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

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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
Protection for spawning barramundi is achieved through the following: a minimum legal size to protect the smaller individuals until a significant proportion are able to spawn as males a maximum legal size to protect larger females that may contribute disproportionately to egg production restrictions on minimum and maximum mesh sizes of commercial fishing nets to selectively fish for these legal-size fish a seasonal closure to fishing when barramundi are likely to be aggregating and spawning. While this general approach applies throughout Queensland, barramundi stocks from the Gulf of Carpentaria and the east coast are managed separately under slightly different regimes. These differences are summarised in Table 1. Table 1 Differences in seasonal closure (SC) and other management measures for barramundi in east coast and Gulf waters Area Duration of SC Legal size Net mesh Inshore netting sizes East coast 1 Nov – 1 Feb 58–120 cm 150–215 mm Restricted in SC Gulf Variable (see below) 60–120 cm 160–245 mm Prohibited in SC The seasonal closure in the Gulf of Carpentaria prohibits fishing for barramundi as well as the use of any commercial fishing nets under the N9 and N3 fishery symbols throughout late spring and summer. Gulf barramundi are known to spawn from about 6 pm to 10 pm on new moon and full moon periods around the high tide. Spawning occurs from about early October to early December in the northern Gulf and from about mid-November to late January in the southern Gulf. The spawning closure is set to cover as many of these peak spawning night-time high tides as possible across this time period throughout the Gulf. The start and end dates of the closure therefore vary from year to year. While the closure generally covers about 118 days, it may start as early as 25 September and finish as late as 4 February. Until 1996 the Gulf seasonal closure was the same as the east coast barramundi seasonal closure, which runs for a period of three months from 1 November to 1 February. The change was made in order to better align the closure with the known physical and biological drivers for barramundi spawning in the Gulf. The minimum legal size of barramundi in the Gulf was changed from 58 cm to 60 cm in 1999. The reasons for this change were to: improve the overall yield from the fishery move closer to the size at sex reversal simplify the size to aid recall. The recent review of the East Coast Inshore Finfish Fishery considered also moving to a lunar cycle-based spawning closure and a minimum legal size of 60 cm in that fishery. This was ultimately not supported for the following reasons: There is an even greater geographical spread, and therefore greater variability, in seasonal spawning peaks on the east coast than there is in the Gulf. Rainfall is generally less predictable on the east coast, meaning that recruitment is generally more variable. It was considered that there is little point Technical report prepared for the Gulf finfish review. Not government policy. 5
Page 1 and 2: Seasonal closure and minimum legal
Page 3 and 4: Contents Contents 1 Purpose of the
Page 5: Summary The current minimum legal l
Page 9 and 10: While the significance of this to m
Page 11 and 12: seasonal closure. For this reason t
Page 13 and 14: Modelling of impacts As discussed p
Page 15 and 16: Table 4 Definitions of symbols Nota
Page 17 and 18: Relative proportion 1 0.8 Commercia
Page 19 and 20: Discussion on analysis The analysis
Page 21 and 22: Appendix Model settings Table A1 Ba
Page 23 and 24: 12 10 log(ML/month) 8 6 4 2 0 2 4 6
Page 25 and 26: 0.5 0.4 Average spawning pattern Ea

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