Northern Prawn Fishery
Research 1997-1998
This page list 1997-1998 research regarding the Northern Prawn Fishery.
Links to further research and surveys can be accessed from the research archive list in the top right of your screen.
CSIRO, FRDC ($55 014)
1996-1997
Summary
Tagging did not affect either the growth or survival of Penaeus indicus
under laboratory conditions. The results of this study show that a large scale
field tag/release experiment in Joseph Bonaparte Gulf should be feasible.
There was no difference in growth or mortality between the
tagged and control prawns. Growth of all prawns was higher in the large 5000
l tanks than in the much smaller 50 l tanks. The mortality of both tagged and
control prawns was higher in small tanks than large tanks. Female P. indicus
grow faster than males.
A prototype release cage was developed and tested under calm
conditions in shallow water, the release mechanism activated successfully and
the prawns left the cage. However, under extreme conditions in Joseph Bonaparte
Gulf, the cage was less successful. The release cage reached depths greater
than 60 m, however, upon retrieval, some prawns remained in the release cage
to within 3 m of the surface.
Tagging in Joseph Bonaparte Gulf would be very dependent
on weather. It would be impossible to tag and release prawns under the conditions
that the cage was tested. An improved release cage design is currently being
developed.
Log book data were used to examine effort patterns in the
Joseph Bonaparte Gulf during different seasonal closures. In the early years
of the fishery (1981-84), effort was concentrated at the end of the year (September
to December), in 1985-86 effort was more in the middle of the year (June to
September), whereas in recent years (1988-1995) effort peaks in May and June.
Length frequency data and data on the maturity of red-legged
banana prawns collected by Rik Buckworth of NT Fisheries were analysed to estimate
parameters of growth and reproduction, and estimate the seasonal pattern of
recruitment to the fishery. The estimates of growth rate were unreliable and
as a consequence best estimates were obtained from the literature for P. merguiensis.
The seasonal pattern of recruitment from the length frequency data suggests
that most prawns recruit to the fishery between February and April. However,
no length frequency data are available for the months between December and February.
The yield, value of the catch and egg production were estimated
for different seasonal patterns of effort using a per-recruit model (SIMSYS).
The results from this model show that both yield and value can vary by as much
as 15% depending on the pattern of effort. However, the results of the model
were sensitive to the estimates of growth and mortality, which highlights the
need to obtain better estimates of these parameters.
QDPI, AMC, Qld and NPF Industry ($473 688)
1996-1999
Edited draft of non-technical summary
The current project aimed to inform, develop and encourage
the use of turtle excluder devices (TEDs) and bycatch reduction devices (BRDs)
by working collaboratively with the prawn trawling industry of northern Australia.
The project also examined the possibility of modifying the headline height of
trawl nets to reduce bycatch. We used several strategies to disseminate the
relevant information about TEDs and BRDs. Methods included:
- Informal, hands-on workshops at ports throughout
northern Australia, these demonstrated the various gears available
- Attending industry meetings and informally visiting
the wharfs to discuss gear with fishers
- Distribution of dedicated bycatch newsletters
and videos summarising TED and BRD issues
- Loans to skippers of TEDs and BRDs custom-built
to suit individual needs
- At-sea assistance with testing of TEDs and BRDs
- An incentive award, the Prawn Trawling Innovation
and Adoption Award to recognise the contribution of individuals within the
northern Australian trawl industry to the development and adoption of TEDs
and BRDs.
Tangible outcomes included face-to-face contact by project
staff with about 30% of the prawn trawl operators in the Queensland East Coast
Trawl Fishery and about 60% in the Northern Prawn Fishery. TED and BRD workshops
were attended by over 400 fishers, netmakers, conservationists and other industry
personnel. Sixty-eight TEDs and 13 BRDs were lent to commercial fishers. Supervised
field tests of TEDs and BRDs occurred on 37 vessels. Research staff spent over
418 days in the field, and recorded performance data on over 828 tows during
which a TED or BRD was fitted to a trawl net.
TEDs were very effective at excluding sea turtles and other
large animals. In total, 14 turtles were caught in standard nets, while two
turtles were caught in TED-equipped nets (i.e. the net was winched in with the
turtle positioned at the base of the grid). Generalisations about the effects
of TEDs on prawn catches were difficult to make, because of variable results.
A reduction in prawn catch of between 4% and 10% occurred during many of the
supervised at-sea testing of TEDs. However, prawn catch rates were maintained
or increased (average 7%) during several supervised TED tests. On some vessels,
prawn loss in the TED equipped net was excessive (e.g. 50%), but could be attributed
to a particular cause such as shallow grid angle. On other vessels, excessive
prawn loss occurred (e.g. 29%) no obvious cause could be found.
BRDs had a varied effect on unwanted fish bycatch. Exclusion
rates depended on the design of the BRD, the composition and quantity of bycatch,
and whether trawling was undertaken during the day or night. In most cases,
bycatch reduction averaged
about 20% during night trawling and about 40% during day
trawling. The data collected suggested that BRDs had little impact on prawn
catches.
The recipients of the Prawn Trawling Innovation and Adoption
were John Olsen in 1997 and Garry Anderson in 1998. Both recipients actively
promoted TED and BRD use amongst their fellow fishers and were ambassadors for
progress industry had made in reducing unwanted bycatch.
Results from the multi-level beam trawl work suggested that
about 96% of most commercial prawn species and 90% of the bycatch entered the
trawl within 600 mm of the seabed. This suggests that the majority of the unwanted
bycatch lives close to the seabed like prawns. As such, the potential for reducing
bycatch simply by reducing the headline height of the trawl seems to be poor.
Many fish species demonstrated strong upward escape responses to the approach
of the trawl and the strategic placement of BRDs in the top panel of the trawl
may be required to exclude these species successfully.
Less tangible outcomes of the project were the exchange of
knowledge and information between project staff and individuals within the trawl
industry. Information distributed by the project provided an important starting
point for the manufacture and use of TEDs and BRDs by fishers and netmakers
of northern Australia. First-hand experience using TEDs and BRDs led many individuals
to begin developing their own designs. This was assisted by providing fishers
with information that would allow then to understand the underlying principles
of fish exclusion
Less than 2% of the Queensland East Coast Trawl fleet used
BRDs, and only two vessels (out of 920) regularly used TEDs when the project
began in 1996. A similar situation prevailed in the Northern Prawn Fishery.
No NPF vessels were known to regularly use TEDs in the NPF in 1996, but seven
vessels were known to have tested a TED previously. TEDs and BRDs were not commercially
available and most of the devices in use were made by the skipper or owner of
the vessel. A wide variety of TED and BRD designs are now commercially available
from at least 20 commercial suppliers in ports throughout northern Australia.
From the beginning 2000, TEDs and BRDs were compulsory in all NPF trawlers.
While the project targeted otter trawl operations, the concepts and designs
for fish exclusion from trawl nets have been utilised by several operators in
beam trawl fleet of the Queensland east coast. This is an example of the change
in industry attitudes towards bycatch reduction amongst many trawl fishers.
This project clearly demonstrated that a focused extension
program can effectively raise the awareness of the fishing industry to sensitive
issues, such as sea turtle bycatch, and encourage the use of "environmentally
friendly" fishing practices. It also clearly demonstrated that the provision
of research and extension information does not necessarily cause or induce all
industry operators to change their practices.
CSIRO, QDPI, AMC, FRDC ($1 477 390)
1996-1999
Edited draft of summary of final report
This project covered bycatch issues in the NPF, the Torres
Strait Prawn Fishery and the Queensland Banana Prawn Fishery. This edited
summary deals only with the NPF.
1) Description of the bycatch of the NPF
Areas of high trawl effort were sampled by scientific surveys
and by an observer on commercial boats to describe the bycatch. The bycatch
was very diverse; 390 species of fish, 47 species of elasmobranchs (sharks,
rays and sawfishes) and 234 invertebrate taxa were recorded. Fish species made
up about 73% of the bycatch weight. Because most fish die after capture, most
bycatch does not survive trawling. Three families, Bathysauridae (lizard fish
or grinners), Leiognathidae (pony fishes) and Nemiteridae (monacled bream),
made up 41% of the weight. Most of the fish species were rare. The bycatch differed
across the areas of the fisheries and with time of year. NPF fishing areas can
be divided into two on the basis of the bycatch composition. These two regions
were dominated by different species of prawn and both should be included in
any bycatch monitoring program
2) To assess the impact of trawling on the sustainability of vertebrate
bycatch species
Stock assessments for bycatch species are not feasible because
bycatch is very diverse and little is known about the biology of most species.
Hence, we developed an approach to examine the likely impact of trawling on
vertebrate bycatch species and applied this to the NPF. Two overriding characteristics
determine the sustainability of bycatch species: the susceptibility of a species
to capture and mortality in a prawn trawl (susceptibility) and the capacity
of a species to recover once depleted (recovery). A number of biological criteria
were assessed for each characteristic. Species were ranked on each characteristic
and the ranking reflects their ability to resist fishing pressure and therefore
their priority for management, monitoring and research. The fishes, elasmobranchs
(sharks, rays and sawfishes) and sea snakes were dealt with separately due to
taxonomic and biological differences.
Since the 1980's, 411 fish species have been recorded in NPF
bycatch and on average the fishery removes about 6% of their total biomass annually.
Thirteen species, from four families ranked as high priority for management,
monitoring and research; these are the least likely to be sustainable. They
are highly susceptibility to trawls because they are benthic or demersal, their
main habitat is soft sediments and their diet may include prawns. Their recovery
capacity is low. In applying this process we have highlighted important gaps
in current knowledge of bycatch species but the ranking must be used with caution.
Future research should be aimed at developing a greater understanding of the
biology of species and their distribution in the region of the fishery.
The biology of elasmobranchs makes them more susceptible to
overfishing than bony fishes because they are long lived, slow growing, reach
maturity at a later age and have few young. Fifty-six species of elasmobranchs
have been recorded in the bycatch of the NPF and the average estimated removal
by the fishery is 14% of the total biomass. Most are dead when landed on deck
(56%) and survival is lower for smaller individuals. Twenty-seven species are
the least sustainable, including stingrays (Dasyatidae), sawfishes (Pristidae),
angel sharks (Squatinidae), zebra sharks (Stegastomatidae), shovelnose rays
(Rhinobatidae) and nurse sharks (Ginglymostomatidae). They are all bottom dwellers
which increases their susceptibility to capture. Research focusing on these
high priority species is vital to ensure their long term sustainability. We
need to know more about the basic biology, distribution, movement patterns and
stock structure of these species. The introduction of compulsory Turtle Excluder
Devices (TEDs) and BRDs in 2000 will result in the exclusion only of large elasmobranchs.
Most elasmobranchs caught by trawlers are small and would fit through the bars
of the TED and be caught.
The biology of sea snakes also makes them more susceptible
to overfishing than bony fishes. Thirteen species of seasnake are caught in
the NPF. About 49% of seasnakes caught in trawls die. Most snakes caught are
mature. Our estimates of sea snake catch and biomass of each species indicate
that fishing mortality could be 5-6% per year, which appears sustainable for
all but 2 species, Hydrophis pacificus (Large headed sea snake) and Disteira
kingii (spectacled sea snake). In the Gulf of Carpentaria, these
two species should receive high priority for further study on the effects of
trawling. TEDs and BRDs appear effective at reducing sea snake catch.
3) To assess the effects of prawn trawling on the biodiversity of vertebrate
bycatch communities
The vertebrate bycatch community was compared between areas
open to trawling and areas that have been protected for 15 years in the western
Gulf of Carpentaria. If trawling had a large impact on biodiversity we would
expect to see fewer species, lower catch rates and smaller individuals in the
open areas. This was not the case; there was no consistent difference in the
number of species between open and closed areas or in catch rates between open
and closed areas. In general, the mean size of species was greater in the open
areas. Although the results were equivocal with respect to the impact of trawling
on biodiversity, this does not imply that trawling has no impact. Any differences
between open and closed areas may be reduced by the low commercial effort in
the open area, aggregated trawling, possible illegal trawling in the closure,
and the mobility of species. This combined with high natural variation may obscure
any impacts of trawling.
4) To develop cost-effective, accurate and feasible methods of describing
and monitoring bycatch.
We carried out studies of sampling and monitoring methods
to assist management. Most species are rare, and a sample of 10% of the catch
of a single trawl contains about half of the species in the catch and has an
80% sampling error for the rare species. This sample size is the minimum recommended
for monitoring. The results suggest that it is not feasible to monitor at a
level that will detect a 50% change in catch rate for all taxa, including rare
species. However, it may be possible to monitor more common species. The high
level of variation in bycatch is contributed to by factors such as moon phase
and should be taken into account when developing monitoring programs.
We compared three methods for monitoring NPF bycatch: crew-member
observers; trained observer collections; and scientific surveys. The fishery-dependent
strategies are the least costly and the best for monitoring rare species. However,
crew-member observers cannot collect data on all bycatch without affecting fishing
operations. In addition there is a real problem of identification of species,
this is difficult given the large number of species in the bycatch. Scientific
surveys are most costly, but provide reliable, accurate and immediately available
data. They are also the only method of collecting data on bycatch in unfished
areas. The design of a monitoring program will depend on the specific objectives.
However, any monitoring program should aim to collect information on a suite
of bycatch species and be able to detect changes in populations that may be
at unsustainable levels. Other features of a monitoring program are also defined
in the report. A monitoring program will be critical to assess whether the bycatch
is sustainable or not.
Conclusions
The high diversity of the bycatch of these tropical prawn
fisheries and the fact that most species are rare means that managing the sustainability
of the bycatch is a significant challenge. There are clearly some species that
are more susceptible to trawling and are unlikely to recover if they are depleted;
these species are the least likely to be sustainable. Future research and management
should concentrate on these species. The development of a monitoring program
for bycatch is not straightforward; the available methods differ in aspects
such as data accuracy, reliability and cost. This project provides guidelines
that can be used in the development of a monitoring program.
CSIRO, FRDC ($850 541)
1996- 2000
Summary of ongoing work
1. Estimates of natural and fishing mortality and growth rates for the
red-legged banana prawn in the Joseph Bonaparte Gulf.
About 1016 tonnes of red-legged banana prawns were caught
in Joseph Bonaparte Gulf 1997, while about 261 tonnes were caught in 1998. The
population estimates from our tag-release-recapture experiments reflect the
fishery catch. We estimate the population was 65 million prawns in 1997 and
20 million in 1998. During 1997, about one tagged prawn was taken per tonne
of catch, while during 1998, two to three tagged prawns were taken per tonne
of catch.
Using the data from 1997, we estimated the natural mortality
of red-legged banana prawns to be about 5% per week (M = 0.11), similar to the
common banana prawn, P. merguiensis, in the Gulf of Carpentaria.
Analysis of our model showed that some aspects of the tag-release
experiment including the size of the prawn (in 1997 only), sex of the prawn
(not a strong effect), time of release and date of release affected prawn survival.
The tag type, tagger, reward paid and location of release had no effect on prawn
survival.
An analysis of growth parameters has been undertaken. Fits
to the tagging data show that there were significant differences in growth between
sexes, but not between years. Our model showed that P. indicus grew at a greater
rate at 50 days of age (males - 1.42 mm wk-1; females - 1.53 mm wk-1)
than later at 100 days (males - 0.82 mm wk-1; females - 1.09 mm wk-1)
and 150 days of age (males - 0.48 mm wk-1; females - 0.77 mm wk-1).
2. The distribution of juvenile red-legged banana prawns in coastal Joseph
Bonaparte Gulf and the most important habitats used.
Juvenile red-legged banana prawns (P. indicus) are
found predominantly in the eastern JBG (>90% of all banana prawns in the
Fitzmaurice, Victoria and Keep Rivers) and Cambridge Gulf (>73% of all banana
prawns). They are found in high abundances (>1000 m-2) in some
small creeks and gutters at low tide. The western JBG (Berkeley and King George
Rivers and many small creeks) have similar high abundances of banana prawns,
but they are predominantly P. merguiensis (>90%). This shows that
the distribution of the two species is quite separate and distinct.
Juvenile P. indicus are found associated with mangrove-lined
mudbanks in estuaries and rivers, similar habitats to those identified for P.
merguiensis in the Gulf of Carpentaria. Repeated trawls among different
riverine habitats in close proximity showed that P. indicus are most
abundant in small side-creeks and gutters, compared to large creeks and rivers.
They are found in only very low numbers around mid-river mudbanks and channels.
3. Mapping of the juvenile nursery habitats on the ArcVIEW and ArcINFO
GIS database
AUSLIG electronic map data were incorporated into a JBG GIS.
Three potential juvenile red-legged banana prawn habitat types - mangroves,
salt-flats and land-subject-to-inundation - were identified from these data
and mapped. The area of each habitat and the linear extent of the habitat/water
interface of likely habitats of P. indicus have been calculated. The
abundances of juvenile P. indicus have been incorporated into the GIS, and relationships
between prawn abundance and habitat have been investigated. No distinct relationship
was found between prawn abundance and any aspect of habitat.
4. Other aspects of GIS that have been investigated
We used three methods (topographical data, aerial photography
and Landsat Thematic Mapper satellite imagery) to estimate the area and linear
extent of juvenile banana prawn habitats (mangroves and salt-flats) in the Berkeley
River and the Lyne River in Joseph Bonaparte Gulf. The Australian Land Information
Group (AUSLIG) topographical dataset is digitised from topographical maps at
a scale of 1:250,000, and includes such features as mangroves, salt-lats, land
subject to inundation, and rivers. Black and white aerial photographs were obtained
from the WA Department of Conservation and Land Management (CALM). These were
classified to provide a coverage of mangroves and salt-flats. Landsat TM satellite
imagery was purchased, and used to classify the mangroves and salt-flats.
For each method, the area and linear extent of each type
of habitat was calculated and compared among methods to gauge the best method
to use to estimate habitat. The results show that, at this scale, the aerial
photographs provide the most accurate estimates of both area and linear extent
of habitats, as verified by ground-truthing, while the topographical data was
the least accurate. Landsat TM imagery gives good estimates of the area of habitats,
but underestimates the linear extents. The differences in the estimates are
attributed to differences in the resolution of each of the methods. The mangroves
of the Joseph Bonaparte Gulf region typically form narrow fringes that can be
detected only at a high resolution. While aerial photographs give the most accurate
results for individual river systems, Landsat TM imagery and topographical data
can be useful tools in broader-scale studies.
5. Other findings
Freshwater discharge from the Ord River Irrigation Scheme
affects the distribution of P. indicus and P. merguiensis in the
Ord River. The all-year-round discharge from the irrigation scheme lowers the
salinity in the Ord River. At low tide, the salinity is 0-5 ppt just upstream
from its confluence with Cambridge Gulf. The abundance of juvenile banana prawns
at sites in the Ord River was low to non-existent, and much lower than in comparable
rivers and sites flowing into Cambridge Gulf that we sampled at the same time.
We stopped travelling up the Ord to sample as there were no prawns at several
sites and the water was fresh.
Changes to river flows (i.e. inputs of fresh water) elsewhere
in the Joseph Bonaparte Gulf system may have an affect of reducing the estuarine
habitat available to juvenile prawns and other estuarine fauna.
CSIRO, FRDC ($69 524)
1995-1998
Non-technical summary:
One of the main objectives of fisheries management is to
ensure the sustainability of fished stocks. To reach this objective scientists
have to adequately assess the status of fished populations with quantitative
models of the fishery systems. Most of these models require estimates of population
parameters such as growth rates, mortality rates and catchability (the proportion
of the population caught by a single vessel each day). Most of these parameters
are unique for each stock; unfortunately they are not easily estimated because
marine organisms are inherently difficult to observe and study. Estimation is
generally done through statistical analysis of catch data, either from the fishery
or from research surveys.
Tropical prawns are fast growing organisms that reach maturity
in a few months and tend to be predated upon or caught before they reach a year
of age. Prawns are also animals for which age can not be easily determined because
they have no hard structures that are retained through their life. As a result
age cannot be estimated and can be inferred only indirectly from their size.
The combination of a short life-span and the inability to age individuals is
a major difficulty in developing estimation methods for populations of tropical
prawns. This is especially the case for those parameters that are time dependent
(such as mortality and growth rates).
This document reports on two years of work devoted to developing
new statistical methods for the estimation of population parameters in tropical
prawn fisheries. The work was divided into five components:
1. Review of current methods of estimating growth and
mortality rates
2. Development of new methods for the estimation of
growth and mortality rates
3. Development of a method for the estimation of size-specific
mortality rates
4. Study of the dynamics of prawn aggregations
5. Estimating the effects of effort and aggregation
dynamics on catchability
In the review of current methods we investigated three models
that use length frequency data to estimate growth and mortality. The first two
of these methods ignore differences in size for individuals of the same age
and assume all prawns recruit at the same time. As a consequence these methods
provide substantially biased estimates of population parameters. The last method
considered did accommodate different sizes at age and gave unbiased results.
This last method, however, provided very uncertain estimates, with large confidence
limits suggesting that estimates were accurate but not precise. This review
concluded that it was imperative to develop new methods, more appropriate for
the life history and fishery characteristics of tropical prawn fisheries.
A new method was then developed for estimation of mortality
rates and growth parameters from length frequency data by incorporating individual
growth variability within the model. The method is flexible enough to accommodate
for different recruitment patterns, length-specific gear selectivity and varying
fishing effort over time. This method is statistically robust and was tested
with data for grooved tiger prawns from Northern Australia.
All the methods mentioned above make the fundamental assumption
that the natural mortality rate does not change with the age or size of the
prawn. We used data for common banana prawns to show this assumption is certainly
not correct for juvenile prawns. We found considerable changes in the natural
mortality of juvenile prawns, from 40% mortality per week for the smallest juveniles
(4mm carapace length) to only 5% mortality per week for the larger ones (12mm
carapace length). This suggests that there is a need to revise the evidence
for size-independence of natural mortality rates for larger prawns.
Schooling is a well-known behavioural trait in fish but it
is less common in prawns. In Australia, there is at least one group of prawns
that form dense schools, the banana prawns. Other prawn species aggregate but
in much smaller densities. We have used logbook data to describe the dynamics
of prawn schools. We found that the apparent biomass of schooling banana prawns
decreases due to fishing more rapidly than that of non-schooling banana prawns.
This implies that the density and catchability of banana prawn stocks decreases
as the season progresses. This possibly invalidates earlier assessments of banana
prawn stocks which assumed catchability was constant throughout the season.
At larger spatial scales we examined the relation between
the effects of non-random distribution of fishing effort and abundance. Models
used to analyse the catch and effort data from the Northern Prawn Fishery suggest
that there has been an increase in catchability due to the reduction in abundance
of tiger prawns and the tendency of tiger prawns to aggregate.
In conclusion this project has made substantial progress
in developing appropriate methods for parameter estimation for tropical prawn
stocks. Some of these methods have been successfully used to show that previous
estimates of growth parameters, mortality rates, and catchability may have been
subject to substantial bias or relied on untenable assumptions. This research
has therefore contributed to correct such estimates at the same time as providing
a set of new statistical tools that can be used for other Australian prawn stocks.
ABARE, FRRF (Cost not available) 2000.
Summary
Implications of alternate management instruments for the
sustainability of target stocks, the commercial fishery and the marine environment
were examined in this report. The sustainable management of the marine environment
is discussed, with consideration of the impacts that management of the commercial
fishery may have on the marine environment. The sustainability of the principal
commercial prawn species is examined in the light of alternative measure to
control fishing efforts. The feasibility of monitoring and adjusting fishing
restriction in real time in response to the observed state of stocks is also
investigated. Where appropriate, moving images show the temporal and spatial
pattern of fishing and are used to assess the likely effects of different management
options.
CSIRO, DJ Sterling Trawl services, NPF industry, AFMA Research Fund ($14
000)
1998-1999
Summary
The survey aimed to establish a sound basis for tracking
of changes in fishing power in the Northern Prawn Fishery by collecting data
on key aspects of configuration of all vessels in the Northern Prawn Fishery.
As vessels are reconfigured, or new technology is adopted by a fleet, changes
in fishing efficiency follow.
Methods: All owners of Statutory Fishing Rights in the NPF were
requested to take part, and interviews were completed with representatives of
92% of the fleet of 1998. Half (55%) of the interviews were with single boat
owners, a third (34%) were with owners of 3 or 4 boats, and the remaining 10%
were with representatives of companies with more than 10 boats. Most (78%) of
those interviewed were owners or managers, and the other 12% were skippers or
fleet engineers. A third (31%) of the interviews were conducted on board, and
the remainder were conducted in an office or elsewhere.
Opinions about the importance of some factors in relation to fishing performance:
Try nets, plotters and echo sounders were rated as having the greatest impact
on fishing performance, followed by engine power, communication equipment and
GPS error. Otterboard size, net size and vessel dimensions were rated as having
some impact, while radar and stabilisers were rated as having low impact.
Engineering performance of the vessel: Median rated engine power
in the fleet was 300kw (interquartile [iq] range 272-336). According to the
opinions of interviewees, over a third of the surveyed vessels (38%) had motors
that could potentially produce more power than stated on the SFR register, (iq
range 45-100 kw more). Median prop diameter was 1.52 m (iq range 1.5-1.7). Bollard
pull (the maximum thrust capability of a vessel while held stationary) was estimated
by a deterministic engineering prediction model (Prawn Trawl Performance Prediction
Model, (PTPPM, Sterling DJ 1996) for a subset of the vessels from rated power,
reported propeller diameter, maximum trawling rpm, operating trawling rpm, and
rated rpm. Median estimated bollard pull in the fleet was 5523 kgwt (iq range
5023-6312).
Characteristics of fishing gear that affect fishing efficiency: median
twine thickness was 36 ply (iq range 30-36); knotless netting was currently
used in 7% of tiger prawn nets and none of banana prawn nets and none used spectra
netting; Bison no 9 were the most common boards; median headrope length was
28 fathoms (iq range 24-28); and most common body taper was 1P4B (reported by
49% of vessels. Swept area performance was calculated for a subset of vessels
by the PTPPM based on trawl gear specifications, calculated bollard pull. Assuming
a tiger prawn fishing night was 12 hours, and each vessel fished for tiger prawns
for 138 nights in 1998), the median swept area per vessel per night was 2.3
square kilometres (iq range 2.0-2.5) and the median swept area per vessel for
the year in 1998 was 319 square kilometres (iq range 281-341). Note that these
estimates of swept area do not take into account any degree of retrawling over
the same grounds.
Innovations with potential to improve catch efficiency of the trawl design
include groundgear type (all had Texas); Stainless steel groundline (used
on 88% of vessels); square mesh codend (no vessels); codend mesh size (median
50mm, iq range 47.6-50.8). Information relevant to bycatch reduction was also
collected.
Innovations with potential to improve targeting ability of the vessel and
crew and possibly trawling time include differential GPS (currently
65% of vessels with active DGPS mode), automated try gear (on board 3% of vessels);
communications improvements (currently satellite phone 97%; personal Computer
linked to email facilities 42%). The most common plotter type was non-PC based
(Furuno or JRC, 75%); autopilot was linked to GPS on board 19% of vessels.
References and further information:
Sterling D 1996 "Abridged Description and Performance
Appraisal of Australian Prawn Trawl Systems" AMECRC (Perth Node) Report,
DJ Sterling Trawl Gear Services Brisbane.
Bishop J and Sterling D. "Measuring Effort Creep in the
Northern Prawn Fishery", 1999 NPF Preseason Forum, Cairns 1999.
Bishop J. & Sterling D. 1999. " Survey of technology
utilisation in the NPF fleet", (AFMA, Canberra, Aus).
CSIRO, FRDC ($972 283)
1995-1998
Non Technical Summary
In the mid to late 1980s NORMAC began to suspect that tiger
prawn spawning stocks in the NPF may have been reduced by fishing to levels
that reduced recruitment to the Fishery. A vessel buy-back scheme and other
effort reductions were introduced to reverse this trend but by the mid-1990s,
the desired recovery of tiger prawns had not occurred. To understand the reasons
for this, and with the support of FRDC, CSIRO began a 3-year research project
in 1995 to improve our understanding of the relationship between the spawning
stock and recruitment in the Tiger Prawn Fishery. This report presents the results
of this project.
This project has relied heavily on data gathered by the NPF
fleet. Some of the data is provided to AFMA as a condition of the NPF permit
(logbook data), but other data were provided to us voluntarily by fishers (GPS
plotter disks with maps of the fishing grounds and the fisher's trawl tracks).
Several fishers also took us on board their vessels to collect information on
try net catches and plotter tracks. We have shown how valuable these data are
and how much is to be gained by enlisting the cooperation of the fleet.
One of the main research issues addressed by this project
was to understand the spatial dynamics of tiger prawn stocks and the fleet that
fishes them. We first developed electronic maps of fishing grounds using data
from fishers' GPS-plotter systems. These maps identify the areas where trawling
occurs and the areas where the substrate is such that trawling cannot occur
(untrawlable areas). These maps have a resolution of 0.2 km - much finer than
the 10.8 km scale provided by the logbooks. Fishing effort and the untrawlable
grounds are distributed unevenly throughout the NPF. Some areas within fished
logbook grids are intensively fished whereas others are untrawlable. The fishers
tend to search for aggregations of tiger prawns early in the night. Once an
area of higher catch has been located, the vessel targets that same area for
the remainder of the night. As a result, some of the fishing ground is trawled
several times in the same night but nearby areas may not be trawled. This information
is essential for understanding seasonal and historical changes in the efficiency
of trawling
In the past, population indices of spawning stock and recruitment
for tiger prawn stocks were assumed to represent the entire NPF stock. However,
given the geographical extent of the NPF, it is unlikely that adult prawns would
mix thoroughly through the whole area. It was unclear, however, whether water
circulation could contribute to the mixing of tiger prawn larvae. To address
this question a simulation model of the currents in the Gulf of Carpentaria
and the behaviour of larval prawns was developed. The model was used to predict
the offshore spawning regions from which larvae could be expected to reach the
seagrass nursery areas along the coast. The model shows that there are large
gaps between these effective spawning areas and this suggests limited mixing
of tiger prawn larvae within the Gulf of Carpentaria.
On a large scale, we have used logbook data to examine trends
in the spatial extent of the fishery. Although the total area fished has been
decreasing since 1983, the area searched continued to increase until 1987. Areas
of high catch have remained unchanged throughout the history of the Fishery.
The area currently fished has contributed to the majority of the historical
catches, but in the early 1980s only 60% of the total catch came from this area.
Some areas that were fished in the 1980s are not fished today - some because
they are now inside trawl closures, but others because of low catch rates. Commercial
count data provided by skippers in their logbooks is accurate and was used to
identify the location and seasonal pattern of recruitment of small prawns onto
the fishing grounds. Many of the areas located as recruitment areas are on the
edges of current area closures, supporting the hypothesis that these closures
are protecting pre-recruits from harvesting. The analyses suggest that October/November
tiger prawn catch rates of the 20-30 count category may be a good predictor
of recruitment of tiger prawns in the following fishing season.
The analyses of logbook, plotter and larval advection data
have led us to propose a new stock structure for tiger prawns inside the Gulf
of Carpentaria. Seven new stock areas have been defined: some contain a stock
of both grooved and brown tiger prawns, and others contain a stock of only one
of these species. It is likely that other stock areas occur outside the Gulf
of Carpentaria.
Spawning/stock recruitment relationships (SRR) have been
defined at two spatial scales; firstly at an NPF-wide scale and secondly at
each of the regional stock levels of the Gulf of Carpentaria. At the NPF-wide
scale, the model suggests a strong influence of spawning stock on recruitment,
and also increased recruitment, independent of the spawning stock, every 3 to
4 years. At regional scales the influence of spawning on recruitment is less
apparent. However, simple biomass dynamic models suggest that, from 1993 to
1998, tiger prawn stocks in the Gulf of Carpentaria remained below the levels
required to produce maximum sustainable yield, thus implying that these stocks
were overfished. Similarly, standardised fishing effort for that same period
remains above the fishing effort required to achieve maximum sustainable yield,
thus implying that overfishing continued to occur. Spatial analysis suggests
that there are differences in the levels of overfishing between regional stocks.
The assessments seem to confirm the perceptions of some members of the fishing
industry who have suggested that the tiger prawn stocks in the Groote Eylandt
region have not recovered from the overfishing that occurred in the early 1980s.
They also suggest that the stocks in the Vanderlin and Mornington Island areas
were further depleted during the later 1980s as the fishing fleet fished more
in these areas in response to the depletion of the stocks around Groote Eylandt.
At smaller spatial scales our research has shown that fishing
is highly aggregated. This reflects differences in the abundance of prawns and
the type of bottom present within the fishing grounds. We believe that this
knowledge will be critical in supporting the establishment of marine protected
areas in northern Australia. To make this information more useful, however,
we will need to quantify fishing impacts on prawn populations and benthic habitat
at these small scales. This will require further research to characterise the
reasons for these aggregations and the relationship between bottom type and
benthic habitat.
In summary, this project has confirmed that tiger prawn stocks
remain overfished at both a large (NPF-wide) scale and a regional scale. This
implies that for NORMAC to put in place management measures that will recover
the stocks from their overfished state, these measures will have to be effective
in all regions of the fishery.
ABARE (Cost not available)
1998-1999
Summary
- Following higher per unit prawn prices in 1997-98, average receipts per
boat across the fishery increased by almost $170 000 in 1997-98 to over $1.1
million per boat
- Costs per boat are estimated to have increased by 6% across the fishery
in 1997-98. Cost increases were heaviest for boats with less than 375 A statutory
fishing rights
- Estimated average boat business profit across the fleet increased to $256
300 in 1997-98 from just under $135 000 in 1996-97
- Average debt across the fishery is estimated to have declined slightly
in 1997-98 to $416 000 per boat. The boat business equity ratio for 1997-98
was 87 per cent.
- Net returns to the fishery (excluding any changes to stocks) rose from
an estimated $14 million in 1996-97 to around $30 million in 1997-98
CSIRO, LWRRDC, FRDC ($16 148)
1998
A full report was published by Land and Water Resources Research
and Development Corporation as LWRRDC Occasional Paper 05/99. Only the objectives
of the study are given here.
Objectives
- Undertake a data and information review that:
1. develops a metadatabase for available regional
data
2. determines information needed to support
management
3. outlines planned development(s)
4. summarises current legislative, jurisdictional
and institutional boundaries documenting the scales of management needed
5. summarise existing planning processes
6. identifies the key aquatic resources
and key attachment areas
- Document the activities, skills and resources
of the research providers
- Consider the spatial extent of a potential study(s)
- Develop research proposal(s) aimed at supporting
the sustainable development of land, water and marine resources in tropical
Australia
- Consider ways of approaching an integrated multidisciplinary
study(s) and identify potential obstacles and risks to research projects.
CSIRO, MG Kailis Company, FRDC ($6 943)
1998-1999 (Stage 1)
Non-Technical Summary
The prawn trawl fishery in Exmouth Gulf, Western Australia,
harvests a mixture of penaeid prawns. Catches of the high value, brown tiger
prawn Penaeus esculentus comprised about 36% of the annual catch in the
1990s. However, annual catches of tiger prawns are now about half the level
they were in the 1970s and fluctuate markedly, from about 200 to 680 t. These
changes in catch create uncertainty in the supply of prawns for export markets
and force fishing and processing operators to have excess capacity to deal with
good years. Managers, fishing industry and researchers are considering the option
of enhancing the natural recruitment of brown tiger prawns by releasing juveniles
in wild nursery areas to reduce natural fluctuations and increase the average
annual catch.
This collaborative project (CSIRO, Fisheries WA, MG Kailis
Group of Companies) assessed the feasibility of stock enhancement of tiger prawns
in Exmouth Gulf by:
- developing a bioeconomic model
- examining the risks of changes in the genetic composition and introducing
disease to the wild population of tiger prawns, and
- identifying further work that would be needed before stock enhancement
should proceed.
This is the first of several stages that may lead to stock
enhancement of tiger prawns in Exmouth Gulf. The project was initiated through
a workshop of all project participants in Perth in July 1998.
Bioeconomic model
A bioeconomic model was developed in EXCEL to make it accessible
to managers and industry. This model contains independent modules (linked worksheets)
for the hatchery, production, nursery and fishery. The results from the model
suggest that a release of 7 million juvenile prawns (1g) would increase catches
of brown tiger prawns by about 100 t and that the median marginal revenue for
this level of enhancement would be $1.2M (range = $0.8M to $1.7M). The marginal
revenue was affected mainly by variation in prawn prices and secondarily by
the densities used to grow juvenile prawns. The variation in prawn prices should
be considered a risk in any future enhancement project. The uncertainty about
the best densities for producing juvenile prawns and the production environment
(i.e. ponds or raceways) is an important area for future research and development.
A further source of uncertainty in the model is the survival rates of prawns
and how they vary at different stages of the enhancement (e.g. survival during
transport and release, whether survival is density-dependent in the nursery).
The median difference between production costs and the median
marginal revenue was about $540, 000. This cannot be interpreted as strict
profit because the model did not include all capital costs and because it used
some assumptions that may not be realistic (e.g. 100% survival of juveniles
during harvest and release). Sensitivity analysis showed that provided the mortality
associated with the harvest, transport and release of juvenile prawns is less
than 30%, the enhancement has a greater than 90% chance of still being profitable.
Although the model did not include the costs of monitoring, the current results
indicate how much can be spent on capital and monitoring for enhancement to
be profitable. The predictions of the current model therefore represent "best-case"
scenarios for stock enhancement and would be refined, as new information becomes
available. The model also provides a rigorous framework for evaluating the possible
success of other enhancement projects.
Nursery habitats
Participants at the stock enhancement workshop recognised
that little was known about the nursery habitats of brown tiger prawns in Exmouth
Gulf, the dynamics of juvenile prawns in the nurseries, and predation rates
on them. This information is needed to develop the best release strategies to
ensure the success of a stock enhancement.
Risks of affecting the gene pool and introducing disease through stock enhancement
The risks of affecting the genetic composition of the wild
stocks from the stock enhancement of penaeid prawns were discussed by a panel
of industry representatives, research scientists, research managers and policy
managers at the FRDC sponsored workshop on "Genetics in the Aquaculture
Industry" held in Perth in October 1998. In this case the risks were considered
low and recommendations were made for minimising the genetic risks. These were:
- Determine the genetic structure of the wild population prior to stock-enhancement
- Use only broodstock from the target population selected for enhancement
- Randomly collect broodstock (to avoid family groups)
- Trace individual families through rearing using genetic markers
- Release the same number of individuals from each of the captive-bred families
- Monitor the effects of the release using molecular methods (e.g. microsatellite
DNA markers)
The risks of introducing disease into the wild population
were also considered at the stock enhancement workshop and in further discussions
with Dr Brian Jones of Agriculture WA. The protocols for assessing diseases
in prawns have been developed as a part of the Fisheries WA program on Disease
and Hatchery testing and a component of the "Tropical Prawn Diseases"
project (FRDC 98/212). Any juvenile prawns produced for stock enhancement would
be tested for disease using these protocols.
Conclusions
The bioeconomic model has shown that the stock enhancement
of tiger prawns in Exmouth Gulf can be profitable. However, further information
is needed on the production of juvenile prawns and the survival of juveniles
during the release (i.e. harvest, transport, release), to make better predictions
about the likely success of an enhancement. The technology for the production,
harvest, transport and release of juvenile prawns needs to be developed. Further
information is also needed on the nursery habitats of the juvenile prawns to
develop release strategies that give the maximum chance of a successful enhancement.
These information needs are the basis for a three year FRDC project "Developing
techniques for enhancing prawn fisheries, with a focus on brown tiger prawns
(Penaeus esculentus) in Exmouth Gulf" (FRDC 1999/222), which is
the second stage in the overall plan for the stock enhancement of tiger prawns
in Exmouth Gulf. If the results of Stage 2 are favourable, it would be followed
by trial stock enhancements (Stage 3, 1 to 2 million juveniles) and a commercial
scale enhancement (Stage 4, 7 to 10 million juveniles) that would attempt to
increase the commercial catch by at least 100 t.
Page last updated
26 July, 2005
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