Seasonal closure and minimum legal size for barramundi in the Gulf ...

Seasonal closure and minimum legal size
for barramundi in the Gulf of Carpentaria
Technical report prepared for the Gulf of Carpentaria
Inshore Finfish Fishery review
September 2009
Queensland Primary Industries and Fisheries 1

Modelling analysis undertaken by:
Fisheries Resource Assessment
Animal Science, DEEDI
M.Tanimoto,
M. F. O’Neill
J. Robins
I. Haliday
Report collation and comments on management measures provided by the Fisheries Harvest Management
group of DEEDI
On 26 March 2009, the Department of Primary Industries and Fisheries was amalgamated with other
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Contents
Contents 1
Purpose of the paper 2
Summary 3
Background 4
Life cycle of barramundi 4
Management measures 4
Discussion 6
Summary of analysis 6
Minimum legal length 6
Length of the Gulf seasonal closure 7
Timing of the Gulf seasonal closure 7
Modelling of impacts 11
Methods 11
Results 14
Yields per recruit 14
Spawning per recruit 15
Discussion on analysis 17
References 18
Appendix 19
Model settings 19
Technical report prepared for the Gulf finfish review. Not government policy. 1

Purpose of the paper
This paper provides some additional background information for an ongoing review of
the Gulf of Carpentaria Inshore Finfish Fishery. It should be considered in
conjunction with the Gulf of Carpentaria Inshore Finfish Fishery review issues and
options paper and the Gulf of Carpentaria Inshore Finfish Fishery review response
form.
A call for stakeholders to identify issues to be considered as part of the review
resulted in a request for a reassessment of the current management arrangements
for Gulf barramundi (Lates calcarifer) on the basis that:
1. The variable seasonal closure to fishing for the species often overlaps the
spring school holiday break reducing family-based recreational fishing
opportunities.
2. A variable closure is more confusing than one starting and ending on fixed
dates and may result in lower compliance.
3. It is confusing to have a different minimum legal length limit for the same
species in two areas of the state, which may result in lower compliance.
This paper provides commentary on the possible impacts of changing these
management arrangements, as well as how such impacts could potentially be
mitigated, if that is deemed necessary. The Gulf of Carpentaria Inshore Finfish
Fishery review issues and options paper explains that a radically different approach
to management of Gulf inshore finfish resources is not considered necessary. This
paper therefore does not contemplate any major shifts in the type of management
tools used in the barramundi fishery.
It should also be noted that, at this early stage of the review, no decisions have been
made on what issues will be addressed or on the preferred options to deal with those
issues. This paper does not present preferred options. It is intended to help you think
about some of the issues that have been identified. It should not be interpreted as
pre-empting if or how these issues will be dealt with.
If a recommendation to amend any aspect of the management arrangements for
barramundi comes from the review, then that recommendation will be subject to full
consultation through the release of a Regulatory Impact Statement (see the Gulf of
Carpentaria Inshore Finfish Fishery review issues and options paper for further
details).
Technical report prepared for the Gulf finfish review. Not government policy. 2

Summary
The current minimum legal length of barramundi in Queensland waters of the Gulf of
Carpentaria is 60 cm. The species is also subject to an annual seasonal closure that
starts and finishes on variable dates that are determined by lunar and tidal cycles.
A monthly size and age structured model was used to predict the impact of changing
both the minimum legal length of barramundi and the seasonal closure in the Gulf to
be consistent with the east coast (58 cm and 1 November to 1 January respectively).
The modeling results indicated that reducing the minimum legal length from 60 cm
back to 58 cm was unlikely to have a detrimental impact on the Gulf barramundi
stock. However, reducing the duration of the closure by one month would be likely to
have a negative impact on the size of spawning stocks as well as overall yield.
While the analysis could not compare the use of a variable closure to one fixed in
time, it appears unlikely that such a move would have any detrimental effect on
barramundi productivity, particularly in the southern part of the Gulf, as long as the
overall length of the closure is not significantly reduced.
The potential changes to the management regime for Gulf barramundi to address the
issues that appear most suitable for consideration are:
setting the Gulf seasonal closure to start on 7 October of every year and end on
1 February of the next year
changing the minimum legal size of Gulf barramundi from 60 cm to 58 cm.
These do not represent preferred management arrangements, but are simply a
starting position for discussion of these issues through the review.
While the current harvest level of Gulf barramundi is considered sustainable, there is
a significant level of fishing pressure on these stocks. It may therefore be worth
considering, even if the risk of impacts on southern Gulf stocks from such changes is
regarded as small, those measures that could even further reduce that risk. Possible
mitigating measures that have been identified are:
Restrict fishing access (prior to 7 October) to a significant estuarine area in the
north western part of Cape York to protect aggregating barramundi and other
spawning species.
Decrease the maximum legal size of barramundi to protect a greater proportion
of female fish and increase egg production.
Amend commercial barramundi fishing net mesh sizes to select for a more
restricted size range of fish and thus increase the spawning biomass.
Introduce another mechanism to constrain the catch of barramundi below
historical levels.
Again it should be noted that these measures do not represent preferred options,
only options for consideration if any change to current management measures is
supported through the process of the review.
Technical report prepared for the Gulf finfish review. Not government policy. 3

Background
The Queensland Inshore Finfish Fishery is a multi-species commercial and
recreational fishery. According to recreational and commercial fisheries data
collected by Queensland Primary Industries and Fisheries (QPIF)—part of the
Department of Employment, Economic Development and innovation (DEEDI)—the
annual harvest of barramundi across the state is in excess of 1000 t. On a regional
scale, about 60% of this total catch generally comes from the Gulf of Carpentaria.
A stock assessment conducted by Welch et al. (2002) indicated that Gulf barramundi
stocks appeared to be healthy and that the fishery is sustainable. The latest stock
assessment of barramundi, however, indicated less conclusive findings on the status
of Gulf stocks due to concerns over the reliability of the commercial catch and effort
data used in the analysis (Campbell & O'Neill 2008).
Life cycle of barramundi
The barramundi is a large predatory species that can grow to 60 kg, but is usually
caught around 5–6 kg. The species is a protandric hermaphrodite, which means it
matures first as a male and becomes female later in life, usually after contributing to
at least one spawning season as a male. Gulf barramundi first reach sexual maturity
as males at about 55 cm, with 50% producing viable sperm at 60–65 cm (Davis
1982). Males then transition into females at 68–90 cm (mean 82 cm) (Davis 1982);
most of the larger specimens in the population are therefore female.
As the water warms and day length increases in spring, sexually mature barramundi
begin to aggregate in estuarine areas to spawn. The species is a broadcast
spawner, which means male and female fish release their eggs and sperm
simultaneously for fertilisation to occur within the water column. Individual fish will
spawn multiple times during a single spawning season, with each female releasing
tens of millions of eggs at each spawning event.
The eggs and larvae require brackish water to develop; however, the young fish are
very soon able to survive equally well in fresh or salt water. A significant proportion of
juvenile fish (generally within their first year of life) will tend to move upstream into
freshwater areas of rivers, creeks and billabongs if they are able to do so. Not all fish
move into freshwater however, and the general upstream migration pattern may vary
between areas and seasons. Those individuals that do move upstream tend to
remain in freshwater until they are mature males, usually returning to the estuary at
four (or less) years of age, although a small number can be up to seven years old.
Management measures
The management measures for barramundi are generally focused on ensuring that
the fish has an opportunity to spawn successfully. Ultimately, however, recruitment
for the species and fishery productivity is strongly influenced by freshwater flows, and
the ability of the juveniles to otherwise access freshwater areas. It appears that fish
that have accessed freshwater are bigger for their age than estuarine resident fish.
Therefore, access to freshwater habitats appears to promote a greater fishable
biomass than what could be achieved in the estuary alone (Julie Robins, pers.
comm.).
Technical report prepared for the Gulf finfish review. Not government policy. 4

Protection for spawning barramundi is achieved through the following:
a minimum legal size to protect the smaller individuals until a significant
proportion are able to spawn as males
a maximum legal size to protect larger females that may contribute
disproportionately to egg production
restrictions on minimum and maximum mesh sizes of commercial fishing nets to
selectively fish for these legal-size fish
a seasonal closure to fishing when barramundi are likely to be aggregating and
spawning.
While this general approach applies throughout Queensland, barramundi stocks from
the Gulf of Carpentaria and the east coast are managed separately under slightly
different regimes. These differences are summarised in Table 1.
Table 1
Differences in seasonal closure (SC) and other management measures for
barramundi in east coast and Gulf waters
Area Duration of SC Legal size Net mesh Inshore netting
sizes
East coast 1 Nov – 1 Feb 58–120 cm 150–215 mm Restricted in SC
Gulf Variable (see below) 60–120 cm 160–245 mm Prohibited in SC
The seasonal closure in the Gulf of Carpentaria prohibits fishing for barramundi as
well as the use of any commercial fishing nets under the N9 and N3 fishery symbols
throughout late spring and summer. Gulf barramundi are known to spawn from about
6 pm to 10 pm on new moon and full moon periods around the high tide. Spawning
occurs from about early October to early December in the northern Gulf and from
about mid-November to late January in the southern Gulf.
The spawning closure is set to cover as many of these peak spawning night-time
high tides as possible across this time period throughout the Gulf. The start and end
dates of the closure therefore vary from year to year. While the closure generally
covers about 118 days, it may start as early as 25 September and finish as late as 4
February.
Until 1996 the Gulf seasonal closure was the same as the east coast barramundi
seasonal closure, which runs for a period of three months from 1 November to 1
February. The change was made in order to better align the closure with the known
physical and biological drivers for barramundi spawning in the Gulf.
The minimum legal size of barramundi in the Gulf was changed from 58 cm to 60 cm
in 1999. The reasons for this change were to:
improve the overall yield from the fishery
move closer to the size at sex reversal
simplify the size to aid recall.
The recent review of the East Coast Inshore Finfish Fishery considered also moving
to a lunar cycle-based spawning closure and a minimum legal size of 60 cm in that
fishery. This was ultimately not supported for the following reasons:
There is an even greater geographical spread, and therefore greater variability,
in seasonal spawning peaks on the east coast than there is in the Gulf.
Rainfall is generally less predictable on the east coast, meaning that
recruitment is generally more variable. It was considered that there is little point
Technical report prepared for the Gulf finfish review. Not government policy. 5

in capturing all peak spawning moons if rain doesn’t subsequently fall at the
right time.
The current size limit and seasonal closure on the east coast is very well
established—changing either may have resulted in difficulties with compliance.
Discussion
Summary of analysis
In order to examine the impact of shortening the seasonal closure, a monthly size
and age structured model was developed. This analysis was undertaken in
conjunction with modelling the impact of amending the minimum legal size to 58 cm.
Details of this analysis can be found in the ‘Modelling of impacts’ section of this
paper; however, it should be noted that the analysis does not provide an absolute
estimate of yield or egg production; it simply provides a comparison between closure
and size limit scenarios.
Two alternative lengths of seasonal closure and two levels of minimum legal size
were examined through the model. The two spawning lengths were four months, to
reflect the current Gulf closure, and three months to reflect the impact of aligning the
closure with the east coast closure that runs from 1 November to 1 February. There
is no quantitative information, however, to assess the relative importance to fishery
productivity of the lunar-based seasonal closure compared to one that is fixed in
time.
Under the model overall yield tended to be higher (up to 6%) with a four-month
closure compared to a three-month closure, but there was no noteworthy overall
difference between the alternative minimum legal sizes.
Minimum legal length
The model indicated that decreasing the minimum legal length limit by 2 cm has a
positive effect on recreational yield but almost no impact on commercial yield. This
was because recreational fishers tend to harvest smaller fish than the commercial
sector, due to a low selectivity of 58–60 cm fish in commercial nets. However, as the
scale of fishing pressure from recreational fishing for barramundi in the Gulf is a lot
smaller than from the commercial fishery, the impact of changing the minimum length
on overall yield would be expected to be relatively small.
The model also indicated that a size reduction was unlikely to have a detrimental
impact on the Gulf barramundi stock in terms of abundance of mature males. Davis
(1982) showed that 50% of southern Gulf barramundi reach sexual maturity at or
around 60 cm. Such a 50% maturity level is often used to set the minimum legal size
of fish, so from that perspective the current size appears appropriate.
Barramundi growth rates tend to be very high, however, particularly as the water
begins to warm. As an example a 58 cm fish from the Fitzroy River in central
Queensland can be expected to grow at a rate of up to 15 mm per month in the
period from October to December (see Figure 7). At this rate a fish just below the
minimum legal size at the start of the seasonal closure could be expected to grow to
sexual maturity before the end of the closure.
Technical report prepared for the Gulf finfish review. Not government policy. 6

While the significance of this to male spawning rates cannot be determined, the
results of the model suggest that amending the size limit to 58 cm may be of
relatively low risk as long as there is reliable annual recruitment. Again, it is also
worth noting that after a recent review of fish size limits for the east coast fishery,
barramundi minimum legal size limit remained at 58 cm.
Length of the Gulf seasonal closure
The seasonal closure has an overall benefit to a wide range of fish species, not just
barramundi. This benefit includes constraining the overall level of effort exerted in the
commercial net fishery by a third compared to allowing net fishing throughout the
year. It also has a benefit in protecting other species during spawning, including
sharks that undergo pupping in the warmer period.
While it is not possible to quantify this benefit, it is likely to make a significant
contribution to the overall sustainability of Gulf fisheries resources. It is therefore
unlikely that any significant reduction in the length of the current closure would be
supported by any stakeholders in the fishery.
Results of the analysis show that there would be a decrease in barramundi yield from
aligning with the east coast’s three-month closure. Such results also make it difficult
to justify reducing the closure by any significant amount, as this would possibly have
both economic and biologically adverse impacts.
Timing of the Gulf seasonal closure
Apart from any interest in aligning the Gulf closure to the east coast closure, the
primary aim of any change to the variable closure would be to avoid overlapping with
the spring school holiday period. Moving to a fixed date may also have secondary
benefits in aiding planning within the commercial fishery, as well as in improving
overall compliance by moving to a simpler and more certain regime. Any change to
the seasonal closure therefore requires a balance between the social and economic
impacts of the type of closure (i.e. fixed or variable) with the need to ensure that the
biological requirements of the species are being adequately met.
If it is decided through the review that there are significant benefits to having fixed
opening and closing dates for the seasonal closure, then a decision will also need to
be made on what those dates should be. As there is no biological criteria that could
be used for setting those dates, any proposal should be determined initially on the
basis of social and economic impacts. These proposed dates should then be
checked against the biological requirements of the species to ascertain whether they
are likely to cause a significant impact on barramundi stock status. If there is believed
to be any significant risk then measures to mitigate such risks may also be
considered.
In regards to setting a date to end the seasonal closure, it would be logical to
consider aligning with the east coast closure, which ends on 1 February. This would
have the following potential benefits:
minimal impact on Gulf barramundi stocks, as the latest the season is currently
scheduled to reopen is 4 February (in 2017)
greater simplicity and consistency and thus possible benefits for compliance
Technical report prepared for the Gulf finfish review. Not government policy. 7

greater equity between east coast and Gulf commercial fishers. Currently the
closure in the Gulf ends just before the east coast in some years and just after it
in others. Aligning the closure end dates would put both groups of fishers on an
‘equal footing’ in relation to marketing their early season product.
In terms of proposing a set date to start the seasonal closure, as discussed
previously the primary issue for consideration is the timing of the two-week spring
school holiday. The benefit of certainty about the closure would apply regardless of
the actual date that is set. The only set date simpler than any other would be the start
of the month (i.e. 1 October), as this may be easier to recall.
The two-week spring school holiday starts on a Saturday and is generally set to
cover at least one week in September, and usually part of the first week in October.
The closure ends on a Sunday, which is normally the first Sunday of October. It
should be noted, however, that at the time of writing Education Queensland has only
confirmed holiday dates until 2010.
Both the timing of the seasonal closure and the timing of the holiday period therefore
vary. The scheduled starting dates of the seasonal closure and the most likely dates
of the school holidays are set out in Table 2. This table also shows the number of
day’s impact on both the school holidays and the seasonal closure under three
different scenarios:
1. Maintain the current variable closure.
2. Change the start date of the closure to 1 October.
3. Change the start date of the closure to 7 October.
Table 2
Number of days impacted for the spring school holidays (SH) and the Gulf
barramundi seasonal closure (SC) under three different scenarios
Days impact if SC started as indicated:
Leave SC as is Start SC 1 Oct Start SC 7 Oct
SC start School holidays SH SC SH SC SH SC
25 Sept 2009 19 Sept – 4 Oct 1 -9 0 -3 -6 0 -12
30 Sept 2010 18 Sept – 3 Oct 1 -3 0 -2 -1 0 -7
4 Oct 2011 17 Sept – 2 Oct 2 0 0 -1 +3 0 -3
7 Oct 2012 22 Sept – 7 Oct 2 0 0 -6 +6 0 0
26 Sept 2013 21 Sept – 6 Oct 2 -10 0 -5 -5 0 -11
1 Oct 2014 20 Sept – 5 Oct 2 -4 0 -4 0 0 -6
4 Oct 2015 19 Sept – 4 Oct 2 0 0 -3 +3 0 -3
8 Oct 2016 17 Sept – 2 Oct 2 0 0 -1 +7 0 +1
28 Sept 2017 16 Sept – 1 Oct 2 -3 0 0 -3 0 -9
The impact on the school holidays shown in Table 2 relates to actual days during the
holiday when barramundi fishing could not be undertaken. For this reason the figures
in the SH column of the table are negative if the closure scenario would prevent
fishing, or zero if it would not.
The impact on the spawning closure, however, relates to potential spawning days
lost or gained compared to the current variable closure, but only at the start of the
1 Approved
2 Likely but not yet approved
Technical report prepared for the Gulf finfish review. Not government policy. 8

seasonal closure. For this reason the impact is shown as positive if the closure starts
earlier under the scenario than it otherwise would have, or negative if it starts later
under the scenario than it otherwise would have.
It is worth noting that the impact on total potential spawning days would only be the
same as the values shown in the SC column of Table 2 if the end date of a changed
closure remained as it is under the current regime. Moving the end date to 1
February, however, would mean that in those years when there is a ‘loss’ of potential
spawning days at the start of the seasonal closure, there would also generally be a
‘gain’ at the end.
The current closure lasts 118 days. Moving the closure to start on 1 October and end
on 1 February would cover 123 days. While such an increase in the closure of five
days might theoretically affect the overall yield of the fishery, this was not tested due
to limitations of the monthly based model; however, any marginal changes would, in
reality, be undetectable anyway. Such a change would not completely remove the
impact on the school holidays anyway, as there would still be a maximum of six days
of holiday fishing lost in 2012.
Moving the closure to start on 7 October and end on 1 February would cover 116
days—a decrease in the length of the seasonal closure by two days. Again, this
scenario could not be tested under the model and any changes would be
undetectable. A decrease of two days would not be significant in terms of the
protection afforded to barramundi and other fish throughout the total length of the
closure.
The loss of early season potential barramundi spawning days represents the greatest
possible impact from moving to a 7 October closure starting date, with a maximum
loss of 11 days at the start of the 2013 seasonal closure compared to five under a 1
October starting date.
There is no quantitative information available to assess the relative importance to
fishery productivity of the lunar-based seasonal closure compared to one that is fixed
in time. Pre-spawning fish are known to aggregate leading up to the new and full
moon phases, therefore moving to a fixed date 11 days after the earliest starting date
for the current closure would, at most, lose one potential night-time spawning moon a
year. This would be unlikely to affect the productivity of southern Gulf barramundi,
because spawning does not occur in that area until mid-November.
The impact in the northern Gulf may be greater, as spawning is known to occur from
early October. This may make barramundi more vulnerable to fishing as they
aggregate leading up to the first night-time full or new moon tide in October. This
means that it might be worth considering mitigating measures that apply to northern
stocks of barramundi (and possibly other fish).
One obvious approach would be to have an earlier seasonal closure in the north of
the Gulf than in the south. Having two different closure periods, however, would be
contrary to the aim of simplifying the current regime, as it would mean there would be
three different seasonal closures in the state. It would also be likely to result in
significant compliance difficulties, particularly on the boundary between the two
closure areas.
An alternative approach would be to introduce an area closure to a significant
estuarine region in the north-western part of Cape York to protect aggregating
barramundi and other actively reproducing species. Such a closure could prohibit all
Technical report prepared for the Gulf finfish review. Not government policy. 9

arramundi fishing and/or all commercial netting for at least a period in late
September until the start of the seasonal closure. Of course the previous comments
regarding compliance difficulties would still need to be addressed.
Other possible mitigating measures for changing the Gulf seasonal closure and
amending the minimum legal length of barramundi to 58 cm are as follows:
1. Decrease the maximum legal size of barramundi from 120 cm to protect a greater
proportion of female barramundi and thus increase egg production. This would,
however, be inconsistent with the aim of amending the minimum legal size to
achieve uniform size limits throughout the state.
2. Amend commercial barramundi fishing net mesh sizes to select for a more
restricted size range of fish (e.g. maximum 215 mm and minimum 165 mm) in
order to increase spawning biomass. While these changes would not affect
recreational fishing, over 90% of Gulf barramundi are taken commercially. Unlike
introducing a lower maximum size, this approach would mean that any larger fish
taken unintentionally could be retained. It is also worth noting that the current
commercial fishing net minimum mesh size does not select for barramundi less
than 60 cm (see Figure 12).
3. Introduce another mechanism to constrain the catch of barramundi below
historical levels (e.g. total catch or effort controls in the commercial fishery and/or
a reduced bag limit in the recreational). This would not, however, be consistent
with the aim of simplicity and uniformity of bag limits between the east coast and
the Gulf.
These mitigating measures will of course have a greater number of positive and
negative aspects than have been addressed here. It is important to remember that
these measures are not being presented here as proposals, but simply as examples
of the type of mitigating measures that could be considered should a decision be
made to amend the seasonal closure and/or the minimum legal size of barramundi.
The requirement for such mitigation and which (if any) of these measures might be
employed would need to be more fully explored as part of the fishery review.
Technical report prepared for the Gulf finfish review. Not government policy. 10

Modelling of impacts
As discussed previously, this paper aims to test the impact of introducing a fixed
seasonal closure and a minimum legal barramundi size of 58 cm in the Gulf. The
alternative closure periods suggested in order to balance the social impact of the
closure with the biological sustainability of the stocks are:
1. a fixed spawning closure from 1 October to 1 February
2. a fixed closure from 7 October to 1 February.
As the most up-to-date barramundi model currently available in Queensland is a
monthly based model, it was unable to detect seven days difference of two
alternative spawning closures. Therefore, a fixed four-month closure (October to
January inclusive), which mimics the current variable closure applied in the Gulf, and
a fixed three-month closure, currently applied on Queensland’s east coast, were
tested to demonstrate the impact of changing the spawning closure. The impact of
changing the minimum legal size by 2 cm was assessed using the size and age
structured model.
Methods
Size and age structured model
In order to examine the impact of changing the minimum legal size and seasonal
spawning closure, a monthly size and age structured model currently being
developed for the Fitzroy River was modified for the Gulf stocks.
As there are no estimates of spawner–recruitment relationship available for the Gulf
barramundi, the model was designed as a yield and spawning per-recruit model with
constant recruitment. There was no relationship between the size of parental stock
and subsequent recruit numbers. It is important to note that this assumption may not
be valid, particularly under the condition of high fishing pressure, and is likely to
underestimate the true impact of fishing on spawning stock sizes.
Table 3 lists the equations used in the yield and spawning per-recruit model. The
symbol definitions are listed in Table 4 and the biological parameters and other
model settings are defined in the appendix.
The model assumes constant recruitment and constant fishing mortality throughout
simulations. The dynamic of the population followed Eq 1.1. Note that Eq (1.1) was
initially run for 32 years without fishing pressure (Z=M) to build stock size to its
equilibrium. The constant annual recruitment R was allocated into the recruitment age
of 12 months according to the within-fishing-year recruitment pattern ( m
). The
growth of fish older than the recruitment age of 12 months was determined by a sizetransition
matrix (P l,l’ ), which was calculated from a normal probability density
function for a fish in size class l’ growing into size l over one month period (Sadovy et
al. 2007).
The expected monthly growth of each size class was based on the extended von
Bertalanffy growth curve developed by Halliday and Robins (2007), which
incorporated the effect of freshwater flow. This assumes that Gulf barramundi have
similar growth pattern as barramundi in the Fitzroy River, central Queensland.
Variable seasonal growth is a feature of Gulf barramundi (Dunstan 1959; Davis &
Technical report prepared for the Gulf finfish review. Not government policy. 11

Kirkwood 1984; Davis 1987; Xiao 1999, 2000) and is probably a reflection of different
environmental conditions (Davis 1987).
Table 3 Equations used in the YPR model
Eq
(1.1)
(1.2)
(1.3)
Equations
N
lam , ,
RPla , , A
for
min m
a Amin
() t
P N ( t1) e for a
A ,..., A
12
L
max max
min
Zm
ll ,' l', a1 min1 max
A
E Sp N sex mat fec
y m l, a,
m l
m1
lL aA
min
min
min
l
l
f F p S D 1
p S D
C t N HW e
Lmax
Amax
m com com, l com, l com rec, l rec,
l Z
tot, m
()
la , l l
1
lL aA Zm
Lmax
Amax
m com com, l com,
l Zm
(1.4) Ccom, m
() t
Nl,
aHW l l 1e
(1.5)
lL aA m
min
min
f F
p S D
f F 1
p S D
m
C () t N HW 1 e
Z
Lmax
Amax
m com rec, l rec,
l Z
rec, m
l,
a l l
lLmin
aA Z
min
m
(1.6) 1
Z M f F p S D p S D
m m m com com, l com, l com rec, l rec,
l
m
Applying flow-related growth curves to the yield and spawner-per recruit model
allowed for the incorporation of environmental variability during assessment of the
management scenarios. Environmentally driven variability in monthly growth related
to flow was approximated using data from the Fitzroy River, as this relationship is not
quantified for the Gulf. Monthly expected growth rate varied as a consequence of
median monthly flow of the Fitzroy River between 1945 and 2005 (see Figure 6 of the
Appendix). The standard deviation of the growth increment was calculated as square
root of mean square error (MSE) of the growth model adjusted for a monthly growth
rate.
The number of barramundi eggs E y was calculated as the sum of the products of the
number of mature females and the fecundity index across the size and monthly
spawning pattern Eq (1.2). The predicted total yield (catch), commercial yield, and
recreational yield were given by the Baranov equations Eqs (1.3), (1.5) and (1.6),
respectively. Monthly fishing mortality was calculated by multiplying annual fishing
mortality F by monthly fishing pattern f m . This monthly fishing mortality was then split
into each fishing sector by p com . Note that monthly fishing pattern summed to 1 for the
three-month closure but not for the four-month closure (0.92). This is because the
overall fishing pressure is likely to be higher with a longer opening of the fishing
season (i.e. three-month closure). While a range of fishing mortality (F) was tested,
scenarios with the four-month closure corresponded to 8% lower F than scenarios
with a three-month closure.
The following assumptions were made to construct the model:
constant annual recruitment
constant natural mortality
constant monthly growth
constant fishing pattern for each scenario
constant fishing mortality F.
Technical report prepared for the Gulf finfish review. Not government policy. 12

Table 4 Definitions of symbols
Notation Definition
a Age class in months (A min = 12; A max = 384; 32 years)
l, l’ Size class in months (L min = 30 cm; L max = 182 cm; cm size classes)
N l,a (t)
The number of fish of age class a in size class l at time t
P l,a=Amin The fraction of fish in length class l for the first recruited age class (A min )
P l,l’
M m
F
Z m
m
fec l
mat l
The fraction of fish in size class l’ that grow into size class l in one month
Instantaneous monthly natural mortality
Instantaneous annual fishing mortality
Total fishing mortality in month m
Monthly proportion of annual recruits in month m
Fecundity at length l
Proportion of fish sexually mature at length l
sex l Sex change relationships (proportion of female at length l)
Sp m
Monthly spawning pattern in month m
W l Length–weight relationships (weight at length l)
f m
S com,l
S rec,l
D com,l
D rec,l
p com
C tot,m (t)
C com,m (t)
C rec,m (t)
H l
Monthly pattern of fishing effort
Selectivity at length l for the commercial sector
Selectivity at length l for the recreational sector
Discard mortality at length l for the commercial sector
Discard mortality at length l for the recreational sector
Proportion of monthly F contributed by the commercial sector
Total catch (yield) in month m at time t
Commercial catch (yield) in month m at time t
Recreational catch (yield) in month m at time t
Fish retainability at length l
Scenarios tested
Two seasonal closures and two levels of minimum legal size were examined through
the model (Table 5). Two types of spawning patterns (i.e. early spawning pattern and
normal spawning pattern) were included in scenarios as sensitivity analyses (Figure
11).
Technical report prepared for the Gulf finfish review. Not government policy. 13

Table 5 Scenarios tested in the model.
Scenario no. Closure period Minimum legal size Spawning pattern*
1 3 (Nov–Jan) 580 Early
2 3 580 Normal
3 3 600 Early
4 3 600 Normal
5 4 (Oct–Jan) 580 Early
6 4 580 Normal
7 4 600 Early
8 ** 4 600 Normal
* Figure 11 in appendix
** Current management arrangement: status quo
Results
Yields per recruit
The total yield curve is shown in Figure 1 and yields for each fishing sector are
shown in Figure 2 and Figure 3 respectively. Commercial yield tended to be
marginally higher (up to 8%) with a four-month closure (Scenarios 5 to 8) compared
to a three-month closure (Scenarios 1 to 4). Within each closure, commercial yield
was consistently slightly higher (up to 2%) with a minimum legal size of 60 cm
compared to a minimum legal size of 58 cm.
Recreational yield tended to be higher (up to 12%) with a minimum legal size of 58
cm (Scenarios 1, 2, 5, 6) than 60 cm (Scenarios 3, 4, 6, 7). Within each minimum
legal size, recreational yield was consistently higher with a four-month closure than a
three-month closure, but the difference was minimal (up to 1%).
Overall yield was similar to that of the commercial sector. A higher yield of up to 6%
was observed with a four-month closure compared to a three-month closure, but
there is no noteworthy difference among minimum legal size and spawning patterns
(

Relative proportion
1
0.8
Commercial yield
0.6
Scenario1 0.8
Scenario2
Scenario3
0.79
0.4
Scenario4 0.78
Scenario5
0.2
Scenario6
0.77
Scenario7 0.76
Scenario8
0
0 0.5 1 1.5 2
1.85 1.86 1.87 1.88 1.89 1.9
F
F
Figure 2 The relative commercial yield shown as proportion scaled to 1 at the maximum
yield (F=0.7985, Scenario 7)
Recreational yield
Zoom
1
1
0.82
0.81
Zoom
Relative proportion
0.8
0.6
0.4
0.2
Scenario1
Scenario2
Scenario3
Scenario4
Scenario5
Scenario6
Scenario7
Scenario8
0 0.5 1 1.5 2
F
Figure 3
0.95
0.9
0.85
1.6 1.7 1.8 1.9
F
The relative recreational yield shown as proportion scaled to 1 at the maximum
yield (F=1.8971, Scenario 5)
Spawning per recruit
Total number of eggs per recruit was higher with a four-month closure than a threemonth
closure and this difference became larger with greater fishing pressure (Figure
4).
Figure 5 shows the ratio of the spawning index with a three-month closure to the
index with a four-month closure. With the highest F applied in the model (F =1.8971 ~
85% harvest rate), the spawning stock would be about 30% lower with a three-month
closure than with a four-month closure. Under moderate to high fishing pressure (F =
0.4308 to 0.7985 ~ 35–55% harvest rate), the spawning index would be about 15%
lower with a three-month closure.
Reducing the minimum legal size to 58 cm resulted in up to 3% less abundance of
mature males and up to 3% less egg production (under the assumption of constant
recruitment though time).
Technical report prepared for the Gulf finfish review. Not government policy. 15

1
2
1
0.95
3
0.9
Scenario
4
5
0.85
0.8
6
7
8
0.75
0.7
0 0.5 1 1.5 2
F
Figure 4
The relative spawner-per-recruit for different levels of fishing pressure (proportion
scaled to 1 with Scenario 8 for each level of F)
1
0.9
Spawning Ratio (Sp 3mon
:Sp 4mon
)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Figure 5
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
F
Comparative spawning ratio defined as the spawning index with a three-month
closure divided by the spawning index with a four-month closure
Technical report prepared for the Gulf finfish review. Not government policy. 16

Discussion on analysis
The analysis showed that the fixed four-month closure from October to January
resulted in better total yield (i.e. catch) than the fixed three-month closure, particularly
when the fishing pressure was high. It is considered that an extra one-month closure
protects a proportion of barramundi from fishing activities and allows them to grow
quickly during the summer season, which results in an increase in overall yield.
Decreasing minimum legal size limit by 2 cm has a positive effect (i.e. up to 12%
increase) on recreational yield but almost no impact on commercial yield (up to 2%).
This was because recreational fishers tend to harvest smaller fish (i.e. high selectivity
around the minimum legal size) than commercial sectors, which have a low
selectivity of 58–60 cm fish in commercial nets. However, as the scale of fishing
pressure of the recreational fishing is a lot smaller than commercial fishery (1/10 of
commercial), the impact of changing the minimum legal size on overall yield was
relatively small. Consequently, changing the duration of the closure was the most
significant factor affecting overall yield.
The fixed three-month closure resulted in a decline in egg production of up to 30%.
However, the current analyses assumed constant recruitment (i.e. no spawner–
recruitment relationships). This means that even if the spawning index becomes
small (e.g. only 10% of virgin spawners remain), the same level of new recruits were
added into the population model each year. Therefore, the current model was unable
to detect the impacts of low spawning stock on the subsequent recruitment (i.e.
unable to detect recruitment overfishing). As a consequence, the current analyses
were likely to underestimate any declines in egg production (spawning fish).
In conclusion, reducing the minimum legal size from 60 cm to 58 cm was unlikely to
have a detrimental impact on the Gulf barramundi stock in terms of yield or
abundance of mature males under the model. However, reducing the duration of the
spawning closure by one month was likely to have negative impacts on the size of
spawning stocks as well as overall yield under high fishing pressure.
Technical report prepared for the Gulf finfish review. Not government policy. 17

References
Campbell AB, O'Neill MF (2008) 'Assessment of the barramundi fishery in
Queensland, 1989-2007.' Queensland Department of Primary Industries and
Fisheries, Brisbane.
Davis TLO (1982) Maturity and sexuality in Barramundi, Lates calcarifer (Bloch), in
the Northern Territory and south-eastern Gulf of Carpentaria. Marine and
Freshwater Research 33, 529-545.
Davis TLO (1985) Seasonal changes in gonad maturity, and abundance of larvae
and early juveniles of barramundi, Lates calcarifer (Bloch), in Van Diemen
Gulf and the Gulf of Carpentaria. Australian Journal of Marine and Freshwater
Research 36, 177-190.
De Lestang P, Griffin R, Allsop Q (2004) 'Assessment of the post-release survival
and stress physiology of barramundi (Lates calcarifer).' Department of
Business Industry and Resource Development. Fisheries Report 73, Darwin,
Northern Territory, Australia.
Dunstan DJ (1959) 'The barramundi Lates calcarifer (Bloch) in Queensland waters. .'
Division of Fisheries and Oceanography Technical Paper Number 5,
Commonwealth Scientific and Industrial Research Organisation, Melbourne.
Halliday IA, Robins JB (2007) 'Environmental flows for sub-tropical estuaries:
understanding the freshwater needs of estuaries for sustainable fisheries
production and assessing the impacts of water regulation.' Final Report to the
Fisheries Research and Development Corporation. Project No 2001/022.
213pp.
Henry GW, Lyle JM (2003) 'The National Recreational and Indigenous Fishing
Survey.' Cronulla, NSW: NSW Fisheries, Final Report Series No. 48 for
FRDC Project No. 99/158.
Hyland SJ (2007) 'Mesh selectivity for barramundi.' Department of Primary Industries
and Fisheries, Cairns.
Robins JB, Halliday IA, Staunton-Smith J, Mayer DG, Sellin MJ (2005) Freshwaterflow
requirements of estuarine fisheries in tropical Australia: a review of the
state of knowledge and application of a suggested approach. Marine and
Freshwater Research 56, 343-360.
Sadovy Y, Punt A, Cheung W, Vasconcellos M, Suharti S, Mapstone B (2007) 'Stock
assessment approach for the Napoleon fish, Cheilinus undulatus, in
Indonesia. A tool for quotasetting for data-poor fisheries under CITES
Appendix II Non-Detriment Finding requirements.' FAO Fisheries Circular
1023, Rome.
Welch D, Gribble N, Garrett R (2002) 'Assessment of the Barramundi Fishery in
Queensland 2002.' Queensland Department of Primary Industries and
Fisheries Cairns.
Technical report prepared for the Gulf finfish review. Not government policy. 18

Appendix
Model settings
Table A1 Barramundi biological parameters
Biology Functions Parameter estimates References/data sources Figures
Growth KS( t) S( t)
gl
() t ( L
l)1
e
L
1746.7
ka kb(log
flowkcut) if log flowkcut
K
C 1.06
ka
if log flow
kcut
ts 17.91
CK sin 2 ( t ts)
St ()
ka
0.07
2
kb
0.01
gl
( t) expected growth increment for length class l
kcut
6.92
Number of days in month (t)
(Robins et al. 2005)* Figure 7
Maturity
log pl
0
1l
0
13.19
1 pl
1
0.02
pl
proportion of matured fish in size class l
(Davis 1982, p. 537) Figure 9
Sex change
1
sexl
l
l
50
1expln(19)
l50
925
l95 l50
(Davis 1982, p. 540) Figure 10
l95
1030
sexl
proportion of female fish in size class l
l50, l95
the lengths at which50 and 95% of the fish became females
Spawning pattern
(Davis 1985)
―
―
(Dunstan 1959)
Halliday (pers. comm.
Figure 11
2009)
* Parameters were re-estimated by Mai Tanimoto in 2009.
Technical report prepared for the Gulf finfish review. Not government policy. 19

Table A2 Barramundi model input parameters and settings
Biology Functions/settings References/data sources Figures
Commercial selectivity (S com ) l
k 2
1m
Scom
exp
2
2km
2
(Hyland 2007) Figure 12
k 5.203, k 0.619, m177.8 (mesh size for 7'' net)
Recreational selectivity (S rec )
Retainability (H)
1 2
Linear increase between 440 mm and 580 mm (0% at 440 mm and 100% at
580 mm)
Full selectivity (i.e. 1) between 580 mm and 660 mm
Linear decline between 660 mm and 1200 mm
100% retainability between legal size limits
Halliday & Robins (pers.
comm., 2009)
Figure 12
― Figure 13
Monthly fishing pattern (f m )
Three-month closure: Median historical monthly fishing pattern observed in Gulf
region before 1997
Four-month closure: Same as the monthly fishing pattern for the three-month closure
― Figure 14
except Oct and Nov (apply 0% for both months)
Natural mortality (M) 0.025 month -1 (0.3 year -1 ) ― ―
Instantaneous fishing mortality (F) [ 0.0513 0.1625 0.2877 0.4308 0.5978 0.7985 1.0498 1.3863 1.8971] ― ―
Commercial discard mortality (D com )
Recreational discard mortality (D rec )
30% mortality for fish smaller than minimum legal size (58 cm or 60 cm)
10% mortality for fish bigger than maximum legal size (120 cm)
10% mortality outside legal size
O’Neill (pers. comm.,
2009)
Halliday (pers. comm.,
2009)
(De Lestang et al. 2004)
―
Fishing ratio (P com )
Commercial : Recreational = 10 : 1
Halliday and Robins (pers.
comm., 2009)
(Henry & Lyle 2003)
―
Technical report prepared for the Gulf finfish review. Not government policy. 20

12
10
log(ML/month)
8
6
4
2
0
2 4 6 8 10 12
Month
Figure 6 Median (log scaled) monthly flow between 1945 and 2005 applied to estimate average monthly
growth transition matrix.
Growth Increment (mm/month)
35
30
25
20
15
10
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
5
0
200 400 600 800 1000 1200 1400 1600 1800 2000
Size (mm)
Figure 7 Monthly growth increment (mm/month) estimated for each month based on the median monthly
flow between 1945 and 2005 (fiscal year).
Technical report prepared for the Gulf finfish review.
Not government policy.

Proportion of mature fish
1
0.8
0.6
0.4
0.2
data
best estimate
95% CI
0
200 300 400 500 600 700 800 900
Length (mm)
Figure 8 Maturity curve fitted to Davis’s Gulf of Carpentaria data (1982).
1
0.9
0.8
GoC data only
Pooled data (Goc + NT)
Proportion of mature fish
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 200 400 600 800 1000 1200 1400 1600 1800
Length (mm)
Figure 9 The final maturity curve used in the model (Red).
1
0.9
0.8
Proportion of female
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 200 400 600 800 1000 1200 1400 1600 1800
Size (mm)
Figure 10 The sex transition curve used in the model.
Technical report prepared for the Gulf finfish review.
Not government policy.

0.5
0.4
Average spawning pattern
Early spawning pattern
Proportion
0.3
0.2
0.1
0
1 2 3 4 5 6 7 8 9 10 11 12
Month
Figure 11 Two spawning patterns for the Gulf barramundi.
1
0.8
Commercial (net)
Recreational (hook)
Proportion
0.6
0.4
0.2
0
200 400 600 800 1000 1200 1400 1600 1800
Size (mm)
Figure 12 Selectivity curves for each fishing sector.
Technical report prepared for the Gulf finfish review.
Not government policy.

1
0.8
MLS=580 mm
MLS=600mm
Retainability
0.6
0.4
0.2
0
200 400 600 800 1000 1200 1400 1600 1800
Size (mm)
Figure 13 Fish retainability (H) for two minimum legal size limits.
Proportion of annual F
0.2
0.15
0.1
0.05
3-month closure
4-month closure
0
1 2 3 4 5 6 7 8 9 10 11 12
Month
Figure 14 Allocation of annual fishing mortality (F) into each month under two fishing closures.
Technical report prepared for the Gulf finfish review.
Not government policy.