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Research & Education
Predicting Port Phillip Bay Snapper Seasons |
FN0585
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Paul Hamer, PIRVic, Queenscliff
Updated: August 2007
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Port Phillip Bay is experiencing the best snapper season in the last 10 to 15 years. While keen fishers are catching snapper of all sizes (from small pinkies to large 10kg trophy fish) most are taking 2 to 6 kg snapper.
This abundance of snapper comes as no surprise to DPI’s fisheries scientists who have been monitoring juvenile snapper in Port Phillip Bay for the last decade.
Funded by Fisheries Victoria, fisheries scientists from DPI’s research arm (Primary Industry Research Victoria, PIRVic) survey selected habitats in Port Phillip Bay each March after the snapper spawning season to determine numbers of newly settled juvenile snapper less than 10 cm in length.
The numbers of fish caught in these surveys give the scientists an indication of the success of the previous spring/summer spawning season. This information, in turn, has allowed them to forecast the strength of year classes entering the fisheries several years into the future.
‘Because we know the rate that the fish grow we can predict when the young snapper from a good spawning year will be large enough to be legally caught and retained,’ explains Paul Hamer, PIRVic Queenscliff’s project leader.
By counting the yearly growth rings in the earbones (otoliths) of the snapper now being caught in the Bay, DPI’s researchers can determine the age of the fish and can validate their predictions about strong year classes entering the fishery.
As expected, the large numbers of snapper being caught in Port Phillip Bay this year have mainly originated from several very successful spawning seasons in the 1990s.
The ageing suggests:
- The majority of snapper in the 2 to 2.5 kg size ranges are from the 1997/98 spawning season, so they will be eight years old this summer.
- Snapper around the 4 to 5 kg marks are from two spawning seasons - 1994/95 and 1995/96 - so they will be 10 or 11 years old this summer.
- Many of the snapper around 6 to 7 kg are from the 1992/93 spawning season, so they’re about 13 years old this summer.
Previous ageing suggests that the 10 kg snapper could be up to 30 years old or even older! The oldest snapper aged by PIRVic was a 10 kg specimen from Western Port bay that was 39 years old.
The good news for Port Phillip Bay’s snapper fishers is that DPI’s researchers expect pretty good fishing for a few more years yet!
The juvenile snapper surveys indicate there have been several exceptionally successful spawning seasons in the last five years.
In particular, the 2000/01 spawning season was very successful and fisheries scientists have been monitoring this age group of snapper closely.
‘Fishers will probably remember being plagued by 15 cm pinkie snapper several years ago. These fish will be five years old this year and about 35-40 cm length, and we’ve already seen that these fish are contributing strongly to the current fishery,’ Paul says.
There was also another good spawning season in 2003/04 (see graph below), and provided the spring migrations of adult snapper into the bay from coastal waters remains consistent, the future prospects of Port Phillip Bay’s snapper fishery are bright.
Graph: Spawning Season 2003-04
The PIRVic research has also shown that successful spawning and survival of young snapper varies greatly from year to year. Ten to 20 fold variations in spawning success are a characteristic of snapper, and this variation in turn has a major influence on the strength of year classes entering the snapper fishery several years later.
Being able to forecast strong or weak year classes entering the fisheries is vital to ensure that fishery management arrangements are ‘adapted’ to suit the circumstances. Fishing controls will allow fishers to take advantage of times when spawning has been successful and snapper are abundant (such as now), but may need to be tightened in future to protect declining adult stocks if the surveys indicate persistent poor spawning success.
What influences spawning success and survival of young snapper is still unknown. Researchers believe it could be related to environmental factors such as water temperature and nutrient supply, which in turn influences the food supply, growth and survival of larval and small juvenile snapper from November to February.
Further Information
For more information about this project, please contact Paul Hamer at PIRVic Queenscliff on (03) 5258 0228.
Fisheries Research and Education Notes are available on the DPI website. Follow the prompts to Fishing and Aquaculture and then to Publications and Fisheries Notes. The notes are listed under the heading Research and Education.
The previous version of this Information Note was published in March 2006.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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