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Marine
fisheries research
Preview
Most people agree that we should try to conserve our fisheries
resources. To do this successfully we need information about
the fish living in New Zealand waters. This information is
collected by fisheries scientists, and passed on to the Ministry
of Fisheries (MFish).
What research is done and how is it used?
Fish stock research
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Provides
information about New Zealand's extensive fish stocks. |
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Monitors
or predicts the impact of fishing and the environment
on fish stocks. |
This
research is used to advise the Government on safe Total Allowable
Catch levels for different species.
Exploration
research, aquaculture, and enhancement
Exploration, aquaculture and enhancement research is used to
help the fishing industry develop new fisheries or exploit different
species.
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Exploration
research: Exploring for new fishing grounds |
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Aquaculture
: Developing techniques for marine farming |
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Enhancement
: Developing techniques for building up "wild"
fish stocks, eg, scallops. |
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Why
is research important?
New Zealand has extensive fisheries in its 200 nautical mile
Exclusive Economic Zone. These range from familiar coastal species
such as rock lobster, blue cod, snapper and kahawai to more
recently developed deepsea fisheries such as orange roughy,
hoki and oreos.
We
need to know the biology and behaviour of fish, the size and
productivity of fish stocks, and their relationship with other
species and the environment, so that we can work out sustainable
catch levels.
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Under
the Quota Management System, introduced in 1986, catch
levels for more than 180 fish stocks are set by the Government.
This is done on the basis of information from fisheries
scientists with input from the fishing industry, recreational
fishers, Maori and environmental groups. |
Without
adequate research, catch levels may be set too high. The result
may be overfishing - taking more fish than the stock can sustain.
Once a fishery has been overfished it usually takes a long time
and very careful management to rebuild the fish population to
a sustainable level. Sometimes a fishery may never recover.
How
do scientists research fish stocks?
Fish populations change in response to being fished, and to
changes in their environment. To manage fisheries you don't
study a fish stock once and leave it at that. Stocks need to
be studied over a long period of time, so that changes can be
monitored and catch rates can be altered if necessary. It is
a bit like weather forecasting - to predict the future you need
some knowledge of the past.
Fish
are very difficult to count. Not only do they live up to thousands
of metres below the sea's surface, but most move around a
lot. Scientists use several research techniques to try and
get as good a picture as possible.
| Research
techniques include: |
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Trawl
surveys
Trawl surveys are the bread and butter of fisheries research.
Regular surveys are used to estimate changes in the total numbers
or weight of fish in an area. They are used for bottom-dwelling
fish like orange roughy, hoki, oreo, snapper, and red gurnard.
In
a trawl survey, a specified number of trawls are carried out
over a particular area. The width of the net's opening, the
area of seabed swept by the net and the total survey area
are known, so from the catch taken it is possible to estimate
the total population in that area.
By
carrying out surveys of the area at the same time each year,
scientists can track changes in the stocks. Biological details
such as the length, weight and sex of the fish in the catch
are recorded. Samples such as otoliths (part of the fish's
inner ear) are also collected for analysis of age on land.
| Problems: |
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Some
fish are unevenly distributed. For example orange
roughy live over pinnacles and there may be large
areas with no fish and small areas with a high number. |
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This
method assumes that a constant proportion of all
fish in the path of the net will go into the net.
In fact some escape by going over the top or round
the sides. In some cases fish can be 'herded' into
the net. |
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Acoustic
surveys
Acoustic surveys aim to estimate the biomass. The technique
is mainly used for hoki, orange roughy and southern blue whiting.
Sound
waves which are sent out from the research ship strike schools
of fish and are reflected back. The energy of the sound waves
is measured and it is possible to estimate the biomass.
| Problems: |
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The
strength of the signal varies depending on the size of
the fish and their orientation, that is, whether a fish
has its head up or down or even on a slight angle, and
whether the sound signal is from a cluster or a mix of
different species. |
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Tagging
Tagging is used to work out the movements and age of a range
of species, such as snapper, rig, kahawai, school shark and
groper.
Fish
are caught, their length (and sometimes weight) is recorded,
a small tag inserted, and they are returned live to the sea.
The tag has a unique serial number and an address for recoveries.
If
fishers catch a tagged fish, they are asked to return at least
the tag (and usually the fish as well) with details about
where and when it was caught. A reward is offered for returned
tags.
Scientists
measuring and tagging fish |
As
scientists know where and when the fish was tagged and where
it was caught they can work out its likely movements. Tagging
can also be used to estimate growth rates (although tagged
fish may not grow at a normal rate because of the presence
of the tag).
Stock
size can be estimated from the number of fish that were tagged,
the number of fish caught by commercial fishers and the proportion
of those fish that were tagged. For example if 100 fish were
tagged and 10 tags are returned, it is assumed that 10 percent
of the stock was caught. If 1000 fish (tagged and untagged)
were caught by the fishery, the total population size is estimated
to be 10,000 fish.
| Problems: |
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It
is difficult to get completely accurate catch figures,
some fishers do not return tags, some tags may come out
and some tagged fish die as a result of being tagged.
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Tagging
cannot be used for deepwater species as the fish have
to be caught, tagged and released, and deepwater species
don't survive the trip to the surface. |
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Underwater
filming
Stereo camera equipment provides three-dimensional images of
spawning schools of hoki and orange roughy, taken hundreds of
metres below the sea's surface. The aim is to identify fish
species and provide information about their size, density and
orientation. As a technique it is still in its early stages.
The
stereo camera is lowered to depths of up to 1200 metres by
a connecting cable, which carries data and a photographic
signal back to the vessel.
The
video may revolutionise marine photography. It works on the
same principle as the stereo camera, with two videos mounted
side by side. It provides immediate results, whereas it takes
weeks to process photographs from stereo cameras.
| Problems: |
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The
lights needed for the video cameras may change the behaviour
of the fish, so on occasions still photos are taken instead,
using flash lights. |
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Egg
production
The egg production method is used to estimate the spawning biomass
of fish. In New Zealand this method has been used with snapper
in the Hauraki Gulf, and more recently orange roughy on the
Chatham Rise and on the east coast of the North Island.
When
most finfish spawn, eggs are released, fertilised and float
to the surface. They are collected in a plankton net. Females
are also caught in separate trawls, to assess how many eggs
each produces.
A
probe is attached to the plankton net, to accurately measure
the depth and temperature at which the eggs were collected.
By
knowing how many eggs there are in a given area, and knowing
the number of eggs produced by the average female, it is possible
to estimate the total spawning biomass of the fish under investigation
in the area.
Age
and growth
Age and growth information is important for scientists trying
to predict the impact of fishing on a fish stock.
Some
fish, such as flounder, are fast growing, short lived and
very productive, so may be able to recover quickly from fishing
pressure. Others, such as orange roughy, are slow growing,
long lived and not very productive, so may take a long time
or never recover.
Removing
the otolith (ear bone)
from a fish |
The
age of some fish can be worked out by counting the rings on
otoliths. (Otoliths are part of the inner ear of the fish
and are important for balance and hearing.) They grow in a
series of daily rings and seasonal bands or growth zones.
Age
determination is used to calculate growth curves for fish
in different areas. For example, snapper on the west coast
grow faster and mature earlier than snapper in the Hauraki
Gulf. It is also useful to estimate the age structure of populations.
When
fish are tagged or kept in tanks, the accuracy of ageing from
otoliths can be checked by injecting a harmless antibiotic (such
as tetracycline) into the fish. This leaves a mark on the otolith
which is visible under ultraviolet light, and the growth zones
or number of rings outside this mark can be determined.
Catch
sampling
To supplement information gained using other techniques such
as trawl surveys and tagging programmes, surveys can be carried
out in fish processing factories and on fishing boats, measuring
the length and sex of the fish and the size of the catch.
This
information, with details of where and when the catch was
caught, is used to build up a long-term database, to help
monitor changes in the size and age structure of fish stocks.
Biochemical
techniques
Genetic fingerprints - the same type used by forensic scientists
tracing crime suspects - can reveal important differences
between and within fish species. Often they are differences
that are only apparent when genetic tags or markers are studied
in the lab.
The
technique behind much of this work is electrophoresis, where
molecules are moved through a gel, which sorts them according
to size. Proteins or enzymes which are closely controlled
by genes are used for the analysis.
The
beauty of this technique is that genetic differences can be
studied without time consuming crossbreeding experiments.
It is especially important with deepwater species which can't
be studied by other techniques, such as tagging, because they
don't survive being caught and released.
Another
biochemical technique is the study of mitochondrial (mt) DNA,
which is passed only through the female line. Because it mutates
rapidly, it is a more sensitive population measure than other
genetic markers. This has provided scientists with important
information on the identification of different stocks of orange
roughy. For example, the Puysegur Bank orange roughy stock
is genetically distinct from other stocks, although there
are no obvious physical differences.
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Scientists
are sometimes called upon to use biochemical genetics
to help with some detective work on the part of fish consumers.
Fish retailers have been known to pass off a cheap species
as something more expensive, and this research has been
used to identify the species from samples of the suspect
fillets. |
Modelling
Modelling is a way of asking computers to answer "what
if?" questions about fish populations. To do this, scientists
create a fish population model, which is a computer program
that describes how populations change over time. The model includes
the four main ways in which populations change:
| 1 |
Growth |
| 2 |
Recruitment
(birth) |
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Natural
mortality (deaths from disease or predation) |
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Fishing
mortality (fish caught by fishers) |
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The
first two factors cause populations to increase; while the
second two factors cause populations to decrease.
To
apply the model to a fish population, information has to be
fed into the program. For example, how fast the fish grow,
the maximum size of the fish, the age at which the fish become
sexually mature, what proportion of the population is likely
to die each year from natural causes, and so on.
Catch-effort
(catch per unit of effort or CPUE)
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Catch-effort
values are obtained by dividing the catch (tonnes or
kilograms) by a measure of the fishing effort required
to catch it.
The
effort can be expressed as time (days, hours), fleet
size (number of fishing vessels), number of fishing
operations (trawl tows, set nets, pot lifts, etc), or
size of the fishing gear (vessel, net length, number
of hooks, etc).
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Changes
in CPUE may give an indication of what is happening to the size
of the fish stock. In theory, a level trend of CPUE values represents
a stable stock, a rising trend an increasing stock and a falling
trend a declining stock. A
falling trend does not necessarily indicate overfishing; a fish
stock will always decline from its initial high level when fishing
occurs until it reaches an equilibrium level, when productivity
balances the catch being taken.
Aquaculture
New Zealand, with its extensive coastline and low levels of
pollution, is considered ideal for aquaculture. Our aquaculture
industry is based on three key species: greenshell mussels,
Pacific oysters and chinook salmon. There are other species
being farmed but they are smaller contributors to the domestic
and export market, for example, freshwater prawns.
Most
mussel farming is carried out in the Marlborough Sounds, but
there are also farms in the Bay of Plenty, Coromandel, Northland
and Stewart Island. The mussels are ongrown from spat (immature
mussels) which have been collected from the wild.
Pacific
Oysters |
Pacific
oysters, farmed in Northland and Coromandel, are also
grown from wild spat. This species was introduced to New
Zealand accidentally (possibly on the hulls of visiting
ships) in the early 1970s, and has replaced the native
rock oyster for farming. Recently it has spread to Tasman
Bay and parts of the Marlborough Sounds, where oyster
farming is developing. |
Chinook
salmon are reared for ocean ranching from rivers on the east
coast of the South Island, and for sea cage rearing in the
Marlborough Sounds and at Stewart Island.
Small
scale, and at this stage experimental, operations have been
set up to farm black-foot paua, rock lobster and grey mullet.
Several
other species are being looked at for aquaculture development,
but because of their biology, different techniques are needed
to raise them. The species include dredge (Bluff) oysters,
yellowfoot paua, and marine finfish.
Populations
of wild dredge oysters in Foveaux Strait have been devastated
by the parasite Bonamia so there is a lot of interest in farming
them. The young oysters are relatively easy to grow and scientists
have found they grow faster when suspended above the bottom
of the sea.
Rock
lobsters are not easy to rear from eggs and have a long larval
stage. The current emphasis is on collecting larval rock lobster
from the sea and growing them in tanks. Scientists have to make
sure the rock lobsters have the right conditions, which include
temperature, food, tank size, density and shelter.
Enhancement
Enhancement is a way of boosting stocks by releasing young into
the wild. Like aquaculture, it is expensive. It is only practical
for high-value species, like rock lobsters, oysters, scallops,
paua and snapper, and may, in the main, only be economic for
species which feed naturally from the food within the water,
as food costs are very high.
Enhancement
of scallops has been very successful in Tasman and Golden
Bays. Paua enhancement has been tried at the Chatham Islands.
In addition there is interest in enhancing snapper in Nelson
and the Hauraki Gulf, and dredge oysters in Tasman Bay and
Foveaux Strait.
For
more information click on any link below.
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