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EXECUTIVE SUMMARY
Information on the number of fish in a population is necessary to
determine the effects of fishing and distinguish them from natural changes. The more accurate this information is, the more easily changes in the
populations can be detected. The power of statistical tests to detect
changes in population size depends on the accuracy and precision of population
estimates. Accuracy is the closeness of an estimate to the
true population size; precision is the closeness of repeated
estimates of the population size. Systematic bias in the
counting or sampling of fish can cause very precise estimates to be inaccurate
and most methods of fish sampling have some sources of characteristic
bias. This review aims to evaluate the accuracy, precision and biases
of the sampling techniques most useful for surveying fish populations
in Great Barrier Reef waters.
Tropical fish populations associated with coral reefs are among the
most difficult groups of fish to survey accurately, partly because of
the variety of their behavioural patterns and partly because of the topography
of their habitat. The landed value of individual species is generally
low and has not attracted the expenditure on surveys of their numbers
that have been justified for the industrial fisheries of temperate zones.
However, the need to develop systems for reliably assessing populations
of target groups (coral trout, sweetlip emperors and the sea perches in
particular) is becoming more urgent, as recreational and commercial fishing
effort on the Great Barrier Reef intensifies.
In shallow waters within the limits of safe SCUBA diving (about 20 m
depth) underwater visual survey along strips (transects) of known width
have been clearly demonstrated to be a precise way of obtaining relatively
accurate estimates of coral trout density that are useful in routine monitoring
of their populations. This has not been the case for the sweetlip emperors
and the schooling sea perches. These fish are seen so rarely in counts
along transects, or school up in such clumped numbers, that there are
many zeroes in visual surveys with occasional large numbers when schools
are seen. This type of data is so imprecise that only very large changes
in population size would be detectable. If a technology can be developed
that measures the area covered by a SCUBA diver then large scale searches
of reefs within fixed time intervals may be a way of counting these sweetlips
and sea perches.
Many of the other popular reef fishes such as red emperor, scarlet
and saddle-tail sea perch, spangled emperor and red-throat sweetlip spend
most, or all, of their time in deeper waters between reefs where underwater
visual surveys are not possible. There is presently little knowledge about
the types of habitat and sea floor cover (sponges, corals, sea fans) that
these fishes aggregate around, but once this is determined there are a
variety of sampling methods that will be useful to estimate their abundance.
Underwater visual surveys could be carried out from submersible
vehicles, from which a human observer counts fish, or remotely-operated
vehicles or ROVs, from which video recordings are made for later interpretation
and analysis. Submersibles and ROVs generally can operate over a very
great depth range and both would be very useful in determining the habitat
requirements of the fish between reefs. They are not suitable for routine
monitoring of fish populations as submersibles are extremely expensive,
and presently unavailable in Australia, and ROVs have a field of view
that is too narrow and shallow to match the human eye.
Hydroacoustic techniques operate in the same way as an echosounder
by transmitting pulses of sound down to the seabed and analysing the echoes
returned from fish and other objects in their path. These techniques have
the potential advantage of speed and ease of operation, in that they do
not require deployment of gear or divers. Sonar and other forms of acoustic
echolocation have been used in estimating school size of pelagic fish
stocks for many years, but the technique's application to coral reef stocks
has not progressed very far, primarily because of lack of interest and
difficulties associated with species recognition. Unlike the situation
with large schools of pelagic fish (which tend to be of one species),
reef fish populations comprise a high diversity of species, many of which
have visual similarities and undoubtedly sonic 'fingerprints' that are
almost indistinguishable.
Most other 'capture' methods for sampling reef fish follow the
line fishing, trapping and trawling that form the basis of commercial
fisheries. These methods offer the advantages of deployment at all times
or depths, and of providing specimens for studies of age and reproduction
or for tagging and release. The catch-per-unit-of-fishing-effort (CPUE)
in the sampling gear is assumed to be an index of population density.
Most of the bias and problems with use of these gears concern the way
CPUE relates to fish abundance. Fish can avoid trawls and escape from
traps, and mesh or hook size can select for or against capture of certain
sizes of fish, so that CPUE is uncoupled from fish abundance. Similar
effects are caused by changes in the vulnerability or 'catchability' of
fish. The use of underwater videos to study the way these gears catch
fish, and comparison with visual surveys or other gear types, has allowed
calibration of CPUE as an index of abundance.
The use of baited, wire-mesh fish traps is the best known of these
capture methods for reef fish. Selectivities caused by entrance and
mesh size, trap volume, bait type, soak time and fish behaviour are some
of the factors shown to affect trap CPUE, but the real constraints on
their use as a sampling tool relate to the characteristics of the data
collected. Like the visual surveys, catches of small emperors and sea
perches reflect their clumped distribution with many empty traps and some
large catches when schools enter. Recent studies have also shown that
fish will readily leave traps once the bait is finished. As a consequence,
traps may be a useful monitoring tool only if large amounts of time are
spent sampling in each critical habitat on a reef, or only if very large
(3-fold or greater) changes in CPUE were of interest. Underwater videos
can be attached to monitor the visits, entrance and escape of fish from
these traps to reduce some of the biases and perhaps increase the efficiency
and sampling power of trapping.
The use of bait to attract fish is also fundamental to the use of
line fishing as a sampling tool. A variety of factors will alter the
responsiveness of fish to bait in traps and on hooks, and hence their
'catchability', so that CPUE does not always reflect their density on
a reef. Tidal state and spawning season have been identified as causes
of consistent increases in the catchability of coral trout on handlines,
but there is a need for more experimental fishing to refine our knowledge
of the times and places where handline fishing would best be used as an
index of abundance. This technique is rapid, relatively cheap, and can
cover all habitats and depths on a reef. Catch rates can be high if professional
fishermen are used, but effort is labor intensive and notoriously difficult
to standardise. The coefficient of variation in handline fishing operations
on the GBR ('tinny' fishing) is similar to that for trapping in the same
habitats. With a thorough analysis of the precision and sampling power
of the technique, including existing datasets, we believe handline fishing
will be useful in monitoring coral trout populations. Visual surveys and
spearfishing in the same areas before, after and perhaps during line-fishing
would allow better understanding of the biases associated with handline
fishing.
The similarities in precision between trapping and line-fishing indicate
that extensive sampling will be required to use these techniques. In itself,
this sampling will cause major population declines of some species independent
of any natural change or effects of fishing, unless the catch is released
unharmed. There is an urgent need to assess the short-term and long-term
survival of fish released from sampling with traps and lines - especially
when those gears are being used in the 20-40 m depths around reefs. Only
anecdotal observations and tag recovery data from shallow water (10 m)
fishing is available to infer the mortality of these fish.
For this reason, and failure of most of the assumptions relating recapture
rates of tagged fish to total population numbers, we believe that mark-release-recapture
experiments will not be a practical way of estimating reef fish population
sizes in deeper waters around reefs.
Semi-pelagic trawls, traps and line-fishing have all been successful in catching the inter-reef sea perches and emperors in northern Australia,
but the precision and sampling power of these gears has not been analysed
for these species. A key requirement for developing sampling techniques
in the inter-reef waters of the GBR will be much better knowledge of the
type and extent of habitat favoured by these species here. The performance
of traps and handlines in a sampling role in deep inter-reef waters has
not been fully analysed, but both could be deployed with pin point accuracy
on habitat features and in aggregations or schools of fish. There may
be a role for hydroacoustic surveys of these schools if traps, lines or
trawls can be used to determine the species composition of them. Much
progress has recently been made in developing semi-pelagic fish trawls
that can efficiently operate at 0.4-1 m above the seabed to avoid hookups
and reduce bycatch and destruction of seabed fauna and habitats. These
will not be useful on rough shoals and pinnacles and virtually all the
catch is killed, but trawls offer advantages elsewhere of very large areas
sampled in all sea conditions and some ability to target aggregations
found with hydroacoustics. They also completely avoid the biases and selectivities
of different responsiveness of fish to baits that hamper trap and line
fishing. The effective area fished by trawls is not yet fully understood
and the catch tells nothing about the nature of schooling or clumping
of species along the trawl path. Cameras and hydroacoustics (netsonde)
mounted on trawl headropes could provide this information as fish enter
the net.
There is no single method currently available which will satisfy
all the requirements for population assessment of commercially and
recreationally important fish species on the Great Barrier Reef. Each
method has characteristic advantages and disadvantages, and the selection
of an appropriate suite of methods will clearly depend upon the objective
of the assessment and the need for biological data in addition to the
simple counts of fish numbers. This conclusion is essentially the same
as those from past comparative studies of reef fish assessment methods,
and indicates that the advances in sampling methodology within the past
few years have not clearly identified a particular technique as being
of general applicability to the tropical reef fisheries. It also indicates
that promising leads in the use of side-scan sonar and baited stations
with video cameras have not been developed since they were first suggested.
However, protocols relating to underwater visual surveys and trapping
have been well defined, tested and improved. There remains a need
to extend this to other non-destructive methods (particularly baited video
stations and hydroacoustics) and generally to the capture methods in inter-reef
waters. There is also a need for development of subsidiary techniques
to identify and categorise habitat type, particularly in depths greater
than the SCUBA diving limit, as knowledge of these variables will have
a profound effect on allocation of sampling effort and subsequent power
of statistical tests.
The Great Barrier Reef has been host to these advances in the development
of underwater visual census techniques, which have been shown to be adequate
for monitoring the abundance of coral trout in shallow water habitats,
and which will probably play an increasingly important role if the live-fish
trade expands into the exploitation of species other than coral trout.
However we believe that alternative techniques must be developed and tested
for the emperors and sea perches, as well as for inter-reef populations
of coral trout. Among these we would recommend traps, handlining operations
and baited video stations as immediate solutions, with the possibility
of side-scan sonar and semi-pelagic trawls in the longer term.
THIS PUBLICATION IS CITED AS:
Cappo, Michael and Brown, Ian W. (1996).
Evaluation of sampling methods for reef fish populations of commercial
and recreational interest.
CRC Reef Research Centre Ltd
Technical Report No. 6
Townsville; CRC Reef Research Centre Ltd, 72 pp.
ISBN 1 876054 06 9.
A full copy of this report may be obtained from the author(s),
and through the following libraries:
Agency libraries: Great Barrier Reef Marine Park Authority,
Townsville; Australian Institute of Marine Science, Townsville; James
Cook University, Townsville; Queensland Department of Primary Industries and Fisheries
(Brisbane and regional offices); Queensland Department of Environment
and Heritage (Brisbane and regional offices); CSIRO Division of Marine
Research, Tasmania.
Public libraries: Queensland, New South Wales, Tasmania and South
Australia State libraries; National Library, ACT.
Parliamentary libraries: Queensland, New South Wales and South
Australia parliamentary libraries.
For a hard copy (or pdf file) of the report contact CRC Reef on info@crcreef.com.
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