red tide algal boom is not coral spawn

Trichodesmium blooms are often confused with coral spawn.

Seasoned anglers at one stage or other, will probably have encountered a grey, red, or brown slick on the ocean surface, usually assumed to be coral spawn.

Chances are the culprit is actually a naturally occurring algal bloom called Trichodesmium.

Trichodesmium is often present in the water column of tropical ocean waters, especially during the warmer months, and becomes readily apparent visually (and on the nose) during calm periods when it floats on the surface, collecting in wind rows or slicks, until it rots and disperses.

It is most often encountered by anglers on calm days out on the briny, but can wash into estuaries and onto beaches, often mistaken for an oil slick as it pushes onto the beach or foreshore.

Algal bloom on ocean surface
Brooke Island Great Barrier Reef Trichodesmium slick

Brooke Island Great Barrier Reef Trichodesmium slick

It's seems the first to mistake a Trichodesmium bloom with coral spawn was Captain Cook himself;

"The sea in many places is here cover’d with a kind of brown scum, such as sailors generally call spawn; upon our first seeing it, it alarm’d us thinking that we were amongst Shoals, but we found the same depth of water where it was as in other places. Neither Mr Banks nor Dr Solander could tell what it was although they had enough of it to examine." (Captain's Log, August 28, 1770)

How to tell the difference between coral spawn and Trichodesmium?

There are a few defining characters when used together, can determine the difference.

  1. The most obvious is probably the smell. Trichodesmium floating on the surface is distinctively fishy whereas coral spawn is more benign, a slightly stronger version of the smell you get during a reef walk at low tide.
  2. Shape and size. Trichodesmium is shaped like small grains of rice when viewed with the naked eye. Because coral spawn can be one of many different species, and can include both eggs and sperm, it is a little harder to be definitive about shape, however as a general rule, fresh coral spawn is round.
  3. Colour. Whilst not definitive as both change colour as they age, coral spawn is generally pink, red or milky white, while Trichodesmium can be green, brown, red or purple.
  4. Timing. Typically coral spawning takes place after the full moon in October and November for inshore species, and after the moon in November and December for offshore species. In other words spawning is greatly influenced by moon phase. Once the event has occurred, the spawn slick will persist for no more than 1-2 days especially during windy periods when the ocean is stirred up. Trichodesmium blooms can occur any time during the year and are most common during periods of warm weather from August to December.
Coral spawn confused with algal bloom

Trichodesmium to the naked eye is shaped like miniature grains of rice.

Algal bloom on the ocean surface

I have also heard it being referred to as sea sawdust, or more curiously, whale sperm. That being the case, we must have an ocean full of carpenters and horny masturbating whales as the slicks can often extend for many kilometres!

Interestingly, Trichodesmium is a nitrogen fixing organism, which basically means that these blooms are a major pathway for nitrogen to get from the atmosphere into the water. Which (trust me I'm a marine biologist) is a good thing.

But let's face it. Most of the people reading this page haven't the slightest interest in water chemistry and just wanna know one thing...

Does a Trichodesmium bloom effect the fishing?

Not in our experience.

I have read accounts that coral spawn washing up en-masse has caused fish kills in Western Australia, but in Ryan's experience, blooms of Trichodesmium floating on the surface have no effect on the fishing.

That said, we would love to hear your opinion or comments below regarding it's effect on fishing if you've had a different experience.

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Until next time, tight lines.


About The Author

Karen Rudkin

Karen Rudkin-Moody became hooked on fishing after being introduced to the sport in 1989. Karen is a marine biologist specializing in estuarine marine protected areas, finishing her successful career in Queensland Marine Parks as Ranger In Charge of the Wet Tropics region within the Great Barrier Reef Marine Park. Karen now works with her husband, accomplished fishing guide Ryan Moody, encouraging people to get away from their computers and into the great outdoors.

Facebook comments


  • Bob Wilson

    Reply Reply October 18, 2019

    Often referred to in Hervey Bay Qld as whale sh*t.

  • Noel Akers

    Reply Reply September 24, 2019

    Hi Peter,
    There is currently a Trichodesmium bloom at Grasstree Beach (between Mackay and Sarina)

  • Ian

    Reply Reply September 22, 2019

    Hi Peter, there is a bloom in the NT at Nhulunbuy September 2019

  • peter bell

    Reply Reply January 21, 2019

    I am studying the occurrence of Trichodesmium blooms–I am developing methods to track these with satellite imagery-I need some ground truthing data for this -could you let me know if you are currently experiencing blooms of Trichodesmium and the location of such blooms


    Peter Bell

    • Emma Raymond

      Reply Reply January 25, 2019

      Hi Peter,
      We have not seen any here lately mate. In saying that the weather has been horrific for AGES and we haven’t been out wide for quite a while

  • Kev

    Reply Reply February 25, 2016

    Have had a good bite with spaniards trolling the bloom. I always thought with was coral spawn. Good article.

    • Karen

      Reply Reply February 29, 2016

      Thanks Kev. You’re not the only one. Many people refer to it as coral spawn. And as with any floating debris, the bloom probably acts as attractant/shelter for bait fish, hence the pelagics hanging around too.

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