Seasonal closure and minimum legal size for barramundi in the Gulf ...

Background
The Queensland Inshore Finfish Fishery is a multi-species commercial and
recreational fishery. According to recreational and commercial fisheries data
collected by Queensland Primary Industries and Fisheries (QPIF)—part of the
Department of Employment, Economic Development and innovation (DEEDI)—the
annual harvest of barramundi across the state is in excess of 1000 t. On a regional
scale, about 60% of this total catch generally comes from the Gulf of Carpentaria.
A stock assessment conducted by Welch et al. (2002) indicated that Gulf barramundi
stocks appeared to be healthy and that the fishery is sustainable. The latest stock
assessment of barramundi, however, indicated less conclusive findings on the status
of Gulf stocks due to concerns over the reliability of the commercial catch and effort
data used in the analysis (Campbell & O'Neill 2008).
Life cycle of barramundi
The barramundi is a large predatory species that can grow to 60 kg, but is usually
caught around 5–6 kg. The species is a protandric hermaphrodite, which means it
matures first as a male and becomes female later in life, usually after contributing to
at least one spawning season as a male. Gulf barramundi first reach sexual maturity
as males at about 55 cm, with 50% producing viable sperm at 60–65 cm (Davis
1982). Males then transition into females at 68–90 cm (mean 82 cm) (Davis 1982);
most of the larger specimens in the population are therefore female.
As the water warms and day length increases in spring, sexually mature barramundi
begin to aggregate in estuarine areas to spawn. The species is a broadcast
spawner, which means male and female fish release their eggs and sperm
simultaneously for fertilisation to occur within the water column. Individual fish will
spawn multiple times during a single spawning season, with each female releasing
tens of millions of eggs at each spawning event.
The eggs and larvae require brackish water to develop; however, the young fish are
very soon able to survive equally well in fresh or salt water. A significant proportion of
juvenile fish (generally within their first year of life) will tend to move upstream into
freshwater areas of rivers, creeks and billabongs if they are able to do so. Not all fish
move into freshwater however, and the general upstream migration pattern may vary
between areas and seasons. Those individuals that do move upstream tend to
remain in freshwater until they are mature males, usually returning to the estuary at
four (or less) years of age, although a small number can be up to seven years old.
Management measures
The management measures for barramundi are generally focused on ensuring that
the fish has an opportunity to spawn successfully. Ultimately, however, recruitment
for the species and fishery productivity is strongly influenced by freshwater flows, and
the ability of the juveniles to otherwise access freshwater areas. It appears that fish
that have accessed freshwater are bigger for their age than estuarine resident fish.
Therefore, access to freshwater habitats appears to promote a greater fishable
biomass than what could be achieved in the estuary alone (Julie Robins, pers.
comm.).
Technical report prepared for the Gulf finfish review. Not government policy. 4