As the world’s population increases more and more people are living near the coast.
Offshore, coral reefs are subjected to increasing levels of artificial light at night.
In our homes, rooms that never really get dark cause the same sort of stress.
Sop, does artificial light at night help or hurt wild fish?
Hi guys and girls, I’m ReefMan, and this week we’re talking about a paper published
in the Proceedings of the Royal Society titled “Long-term exposure to artificial light
at night in the wild decreases survival and growth of a coral reef fish”.
There has been a lot of research on this over the years, and the results are widely varied.
With nearly 10% of the world's population - that’s over 600 million people - living
near coastlines, the level of light pollution being spilled into the water will likely increase
over time.
There are studies looking at the effect of keeping fish with light on 100% of the time.
They’ve found that the growth of the fish increases.
They’ve found that the growth of the fish decreases.
Other studies have found no effect on the fish at all, or even different responses in
different species of fish.
In this study, which took place over about 20 months, the team collected clownfish eggs
from a lagoon in French Polynesia, raised the larva in their lab, and then put the juvenile
clownfish into Heteractis magnifica anemones that were spaced out into individual territories
for the fish.
Each anemone received one clownfish, 42 in total.
They used freshly placed anemones to lessen the changes of a new fish being kicked out
of an already occupied anemone.
Three groups of territories received some level of artificial light at night, and then
of course there were matching control groups that had similar reef structure but no artificial
light at night.
Artificial light at night can lower survival rates due to the ability of predators to better
see the fish at night.
It can increase growth because the clownfish would be able to see plankton - their main
food source - throughout the night and therefore get more food.
It can reduce growth and the overall health of clownfish because they might suffer from
sleep deprivation.
Clownfish naturally have a high mortality rate when young.
That’s why they lay so many eggs - if they all survived the ocean would be overrun with
clownfish.
The team monitored the study’s clownfish at several points throughout their lives.
About one month after placement into the anemones, about two months later when they would be
immature adults, and about 15 months into the study when the fish would be just reaching
maturity, and then about 20 months into the study as adult fish.
If an anemone was empty, they counted this as a mortality event.
If the anemone itself was gone, they didn’t count it as a mortality event for the fish
because it was impossible to know if the fish died or if the anemone itself died and the
fish found a new one to host.
The light level in the light polluted locations was, on average, 143 times more intense than
the moonlight at the control locations.
Only about one third of the clownfish survived to the end of the study.
The fish in locations which were subjected to artificial light at night were 36% more
likely to have died.
The fish in these locations also had slower rates of growth - 15% lower compared to the
control fish.
By weight, their growth rate was 51% lower!
So the light polluted fish are certainly not getting more food due to the light at night.
In our aquariums, of course, our fish usually don’t have to deal with natural predators.
So we can be thankful at least that light leaking into our tanks isn’t giving some
other fish a fin up on our prized clownfish while they sleep.
We should perhaps worry about the effects of sleep deprivation though, and certainly
our coral would also appreciate some dark time.
Optimal growth, in people and in fish, requires a period of inactivity.
We have to sleep, and, it turns out, so do fish.
Other studies have looked at the effect of keeping clownfish in tanks with lights always
on - and they found a reduced growth rate.
Maybe this is due to a reduction in metabolism caused by a lack of sleep?
Check out the paper if you’re interested in more of the details.
As always, there’s a link below where you can find it.
Give it a read!
I hope you enjoyed the video, if you did then subscribe and you’ll get more like it.
Be kind to each other and have a fantastic day!
Bye!