Reproduction and Growth of Dusky Flathead

[Si]

Hi,
I just thought i would share some of the findings of a study from the NSW DPI into the Reproduction and Growth of Dusky flathead in NSW estuaries. The reason being is that i found it quite interesting particulary length at sexual maturity vs minimum legal lengths.

study was over five years.

It was determined that the length at which 50% of population is sexually mature was 31.72cm for males and 56.75cm for females.

'The corresponding age at which the 50% of population is reproductively mature was 1.22 years for males and 4.55 years for females. '

'The largest female was 98.5cm (7.5kg) and the oldest was estimated to be 16yrs, whereas the largest male was 61.5cm and 11 years. '

'The current legal length at the time of the study (2008) of 36cm (nsw) appears to protect 5% of female and 75% of male spawning populations'

'To provide greater protection to mature females so to allow 50% of females to spawn at least once would require an increase in legal size to 57cm!! This would mean most legal ctaches would be female. '

female fish grew faster and attained greater length than males.

while its a nsw study i dont think much difference for QLD, its quite interesting to think that the majority of female fish 57cm that we catch are not sexually mature. anyway some interesting facts to note. However, if we did raise the legal length to appro 57cm then females would only be removed from the fishery upsetting the balance, hence the reason why we probabaly have a legal length based somewhere in between.


Cheers.


article can be found here.

http://www.dpi.nsw.gov.au/__data/ass...ort_REPORT.pdf

[Feral]

I read an article published by Qld DPI last year, where one of the ramp studies showed all legal fish, 40 to 70cm at the time ( just under 300 flatties all up from memory) they checked were female. Tried finding it since but have not been able to, it had some other interesting data as well.

I thought the theory was flatties got to a certain size then changed sex?

[Si]

I read an article published by Qld DPI last year, where one of the ramp studies showed all legal fish, 40 to 70cm at the time ( just under 300 flatties all up from memory) they checked were female. Tried finding it since but have not been able to, it had some other interesting data as well.

I thought the theory was flatties got to a certain size then changed sex?

yeah thats what i thought too.

there is some thought for sex changing (protandrous hermaphroditism) for flatheads but this study didnt find any evidence of this at all (looking at gonads - testes and ovaries) and went on to suggest they dont change sex but rather male fish do not grow as large or as fast as females so ...jury still out???? i dunno.

[TheRealAndy]

duskies have an upper size limit of 75cm in qld, so the old girls can breed hard.

[Si]

duskies have an upper size limit of 75cm in qld, so the old girls can breed hard.

thank god too.

cos if this study is anything to go buy then we are taking fish (females) between all legal minimum limits to around mid fifties (cm) that the majority have not had a chance to reproduce yet.

[Fishbait]

Did they say anything on growth rates for flathead - ie. age in years and length in cm? I'd be intersted in that too. Thanks for the report.

[TheFishRaider]

I read an article published by Qld DPI last year, where one of the ramp studies showed all legal fish, 40 to 70cm at the time ( just under 300 flatties all up from memory) they checked were female. Tried finding it since but have not been able to, it had some other interesting data as well.

I thought the theory was flatties got to a certain size then changed sex?

Same here i thought that they changed when they got to 60cm. well i guess i was wrong

[ifishcq1]

all flathead start as males just like barras, why do you thimk we never see a small female flatty? in the study they probably either forgot or never knew
cheers

[Toddy_again]

all flathead start as males just like barras, why do you thimk we never see a small female flatty? in the study they probably either forgot or never knew
cheers

They knew about it as its mentioned a couple of times in the study.
A 61.5cm male suggests to me that they dont and of all the fish that they surveyed(7800 odd) there was nothing to suggest that this happens either.
I will be letting them go above 55cm after reading that.


Toddy

[Lovey80]

all flathead start as males just like barras, why do you thimk we never see a small female flatty? in the study they probably either forgot or never knew
cheers

This is as far as ALL evidence suggests is totally wrong and was only assumption made by anglers after the slot limit was introduced. (people just assumed they did the same as barra) Females and Males are like that through out their lifetimes but the females grow faster and much larger. As it is rare to find a male that gets into the no take zone. The big females are left as the most productive breeders and can roe thousands of eggs. There is no sex change in flat head at all.

I reproduced a letter to the editor of Modern Fishing Magazine from 2007. Below is the link.

http://www.ausfish.com.au/vforum/sho...vey80+flathead

I don't know about you lot but I think that the humble flathead has made a big comeback in the last 10 years or so. I think the slot limit is working what about you?

Cheers

Chris

[tunaticer]

"It was determined that the length at which 50% of population is sexually mature was 31.72cm for males and 56.75cm for females."

Does that mean that all of those flathead in the 40-50cm range I have cleaned in years gone by with roe are not sexually mature?If that is the case then how to they depict the difference between immature roe and mature roe?

Apart from sizes, is there any other distinguishing feature between female and male flathead, ie fin count or spike size/location or anything? I have never taken too much notice when taking the fillets to know.

[Jono_SS]

"It was determined that the length at which 50% of population is sexually mature was 31.72cm for males and 56.75cm for females."

Does that mean that all of those flathead in the 40-50cm range I have cleaned in years gone by with roe are not sexually mature?If that is the case then how to they depict the difference between immature roe and mature roe?

Tunaticer, if you look at your first words you'll see that you say "50% of population is sexually mature". In other words, it is quite possible you have seen mature females in the 40-50cm range. However, the comments would suggest that females in the 40-50cm range are more likely to be immature than mature. Note that immature females still have ovaries containing oocytes (developing eggs).

The criteria for distinguishing "mature" from "immature" or "not quite mature" fish can vary between studies, but generally are to do with the stage of development of the individual oocytes (viewed microscopically) which collectively give a certain "macroscopic" appearance to the ovary.

Do a search for
"Mackie, Michael. & Lewis, Paul. & Western Australia. Fisheries Research Division. 2001 Assessment of gonad staging systems and other methods used in the study of the reproductive biology of narrow-barred Spanish mackerel, Scomberomorus commerson, in Western Australia / Michael Mackie and Paul Lewis Department of Fisheries, Perth" - it's a fantastic document with some good photos.

cheers.

Jonathan.

[Si]

Tunaticer, if you look at your first words you'll see that you say "50% of population is sexually mature". In other words, it is quite possible you have seen mature females in the 40-50cm range. However, the comments would suggest that females in the 40-50cm range are more likely to be immature than mature. Note that immature females still have ovaries containing oocytes (developing eggs).

The criteria for distinguishing "mature" from "immature" or "not quite mature" fish can vary between studies, but generally are to do with the stage of development of the individual oocytes (viewed microscopically) which collectively give a certain "macroscopic" appearance to the ovary.

Do a search for
"Mackie, Michael. & Lewis, Paul. & Western Australia. Fisheries Research Division. 2001 Assessment of gonad staging systems and other methods used in the study of the reproductive biology of narrow-barred Spanish mackerel, Scomberomorus commerson, in Western Australia / Michael Mackie and Paul Lewis Department of Fisheries, Perth" - it's a fantastic document with some good photos.

cheers.

Jonathan.

yep, thats it.
in the article referenced it has a photo log of the stages of ovary development.