Photo by Dana Tentis from Pexels

What is the Intimate Connection Between Delicious Spot Prawns and Diseased Starfish?

Is the loss of starfish affecting the supply and price of our delicious spot prawns?

Rich Sobel
Feb 21 · 9 min read

Do you enjoy eating seafood? I do and I really love Spot prawns.

They are an especially large prawn; a local delicacy. And the season that they become available on the market, where I reside, is a very short one. So we get em, eat em and wait until next year to do it all over again.

There is a high demand for them both locally and internationally. Unfortunately, it seems that the supply has been decreasing. Why is that? Overfishing? Diseases affecting Spot prawns? Loss of starfish?

Wait a minute, did you just say “loss of starfish”?

What has the loss of starfish got to do with the supply of spot prawns?

In this article, I’ll show you how the populations of both starfish and spot prawns are intimately connected by way of 2 other species, sea urchins and kelp. And how the decline in the starfish population due to a wasting disease is affecting the supply of our spot prawns.

All About Spot Prawns

Spot prawns (Pandalus platyceros) are rather interesting creatures in their own right. They range from the waters in Unalaska Island in Alaska to San Diego, California.

BC spot prawns are the largest of the seven commercial species of shrimp found on the west coast of Canada. They vary greatly in size, with some larger females reaching 25 cm (almost 10 inches!!) in total length.

They are called spot prawns because when they are cooked, characteristic white spots show up on their tails. They also have unique white stripes or bars on the main body shell, too.

Spot prawns live for 4–6 years and they are hermaphrodites. That means that they switch genders! For the first 2 years, they are males and then they switch and become females. That is when they reach full size so it is unlikely that you will ever eat male spot prawns. Too small.

The juveniles tend to live in the shallower subtidal regions and can often be found “hiding” under sieve kelp blades (the long flat parts we would call leaves if they were land plants)

In British Columbia, where I live, about 2,450 tons of spot prawns are harvested in the waters between the mainland and Vancouver Island every year!

The harvest season usually starts in May and lasts from 6–8 weeks. Like I said, get em, eat em and wait until next year. And they’re so popular that there’s even a festival in honour of them.

They are caught by spreading baited traps along the rocky ocean floor at depths ranging from 40 to 100 metres. Fishermen tell us this method has minimal impact on ocean habitat and very low levels of bycatch of other species. But…trap fishing takes place over rocky or hard bottom habitat, and the gear can damage vulnerable glass-sponge reefs and coral beds. So as is usually the case, it’s not as simple and straightforward as the harvesting folks might like us to believe.

Here’s a bit more about the actual trapping methods, from the Ocean Wise site.

“Traps and pots are baited, immobile enclosures placed on the ocean floor and typically target crustaceans, however they can also target bottom dwelling fish such as cod or sablefish. The animals are able to enter the trap/pot through an opening of a specific size and shape, but are unable to leave. Traps and pots are selective as they are designed to catch specific species. The mesh size on the traps allow for undersized or immature individuals to escape. Additionally, any bycatch accidentally caught e.g. non-target species like octopus, undersized individuals, and females carrying eggs can be released live and uninjured. Traps and pots are generally a sustainable way of fishing, unless they are placed in areas where the main fishing line which ties them together has the potential to entangle migrating marine mammals, or if they are placed on top of sensitive habitat.”

Because the current demand is so high, getting enough spot prawns to supply the market often requires the use of trawl nets. These nets can result in significantly more damage than the trapping methods, especially when dragged over glass sponge reefs and other sensitive ocean environments.

But I digress.

As interesting as all that is, let’s focus our attention back on the other creatures that play a role in the life and supply of spot prawns; starfish, sea urchins and kelp.

Ochre Sea Stars By D. Gordon E. Robertson

The Starfish

In this part of the story, the two main starfish players are Ochre Sea stars (Pisaster ochraceous) and Sunflower stars (Pycnopodia helianthoides). These two species play a pivotal role in maintaining and stabilizing diverse benthic communities. (the benthos is the community of organisms that live on, in, or near the seabed, river, lake, or stream bottom, also known as the benthic zone.)

In 1969, Dr. Robert T. Paine, an ecologist and zoology professor at the University of Washington was studying intertidal zones on the coast of British Columbia. Dr. Paine invented a term, keystone species, for organisms whose loss would drastically alter the composition of the community they had been a part of. These species are predators at the top of the food chain and their disappearance results in a loss of a number of different species that make up the community. He designated the Ochre stars as a keystone species and the first organisms in the world to be so designated.

Why the Ochre stars?

The Ochre stars feed on mussels and thus control their population. This allowed limpets, chitons, barnacles and other species that eat the same food as mussels to survive and produce greater numbers.

The Ochre star is a generalist feeder and also eats limpets, snails, chitons, hermit crabs and marine worms. So by eating the mussels, they ensure that lots of other kinds of food are also available for them. Win-win for the Ochre star!

Sunflower stars Image by skeeze from Pixabay

What about the Sunflower stars?

This is another remarkable creature and it is also developing a reputation as a keystone species.

It’s one of the world’s largest predatory sea stars. It can reach sizes up to 1 metre across (3 feet), can weigh up to 5 kg (11 lbs) and ranges from the intertidal zone to depths of about 450 metres. It has up to 25 arms with almost 15,000 tube feet. These are the little sucking structures on the underside of the arms that starfish use to move. These stars can move up to 3 metres per minute. Your average starfish moves at about 15 cm per minute.

Let’s put this speed difference in terms you can easily understand. Imagine you’re driving a car at 5 km/hour and another car passes you travelling at 100 km/h! Got the picture?!

Not only are the Sunflower stars fast and big, they are also voracious! They’ll eat pretty much anything; snails, mussels, chitons, octopus, sea urchins, other stars, sea cucumbers, sand dollars, crabs, barnacles, squid and the list goes on.

In the intertidal zone, which is the one we’re really interested in because that’s where our spot prawns live, the juvenile Sunflower stars feed on the mussels and sea urchins.

And guess what the mussels and sea urchins eat a lot of?

Yup. Kelp!

So by eating mussels and sea urchins, these two starfish species helped maintain a plentiful kelp population. And everything was just peachy keen for everyone.

Until the starfish started getting sick. Really sick. And dying.

Starfish Wasting Disease

In 2013, wasting disease came into the area big time. It was affecting more than 20 species of starfish from Baja, California all the way up to Alaska.

Ochre star with wasting disease

Not a pretty sight.

And it was responsible for a massive die-off that was almost eliminating the starfish populations.

Since it was discovered, there have been several agents, including a particular kind of virus, that were suggested to be responsible. But none of them have been definitively shown to be responsible for the disease. So, the causes of sea star wasting disease are still waiting to be determined.

Lately, some microbiologists, like Ian Hewson, think that the presence of the virus in sea stars affected by wasting disease is just a side effect. He notes that the timing of wasting disease corresponds with runoff events in the ocean — think dying algal blooms or massive storm events that put a ton of organic matter into the ocean. That “dissolved organic matter” eventually covers sea stars and is broken down by bacteria.

So bacteria utilized all the oxygen and basically deprives the starfish of that oxygen,” Hewson said. And that starts the wasting cascade.

Hewson also suggested that this could be a direct result of climate change.

“If our hypothesis is correct, that would actually cause wasting disease to get worse” as the ocean warms up.

Interesting note: In the past year, Alaskan ecologist Brenda Konar and her students have observed that starfish populations seem to be recovering somewhat. Only time will tell if this is a true recovery or just a glitch in the overall trends.

So there are no proven causes to target for possible intervention and if it really is due to climate change, then it could be some time before any real progress can be made to alleviate the situation.

How Do Dying Starfish Affect the Kelp?

Simple. The massive disappearance of the keystone starfish predators resulted in a rapid expansion of mussels and especially sea urchins.

Live and dead green sea urchins

In the area of British Columbia that I call home, the Green Sea Urchin (Strongylocentrotus droebachiensis) population quadrupled! And they ate. And ate. And ate.

Sea Colander Kelp aka Devil’s Apron or Shotgun kelp. The holes in this blade are how it normally looks and not from sea urchins!

All that urchin eating reduced the Sea Colander Kelp population by 80%.

So what’s all this got to do with Spot prawns?

Simple.

The Spot prawns use Sea Colander Kelp “forests” as their nursery habitat. The kelp blade cover helps to protect the developing prawns.

Less kelp forest cover results in a greater loss of prawn larvae from predation which translates into fewer adults.

And that brings us back to the economic impacts. Fewer adults per a given area mean that harvesting the same amount takes more effort and greater area fished. This costs more and leads to higher prices. Without consumer demand levels shifting, there is greater pressure to harvest the same amount.

To do that, the original trapping system cannot supply the demand fast enough. Which means more efficient harvesting methods like trawling. As we noted at the beginning of this article, trawling is much more disruptive to surrounding ecological environments.

And if you remove more of a waning population, well you know what happens.

So there you have it. The connection between Starfish Wasting disease and the supply of Spot prawns.

Are there any solutions to this dilemma?

As I noted above, some scientists think that the wasting disease is on the way out and the starfish are recovering. While I’m a very optimistic person myself, I do think it’s a bit too soon to celebrate because we see a few starfish increasing again. Only time will tell.

Which brings me back to a personal dilemma. Should I stop eating spot prawns? Or should we be asking our BC Ministry of Environment and Climate Change to impose harvest limits on this valuable resource? If that’s what it takes to save our spot prawns, I’m all for it.

In the meantime, I’m keeping my fingers crossed that it will all work out somehow because as I said at the beginning, I love to eat Spot prawns!

Until next time,

Rich

Hey! If you enjoyed this article then please subscribe to my newsletter to hear about my latest articles and grab my free ebook here.

Sources

Although I researched a fair number of sites to put this article together, my primary inspiration and resource was an article by marine biologist Sheila Byers in Volume 47 (2019) of Discovery. Discovery is published annually by the Vancouver Natural History Society (Nature Vancouver) as a service to its members and subscribers. And it’s an absolutely wonderful publication. Gets 5 stars from me, even if you don’t live anywhere near Vancouver, BC area!

Rich Sobel

Written by

Rich writes about fascinating creatures and biological issues that affect our everyday lives. Get his free ebook at www.biology4everyone.com.

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