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).
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.
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.
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 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).
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.
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Tropical tuna are one of the few wild animals we still hunt in large numbers, but finding them in the vast Pacific ocean can be tremendously difficult. However, fishers have long known that tuna are attracted to, and will aggregate around, floating objects such as logs.
In the past, people used bamboo rafts to attract tuna, fishing them while they were gathered underneath. Today, the modern equivalent – called fish aggregating devices, or FADs – usually contain high-tech equipment that tell fishers where they are and how many fish have accumulated nearby.
It’s estimated that between 30,000 and 65,000 man-made FADs are deployed annually and drift through the Western and Central Pacific Ocean to be fished on by industrial fishers. Pacific island countries are reporting a growing number of FADs washing up on their beaches, damaging coral reefs and potentially altering the distribution of tuna.
Our research in two papers, one of which was published today in Scientific Reports, looks for the first time at where ocean currents take these FADs and where they wash up on coastlines in the Pacific.
Attracting fish and funds
We do not fully understand why some fish and other marine creatures aggregate around floating objects, but they are a source of attraction for many species. FADs are commonly made of a raft with 30-80m of old ropes or nets hanging below. Modern FADs are attached to high-tech buoys with solar-powered electronics.
The buoys record a FAD’s position as it drifts slowly across the Pacific, scanning the water below to measure tuna numbers with echo-sounders and transmitting this valuable information to fishing vessels by satellite.
Throughout their lifetimes FADs may be exchanged between vessels, recovered and redeployed, or fished and simply left to drift with their buoy to further aggregate tuna. Fishers may then abandon them and remotely deactivate the buoys’ satellite transmission when the FAD leaves the fishing area.
Fishing licence fees can provide up to 98% of government revenue for some Pacific Island countries and territories. These countries balance the need to sustainably manage and harvest one of the only renewable resources they have, while often having a limited capacity to fish at an industrial scale themselves.
FADs help stabilise catch rates and make fishing fleets more profitable, which in turn generate revenue for these nations.
The abandonment or loss of FADs adds to the growing mass of marine debris floating in the ocean, and they increasingly damage coral as they are dragged and get caught on reefs.
Perhaps most importantly, we don’t know how the distribution of FADs affects fishing effort in the region. Given that each fleet and fishing company has their own strategy for using FADs, understanding how the total number of FADs drifting in one area increases the catch of tuna is crucial for sustainably managing these valuable species.
Where do FADs end up?
Our research, published in Environmental Research Communications and Scientific Reports, used a regional FAD tracking program and fishing data submitted by Pacific countries, in combination with numerical ocean models and simulations of virtual FADs, to work out how FADs travel on ocean currents during and after their use.
In general, FADs are first deployed by fishers in the eastern and central Pacific. They then drift west with the prevailing currents into the core industrial tropical tuna fishing zones along the equator.
We found equatorial countries such as Kiribati have a high number of FADs moving through their waters, with a significant amount washing up on their shores. Our research showed these high numbers are primarily due to the locations in which FADs are deployed by fishing companies.
In contrast, Tuvalu, which is situated on the edge of the equatorial current divergence zone, also sees a high density of FADs and beaching. But this appears to be an area that generally aggregates FADs regardless of where they are deployed.
Unsurprisingly, many FADs end up beaching in countries at the western edge of the core fishing grounds, having drifted from different areas of the Pacific as far away as Ecuador. This concentration in the west means reefs along the edge of the Solomon Islands and Papua New Guinea are particularly vulnerable, with currents apparently forcing FADs towards these coasts more than other countries in the region.
Overall, our studies estimate that between 1,500 and 2,200 FADs drifting through the Western and Central Pacific Ocean wash up on beaches each year. This is likely to be an underestimate, as the tracking devices on many FADs are remotely deactivated as they leave fishing zones.
Using computer simulations, we also found that a significant number of FADs are deployed in the eastern Pacific Ocean, left to drift so they have time to aggregate tuna, and subsequently fished on in the Western and Central Pacific Ocean. This complicates matters as the eastern Pacific is managed by an entirely different fishery Commission with its own set of fisheries management strategies and programmes.
Growing human populations and climate change are increasing pressure on small island nations. FAD fishing is very important to their economic and food security, allowing access to the wealth of the ocean’s abundance.
We need to safeguard these resources, with effective management around the number and location of FAD deployments, more research on their impact on tuna and bycatch populations, the use of biodegradable FADs, or effective recovery programs to remove old FADs from the ocean at the end of their slow journeys across the Pacific.
Light pollution occurs when artificial light interferes with ecological systems or processes, usually at night.
Natural light at night, produced by the moon, stars, and other celestial bodies, is minimal. A full moon creates only 0.05-0.1 lux, which pales in comparison to the artificial light produced by humans, which can range from around 10 lux from an LED or low-pressure sodium streetlight, up to 2,000 lux from something like stadium lighting.
Because nearly all organisms on Earth have evolved with a stable day-night, light and dark cycle, many biological events are now highly attuned to the daily, lunar, and seasonal changes in light produced by the reliable movements of the Earth and Moon around the Sun.
But artificial light can mask these natural light rhythms and interfere with the behaviour and physiology of individual creatures, and ecosystems as a whole.
The ocean is not exempt from these problems. Light pollution is spreading to marine habitats through urbanised coastlines and increasing marine infrastructure such as piers, harbours, cruise ships, and tropical island resorts where bungalows extend out into the lagoon, directly above coral reefs.
Why are clownfish at risk?
Clownfish, like many reef fish, are particularly vulnerable to light pollution because they don’t move around much in their adult stage. Clownfish can travel long distances in the first 2 weeks after hatching, but at the end of this period the young fish will settle in a suitable sea anemone that becomes their forever-home.
This means that if a fish chooses an anemone on a shallow reef in an area that is heavily lit at night, they will experience chronic exposure to light pollution throughout their life; they won’t just move away.
Clownfish also lay their eggs attached to rock or other hard surfaces, so in areas exposed to light pollution the eggs will experience continuous artificial light (as opposed to many fish that lay and fertilise eggs in open water, so they are immediately carried away by ocean currents).
What we found
To test how artificial light affects clownfish reproduction, we examined the common clownfish (Amphiprion ocellaris) in a lab experiment.
Five breeding pairs of fish experienced a normal 12-hour daylight, 12-hour dark cycle, while another five pairs of fish had their “night” period replaced with 12 hours of light at 26.5 lux, mimicking light pollution from an average coastal town.
For 60 days, we monitored how often the fish spawned, how many eggs were fertilised, and how many eggs hatched. While we saw no difference in spawning frequency or fertilisation rates between the two groups of fish, the impact of the artificial light treatment on hatch rate was staggering. None of the eggs hatched, compared with an average of 86% in the control group.
At the end of the experiment we removed the artificial light and monitored the fish for another 60 days to see how they would recover. As soon as the light at night was removed, eggs resumed hatching at normal rates.
Clownfish, like many reef fish, have evolved to hatch after dusk to avoid the threat of being eaten. Newly hatched baby clownfish, like most coral reef fish, are small (about 5mm long) and transparent. Hatching in darkness likely means they are less visible to predators as they emerge from their eggs.
Our findings show that the presence of artificial light, even at relatively low levels, can disrupt this crucial process, by masking the environmental cue – darkness – that triggers hatching. As many reef fish share similar reproductive behaviours to clownfish, it is likely artificial light will similarly interfere with the ability of other fish species to produce viable offspring.
And the problem is only growing. The reach of light pollution across all land and sea is expanding at an estimated rate of 2.2% per year, and this will only increase with the rising global human population.
Although research on the ecological impacts of light pollution is arguably only in its infancy, the evidence for negative consequences for a range of insects, birds, amphibians, reptiles, and mammals, including humans, is stacking up.
Our new research adds another species to the list, and highlights the importance of finding ways to manage or reduce artificial light, on land and below the waves.
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.
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.
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.
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.
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.
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.
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.
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How would the disappearance of anglerfish affect our environment? – Bella, age 6, Sydney.
As I am sure you know, anglerfish live deep in the ocean. The females have an enlarged fin overhanging their eyes and their mouth that acts as a lure – much like bait on a fishing line – and this explains their name. (“Angling” is a method of fishing.)
The fact is we understand very little about the deep sea and how its inhabitants, including anglerfish, will respond to change. In fact, more people have walked on the Moon than have been to the bottom of the ocean.
Close your eyes and imagine a spider’s web. All parts of it are connected, and if a bug gets tangled in one part, it can cause a completely different part of the web to wobble or break.
It helps to remember that all species are interconnected via something called the “food web”. The food web is not a real web like a spider’s web. It’s just a way of thinking about how species are connected to each other. Basically, the food web tells us who eats whom.
If you make a change to one part of the food web, that can have an ripple effect that can cause changes on another part of the web.
Less of one animal can mean more of another
Anglerfish usually eat small fish, as well as relatives of shrimp.
It is likely that if all the anglerfish in the ocean disappeared, their prey would explode in number and another predator would then “step in” to replace them.
And any species that likes to eat the anglerfish would have to start eating another species instead – or risk dying out.
At the height of the whaling industry, about 100 years ago, whales nearly disappeared. That meant that the number of krill (the tiny animals that whales eat) exploded, providing a feast for other animals that also eat krill – such as seals. That is how a food web works.
Weird and wonderful
There are around 200 different types of anglerfish. Although one giant species grows to over a metre, most anglerfish are tiny – less than 10cm long.
Only female anglerfish have lures. These lures often glow in the dark, thanks to the bio-luminescent bacteria inside them, which presents a tempting (but fake) meal to their unsuspecting prey.
Anglerfish don’t form large schools like many other fish and this represents a problem for them – they need to find a mate. The tiny males have found a solution: if they do happen to find a female, they grasp onto her with their mouths and never let go.
These males tap into the females’ blood stream and never have to eat again. Scientists call this behaviour parasitic. Sometimes more than one male can be attached to a single female. Imagine someone’s father being 100 times smaller than their mother and being permanently attached to her.
Among the biggest problems for a lot of fish species are disease and overfishing by humans. But it’s highly unlikely that these threats could wipe out anglerfish.
Anglerfish are found between 300 and several thousand metres of water. At this depth, it is constantly dark and the water is cold.
As they live in such deep water and do not form schools, they are not targeted by fishermen, a common threat for many shallow water fish.
And anglerfish are so widely spread across the world’s oceans that any disease is highly unlikely to spread among them.
There is one threat that might affect angler fish – the threat of global warming. Temperatures in the deep ocean are very stable, they simply don’t change much.
Anglerfish live their entire lives at depth with near constant temperatures; hence even small shifts in temperature may affect them. It remains unclear whether increasing temperatures really will threaten angler fish – only time will tell.
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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.
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.
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.
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.
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.