Spawning and recruitment of barramundi in estuaries.

[Douglas]

I have some good info on the subject that I'd be happy to share.

Its from Bill Sawynok (Info-Fish Services and the Suntag tagging scheme) and he doesn't mind me passing on.

It opened my eyes and mind. Explained a lot.

Sure puts some NT big barra fishing in a different light as well.

[Douglas]

After a stuttering start, lets have a look at the spawning element which is different to the recruitment element although one is a consequence of the other.

Firstly I suggest that you have a look at the story of coral spawning on the GBR.

There are some similarities to be seen in that both are mass events and the timing is different for different parts of the coastline. This is reflected in differing spawning closures between the east coast and the Gulf of Carpentaria spawning periods for barramundi due to differing local circumstances.

Lets simply look at the east coast area and the Fitzroy in particular although places like Burnett and Mary will have differences.

What's been found to date.

Spawning (release of eggs from the big females) coincides with slack water on a big tide usually at night as the tides tend to be bigger at night.

Big tides mean stronger run to distribute the eggs.

The females are attended by thousands of males (younger and smaller) whose job it is to fertilise the released eggs. They have 1 hour to do that job. After that, the eggs "harden" and will not produce.

What that means is that both the females and the males have to be close together due to the 1 hour constraint on fertilisation.

That means they have to congregate.

Have an interruption here. Be back in a minute.

[Douglas]

Picking up again:

We've found that the first ingredient is slack water and the reason for that is so the eggs basically stay in the one place as the males zoom around doing their fertilising task. A single female can produce up to a couple of million eggs per spawning and there are thousands of females doing their thing. Hence the huge task of the males. So many eggs to fertilise and 1 hour to do it. No wonder they don't stay males for long as they couldn't keep up the pace over a lifetime!

The second ingredient is a tidal run to sweep the fertilised eggs up into the mangroves and inlets to give them the best chance of survival. To do that its obvious that the slack water period is on the bottom of the tide (low water) as high water slack means the eggs would be swept the other way (out to sea) by a ebbing tide and perish. Maximum distribution means the spawning takes place at the mouth of the river as that allows access to the full river mouth and mangrove flats.

On that note, have a look at the Burnett River mouth. There's been a lot of development work there with seawalls extending out to sea. Great for safe navigation and access to the river but disastrous for barramundi spawning. The once easily accessible mangrove flats of Skyringville for fertilised barramundi eggs have had the access restricted and spawning grounds significantly reduced. Once that happens, natural restocking cannot keep pace with the predation of man and the barramundi stocks reduce especially the bigger breeding females. Meanwhile up the river is the weir at Bundaberg city that limits the upstream migration of the surviving spawning population to only a few kms. Along the way there are few places for the small fingerlings to hide.

Larger river systems along the coast have similar barramundi spawning problems where development work at the mouth or a city close to the mouth have the same effect. Mackay stands out as one with the pioneer river though not as bad as the Burnett/Bundaberg.

Meanwhile river systems which have large mangrove areas at mouth and little development works at the mouth stand out as ideal spawning grounds. The Fitzroy delta is an obvious one and the Mary with their river mouths providing basically unrestricted access.


Both Rockhampton and Maryborough are many km upstream along with any weir or barrage restrictions. Although a smaller river with its mouth nowhere near as large as the Fitzroy, the Mary has the near proximity of Fraser Island and Sandy Straits increasing the size of the mangrove spawning protection zones. Hence the Mary has the potential for better spawning than a lot of the other larger rivers.

Gotta go again. We haven't finished with the main spawning ingredients yet.

[Douglas]

Here again

Attached are some google earth shots of the 3 river systems Fitzroy, Burnett and Mary along with what is thought to be the spawning story areas and subsequent distribution by flooding tide.

The Burnett sure stands out as a hard one to live in if you were a female barramundi. See the close-up of the Burnett for a glimpse of what rivermouth development can do to a fishery like barramundi.

The first 3 are shots of the Fitzroy, Burnet and Mary Rivers from an eye-height of 75km to give same scale comparisons and layout of relevant city, river obstructions and wetlands.

There's also a close-up of the Burnett. Not a lot in the way of mangroves for fertilised eggs to be washed into. A circuitous path in both directions plus the second entrance next to the northern breakwater wall has been closed off by the build-up of sand that the breakwater wall has contributed to. That opening was closing up in 1995 when I left Bundaberg with only top half of tides getting in and now its blocked completely.

Remember that we're only talking spawning here (not yet finished) and not recruitment.

The flooding tide at the Mary mouth is influenced by both ends of Sandy Straits. Would like any info anybody could offer along those lines. A look at the tide times and heights from the nearby standard ports would be the starting point.

Burnett River.jpgFitzroy River.jpgMary River.jpg





Burnett close-up.jpg

[Douglas]

So...... what are the other ingredients necessary for spawning to take place?

Imagine you were one of these young ardent barramundi. What details would need to be in place for you to have the most romantic night of your life?

Location.....yes we have that. Adjacent to mangrove sanctuary .

Timing...... Yep. Slack water to allow maximum fertilisation leading to max tidal flow in direction of sanctuary for fertilised eggs.

Water temperature.... Don't know. Haven't thought about that. Maybe don't care.

Salinity...... Bit of a gimme as I can't do anything about that. Be good if it was stable I think. No excessive river flows at that time. Prick up ears about excessive rainfall leading to changes in salinity at river mouth. Too much rain (meaning increased river flows) and we can call off the romantic night out as the females control the reproduction process. Have to wait for next cycle of when "the planets align". So look at river flows. Maybe extra fresh water will affect temp as well as salinity.

Big tides for max distribution of fertilised eggs.......... Night tides better for that at full or new moons.

So what do we end up with?

Have a look at tides for Nov/ Dec 2017 in the Port Alma area. We'll look at Mary after that.

[Douglas]

Well the full moon night of 4th December promises a low of .39m at 3.34 am followed by a high of 5.65 at 9.26 am.

That's a tidal flow of 5.17m in the right direction (towards the sanctuary of the mangroves and accessible wetlands) at the right time.

5.17 metres is the largest rise of tide for the period Nov - Dec 2017. If I was a betting man, that night would have very short odds as being spawning night unless something intervenes closer to the date (like river flow changing the salinity). To put it bluntly - rain.

In addition to the tidal height, the Fitzroy upstream has a tidal period of 5 in and 7 out. The Mary is similar.

That means that the flooding tide goes like the clappers to move all the water upstream in 5 hours. The fertilised eggs hitch a ride on the fast flowing up tide for wider distribution. With a 7 hour ebb, the run is slower so the "pull back" of the ebb is less than the "push forward" effect of the flood tide. Over a couple of tides the fertilised eggs still looking for sanctuary may travel 3 steps upstream and 1 step back with changes in run direction.

The 5in - 7 out timing gradually changes back to 6-6 the closer that you get to the mouth.

At the Mary River barrage above Maryborough, the tidal cycle is about 4.45 hrs in and 7.15 hours out so you could expect a bit more run on the incoming Mary tide than the Fitzroy.

Both rivers are ideal for distributing fertilised barramundi eggs upstream.

Now looking at tides for Nov/Dec 2017 for the Mary.

Mon 4th Dec is a standout again promising a low of .28m at 3.37 am and a high of 3.44 at 10.02 am at River Heads. That makes a flood tide rise of 3.16m (largest in Nov/Dec). The Standard Port for River Heads and most of the Sandy Straits is Burnett Heads and the times given are adjusted for River Heads. There is no published height ratio information for the northern end of the Straits like there is for the southern end.

However, the mouth of the Mary is 15km from the top of the Straits and 50km from the bottom of the Straits. Both River Heads and Boonooroo (to the south) have similar adjusted low and high tide times so I've discounted any difference in influence by flow from either direction in flood tide destined for the Mary river. There may be some north-bound push of flooding tide from Inskip countering the southbound distribution of fertilised eggs in the mangrove areas of the Straits to some degree but not enough (I think) to have significant impact on egg distribution.

So that just about sums up the spawning cycle in a perfect season. However, changes in salinity due to unwanted river flow pose the greatest threat to a perfect spawning season.

In the real world, the timing can be thrown out of kilter which usually still leads to spawning but not at the ideal time which leads to mixed results.

You cant go and measure how the spawning is progressing in real time. The best you can do is look for the results over a long period of time injecting all of the known variables into the observation. Observations over 20 - 30 years are the only way to iron out the effect of short term events. That's exactly what Bill Sawynok has done. I'll fill you in a bit later with more detail.

Back to measuring the success (or otherwise) of a spawning season. How can you tell?

Well, the best way is to find the results of the spawning (fingerlings) and compare those results with previous years looking at any differences year to year.

That now brings us to the other important part called recruitment.

Lets say we had a thousand female barramundi each pumping out 1 million eggs in an hour or so. 1000 x 1 million = 1 billion eggs!

If they all survived, you wouldn't be game to stick your toe in the river. Although they don't have teeth, they have been known to give a nasty suck at times.

In addition, what a task for the poor little males. 1 billion eggs to fertilise and only 1 hour in which to do it. So they never get the job done fully and a lot of those 1 billion eggs harden and don't produce.

Anybody got anything they'd like to add or info to contribute?

We'll have a look at recruitment next.

[Douglas]

OK lets look at recruitment.

What's the difference between spawning and recruitment? Spawning is when the barramundi do their reproduction activity.

Recruitment is a measure of how many barra larvae and fingerlings survive to the next stage.

First off we need to look at the breeding cycle.

In the attachment look at the left hand side of the little map. The spawning phase.

Barra-Life-cycle.jpg

The little arrows pointing upward show where the eggs ago to turn into larvae. Up into the mangroves and wetlands. They are 1.5mm long at this stage. Within a month they are about 20mm long and that is when the fun starts.

Everything (including bigger barramundi) want to eat them so they must find sanctuary. By this time its a month later and the tides aren't so high so where can they go?

The answer is nowhere unless rain and runoff give access to nearby lagoons and waterways out of the river. The longer they stay in the river, the fewer survive.

To assist their survival, rains/runoff need to come within a month. The alternative is a higher mortality rate and the fewer join the river barramundi bio-mass. The odds of survival to a size large enough to be safe are much much greater in a freshwater lagoon with weedbanks than in the main river. If the rains/floods come late, then fewer have survived to seek the sanctuary. Recruited into the bio-mass.

As its impossible to count larvae or fingerlings, the only way is by sampling with a castnet in specific places at specific times and comparing results to previous years.

Then you can actually calculate recruitment success for the spawning cycle and also the state of river stocks when those recruits get back into the river. Keep this in mind if you've ever heard of a term called Crystal Bowl.

Castnetting is only carried out until the recruits are big enough to take lures (200mm or so). The sampling is continued by line capture, tagging and release. If the returns by both castnet and line are poor then recruitment was poor. etc..etc.

Bill has about 20 sites that he samples in Jan, Feb and March each year with 20 casts per site each visit. He finishes cast net sampling at end of March.

I've been out with doing the sampling with him and have learned a lot about the subject over the trips.

Barramundi aren't the only species he surveys.

So one other necessary ingredient to recruitment is somewhere for the little fellas to go where they'll be safe.

You'll see that the Burnett simply has insufficient sanctuary areas in addition to all the other things impeding successful spawning and recruitment.

The Mary has better sanctuary areas and the Fitzroy is absolutely choc-a-block with them.

I'd really like some info on the sanctuaries and refuges that the Mary provides if anybody has that info, please.

The Fitzroy floodplain lagoons are safe and reliable refuges and won't dry up but the little guys have to get there first and that can only happen via a flood. Once there however, they can stay until the next flood even if its a couple of years away.

In other refuges, some of the bigger than usual tides make it in to them so access and escape isn't at much of a problem.

Time for a break.

Apologies for the attachment in this post. No matter what I did I couldn't get it the way I wanted.

[Douglas]

Thanks for persevering guys.

I'm not a marine biologist or anything so have attempted to put the terms that I was exposed to into plain English. I still pull Bill up at times when he uses big technical words (like omnibusconductor) by asking him what it means in English.

I reckon I've got a pretty good working understanding of what goes on and that is really what I want to know. The technical terms escape me.

[Didley]

Well done Douglas, that's a very interesting series of posts, many thanks for all your work

[gecko]

great work, thanks heaps. A fantastic read.

"No wonder they don't stay males for long as they couldn't keep up the pace over a lifetime!"
I didnt know they changed sex?? I read that the Flatty myth had been debunked.

Just curious about this whole concept of changing sex in fish.