BRISBANE RIVER KING THREADFIN ASSESSMENT 2016

An assessment was made of available data for King Threadfin in the Brisbane River over the past 10 years from 2007-2016. This will be available from the Crystal Bowl website www.crystal-bowl.com.au.

Summary

  • There were only 4 years when there were more than 20 Threadfin tagged in the Brisbane River, 2010 and 2014-2016
  • The highest number of fish caught, the most effort and the highest catch rate was in 2015
  • Most fish were legal with very few undersized fish caught except in 2015
  • In 2014 trophy sized fish over 1m made up 53% of the catch and the average length of the fish was 935mm
  • The size range of fish from 2014-2016 suggests that these fish were likely to have recruited from 2011-2013
  • The lack of undersized fish in 2016 with almost all fish being over legal size suggests poor recruitment in 2014-2015 and that is consistent with the low river flows over that time
  • Catch rate comparisons between the Brisbane and Fitzroy Rivers show a higher catch rate IN 2015 however the overall effort was less in the Brisbane River
  • Annual growth was 115mm/year in the first year after recapture
  • Movement was mostly within the river with some movement of fish into northern Moreton Bay where they are caught commercially
  • There was no commercial catch from 2007-2010 and then it rose sharply to 22.1t in 2014 with both catch and catch rate falling since then

Observations

  • Both the recreational and commercial fishery are targeting legal sized fish
  • Movement into the bay is likely to be associated with spawning
  • An unknown number of tagged fish were caught commercially in Moreton Bay but were not reported
  • The fishery is likely to decline significantly in 2017 as there appears to be little recruitment from 2014-2016 to replace fishing and natural mortality

Environmental conditions and Outlook

  • Major flooding in early 2011 and moderate to good flows in 2012 and 2013 are likely to have aided recruitment
  • BOM POAMA34 forecasts for 2017 are for neutral conditions over the recruitment period (Dec-Mar) tending towards El Niño again later in the year
  • The BOM rainfall outlook is for a 35-40% chance of exceeding median rainfall from Jan-Mar in south-east Queensland suggesting moderate recruitment however this will depend on actual river flows

Infofish Australia
January 2017

MEASURE

ACTUAL

GRAPH

BRISBANE RIVER KING THREADFIN
Catch and effort
number of fish
fisher days

Highest  Lowest
CATCH
2015    2007
399    0

EFFORT
2015    2013
288    6

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Catch rate
Fish/fisher/day

Highest  Lowest
ALL
2015    2010
1.39    0.32

LEGAL
2015    2013
1.22    0.28

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Average length mm
and size range

Highest   Lowest
2014   2015
935mm   800mm

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Percentage sizes
undersize <600mm
legal-trophy 600-999mm
trophy 1000mm+

Highest   Lowest
LEGAL-TROPHY
2016   2014
77%   53%

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Comparative catch rates
Brisbane River
Fitzroy River
Fish/fisher/day

Highest   Lowest
BRISBANE
2015   2007
1.39   0

Highest   Lowest
FITZROY
2016/2013   2010
1.12   0.04

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Threadfin growth
MM/year

AVERAGE GROWTH FIRST YEAR
115mm
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Threadfin movement kms

AVERAGE DISTANCE MOVED 9.5KM
RECAPTURES OUTSIDE RIVER
9 (all in northern Moreton Bay)
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River flows and Threadfin lengths (MM)

FLOODS
2011
2013
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Commercial catch
Catch tonnes
Catch rate kg/day

Highest   Lowest
CATCH
2016   2007-2010
22.1t   0

Highest   Lowest
CATCH RATE
2014   2007-2010
117.6kg   0

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NINO34 forecast for 2017 Neutral conditions for recruitment period Dec-Mar and towards El Niño later in the year
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Rainfall outlook
Jan-Mar 2017
BOM forecast 35-40% chance of exceeding median rainfall
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DATA SOURCES

Data used in the Brisbane River King Threadfin Crystal Bowl are:

BASIS FOR THE ANALYSIS

The analyses of the data were based on the following:

  • Catch and effort data for taggers were the total catch and number of days fished in the Brisbane River estuary for all species in Suntag map MB05 and MB02 grids O10, O12, P11, P12, Q11, R11 and R12
  • Not all of those trips targeted Threadfin however as there were no trips recorded that targeted Threadfin that were zero trips this provides an approximation for zero trips
  • Catch rates for taggers was calculated as fish/fisher/day
  • Catch rates for taggers were only calculated for years where 20 or more fish were recorded