| An opinion
piece by Chris Paulin first published in the Wellington Flyfishers
Club Newsletter Feb 2009
Because of its close proximity to major urban
centres, the Hutt River has always been a popular local fishing
destination. Brown trout (Salmo trutta) were first liberated into
the Hutt River by the Wellington Acclimatisation Society in 1874,
and until 1941, tens of thousands of trout were liberated annually.
After 1941, artificial stocking became irregular and ceased in 1976.
Angler use of the river has varied significantly
over the past twenty years. More than 6,000 angler visits were recorded
during the 1985/86 fishing season and this rose sharply in the 1994/95
fishing season when nearly 20,000 visits were recorded. This increase
in fly-fishing activity coincided with several major movie releases,
such as “A river runs through it” and “Bridges
of Madison County” which featured flyfishing, and possibly
encouraged many non-anglers to take up the sport. Shortly after
this increase however, a series of major floods on the Hutt River
in the late 1990s were followed by in a significant decline in fish
numbers, and numbers of angler visits also sharply declined as catches
dropped: the 2001/02 season recorded just over 6,000 visits.
The Hutt River is normally open to fishing all
year and recently, a comparatively
low daily bag limit (2 fish) was imposed. Additionally, in years
when the trout population is very low, a 450mm maximum size limit
has been imposed to protect the breeding stock. Fisheries management
agencies claim that the Hutt fishery is vulnerable to the impact
of floods, and consequently, fish numbers can vary greatly from
year to year.
Questions:
Why are trout numbers in the Hutt River so vulnerable to the impact
of floods?
Can fish stock management through manipulation of allowable catch
and size limits protect the fishery long-term when other factors
are involved?
Observed declines in trout numbers
A substantial reduction in the trout population
in the Hutt River was recorded in the summer of 2007/08. The decrease
was most noticeable in the medium size class of fish (200-400mm)
and was attributed (by Fish & Game) to large spring floods in
September - November 2006 severely impacting on trout fry and fingerling
survival. Large, adult trout (>400mm), were also found to be
in much lower numbers, however, the cause of this decline was reportedly
unknown. These declines were similar to the declines following the
1997/98 spring floods, which were preceded by major cross-blading
and re-alignment flood control works near Harcourt Park in Upper
Hutt, resulting in the disturbance, and subsequent downstream transportation
of millions of tonnes of gravel and sediment.
Studies of trout ecology and floods
Trout are freshwater fishes, and have evolved
in habitats and climates where frequent and often large flood events
are a common feature of the environment. A review of trout population
studies around the world suggests that in most river systems, total
trout biomass does not differ significantly before, during, or after
major flood events. Net movement and habitat use by radio-tagged
adult trout during floods was not found to be significantly different
from observations made prior to the flood events. Most studies show
that young trout are displaced downstream by major floods, although
displacement distance is generally hard to determine since some
fish could have originated from local populations associated with
intervening tributaries.
Most studies found that abundance, catch rate,
body condition, and diet of adult trout in river systems were not
significantly affected by floods, and the floods did not detrimentally
affect spawning success. In fact one study showed that young-of-year
fish increased by 20% in summers following spring floods, which
was attributed an increase in food supply being swept into the river
from swollen tributaries.
If these numerous studies throughout Europe and
North America show that floods do not have a significant effect
on trout numbers, why is it that the Hutt River fishery in New Zealand
is so different, and subject to such variable numbers of fish following
floods as management agencies claim?
Trout behaviour during floods and habitat degradation
Observations of trout behaviour during flood
events indicates that fish avoid the direct effects of strong flood-waters
by taking shelter in areas of reduced flow such as close to banks
under overhanging vegetation, or by taking refuge below the leading
‘lip” of deep pools where the water may remain comparatively
clear and calm, even as floodwaters rage above.
Studies of trout populations in New Mexico, USA,
found that declines in trout populations were associated with habitat
degradation. Principally, mining, logging, and cattle-grazing activities
that altered habitat through increased erosion and sedimentation,
as well as reducing riparian cover (riverbank vegetation). A dramatic
change in population abundance of trout in one catchment following
a catastrophic flood event was investigated. Trout population density
declined by about 98% after the flood in a river section affected
by sedimentation and debris flow, whereas population numbers in
another flooded section of the same river, unaffected by debris
flow or sedimentation remained relatively stable.
Mining, logging, and cattle-grazing are not activities
associated with the Hutt River which now flows through an urbanised
catchment. The wide, clear banks have few areas where overhanging
vegetation could provide refuge for adult or juvenile trout in a
flood event. However, in unmodified sections of the river, relatively
stable deeper pools could provide a refuge. Unfortunately, current
flood management practices carried out by the local regional council
results in the worst possible scenario for trout survival in floods:
stretches of riverbed are now frequently “cross bladed”
by bulldozers, and straightened – a process which not only
destroys the naturally occurring deeper pools, it increases the
speed of the water flow, and has a significant effect in greatly
increasing sediment and debris loads carried downstream.
Effects of cross blading on trout survival
The Hutt River is a “freestone” river;
and under natural conditions, quantities of stones and gravel are
gradually transported downstream, slowly filling deeper pools while
other pools are created – a process which make take several
months or even years, allowing trout to move to new pools between
flood events. (For example, the lower Motueka River is also a “freestone”
river, and has trout population numbers that are among the highest
of any river in New Zealand). Cross-blading exacerbates the trout
survival problem in the Hutt River by loosening the stones in the
riverbed: the relatively stable stony riverbed is torn up, leaving
numerous ridges of stones and gravel at right angles to the water
flow.
Because cross-blading loosens the rocks and gravel
of the riverbed, the next flood event is able to pick up the loose
gravel and rocks, and transport them downstream rapidly, depositing
them in pools of slower flowing, deeper water, often destroying
the pools within a few hours. The rapidly changing riverbed and
destruction of the pools forces trout out into the current where
they are unable to survive, and also destroys valuable streambed
habitat required for the insect nymphs that trout depend on for
food.
Fisheries managers reported that in 2008 no drift
dives were carried out in the Melling reach of the Hutt, due the
discolouration of the water caused by a bulldozer operating in the
river in the vicinity of Avalon. Cross-blading with bulldozers is
destructive and the chosen method of local flood control –
in Europe mechanical control of many river systems is now illegal,
and allowing naturally occurring swamps and marshlands to re-establish
has solved many problems in flood-prone areas. The urbanised landscape
surrounding the Hutt River means that flood control by allowing
wetlands to re-develop cannot occur, and mechanical measures currently
in use are unlikely to change in the near future without a reassessment
of flood management practices and priorities.
Trout population protection and recovery
If adult trout are being lost as a result of
flood management regimes, protection of the fishery must rely on
management of the trout population dynamics and be aimed at protecting
the remaining breeding fish, as well as increasing juvenile survival.
As this is incompatible with current flood control measures, replacing
juveniles through restocking when the trout population collapses
after a major flood is essential. This would allow the population
with few remaining reproductive individuals to quickly recover back
to stream carrying capacity.
Restocking
Overseas studies show that complete recovery
of trout populations can be achieved in as little as eight months
following a flood where sedimentation or debris has affected a limited
section of river, provided there are protected populations in nearby
tributaries. However, virtually all of the smaller tributaries of
the Hutt (such as the Belmont Stream, which was reportedly an ideal
trout stream in the 1920s with catches of up to 25 fish per day),
have been turned into nothing more than piped drains. Given the
frequent and repeated destruction of habitat caused by cross-blading
throughout the Hutt River, and the loss of side-tributaries, there
is a good argument for artificial restocking, either on an annual
basis or following major flood events.
Management agencies cite evidence to show that
the release of hatchery fish is unproductive and wasteful in streams
already stocked to capacity, and for this reason most rivers in
New Zealand (including the Hutt), are no longer artificially restocked.
However, this management scenario should not be applied to the Hutt
River, where the fish stock is not at capacity, and is regularly
decimated as a direct result of flood control procedures!
Other impacts
Cross-blading not only affects fishing: vehicular
access to the Hutt is easy and cross
blading allows the drivers of 4WD to drive up and down the river
for no reason other than their own vicarious pleasure, further disrupting
trout lies and destroying breeding redds. At the same time swimming
holes, part of New Zealand's youthful tradition, are being lost
by the same modification to the river beds. Straightening the riverbed
by cross-blading creates a steeper gradient, and although the flood
risk may be reduced by lowering the riverbed, the increased speed
of flow can increase the risk of flood damage to banks and structures
downstream.
Conclusion
Without changes to existing mechanical flood
control policies, management of the fish stocks through bag limits
and size limits will always be a stop-gap measure, and is unlikely
to see a return to a permanent, high, or stable trout population
in the Hutt River.
Freestone rivers such as the Motueka and the
Hutt can support large populations of trout, however, better management
of flood control works on the Hutt River to protect both juvenile
and adult trout, supplemented by artificial restocking, is required
to protect the fishery long-term and maintain the fish stock at
capacity.
Combined with the creation of a harbour salmon-fishery
through releases of salmon fry into the lower reaches of the river,
this would result in one of the best and most accessible salmonid
fisheries in the world. |
