies, the greatest hope in the shorter term lies in further establishment and expansion of MPAs, which have been subject to increasing research in recent years. It has been a mixed picture so far, with some successes but also questions over effectiveness in areas where overfishing is still occurring nearby, with debates over different approaches and how best to resource them.

MPAs are not a new concept; they emerged well over half a century ago for reversing some of the damage resulting from centuries of treating oceans as infinite resources both for food and dumping waste, and since then MPAs have expanded rapidly at a rate of over 8% per year by area (Worm, 2017). Furthermore, the rate of MPA expansion has accelerated during the past decade, including large zones in remote ocean areas where commercial extraction of natural resources such as fish, minerals or oil is banned completely. Nine of the world’s 10 largest protected areas are now marine, the biggest of all in the Ross Sea of Antarctica occupying 1.55 million square Kms, almost three times the size of France.

… continued global expansion of MPAs without similar investment in human and financial capacity would waste resources, but could be successful if it were given the required commitment.

There are caveats still, including controversy over further expansion, especially where MPAs intersect with commercial fishing areas. There are also questions over policing MPAs, which is more enforceable in waters under the jurisdiction of countries that have signed up than on the “high seas” that are subject to international agreements. A variety of technological systems can now monitor MPAs and detect abuses, but unless violations can be enforced through effective agreements and collaborations, these will be of limited value.

The most comprehensive assessment so far, conducted in 2017, found that, although 71% of MPAs positively influenced fish populations, conservation impacts were highly variable. Many MPAs failed to meet targets for effective and equitable management, largely through lack of sufficient money and experienced staff (Gill et al, ²⁰¹⁷); MPAs with adequate staff capacity and management had ecological effects 2.9 times greater than those without. This led to the conclusion that continued global expansion of MPAs without similar investment in human and financial capacity would waste resources, but could be successful if it were given the required commitment.

It is also important to recognize that MPAs on their own cannot achieve sustainable fishing on a global scale, because they will only ever account for a small proportion of the total ocean surface and fish will not stay conveniently within their boundaries. Therefore, sustainable fishing has to be achieved outside MPAs, where at least some progress has been made through enforcement of quotas for specific species. In the UK for example, the percentage of UK quota‐fish stocks increased from 9% in 1990 to 51% in 2019, according to the country’s Joint Nature Conservation Committee (JNCC) (https://jncc.gov.uk/our‐work/ukbi‐b2‐sustainable‐fisheries/).

Achieving sustainable fishing

It is worth pointing out that the concept of overfishing is not quite the same as unsustainable fishing. The UN Food and Agriculture Organization (FAO), the main global authority on fisheries, defines a target biomass and considers overfishing to have occurred when the population is reduced to 80% or less of that level (https://sustainablefisheries‐uw.org/fact‐check/how‐many‐fisheries‐are‐overfished/). However, that definition does not take account of the dynamic situation determined by the level of fishing. A fish stock above target biomass might still be unsustainable if it was seriously overfished at the time. Conversely, a severely depleted stock below that 80% threshold might be in much better shape if it was well managed and recovering in numbers year by year.

In that UK study, another measure has been used—the spawning biomass of each stock—which is considered essential for maintaining a sustainable yield of that fish. By that definition, the percentage of UK fish considered sustainable rose from 30% in 1990 to 58% in 2019. Other countries, such as Norway and Iceland, have also made progress in sustainable use of their fisheries, demonstrating that progress can be made outside MPAs, as indeed it must.

There is little room for complacency though around European waters, as 39% of fish stocks are classified as overfished in the North East Atlantic and nearby seas (https://www.fishforward.eu/en/topics/facts‐figures/). The EU is also the world’s primary importer of fish, over 50% of which comes from developing countries. This means that Europe is outsourcing the problem of overfishing to areas where sustainable practices have yet to be established. This comes at a time when global consumption of fish and seafood is still rising, having quadrupled during the past 50 years. The situation is not quite as dire as that statistic suggests because aquaculture, or fish farming, has also expanded enormously, accounting for 46% of the world’s fish consumption by 2020, which has helped to check the output of wild fisheries (https://reliefweb.int/sites/reliefweb.int/files/resources/The%20State%20of%20World%20Fisheries%20and%20Aquaculture%202020.%20In%20brief.pdf). Meanwhile, more than half of the global wild‐caught fish comes from developing countries such as Indonesia, Peru, Vietnam, India or China, being the world’s top producer (https://www.fao.org/publications/sofia/2020/fr/). Some species suffer more from overfishing than others, including Atlantic cod and Haddock in the Northeast Atlantic, Whiting in the Southwest Atlantic and Atlantic bluefin tuna in the Indian Ocean https://www.fao.org/Newsroom/common/ecg/1000505/en/stocks.pdf).

One factor that has contributed locally towards more sustainable fishing has been through integrating social sciences into the study of human‐dominated ecosystems, a factor Worm picked up on from the Gill et al report. This again should extend to the whole ocean and not just MPAs, bringing in all stakeholders, including the fishing communities. More focus on leadership and social cohesion within the fishing community, rather than just on biophysical aspects of the system, would be more likely to lead to sustainable use of the oceans.

Buffer against climate change

Meanwhile, the role of MPAs themselves has expanded beyond pure conservation or sustainable fishing to include climate change. Another 2017 study argued that well‐managed marine reserves may help marine ecosystems and people adapt to five prominent impacts of climate change: acidification, sea‐level rise, intensification of storms, shifts in species distribution and decreased productivity and oxygen availability, as well as their cumulative effects (Roberts et al, ²⁰¹⁷). The paper argued that healthy ocean ecosystems could help mitigate these aspects by promoting carbon sequestration and storage, buffering against environmental fluctuations and the biological responses to protection.

With the role of fish in the ocean carbon cycle now being better calibrated, this knowledge generates opportunities for devising more sustainable approach to fisheries.

While this is rather speculative, the authors made a strong case for using marine reserves to help rebuild populations of teleost fish that play a significant role in the inorganic carbon cycle. Teleost fish drink seawater for osmoregulation and precipitate almost all the ingested calcium and some ingested magnesium as carbonate minerals in their alkaline intestine, excreting high‐magnesium calcite crystals from their gut. These fish carbonates dissolve at shallower depths than the calcite and aragonite produced by marine calcifiers such as coccolithophores, foraminifera and corals, which has the effect of raising alkalinity. The accumulation of high‐magnesium calcite in shelf sediments, a large proportion of which comes from fish, could therefore provide a first line of defence against the impact of ocean acidification resulting from increased atmospheric CO₂ concentrations.

With the role of fish in the ocean carbon cycle now being better calibrated, this knowledge generates opportunities for devising more sustainable approach to fisheries. The biggest challenge lies in bringing relevant stakeholders along and striking a balance between the benefits of fishing and the environmental cost of extracting them from their ecosystems. As Worm noted, the biogeochemical aspects are only now coming into focus and need to be taken account of in discussions around sustainable fisheries.

EMBO Reports (2022) 23: e54514.