Fish larvae float across national borders, binding the world’s oceans in a single network


Larval black sea bass, an important commercial species along the US Atlantic coast.
NOAA Fisheries/Ehren Habeck

Nandini Ramesh, University of California, Berkeley; James Rising, London School of Economics and Political Science, and Kimberly Oremus, University of Delaware

Fish populations are declining around the world, and many countries are trying to conserve them by regulating their fishing industries. However, controlling fishing locally may not do enough to strengthen fish populations. Often one nation’s fish stocks depend on the spawning grounds of a neighboring country, where fish release eggs and sperm into the water and larvae hatch from fertilized eggs.

We do research on oceans, climate and fisheries. In a recent study, we showed that global fisheries are even more tightly connected than previously understood. The world’s coastal marine fisheries form a single network, thanks to the drift of larvae along ocean currents.

This suggests that country-by-country fishery management may be fundamentally insufficient. If a fish species that provides food to one country should decline, the amount of fish spawn, or eggs and larvae, riding the ocean currents from there to other countries would also decline dramatically, resulting in further loss of fish elsewhere.

Many countries live with this risk, although they may not realize it. To manage fisheries effectively, nations must understand where the fish in their territories originate.

Ocean currents affect the speed at which fish eggs and larvae drift and vary through the year. This map shows surface current speeds for January: yellow = fastest, dark blue = slowest. Each country’s territory is highlighted with red dots during the month of maximum spawning activity in that country. In each territory, a different number of species spawn in each month of the year. The red dots appear in the month during which the largest number of species spawn in that territory.

Crossing national borders

Fish don’t recognize political boundaries, and regularly travel internationally. Scientists have tracked adult fish movements using electronic tags, and have shown that a few species migrate over long distances.

Countries and territories have negotiated agreements to ensure sustainable sharing of migratory fish. One such agreement joins several nations in the Western and Central Pacific Fisheries Commission to ensure that the territories fish cross share them sustainably.

But fish eggs and larvae are much harder to follow. Many species lay eggs in large numbers that float near the ocean surface. When they hatch, larvae measure a few millimeters long and continue to drift as plankton until they grow large enough to swim. During these stages of the life cycle, ocean currents sweep fish spawn across international boundaries.

Simulating the journeys of eggs and larvae

Like weather on land, the pattern of ocean currents varies with the seasons and can be predicted. These currents are typically sluggish, traveling about an inch per second, or less than 0.1 miles per hour.

There are a few exceptions: Currents along the eastern coasts of continents, like the Gulf Stream in North America or the Kuroshio in Asia, and along the equator can be significantly faster, reaching speeds of 2 miles per hour. Even a gentle current of 0.1 miles per hour can carry spawn 40 miles over a month, and some species can float for several months.

Government and academic scientists use a vast network of satellites, moored instruments and floating buoys to monitor these surface flows. Using this information, we performed a computer simulation of where drifting particles would be carried over time. Scientists have used this type of simulation to study the spread of marine plastic pollution and predict where debris from plane crashes at sea could have washed ashore.

Different fish species spawn in different seasons, and a single species may spawn in several months at different locations. We matched the seasons and locations of spawning for over 700 species with ocean current data, and simulated where their spawn would drift. Then, using records of where those species have been fished, and information about how suitable conditions are for each species in different regions, we deduced what fraction of the fish caught in each country arrived from other countries because of ocean currents.

A small-world network

Scientists and policymakers can learn a lot by studying these international connections. Each species that floats across international boundaries during its plankton stage represents a linkage between countries. These linkages span the world in a dense, interconnected network.

Each color represents a region in the network of fish larvae connections. This map shows the strongest 467 connections among a total of 2,059 that the authors modeled.
Nandini Ramesh, James Rising and Kimberly Oremus, CC BY-ND

At a global level, this network of connections has an important property: It is a small-world network. Small-world networks connect regions that are far apart to each other by just a few steps along the network. The concept is rooted in social scientist Stanley Milgram’s 1960s experiments with social networks, which found that it was possible for a letter to reach almost any total stranger by passing through six or fewer hands. Milgram’s work was popularized in the 1990 play “Six Degrees of Separation.”

Among fisheries, the world seems even smaller: We found that the average number of degrees of separation among fisheries is five. This means that local problems can become global risks.

For example, imagine that a fishery collapses in the middle of the Mediterranean. If the population in one spawning region collapses, it could quickly put pressure on neighboring fisheries dependent upon it. If fishers in those neighboring countries overfish the remaining population or shift to other species, the disturbance can grow. Within just a few years, a fisheries disturbance could travel around the world.

We assessed how countries would be affected in terms of food security, employment and gross domestic product if they were to lose access to fish spawn from other territories. The most affected countries cluster in the Caribbean, the western Pacific, Northern Europe and West Africa. These hotspots correspond to the network’s most clustered areas, because the effects of these flows of fish spawn are most pronounced where many coastal countries lie in close proximity.

International flows of fish eggs and larvae affect countries’ total catch, food security, jobs and economies.
Nandini Ramesh, James Rising and Kimberly Oremus, CC BY-ND

Thinking globally about fisheries

Because the world’s fisheries are so interconnected, only international cooperation that takes flows of fish spawn into account can effectively manage them. Aside from egg and larvae connections, fisheries are linked by movements of adult fish and through agreements among countries allowing them to fish in each other’s waters.

All of this suggests that fishery management is best conducted at a large, international scale. Proposals for doing this include defining Large Marine Ecosystems to be jointly managed and creating networks of Marine Protected Areas that safeguard a variety of critical habitats. Ideas like these, and careful study of interdependence between national fisheries, are crucial to sustainable use of the oceans’ living resources.

[ Expertise in your inbox. Sign up for The Conversation’s newsletter and get a digest of academic takes on today’s news, every day. ]The Conversation

Nandini Ramesh, Postdoctoral Researcher in Earth and Planetary Science, University of California, Berkeley; James Rising, Assistant Professorial Research Fellow, London School of Economics and Political Science, and Kimberly Oremus, Assistant Professor of Marine Policy, University of Delaware

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Ocean warming has fisheries on the move, helping some but hurting more



An Atlantic cod on ice. Cod fisheries in the North Sea and Irish Sea are declining due to overfishing and climate change.
Robert F. Bukaty/AP

Chris Free, University of California, Santa Barbara

Climate change has been steadily warming the ocean, which absorbs most of the heat trapped by greenhouse gases in the atmosphere, for 100 years. This warming is altering marine ecosystems and having a direct impact on fish populations. About half of the world’s population relies on fish as a vital source of protein, and the fishing industry employs more the 56 million people worldwide.

My recent study with colleagues from Rutgers University and the U.S. National Oceanic and Atmospheric Administration found that ocean warming has already impacted global fish populations. We found that some populations benefited from warming, but more of them suffered.

Overall, ocean warming reduced catch potential – the greatest amount of fish that can be caught year after year – by a net 4% over the past 80 years. In some regions, the effects of warming have been much larger. The North Sea, which has large commercial fisheries, and the seas of East Asia, which support some of the fastest-growing human populations, experienced losses of 15% to 35%.

The reddish and brown circles represent fish populations whose maximum sustainable yields have dropped as the ocean has warmed. The darkest tones represent extremes of 35 percent. Blueish colors represent fish yields that increased in warmer waters.
Chris Free, CC BY-ND

Although ocean warming has already challenged the ability of ocean fisheries to provide food and income, swift reductions in greenhouse gas emissions and reforms to fisheries management could lessen many of the negative impacts of continued warming.

How and why does ocean warming affect fish?

My collaborators and I like to say that fish are like Goldilocks: They don’t want their water too hot or too cold, but just right.

Put another way, most fish species have evolved narrow temperature tolerances. Supporting the cellular machinery necessary to tolerate wider temperatures demands a lot of energy. This evolutionary strategy saves energy when temperatures are “just right,” but it becomes a problem when fish find themselves in warming water. As their bodies begin to fail, they must divert energy from searching for food or avoiding predators to maintaining basic bodily functions and searching for cooler waters.

Thus, as the oceans warm, fish move to track their preferred temperatures. Most fish are moving poleward or into deeper waters. For some species, warming expands their ranges. In other cases it contracts their ranges by reducing the amount of ocean they can thermally tolerate. These shifts change where fish go, their abundance and their catch potential.

Warming can also modify the availability of key prey species. For example, if warming causes zooplankton – small invertebrates at the bottom of the ocean food web – to bloom early, they may not be available when juvenile fish need them most. Alternatively, warming can sometimes enhance the strength of zooplankton blooms, thereby increasing the productivity of juvenile fish.

Understanding how the complex impacts of warming on fish populations balance out is crucial for projecting how climate change could affect the ocean’s potential to provide food and income for people.

Warming is affecting virtually all regions of the ocean.

Impacts of historical warming on marine fisheries

Sustainable fisheries are like healthy bank accounts. If people live off the interest and don’t overly deplete the principal, both people and the bank thrive. If a fish population is overfished, the population’s “principal” shrinks too much to generate high long-term yields.

Similarly, stresses on fish populations from environmental change can reduce population growth rates, much as an interest rate reduction reduces the growth rate of savings in a bank account.

In our study we combined maps of historical ocean temperatures with estimates of historical fish abundance and exploitation. This allowed us to assess how warming has affected those interest rates and returns from the global fisheries bank account.

Losers outweigh winners

We found that warming has damaged some fisheries and benefited others. The losers outweighed the winners, resulting in a net 4% decline in sustainable catch potential over the last 80 years. This represents a cumulative loss of 1.4 million metric tons previously available for food and income.

Some regions have been hit especially hard. The North Sea, with large commercial fisheries for species like Atlantic cod, haddock and herring, has experienced a 35% loss in sustainable catch potential since 1930. The waters of East Asia, neighbored by some of the fastest-growing human populations in the world, saw losses of 8% to 35% across three seas.

Other species and regions benefited from warming. Black sea bass, a popular species among recreational anglers on the U.S. East Coast, expanded its range and catch potential as waters previously too cool for it warmed. In the Baltic Sea, juvenile herring and sprat – another small herring-like fish – have more food available to them in warm years than in cool years, and have also benefited from warming. However, these climate winners can tolerate only so much warming, and may see declines as temperatures continue to rise.

Shucking scallops in Maine, where fishery management has kept scallop numbers sustainable.
Robert F. Bukaty/AP

Management boosts fishes’ resilience

Our work suggests three encouraging pieces of news for fish populations.

First, well-managed fisheries, such as Atlantic scallops on the U.S. East Coast, were among the most resilient to warming. Others with a history of overfishing, such as Atlantic cod in the Irish and North seas, were among the most vulnerable. These findings suggest that preventing overfishing and rebuilding overfished populations will enhance resilience and maximize long-term food and income potential.

Second, new research suggests that swift climate-adaptive management reforms can make it possible for fish to feed humans and generate income into the future. This will require scientific agencies to work with the fishing industry on new methods for assessing fish populations’ health, set catch limits that account for the effects of climate change and establish new international institutions to ensure that management remains strong as fish migrate poleward from one nation’s waters into another’s. These agencies would be similar to multinational organizations that manage tuna, swordfish and marlin today.

Finally, nations will have to aggressively curb greenhouse gas emissions. Even the best fishery management reforms will be unable to compensate for the 4 degree Celsius ocean temperature increase that scientists project will occur by the end of this century if greenhouse gas emissions are not reduced.

[ Like what you’ve read? Want more? Sign up for The Conversation’s daily newsletter. ]The Conversation

Chris Free, Postdoctoral Scholar, University of California, Santa Barbara

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Fish kills and undrinkable water: here’s what to expect for the Murray Darling this summer



Dry conditions will make for a difficult summer in the Murray Darling Basin.
AAP/Dean Lewins

Jamie Pittock, Australian National University

A grim summer is likely for the rivers of the Murray-Darling Basin and the people, flora and fauna that rely on it. Having worked for sustainable management of these rivers for decades, I fear the coming months will be among the worst in history for Australia’s most important river system.

The 34 months from January 2017 to October 2019 were the driest on record in the basin. Low water inflows have led to dam levels lower than those seen in the devastating Millennium drought.

No relief is in sight. The Bureau of Meteorology is forecasting drier-than-average conditions for the second half of November and December. Across the summer, rainfall is also projected to be below average.

So let’s take a look at what this summer will likely bring for the Murray Darling Basin – on which our economy, food security and well-being depend.

A farmer stands in the dry river bed of the Darling River in February this year.
Dean Lewins/AAP

Not a pretty picture

As the river system continues to dry up and tributaries stop flowing, the damaging effect on people and the environment will accelerate. Mass fish kills of the kind we saw last summer are again likely as water in rivers, waterholes and lakes declines in quality and evaporates.

Three million Australians depend on the basin’s rivers for their water and livelihoods. Adelaide can use its desalination plants and Canberra has enough stored water for now. But other towns and cities in the basin risk running out of water.




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Paddling blind: why we urgently need a water audit


Governments were warned well before the drought to better secure water supplies through infrastructure and other measures. But the response was inadequate.

Some towns such as Armidale in New South Wales have been preparing to truck water to homes, at great expense. Water costs will likely increase to pay for infrastructure such as pumps and pipelines. The shortages will particularly affect Indigenous communities, pastoralists who need water for domestic use and livestock, irrigation farmers and tourism business on the rivers.

Water in major storages as reported at 13 November 2019.
Murray Darling Basin Authority

As we saw during the Millennium drought, when wetland soils dry some sediments will oxidise to form sulfuric acid. This kills fauna and flora and can make water undrinkable.

Red gum floodplain forests and other wetland flora will continue to die. Most of these wetlands have not had a drink since 2011. The desiccation, due to mismanagement and drought, is likely to see the return of hypersalinity – a huge excess of salt in the water – with river flows too weak to flush the salt out to sea.




Read more:
Murray-Darling report shows public authorities must take climate change risk seriously


If drought-breaking rains do come, as they did in 2010-11, this would create a new threat. Floodwaters would inundate leaf litter on the floodplains, triggering a bacterial feast that depletes the water of oxygen. These so-called “blackwater” events kill fish, crayfish and other aquatic animals.

The risk of blackwater events has largely arisen because government authorities have failed to manage water as they had agreed. In particular, the NSW and Victorian governments have not worked with farmers to allow managed river flows to inundate floodplains.

The prospect of thousands of dead fish in the Murray Darling Basin looms large again this summer.
AAP/GRAEME MCCRABB

How did we get here?

The severity and impacts of this drought should not come as a surprise. In the 1980s, the CSIRO’s first projections of climate change impacts in the basin foreshadowed what is unfolding now.

Despite the decades-old warnings, water management authorities in some catchments favoured water extraction by irrigators over rural communities, pastoralists and the environment. For example, the NSW Natural Resources Commission in September found that state government changes to water regulations brought forward the drying up of the Darling River by three years.




Read more:
We can’t drought-proof Australia, and trying is a fool’s errand


Since the basin plan was adopted in 2012 our federal and state political leaders have reduced the volume of real water needed to keep the rivers healthy, supply water to people and flush salt out to sea. For example, in May 2018 the federal government and Labor opposition agreed to reduce water allocated to the environment by 70 billion litres a year on average, without a legitimate scientific basis.

The basin plan is based on historical river flow records, without explicitly allowing for diminished inflows resulting from climate change. Australian water management has followed what’s been termed a “hydro-illogical cycle” where drought triggers reform, but government leaders lose attention once it rains. This suggests meaningful reform must be implemented when drought is occurring and politicians are under pressure to respond.

Severe drought and mismanagement means a dire summer for the Murray-Darling river system.
Dean Lewins/AAP

How to fix this

Governments must assume that climate-induced drought conditions in the basin are the new normal, and plan for it.

Action should include:

  • Revising water allocations consistent with climate change projections

  • Investing in managed aquifer recharge to supply more towns with reliable and safe water

  • Restoring rivers by reallocating enough water to sustain their health

  • Increasing wetland resilience by reconnecting rivers to their floodplains in wetter years

  • Improving river health, such as by fencing out livestock.

Investing in these adaptation actions now would provide jobs during the drought and prepare Australia for a much drier future in the Murray-Darling Basin.The Conversation

Jamie Pittock, Professor, Fenner School of Environment & Society, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Curious Kids: how do fish sleep?



The Ringtail Unicornfish, which occurs in tropical marine waters of the Indo-Pacific. All fish sleep, even the weird-looking ones.
Bernard Spragg/Flickr

Culum Brown, Macquarie University


How do fish sleep? Do they keep swimming or do they sleep somewhere? – Anna, age 5, Thornleigh, NSW, Australia.



Nearly all animals sleep. Sleep is very important for refreshing the mind and body. When people sleep we close our eyes and lie motionless for a long time. We may be less aware of what is going on around us and our breathing slows down. Some people are very heavy sleepers and it takes a LOT to wake them up!

Fish don’t have eyelids — they don’t need them underwater because dust can’t get in their eyes. But fish still sleep. Some sleep during the day and only wake up at night, while others sleep at night and are awake through the day (just like you and I).

A happy puffer fish.
Flickr

How do fish know when it’s bedtime?

It’s pretty easy to tell when fish are sleeping: they lie motionless, often at the bottom or near the surface of the water. They are slow to respond to things going on around them, or may not respond at all (see some sleeping catfish here). If you watch their gills, you’ll notice they’re breathing very slowly.




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Curious Kids: how are stars made?


People with fish tanks at home will know that when the lights go off at night, the fish become far less active. If you turn a light on in the middle of the night you’ll see how still they are.

Like people, fish have an internal clock that tells them when to do things like sleep and eat. So even if you accidentally leave the lights on at night, the fish may settle down and go to sleep anyway.

A video showing sleeping catfish.

Some scientists have studied sleep in fish that live in caves where it is always dark. Even in some of these species there are times of low activity that look just like sleep. Of course there is no sunrise or sunset in caves so their rhythm is often different to fish that live at the surface in bright sunshine.

Some fish, like tuna and some sharks, have to swim all the time so that they can breathe. Its likely that these fish sleep with half their brain at a time, just like dolphins.

Parrot fish make a mucus cocoon around themselves at night — a gross, sticky sleeping bag which might protect them from parasites attacking them while they sleep.

Fish don’t need eyelids because dust can’t get in their eyes – but they still sleep.
Gavin Leung/Flickr

Fish may dream like people do!

One wonders if fish dream while they are sleeping. So far we don’t have the answer to that question but recent video footage of a sleeping octopus showed it changing colours, which suggests it may have been dreaming about hiding from a predator or sneaking up on its own prey (which is why octopuses change colour when they’re awake).




Read more:
Curious Kids: why is the sea salty?


Believe it or not, fish sleep is being studied to help us better understand sleep in people. Most of these studies use zebrafish and try to understand things like the effects of sleep deprivation (lack of sleep), insomnia (trouble getting to sleep) and circadian rhythm (sleep cycles).

Here is a cool video about sleep in animals, including fish.


Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to curiouskids@theconversation.edu.au — —The Conversation

Culum Brown, Professor, Macquarie University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

No-take marine areas help fishers (and fish) far more than we thought



A juvenile Plectropomus leopardus from the Whitsundays.
David Williamson/James Cook University

Dustin Marshall, Monash University and Liz Morris, Monash University

One hectare of ocean in which fishing is not allowed (a marine protected area) produces at least five times the amount of fish as an equivalent unprotected hectare, according to new research published today.

This outsized effect means marine protected areas, or MPAs, are more valuable than we previously thought for conservation and increasing fishing catches in nearby areas.

Previous research has found the number of offspring from a fish increases exponentially as they grow larger, a disparity that had not been taken into account in earlier modelling of fish populations. By revising this basic assumption, the true value of MPAs is clearer.




Read more:
Protecting not-so-wild places helps biodiversity


Marine Protected Areas

Marine protected areas are ocean areas where human activity is restricted and at their best are “no take” zones, where removing animals and plants is banned. Fish populations within these areas can grow with limited human interference and potentially “spill-over” to replenish fished populations outside.

Obviously MPAs are designed to protect ecological communities, but scientists have long hoped they can play another role: contributing to the replenishment and maintenance of species that are targeted by fisheries.

Wild fisheries globally are under intense pressure and the size fish catches have levelled off or declined despite an ever-increasing fishing effort.

Yet fishers remain sceptical that any spillover will offset the loss of fishing grounds, and the role of MPAs in fisheries remains contentious. A key issue is the number of offspring that fish inside MPAs produce. If their fecundity is similar to that of fish outside the MPA, then obviously there will be no benefit and only costs to fishers.




Read more:
More fish, more fishing: why strategic marine park placement is a win-win


Big fish have far more babies

Traditional models assume that fish reproductive output is proportional to mass, that is, doubling the mass of a fish doubles its reproductive output. Thus, the size of fish within a population is assumed to be less important than the total biomass when calculating population growth.

But a paper recently published in Science demonstrated this assumption is incorrect for 95% of fish species: larger fish actually have disproportionately higher reproductive outputs. That means doubling a fish’s mass more than doubles its reproductive output.

When we feed this newly revised assumption into models of fish reproduction, predictions about the value of MPAs change dramatically.


Author provided

Fish are, on average, 25% longer inside protected areas than outside. This doesn’t sound like much, but it translates into a big difference in reproductive output – an MPA fish produces almost 3 times more offspring on average. This, coupled with higher fish populations because of the no-take rule means MPAs produce between 5 and 200 times (depending on the species) more offspring per unit area than unprotected areas.

Put another way, one hectare of MPA is worth at least 5 hectares of unprotected area in terms of the number of offspring produced.

We have to remember though, just because MPAs produce disproportionately more offspring it doesn’t necessarily mean they enhance fisheries yields.

For protected areas to increase catch sizes, offspring need to move to fished areas. To calculate fisheries yields, we need to model – among other things – larval dispersal between protected and unprotected areas. This information is only available for a few species.

We explored the consequences of disproportionate reproduction for fisheries yields with and without MPAs for one iconic fish, the coral trout on the Great Barrier Reef. This is one of the few species for which we had data for most of the key parameters, including decent estimates of larval dispersal and how connected different populations are.

No-take protected areas increased the amount of common coral trout caught in nearby areas by 12%.
Paul Asman and Jill Lenoble/Flickr, CC BY

We found MPAs do in fact enhance yields to fisheries when disproportionate reproduction is included in relatively realistic models of fish populations. For the coral trout, we saw a roughly 12% increase in tonnes of caught fish.

There are two lessons here. First, a fivefold increase in the production of eggs inside MPAs results in only modest increases in yield. This is because limited dispersal and higher death rates in the protected areas dampen the benefits.




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However the exciting second lesson is these results suggest MPAs are not in conflict with the interests of fishers, as is often argued.

While MPAs restrict access to an entire population of fish, fishers still benefit from from their disproportionate affect on fish numbers. MPAs are a rare win-win strategy.

It’s unclear whether our results will hold for all species. What’s more, these effects rely on strict no-take rules being well-enforced, otherwise the essential differences in the sizes of fish will never be established.

We think that the value of MPAs as a fisheries management tool has been systematically underestimated. Including disproportionate reproduction in our assessments of MPAs should correct this view and partly resolve the debate about their value. Well-designed networks of MPAs could increase much-needed yields from wild-caught fish.The Conversation

Dustin Marshall, Professor, Marine Evolutionary Ecology, Monash University and Liz Morris, Administration Manager, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

We wrote the report for the minister on fish deaths in the lower Darling – here’s why it could happen again


Robert Vertessy, University of Melbourne; Fran Sheldon, Griffith University; Lee Baumgartner, Charles Sturt University; Nick Bond, La Trobe University, and Simon Mitrovic, University of Technology Sydney

Over the recent summer, three significant fish death events occurred in the lower Darling River near Menindee, New South Wales. Species involved included Murray Cod, Silver Perch, Golden Perch and Bony Herring, with deaths estimated to be in the range of hundreds of thousands to over a million fish. These events were a serious ecological shock to the lower Darling region.

Our report for the Minister for Agriculture and Water Resources examines the causes of these events and recommend actions to mitigate the potential for repeat events in the future.

The final report has just been released, summarising what we found and what we recommend.

Causes of the fish deaths

High-flow events in the Darling River in 2012 and 2016 filled the Menindee Lakes and offered opportunities for substantial fish breeding, further aided by the targeted use of environmental water.

The result was very large numbers of fish in the lakes, river channels and weir pools around Menindee. After the lake-filling rains of late 2016, two very dry years ensued, resulting in very low inflows into the Barwon-Darling river.

As the supply of water dried up, the river became a series of disconnected and shrinking pools. As the extremely hot and dry conditions in late 2018 took hold, the large population of fish around Menindee became concentrated within weir pools.

Hot weather, low rainfall and low flows provided ideal conditions for algal blooms and thermal stratification in the weir pools, resulting in very low oxygen concentrations within the bottom waters.

With the large fish population now isolated to the oxygenated surface waters of the pools, all that was needed for the fatal blow was a trigger for the water profile to mix. Such a trigger arrived on three separate occasions, with changes in the weather that brought sudden drops in temperature and increased wind that caused sudden turnover of the low-oxygen bottom waters.

Summary of the multiple causes of the 2018-19 fish death events in the lower Darling river.

With the fish already stressed by high temperatures, they were now unable to gain enough oxygen from the water to breathe, and a very large number of them died. As we write, the situation in the lower Darling remains dire, and there is a risk of further fish deaths if there are no significant inflows to the river.

Fish deaths caused by these sorts of turnover events are not uncommon, but the conditions outlined above made these events unusually dramatic.

So, how did such adverse conditions arise in the lower Darling river and how might we avoid their reoccurrence? We’ve examined four influencing factors: climate, water management, lake operations, and fish mobility.

Key influencing factors

We found that the fish death events in the lower Darling were preceded and affected by exceptional climatic conditions.

Inflows to the water storages in the northern Basin over 2017-18 were the second lowest for any two-year period on record. Most of the Murray-Darling Basin experienced its hottest summer on record, exemplified by the town of Bourke breaking a new heatwave record for NSW, with 21 consecutive days with a maximum temperature above 40℃.

We concluded that climate change amplified these conditions and will likely result in more severe droughts in the future.

Changes in the water access arrangements in the Barwon–Darling River, made just prior to the commencement of the Basin Plan in 2012, exacerbated the effects of the drought. These changes enhanced the ability of irrigators to access water during low flow periods, meaning fewer flow pulses make it down the river to periodically reconnect and replenish isolated waterholes that provide permanent refuge habitats for fish during drought.

We conclude that the Lake Menindee scheme had been operated according to established protocols, and was appropriately conservative given the emerging drought conditions. But low connectivity in the lower Darling resulted in poor water quality and restricted mobility for fish.

Recommended policy and management actions

Given the right mix of policy and management actions, Basin governments can significantly reduce the risks of further fish death events and promote the recovery of affected fish populations.

The Basin Plan is delivering positive environmental outcomes and more benefits will accrue once the plan is fully implemented. But more needs to be done to enhance river connectivity and protect low flows, first flushes and environmental flow releases in the Barwon-Darling river.

Drought resilience in the lower Darling can be enhanced by reconfiguring the Lake Menindee Water Savings Project, modifying the current Menindee Lakes operating rules and purchasing high security water entitlements from horticultural enterprises in the region.

In Australia, water entitlements are the rights to a share of the available water resource in any season. Irrigators get less (or no) water in dry (or extremely dry) years.

A high-security water entitlement is one with a high chance of receiving the full water allocation. In some systems, although not all, this is expected to happen 95 per cent of the time. And these high-security entitlements are the most valuable and sought after.

Fish mobility can be enhanced by removing barriers to movement and adding fish passageways.

It would be beneficial for environmental water holders to place more of their focus on sustaining fish populations through drought sequences.

The river models that governments use to plan water sharing need to be updated more regularly to accurately represent the state of Basin development, configured to run on a whole-of-basin basis, and improved to more faithfully represent low flow conditions.

There are large gaps in water quality monitoring, metering of water extractions and basic hydro-ecologic knowledge that should be filled.

Risk assessments need to be undertaken to identify likely fish death event hot spots and inform future emergency response plans.

All of these initiatives need to be complemented by more sophisticated and reliable assessments of the impacts of climate change on water security across the Basin.

Governments must accelerate action

Responding to the lower Darling fish deaths in a prompt and substantial manner provides governments an opportunity to redress some of the broader concerns around the management of the Basin.

To do so, Basin governments must increase their political, bureaucratic and budgetary support for high value reforms and programs, particularly in the northern Basin.

All of our recommendations can be implemented within the current macro-settings of the Basin Plan and do not require a revisiting of the challenging socio-political process required to define Sustainable Diversion Limits (SDLs).

Successful implementation will require a commitment to authentic collaboration between governments, traditional owners, local communities, and sustained input from the science community.


The authors would like to acknowledge the contribution of Daren Barma, Director of Barma Water Consulting, to this article.

A version of this article has been published in Pursuit.The Conversation

Robert Vertessy, Enterprise Professor, University of Melbourne; Fran Sheldon, Professor, Australian Rivers Institute, Griffith University, Griffith University; Lee Baumgartner, Associate Research Professor (Fisheries and River Management), Institute for Land, Water, and Society, Charles Sturt University; Nick Bond, Professor of Freshwater Ecology and Director of the Centre for Freshwater Ecosystems, La Trobe University, and Simon Mitrovic, Associate Professor, University of Technology Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

More fish, more fishing: why strategic marine park placement is a win-win



File 20190325 36267 6gisnm.jpg?ixlib=rb 1.1
Marine parks are good for fish – especially if they’re in the right areas.
Epstock/Shutterstock

Kerstin Jantke, University of Hamburg; Alienor Chauvenet, Griffith University; Hugh Possingham, The University of Queensland; James Allan, The University of Queensland; James Watson, The University of Queensland, and Kendall Jones, The University of Queensland

Australia has some of the most spectacular marine ecosystems on the planet – including, of course, the world-famous Great Barrier Reef. Many of these places are safe in protected areas, and support a myriad of leisure activities such as recreational fishing, diving and surfing. No wonder eight in ten Aussies live near the beach.

Yet threats to marine ecosystems are becoming more intense and widespread the world over. New maps show that only 13% of the oceans are still truly wild. Industrial fishing now covers an area four times that of agriculture, including the farthest reaches of international waters. Marine protected areas that restrict harmful activities are some of the last places where marine species can escape. They also support healthy fisheries and increase the ability of coral reefs to resist bleaching.




Read more:
Most recreational fishers in Australia support marine sanctuaries


One hundred and ninety-six nations, including Australia, agreed to international conservation targets under the United Nations Convention on Biological Diversity. One target calls for nations to protect at least 10% of the world’s oceans. An important but often overlooked aspect of this target is the requirement to protect a portion of each of Earth’s unique marine ecosystems.

How are we tracking?

The world is on course to achieve the 10% target by 2020, with more than 7.5% of the ocean already protected. However, our research shows that many marine protected areas are located poorly, leaving many ecosystems underprotected or not protected at all.

What’s more, this inefficient placement of marine parks has an unnecessary impact on fishers. While marine reserves typically improve fisheries’ profitability in the long run, they need to be placed in the most effective locations.

We found that since 1982, the year nations first agreed on international conservation targets, an area of the ocean almost three times the size of Australia has been designated as protected areas in national waters. This is an impressive 20-fold increase on the amount of protection that was in place beforehand.

But when we looked at specific marine ecosystems, we found that half of them fall short of the target level of protection, and that ten ecosystems are entirely unprotected. For example, the Guinea Current off the tropical West African coast has no marine protected areas, and thus nowhere for its wildlife to exist free from human pressure. Other unprotected ecosystems include the Malvinas Current off the southeast coast of South America, Southeast Madagascar, and the North Pacific Transitional off Canada’s west coast.

Marine park coverage of global ecosystems. Light grey: more than 10% protection; dark grey: less than 10% protection; red: zero protection.
Author provided

Australia performs comparatively well, with more than 3 million square km of marine reserves covering 41% of its national waters. Australia’s Coral Sea Marine Park is one of the largest marine protected areas in the world, at 1 million km². However, a recent study by our research group found that several unique ecosystems in Australia’s northern and eastern waters are lacking protection.

Furthermore, the federal government’s plan to halve the area of strict “no-take” protection inside marine parks does not bode well for the future.

How much better can we do?

To assess the scope for improvement to the world’s marine parks, we predicted how the protected area network could have been expanded from 1982.

With a bit more strategic planning since 1982, the world would only need to conserve 10% of national waters to protect all marine ecosystems at the 10% level. If we had planned strategically from as recently as 2011, we would only need to conserve 13% of national waters. If we plan strategically from now on, we will need to protect more than 16% of national waters.

If nations had planned strategically since 1982, the world’s marine protected area network could be a third smaller than today, cost half as much, and still meet the international target of protecting 10% of every ecosystem. In other words, we could have much more comprehensive and less costly marine protection today if planning had been more strategic over the past few decades.

The lack of strategic planning in previous marine park expansions is a lost opportunity for conservation. We could have met international conservation targets long ago, with far lower costs to people – measured in terms of a short-term loss of fishing catch inside new protected areas.

This is not to discount the progress made in marine conservation over the past three decades. The massive increase of marine protected areas, from a few sites in 1982, to more than 3 million km² today, is one of Australia’s greatest conservation success stories. However, it is important to recognise where we could have done better, so we can improve in the future.

Australia’s marine park network.
Author provided

This is also not to discount protected areas. They are important but can be placed better. Furthermore, long-term increases in fish populations often outweigh the short-term cost to fisheries of no-take protected areas.

Two steps to get back on track

In 2020, nations will negotiate new conservation targets for 2020-30 at a UN summit in China. Targets are expected to increase above the current 10% of every nation’s marine area.

We urge governments to rigorously assess their progress towards conservation targets so far. When the targets increase, we suggest they take a tactical approach from the outset. This will deliver better outcomes for nature conservation, and have less short-term impact on the fishing industry.




Read more:
More than 1,200 scientists urge rethink on Australia’s marine park plans


Strategic planning is only one prerequisite for marine protected areas to effectively protect unique and threatened species, habitats and ecosystems. Governments also need to ensure protected areas are well funded and properly managed.

These steps will give protected areas the best shot at halting the threats driving species to extinction and ecosystems to collapse. It also means these incredible places will remain available for us and future generations to enjoy.The Conversation

Kerstin Jantke, Postdoctoral Researcher on conservation biology, University of Hamburg; Alienor Chauvenet, Lecturer, Griffith University; Hugh Possingham, Professor, The University of Queensland; James Allan, Postdoctoral research fellow, School of Biological Sciences, The University of Queensland; James Watson, Professor, The University of Queensland, and Kendall Jones, PhD candidate, Geography, Planning and Environmental Management, The University of Queensland

This article is republished from The Conversation under a Creative Commons license. Read the original article.