Sawfish: Why is our work so exciting and how does it work?

I'm reposting this Lab Note here because it's super cool science and I thought you all would like to read this explanation of our work. The new crowdfunding campaign is on the final 4 days of fundraising! Now is the time to donate if you haven't! Funds are currently being matched dollar for dollar, so your donation has double the impact! Come visit the new page if you haven't done so already!

 

The adult sawfish rostrum from our recent manuscript

We've talked before about how we use the tiny differences in chemistry in tarpon scales to understand their migration, but I haven't talked about how this applies to sawfish. This is, I think, the coolest part of the project!

Why is it so cool? It's cool because we're the first in the world to show that this is possible and it feels awesome to step out into the unknown and discover something new. Also, sawfish are so cool looking, so interesting, and so endangered that it feels good to find a scientific method that might help conserve them, even if it's just a small thing.

So, lets talk about what we've found!

Just before I went to Brazil my undergraduate employee at University of Idaho, Breanna Graves, and I managed to get our hands on two sawfish rostrum, in Idaho of all places! We got permission to sample a tooth from each one and analyzed them for growth and chemistry.

 

Breanna Graves sampling genetics from a sawfish rostrum at University of Idaho

Perhaps you're asking, "why did you want to do that?" Fair question.

One of the big questions about sawfish biology and ecology is where they move and what they do. They are really hard to find, so tracking them physically is really hard work and even with radio or GPS tags it takes a ton of work to get a relatively small snapshot into their lives. But, knowing where they are and where they move might mean we could tailor conservation efforts to help them.

THE big problem for sawfish is that that nose has never seen a fishing net it doesn't want to cuddle with...which means that sawfish are accidentally caught and killed in fishing nets. If we knew more about where sawfish move, when they move, and where they tend to congregate, we could do things like limit fishing at places and times when sawfish are most vulnerable. But, we just don't have that information. We need a better way to understand what sawfish do throughout their lives, a better snapshot into their movements.

This is where sawfish teeth come in

Just like scales and otoliths (see the methods of my prior project for more on how cool otoliths are) sawfish teeth are made of calcium minerals. They also continue to grow throughout the life of the fish, so any chemical records might be stored inside them in the same way. We wondered if sawfish teeth might be one way to uncover more information about their migrations. But, nobody had ever really tried to do it before.

It Works!

Breanna and I prepared our two sawfish samples as thin sections to see if we could:

1. See any growth banding like we see in scales and otoliths, and

2. Find changes in chemistry that indicated movement from fresh to salt water.

 

This is what the inside of a sawfish tooth looks like

The long and the short of it was that we were able to show:

1. Sawfish teeth hold chemical records of movement in the same way that an otolith does

2. There are repeated structures inside sawfish teeth, but they aren't as directly related to age as in otoliths and scales. We need to do more research on this.

We've posted the manuscript to BiorXive, a preprint server, so that anyone can read it for free. We'll be submitting it to peer review soon. You can read it HERE.

Here's the Big Results

This plot from a presentation Breanna gave at the recent National American Fisheries Society meeting shows the chemical results from two teeth. I'll walk through what it means below.

 

Ok, so what does this big plot actually mean? First, I should say that the ocean has tons of strontium (Sr) and very little barium (Ba). That's important to know. When we measure things on each tooth we measure the change of Sr or Ba in comparison to the amount of calcium. So, plots B and C are of the ratio of Sr:Ca and Ba:Ca.

So, when Sr:Ca is increasing on the plot that means there is more Sr in the water and a fish is moving into saltier water. Since the ocean doesn't have much barium in it, when Ba:Ca falls it is another clue that the fish is moving into saltier water.

Finally, we also have a third clue. The ocean, everywhere around the world, has the same ratio of the isotope 87Sr to the lighter-weight 86Sr. I'll spare you the details of why, but the entire ocean has the same ratio of 0.70918...everywhere.

So, when the black line in plot A moves away from the blue horizontal line it means that the fish is moving into non-ocean water.

So...when we look at the plot above we can see that the adult fish starts life (on the left of the plot) in non-ocean water. It moves into saltier water in the middle of it's life. But, we know that that water wasn't actually ocean water because Sr:Ca doesn't rise very much, so it can't be fully salt water. The black 87Sr/86Sr line also doesn't quite get close enough to 0.70918 to be pure ocean water but it's hard to see on this plot.

The opposite is true of the embryo tooth. It's chemistry has really high Sr:Ca, really low Ba:Ca and the 87Sr/86Sr signature is even closer to the ocean signature. This fish clearly was growing inside a mother that was living in pure ocean water!

So, this plot shows us something really cool. We can use the chemistry of sawfish teeth to understand where they have been and where they have moved during their life! That's huge!

This means that potentially we can learn about how these fish move using all those dried saws that are sitting in museums and in peoples living rooms all over the world. There's a ton of information there, AND we can access that information without having to bother, harm, or kill any sawfish in the wild. That's potentially big news for sawfish conservation.

But, there is a lot we still need to do. A sample of two fish is not conclusive. We need to analyze a lot more sawfish teeth if we're going to unravel real information about where fish are, and when. The Amazon region is also cited as one of the last refuges for sawfish in the Americas...so our sample represents an opportunity to start learning a lot more about a really important population of sawfish.

But, the only way we can make that happen is to get these samples analyzed. To do that we need money, these analyses are not cheap. This crowdfunding will give us a good head start, and give us more robust data that we can use to secure bigger grants in the future. That is why your donation is so important!