|
Widespread throughout the Murray-Darling system, although
greatly reduced in abundance throughout its range when compared
to the original population. Murray cod were originally extremely
common and supported a substantial commercial fishery in the
19th century and in the early decades of the 20th. Prior to
European colonisation aboriginal people were able to exploit
the species as a major food source using relatively simple
technologies aided by the fish's great abundance.
The decline in Murray cod numbers is reportedly due to multiple
causes. The most significant factor that has been identified
has been the regulation of the majority of the rivers in the
Murray-Darling basin. This has resulted in substantial alterations
to the natural flow regimes of virtually all rivers in the
system. In addition, the dams and weirs themselves provide
a physical barrier to fish migration. Loss of river side vegetation,
physical damage to stream banks and beds by large hard-hooved
animals (especially cattle) and increased silt inflows due
to erosion and road run-offs have led to major changes in
the physio-chemical properties of much of the water in the
system. Also the removal of in-stream debris, such as fallen
trees and so on has resulted in a reduction of suitable habitat
for native fish in general and Murray cod in particular. Angling
and commercial fishing have had an effect, but it is insignificant
next to the other factors and is not considered to be of concern,
under the current regulatory regime.
Murray cod have been stocked into many water reservoirs throughout
the Eastern States of Australia and are a popular fish for
farm dams in warmer areas. Some re-stocking of river populations
has occurred but the most important tool in restoring cod
populations to something like their former glory is the appropriate
management of the river systems. This is a nettle yet to be
fully grasped by Government due to the inherent high costs
involved.
|
|
Individuals known to migrate upstream 100 Km or more before
breeding in spring, returning to original location afterwards
- even the same snag in the river at times. Breeding believed
to be triggered by longer daylight hours, increased water
temperature and flooding. Fry feed on zoo-plankton, etc.,
on newly inundated flood plain and are eventually washed into
billabongs and backwaters where they grow rapidly for 6 to
8 weeks before being recruited into main population by later
flooding. Growth has been estimated in the Murray River as
230, 340, 460, 560 and 640 mm after years one to five respectively.
Will breed in earthen dams in captivity if provided with suitable
spawning sites. 200 litre (44 gallon) drums with the ends
removed are suitable structures in a farm dam. Fisheries and
commercial breeders often use more sophisticated devices to
facilitate the removal of eggs, but the general aim is to
simulate a large submerged hollow log. Generally, spawning
structures are examined for eggs and when present they are
removed to a hatchery for incubation. If left in the structure,
as would usually be the case in a farm dam, a much lower survival
rate is to be expected, although this is not really a problem
in that case.
|
