queensland Brisbane River threadfin research - some results!

[braddo]

Hi all,

As some of you may know I'm running a research project at UQ examining the biology of king threadfin in the Brisbane River. This project forms part of a larger study into the biology and population connectivity / movements of the species across northern Australia. I would like to share a few quick results with you on the reproductive biology such as the length and age threadies mature and change sex, and the growth patterns and age structure of threadfin in the river. These results are preliminary and are based on the 81 fish donated to the project since November 2008. I would like to thank everyone who has donated fish to the project; without these donations and your ongoing support this project would not have been possible. I hope this information is useful and helps to provide a better understanding of this amazing fish found right here on our doorstep!

Reproduction

Spawning

King threadfin spawn in the Brisbane River between October and early March, with peak spawning occurring in October and December. Individual fish are capable of spawning multiple times over the spawning season.

Maturity

King threadfin start out life as males before changing sex to females. Fish generally mature as males before changing sex. Maturity in male fish generally begins at around 700 mm total length (between 2-3 years of age) and by 1000 mm all males are mature (at around 5 years of age). The probability that a fish is mature increases with size:

Total length (mm) = Percentage of fish that are mature

600 = 0

700 = 10

800 = 40

900 = 75

950 = 90

1000 = 100

Thus if you catch a threadfin that is greater than a metre long, there is a 100% chance that fish is mature, and capable of spawning during the spawning season.

Sex change

Sex change from males to females in Brisbane River king threadfin begins when fish are around 900 mm in length (at around 3-4 years of age). By 1200 mm total length (at around 10-11 years of age) all fish have completed sex change and are mature females:

Total length (mm) = Percentage of fish that have changed sex

600 = 0

700 = 0

800 = 0

900 = 10

1000 = 15

1100 = 20

1150 = 40

1175 = 80

1200 = 100

Age and growth

Generally, king threadfin caught in the Brisbane River are between 3 and 10 years old, with the majority being around 3-5 years old (800 – 1000 mm). Younger fish (1-2 years old) are generally smaller than the current minimum size limit (600 mm), so are not represented in the recreational catch (a few fish < 600 mm were donated to the project by researchers working under a general fisheries permit). The oldest fish donated was 14 years old and was 1300 mm (1.3 m) total length. Growth rates of king threadfin in the Brisbane River vary between fish, but generally growth is rapid. Average total lengths at the end of each year of age are as follows:

Age = Average total length (mm)

1 = 300

2 = 550

3 = 775

4 = 925

5 = 1000

6 = 1050

10 = 1175

Subject to ongoing research

This project forms part of a larger research project into the biology and movements/population connectivity of king threadfin populations across northern Australia and is funded in part by funded by the Australian Fisheries and Research Development Corporation (FRDC). The project aims to determine the degree of movement and mixing of threadfin populations between rivers across northern Australia, and establish the key biological parameters of each population, in an effort to better understand the species’ biology to ensure its sustainability.

I am still chasing samples in order to compare the biology of threadfin in the Brisbane River to that of populations in other rivers acorss Queensland, so if anybody would like to donate frames of fish kept for consumption that would be very much appreciated. Frames can be frozen, and I will arrange collection. Please keep the guts and gonads intact (or in a bag if removing for filleting), as these provide valuable information on diets, movements and reproductive biology. Remember, a minimum size limit of 60 cm total length and a bag limit of 5 fish applies for king threadfin caught in Queensland waters. To donate frames give me a call on 0401 055 589 or send me a PM and I’ll arrange collection. If anyone has any questions about the results presented here or would like further information on thready biology in the Brisbane River give me a call or send me a message.

Cheers

Brad

[51MPL]

awsome info keep up the good work!

[rayke1938]

Thanks for the interim report.

Are you also doing any DNA sampling on the Brisbane river fish vs other populations furthur north?

Just interested if they are all part of the same gene pool.

Cheers

Ray

[braddo]

G'day Ray,

It's timely you ask - we got some results back this week that suggest Brisbane fish are gentically different to those sampled from central and north Queensland. Given that in marine fish populations it typically takes 100s to 1000s of years of isolation to be gentically different, this raises a whole heap of questions about whether they have always been in the river or are recent (in terms of decades) arivals from the north. At this stage the locations tested by genetics are too widely spaced to provide conclusive data on this (generally for these types of studies you start with widely geographically separated samples and then, if different, look at a finer scale, so you don't blow all your funding chasing rainbows that don't exist!). We're looking to run some finer scale analyses to see where the genetic 'boundary' is.

Brad

[Cowfish13]

Thanks for all that info Braddo Good luck with the rest of your project. How long is going for?

[Jimmyjack]

THanks Braddo great work

[Angus]

Thanks for the efforts in the updates mate. It really makes all those that have helped feel good I bet!

Angus

[rayke1938]

Thanks Brad. I hope that the funding continues and actually increases.

Cheers

Ray

[shortie]

Awesome thanks for keepng us informed. Its great to see real results unlike the hearsay we have been hearing.

[tugger]

thanks braddo they r quick at growing there must be plenty of tucker in the river for them i'll keep the frames fresh again if i get another 1

mark

[jeff f]

Angus wrote:

Thanks for the efforts in the updates mate. It really makes all those that have helped feel good I bet!

Angus

yeah it does angus

good stuff braddo glad to help

looks like legal needs some rethinking

possibly the same as barra 60 botton 120 top

[adambris]

even as a guy that dosnt realy target them, its interesting to read that sought of info.

you are obviously very passionate about this, well done on your efforts.

adam

[nadders]

good stuff braddo. im glad to have this written down to refer to while discussing the issue with people rather than just having what you told me in person- it means alot more to show someone the words rather than say "brad has told me..."

i hope dpi&f consider putting a slot size in as per barra, flathead etc

cheers

Dan

[tobie10]

very interesting research, thanks for the update! I hope they put on upper limits and smaller bag limits as well to maintain the population. I'll definitely be keeping your maturity data in mind if I ever get on to another threadie - good to have some science behind decisions to release or keep.

[ellicat]

Braddo, Will your research extend to sustainability issues ?

Do you believe the current regulations are adequate for maintaining a sustainable population ?

Will be interesting to see if the Brissie ones are a distinct species and, if so, what has caused their population explosion in recent years.

[Angus]

nadders wrote:

good stuff braddo. im glad to have this written down to refer to while discussing the issue with people rather than just having what you told me in person- it means alot more to show someone the words rather than say "brad has told me..."

i hope dpi&f consider putting a slot size in as per barra, flathead etc

cheers

Dan

I dont get involved with the argument to often as I just dont see the worth as I agree with many points from both sites

But this is one I agree with. Would be happy to see a slot size based on this research alone.

Everyone seems content with the flathead slot size and from many a Pin regulars mouth the size and number of fish has increased in this region alone.

With threaddies, when a LEGAL length could still be 90ish cm's (at least) of flesh, I cant see how the Fillet & Release group and the Responsible Fishing groups could not both be happy with this.

I would also on responsible and conservative front be more than happy with a 2 fish limit. This is not a GREENY view. This is an "Im happy to make small gestures to ensure I can get a feed in the future" view. I also feel that once again 90cm's of flesh is tonnes, let alone 90cm x 2.

I would be unhappy with season closures (one for the F & R's).

Why?

* Selfish reason but meh. I get to fish more in Summer.

* The legal fish would be under the spawning size.

* The population does not seem threatened. Once again ill use the Flathead analogy, the slot approach to conservation seems to have been a sufficient tool.

The only issuse I see repeating regardless of research is the difficulty of release issue. I dont have an answer for this and wont touch it I do think though that even a small chance of survival is worth a chance release. If they die? Well something will eat it. The Lion King summed it up nicely with the song "Circle Of Life" HAhaha.

Anyway take this how you will its merely opinion.

Angus

[nadders]

if i ever get out for a thready session gus (its been a while!) i'll do my very best to video a release weight in action, i made one for the grand cost of $0.00 using a wire coathanger and some old weights. ive said it before but the biggest problem with releasing is firstly the handling the big fish, a large heavy fish does not do well out of water unless supported in a cradle etc, and secondly dealing with the inflated air bladder, both quite easily worked around

but yeah i will try and video a release weighting. its dead easy to make one

[Angus]

nadders wrote:

if i ever get out for a thready session gus (its been a while!) i'll do my very best to video a release weight in action, i made one for the grand cost of $0.00 using a wire coathanger and some old weights. ive said it before but the biggest problem with releasing is firstly the handling the big fish, a large heavy fish does not do well out of water unless supported in a cradle etc, and secondly dealing with the inflated air bladder, both quite easily worked around

but yeah i will try and video a release weighting. its dead easy to make one

Nadders that would be an ideal Vid to upload to the site to use for linking when this question arrises.

Angus

[Feral]

Braddo , have you been getting any fish out of the rivers just north of Brisbane? I can put the word out if your interested in some. The local boys get a few.

Very interesting that they dont have a spawning run, but spawn multiple times over the warmer months.

Angus - a slot size would be interesting, the 100% female population is close to maximum size of the research frames, if they tried to base the slot similar to dusky flathead that would mean the slot would start somewhat larger than 120cm. (All legal duskies that have been caught in DPI research are female, IE fish over 40cm have already changed sex).

[Yellow Peril]

Thanks for the update Brad. Great to see you are getting some interesting results.

It will be good to see some of this science being used for fish management. I for one am not in favour of a slot size due to the difficulty of releasing threadfin from deep water. I know it is easy if you know how, but really very few know how to use release weights or carry them, or how to vent the swim bladder. One thing for sure, 10 fish bag limit is WAY too high.

Jeremy

[braddo]

ellicat wrote:

Braddo, Will your research extend to sustainability issues ?

Do you believe the current regulations are adequate for maintaining a sustainable population ?

Will be interesting to see if the Brissie ones are a distinct species and, if so, what has caused their population explosion in recent years.

Sorry for the late reply on this. Yep - my research will be looking at sustainability issues. The project has received some funding from the Fisheries Research and Development Corporation (FRDC) mainly due to concerns over threadfin as a commercially and recreationally exploited resource. WA, NT and QLD state fisheries are onboard as well due to sustainability concerns (WA Fisheries currently regards threadfin stocks as being overexploited in their state waters). Sorting out their biology and how much mixing occurs between populations is just the first step to ensure sustainability.

From a size / bag limit angle I don’t believe the current regulations are adequate for maintaining a sustainable population. Many who have replied to the post have suggested an upper size limit (like flathead or barramundi). I also believe that an upper size is warranted to protect the breeding females however whilst this idea looks great on paper putting it to practice will be a lot harder . Yellow Peril points out the difficulties in releasing fish from the deep water of the river, though I’m sure this can be overcome with experience and training (got that releasing video ready yet Nadders?). The greatest issue here will be with the commercial fisheries. King threadfin are an important component of the inshore gill net fisheries across northern Australia, which mainly target barramundi. However, in some seasons/years, the catch (in tonnes) or market price for king threadfin exceeds that of barra . However, barramundi (who have a max size of 120 cm) are relatively ‘hardy’; a +120cm fish can be caught in the nets and be able to be released. Unfortunately, king threadfin aren’t as hardy and die almost immediately after hitting a net (I’ve spent a fair bit of time working on commercial boats over the course of this research and seen it happen – if you see a king hit a net it is usually dead by the time you reach it). Putting in a max size with no other restrictions (such as net mesh size) in place in important commercial fishing areas will probably just result in a whole bunch of +120cm threadfin ending up as crab food. The obvious way to stop this is to reduce the size of mesh that can be used, but that may not be a viable option for the commercial fisheries as it will decrease the catch of other species as well. There is a seasonal closure on barramundi over late spring – summer (typically encompassing November, December and Jan but dates vary between areas and years), which effectively includes threadfin and allows them a chance to spawn. Perhaps a regional approach may work – where a maximum size is put in place for the Brisbane population who aren’t subjected to commercial fishing pressure (save for the odd fish in the beam trawls now and then) and a continuation of the current practices in commercially-fished areas. Obviously is debate has a number of points on both sides and this discussion can go on for longer but I’d be interested to hear people’s thoughts on this.

I also believe the minimum size is too low. The minimum size is designed to give each fish at least one chance of spawning. However at 60 cm TL it is too low to protect any threadfin spawning (they don’t start until around 70 cm TL. The minimum size was actually increased from 40 cm in March last year so I suppose that’s an improvement.

Cheers

Brad

[rayke1938]

A regional bag limit of one in waters say south of noosa would be a start.

I do not believe that they are targeted by the pros south of Hervey bay maybe this should also enthroned in legistation in case there are populations accessible to the pros that have yet to be developed.

Nadders has done a lot for angler education on this site but it has to be extended to the angler and tackle retail fraternity as a whole package.There would appear to be a group of tackle retailers and magazine writers who seem to be advocating a go for it attitude.

Cheers

Ray

[nadders]

have not been able to get out for a fish since my last post in this thread braddo but next chance i get i will try and get some footage, wish me luck

[nadders]

i also think if a slot limit were put in a 120max would be pretty useless, in my opinion 100 should be the magic number. most fish ive seen caught are sub 120 anyway, and given the difficulties in handling the big ones i dont think its a good idea for people to be trying to carefully measure an obviously m+ fish which may die as a result. in my opinion a 100 limit would alleviate alot of pressure on the breeding population, just my 2c

[rayke1938]

Currently a thread on ausfish about release weights. This link to Recfishwest is quite good.
/>http://www.recfishwest.org.au/ReleaseWeight.htm

They also show how to make your own or they are avail on the sunshine coast maybe fishyman can advise if avail in Brisbane.In NPD we just used a small snapper lead and barbless hook but this looks much better and less fiddly.Also note they do not seem to be in favor of venting.

Wellsys have 2 models, different weights, 20 bucks.

Wellsystackle.com.au look under fish release products

Release Weight 40oz. (1.132kg)

Code 180356

Attach this weight to a spare rod & reel and use it to return deep water species to their habitat quickly and humanely where they are to be released after capture. This procedure will recompress the expanded gases in the swim bladder and ensure a quick recovery. The release weight has a barbless hook set into a lead weight with an eye located central and on top of the gape to facilitate quick attachment to a rod & reel. Place the hook in the top jaw of the fish to be released, piercing the top lip from the outside into the mouth and then lower it to the bottom. A tug on the line when it reaches the bottom will release the fish from the hook. View instructional DVD footage on www.recfish.org.au/ReleaseWeightDVD.htm

Price AUD 21.90

Qty

Cheers

Ray

[ellicat]

braddo wrote:

ellicat wrote:

Braddo, Will your research extend to sustainability issues ?

Do you believe the current regulations are adequate for maintaining a sustainable population ?

Will be interesting to see if the Brissie ones are a distinct species and, if so, what has caused their population explosion in recent years.

Sorry for the late reply on this. Yep - my research will be looking at sustainability issues. The project has received some funding from the Fisheries Research and Development Corporation (FRDC) mainly due to concerns over threadfin as a commercially and recreationally exploited resource. WA, NT and QLD state fisheries are onboard as well due to sustainability concerns (WA Fisheries currently regards threadfin stocks as being overexploited in their state waters). Sorting out their biology and how much mixing occurs between populations is just the first step to ensure sustainability.

From a size / bag limit angle I don’t believe the current regulations are adequate for maintaining a sustainable population. ...... Is your opinion in relation to the Brisbane population ? If so, why are the current regs inadequate ?...Is this based on data or a "feeling" ?

Many who have replied to the post have suggested an upper size limit (like flathead or barramundi). I also believe that an upper size is warranted to protect the breeding females however whilst this idea looks great on paper putting it to practice will be a lot harder . Yellow Peril points out the difficulties in releasing fish from the deep water of the river, though I’m sure this can be overcome with experience and training (got that releasing video ready yet Nadders?)...... The greatest issue here will be with the commercial fisheries........In Brisbane ?

King threadfin are an important component of the inshore gill net fisheries across northern Australia, which mainly target barramundi. However, in some seasons/years, the catch (in tonnes) or market price for king threadfin exceeds that of barra . However, barramundi (who have a max size of 120 cm) are relatively ‘hardy’; a +120cm fish can be caught in the nets and be able to be released. Unfortunately, king threadfin aren’t as hardy and die almost immediately after hitting a net (I’ve spent a fair bit of time working on commercial boats over the course of this research and seen it happen – if you see a king hit a net it is usually dead by the time you reach it).

Putting in a max size with no other restrictions (such as net mesh size) in place in important commercial fishing areas will probably just result in a whole bunch of +120cm threadfin ending up as crab food. The obvious way to stop this is to reduce the size of mesh that can be used, but that may not be a viable option for the commercial fisheries as it will decrease the catch of other species as well. There is a seasonal closure on barramundi over late spring – summer (typically encompassing November, December and Jan but dates vary between areas and years), which effectively includes threadfin and allows them a chance to spawn. Perhaps a regional approach may work – where a maximum size is put in place for the Brisbane population who aren’t subjected to commercial fishing pressure (save for the odd fish in the beam trawls now and then) and a continuation of the current practices in commercially-fished areas. Obviously is debate has a number of points on both sides and this discussion can go on for longer but I’d be interested to hear people’s thoughts on this.

I also believe the minimum size is too low. The minimum size is designed to give each fish at least one chance of spawning. However at 60 cm TL it is too low to protect any threadfin spawning (they don’t start until around 70 cm TL. The minimum size was actually increased from 40 cm in March last year so I suppose that’s an improvement.

Cheers

Brad

Cheers for the feedback. I've asked a couple more questions in blue above, as if the Bris population is distinct, then it would not have much bearing for the rest of the populated areas ???

Will be interesting to see if the Brissie ones are a distinct species and, if so, what has caused their population explosion in recent years.

Any ideas on that ?

As for opinion on size/bag limits - From what you have said it would make sense to have a minimum size at 80 or 90 cm and forget about a max*. Then give them a 1 month spawning holiday in the peak month (December ??).

*Educate all you want, but the majority of knuckle-dragging neanderthals like fishermen will not be bothered with release weights etc. Most fishermen don't participate on forums and would only buy the odd BnB if there's a good/relevant front cover.

[nadders]

without putting words in brads mouth, i believe he said they are genetically distinct from others sampled (not sure of the sample sites) which indicates they didn't migrate from at least the other sample sites.

anecdotal evidence from people i trust placed the first pop increase around the nineties, my theory is that around this time (also around the time they stopped dredging) the few scattered fish in the river had a good few breeds, possibly due to better water quality from the end of dredging

10 years from then, say mid 2000s u had gav dunne etc publisising big threadies, 120+'s, which by brads scale are around 10 years old, it seems alot of big fish were poppin up around this time which imo could have been from the good breeding seasons had in the mid nineties in my theory.

of course its a theory and speculation but i havent heard any explanations given by researchers so until someone does a specific study on why the pop increase, theories are all we got, and thats the one that makes sense to me

[nadders]

as for no max size, i still think its important for a max size as an 80-90cm minimum and no max doesn't change the fact that lots of 100+s congregate at certain well known areas which happen to get fished pretty hard, the idea of slot limits has to be to protect the important sized fish imo, if nothing is done to protect the mature females then i think the job is only half done.

increasing the minimum size to a size where most fish are mature males is a good idea to protect the chances of the males adding their part, but i don't think having no max size and leaving the females as vulnerable as they are will do much for them as it takes 2 to tango and i havent heard of any surrogacy or adoption programs for alternative lifestyle threadfins to have children

1 month spawning holiday i personally wouldnt mind, but i think a slot size in order to discourage fishing for oversize samples is the go (theres heaps of spots to get smaller ones all througout 70km of river).

as for education, obviously the majority of fishos dont get on forums and education without legal enforcement apparently has not much effect but it has its place, it never hurts for people to gain awareness

[smitty]

Well done on your work Brad, hopefully it is put to good use.

It seems quiet obvious to me there has been a massive increase in effort focussed on these fish in brisbane in recent years, based on your gene pool study they have been in the river a long time and at current levels of effort in 10 years time there may well be a real shortage of breeding sized fish if steps are not taken.

Id put my hand up for a 90-100cm long maximum size to reduce the targetting of these breeding sized fish which do not release well anyway, 1 fish in the 70 to 90cm range is big enough to feed a family of 5.

Hopefully the smaller fish will become more common throughout the river if the stocks are not hit so hard in that, sofar there seems to be only the odd smaller fish being taken in my local area fig tree pocket and jindalee, 1 a trip is a good trip.

[ellicat]

Also,

Any estimates on what the population is ?

What the spawn numbers per fish are ?

What effect they have on other fish species' numbers ?

Nadders,

It's not whether the gay salmon can have babies or not, it's still their right to have babies :silly: :laugh: ..... but good point. Can't really be certain until the questions above are answered though. I'd have thought the larger the population of the up-and-comers the higher the number of fish that make it to the finishing line. If 90 big girls get taken each month from the pipeline (during breeding season) and the population is 1000 big girls around the traps, and 5000 males waiting for the operation in the next 12 months then that sounds like a sustainable equation.

[nadders]

id like to see some population estimates as well, keenly interested in that. i think brad once told me the typical ratio of big girls to males in the species is very much smaller than 1:5 though hence my consistant concern for the big'uns. how are population estimates done anyway, does it rely on tagging and recapture data?

i must admit i've been highly slack in the tagging dept, i tagged a couple when i first got the equip, then left the equip at home for a few more fish, then stopped chasing them during the spawn and havent re-started yet. i'll make an effort to dig up the gear and contribute to that side of the research this year. i found it a bit tough tagging them while still in the water so i defs need more practice

[elops]

Thanks for the post Brad good to see some results from your research. Have you had any returns from the tagging yet ? It would be good to see some more effort go into this.

Dan its pretty much a given that there were a number of large fish around in 95/96 with subsequent recruitment from there. Hopefully we will see some sort of legislation change to protect the sustainability of threadfin in the river and soon enough to reverse what (and I will say it ) has been the wholesale slaughter of a large number of obviously female fish in the lest six months

Steve

[elops]

nadders wrote:

id like to see some population estimates as well, keenly interested in that. i think brad once told me the typical ratio of big girls to males in the species is very much smaller than 1:5 though hence my consistant concern for the big'uns. how are population estimates done anyway, does it rely on tagging and recapture data?

i must admit i've been highly slack in the tagging dept, i tagged a couple when i first got the equip, then left the equip at home for a few more fish, then stopped chasing them during the spawn and havent re-started yet. i'll make an effort to dig up the gear and contribute to that side of the research this year. i found it a bit tough tagging them while still in the water so i defs need more practice

I have found it a bit hard myself mate they are definately the hardest fish to handle and tag I have encountered.