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Department
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Effects
of Netfishing in
Tropical Australian Estuaries
Investigators:
Janet A. Ley1, Ian Halliday2, Rod Garrett2, Neil Gribble2,
Andrew Tobin2 Paul Dixon1
Affiliations:
1. Australian Institute of Marine Science
2. Queensland Department of Primary Industries
May 2000
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Funded
by: Fisheries Research and Development
Corporation |
Introduction
Policy decisions about the ecological sustainability of inshore
commercial netfishing are currently being made without access to quantitative
data on the impacts of various fishing practices both commercial, recreational
and indigenous. To address this lack of information, a case study using fishery
independent techniques is being conducted to compare relative abundances of
fishes among estuaries closed and open to commercial netfishing. The effort has
received the cooperation of both the commercial and recreational fishing
sectors. This is a report on progress to date.
 Objective:
To assess fish biodiversity in tropical estuaries subject to commercial
gill-net fishing
Hypothesis:
Removal of top predators in fished systems leads to differences in fish
assemblages in open versus closed systems
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Aerial Photo of Yellow Gin Creek
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DESIGN
Pairs of mangrove-dominated estuaries in 3 tropical subregions:
Northern pair: Steep rainforest catchment; annual rainfall:
4,000 mm
Central pair: Mixed rainforest, eucalypt; annual rainfall:
1,500 mm
Southern pair: Dry eucalypt forest; annual rainfall: 900 mm
Each pair: 1 closed & 1 open to commercial netfishing
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Gill-net in mangrove creek
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METHODS
Fish surveys using research gillnets (12 nets, 15-150 mm mesh)
Bimonthly for 2 years: Year 1 complete
Nets deployed from 3 hours before sunset to 3 hours after sunset
Fish removed, measured, recorded and returned to the water alive whenever
possible
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Barramundi |
RESULTS
Year
1:
As expected, consistently greater numbers of barramundi
(Centropomidae: Lates calcarifer) have been netted by the research team
in each of the estuaries closed to commercial fishing, indicating that the
standing crop of the target species was reduced by fishing in the open systems.
Barramundi size class distributions have consistently remained
similar between open and closed systems, suggesting that productivity of
barramundi may be affected little by netfishing.
Patterns in the spatial and temporal distributions of other
fish species have also varied independently of fishing.
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Gill-netting at sunset in Nobbies |
PRELIMINARY CONCLUSION
Current netfishing practices may have little effect on overall
fish biodiversity.
FUTURE RESEARCH
Continuing surveys and analyses to further define:
Functional biotic differences between fished
vs. unfished systems.
Influence of seasonal dynamics and fish reproductive cycles.
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For
further
information
Contact: Janet
Ley, AIMS
Phone: (07) 47534343
International: +617 47534343
E-mail:
j.ley@aims.gov.au
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