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DAF Long Term Monitoring Program and Suntag
  |  First Published: May 2016



The Department of Agriculture and Fisheries run a long term monitoring program (LTMP), which monitors the status of key species. The program uses a combination of techniques such as age collection from otoliths as well as the collection of length data from harvested fish in both the recreational and commercial sectors. Length data is recorded from surveying the catch by DAF staff.

In contrast community data is collected at point of catch by the fisher as a part of their normal fishing activity.

Fish are aged by extracting a pair of bones called otoliths (ear bones), then examining them under a microsope and counting what are the equivalent of tree rings on the bones. Otoliths are made up of calcium carbonate and a protein that is laid down at different rates during a fish’s life. This process creates alternating opaque and translucent bands.

Dusky Flathead and the LTMP

Dusky flathead is an important recreational species. There are a number of reasons for that, they are great eating, easily found, targeted from land or boat and there are a lot of techniques that work to catch them. In the last Statewide Recreational survey, flathead was the number 4 reported fish after yellowfin bream, sand whiting and trumpeter whiting. Suntag has records on around 57,300 flathead making it also number 4 on our all time list after barramundi, bass and bream.

The differences between LMTP and Suntag in terms of species is due to Suntag fishers using lure fishing most often as their catch method, where as the Statewide Survey and LTMP report a much broader mix of techniques overall with more bait anglers.

The LTMP has produced a really good Age-Length curve for dusky flathead, and I will be using that in the discussion. Please note that age-length varies from state to state. A number of fish species in Queensland seem to grow faster than their southern cousins, which may be due to the more tropical/energy rich waters providing more food overall. Or it could just be the fish know we make everything better in the Sunshine State.

Are dusky flathead protandrous?

Some species of Flathead are protandrous hermaphrodites, that is they change sex from male to female and it has been put forward that dusky flathead are also, based on the fact that all large flathead are female. A recent report by Barry Pollock has addressed this issue.

The reality is that male dusky flathead are smaller, grow slower and live shorter lives than the females. Males grow to around 50cm as a typical maximum length and live around 6 years. Females can grow to 1.3m and can live longer than 15 years.

Why this is the case isn’t well understood, but it’s a naturally evolved adaptation, so it must have given flathead an advantage at some point.

Dusky flathead status

The LTMP reports that the majority of fish harvested are 3 years in age and that overall the harvest consists of fish of ages 3-5 years (40-60cm). At 5 years of age fish are getting close to the upper 75cm slot limit. Interestingly, most of the fish in this size range are females, so most of the fish harvested are female.

Out of interest, I looked at the Suntag data to see what we have. I used a chart to cross match fish recorded against the median age provided by the LTMP. In terms of method of capture, 21,400 have been caught on lure and 4,900 on bait showing a preference for lure.

I looked at data over 20 years (55,000 fish) and a couple of recent time periods (2004-5, 2014-5) and the results were similar, suggesting that there hasn’t been a lot of change.

Conclusion

The LTMP program is very robust in its design and execution. The current assessment of the dusky flathead fishery is that it’s sustainably fished. Flathead are heavily harvested particularly by the recreational sector, with an estimated catch of 150 tonnes, which shows in the fact that not a lot of Year 6 and beyond fish are recorded in either the Suntag data or the LTMP harvest, though with an upper slot limit that plays a big role in the harvest data.

It is good though that the Suntag data gets similar results as this suggests the fishing experiences reported for all fishers surveyed – commercial, recreational and taggers are the similar.

Yellowfin bream and the LTMP
Yellowfin bream is the most highly harvested recreational species with around 1,156,000 the estimate in the most recent Statewide Recreational survey. They are a good eating fish and reasonably easy to catch on a variety of bait. In more recent years, a large sportfishing scene has built up around bream with a range of specialist lures now available. Steve Morgan recently did a video showing how simple it was to pick up bream on lures. Suntag has records on around 61,000 fish making it our third most reported fish species after barramundi and bass.
The LTMP has produced a really good age-length curve for yellowfin bream and I will be using that in the discussion. Please note that age-length varies from state to state and bream from southern waters have slower growth rates particularly as they get older.
The 2010 change in size limit

Up until march 2010 the size limit for yellowfin bream was 23cm and that was then raised to 25cm in order to give bream more opportunities to spawn before they are harvested. The effect of raising that limit was to 25cm was to have an immediate reduction in fish harvested at 23cm and over time the proportion of fish harvested at 25cm increased in both the recreational sector and the professional sector.

In 2010 around 35% of commercial and 27% of recreational yellowfin bream harvested were in the 25cm class. That grew steadily until in 2013 around 51% of commercial and 30% of recreational yellowfin bream harvested were in the 25cm class.

The interesting thing is the Suntag data tells a similar story. Taking 2 years immediately prior 2007-2009 (8000 fish) and then two years after the bag limits should have had an effect 2012-2014 (4100 fish) there is a definite shift.

The difference is that the Suntag data contains a much more even proportion of sizes. For example, there is not as big a spike in the 25cm class of fish but there is more fish >32cm being reported.

The same population dynamics are detected but the spread of fish reported are quite different.

Yellowfin bream status

The LTMP reports that the majority of fish harvested are 5-6 years in age and that overall the harvest consists of fish of ages 4-10 years (25-40cm). Figure 10 shows the typical age profile of yellowfin bream harvested in Queensland.

In comparing the LTMP with Suntag, I used a chart to cross match fish recorded against the median age provided by the LTMP. In terms of method of capture, 6400 have been caught on lure and 24500 on bait, showing a preference for bait, which is consistent with harvest fishers.

Conclusion

The current assessment of the yellowfin bream fishery is that it’s sustainably fished. An interesting fact from the LTMP is that recreational fishers seem to be more successful at targeting Bream >27cm and the Suntag data supports this. This suggests that larger bream are found in different areas to those targeted by commercials. It’s also possible larger bream are more randomly found in the fishery.

Snapper and the LTMP

Snapper is a popular recreational species with an estimated catch of around 203,000 fish in the most recent Statewide Recreational Survey. They are a good eating fish and reasonably easy to catch on a variety of bait. In more recent years a inshore kayak fishery has sprung up particularly in Moreton Bay using soft plastics, hardbodied lures and hard vibes. Suntag has records on around 21,300 snapper.

Snapper is a long living species with slow growth rates. The LTMP has produced a really good age-length curve for snapper and I will be using that in the discussion.

Suntag and LTMP on snapper

While for bream and flathead there is quite a bit of overlap with the LTMP in terms of fish recorded when it comes to snapper Suntag is seeing quite a different part of the picture. Of the 21,332 recorded 18,116 are <35cm, so 84.9% of the fish recorded are below legal length. The LTMP by contrast sees fish that are above legal length as they are harvested fish.

We are seeing variations in catch rates of undersized fish over time. The detail of that requires quite a bit more work but what we do know is the more juvenile fish we pick up in monitoring programs the better we can predict the future fishing conditions.

I did quite a lot of reading on the causes of variability of recruitment from both Australian and New Zealand researchers, and I can summarise as ‘it happens’. There is quite a sophisticated program in Victoria that monitors very small juveniles but they look at quantity of variation more than the why. Most suspect it is an environment factor, so I did a rough plot of the Sea Surface Temperature Anomaly vs Catch Rate for Juveniles just for interest. While I am not drawing any conclusive links, it does seem to indicate there could be an environmental component.

The larger the juvenile catch the better fishing conditions will be in the future. That is certainly the experience we have in the barramundi fishery where variable recruitment is also a factor. I also did a plot of the undersized catch rate vs the commercial catch. We don’t have reliable data on the recreational harvest, beyond knowing that it’s at least as large as the commercial catch. The interesting point of note is there was a spike in the juvenile catch rate in 1994-95 and then a spike 10 years later in the commercial catch. We know that snapper live long and are slow growing, that there might be as big a gap as 10 years before they are picked up is interesting.

Snapper status

The LTMP reports that the majority of fish harvested are 4-5 years in age and that overall the harvest consists of fish of ages 4-11 years (35-80cm).

Unfortunately, we don’t have a baseline age profile from the LTMP to compare with for snapper.

In comparing the LTMP with Suntag, I used a chart to cross match fish recorded against the median age provided by the LTMP. In terms of method of capture 2,800 have been caught on lure and 7,800 on bait showing a preference for bait, which is consistent with harvest fishers.

Conclusion

Snapper are currently assessed as overfished and while our data isn’t as strong in harvest length snapper that is almost certainly right. There hasn’t been a sustained period of above average undersized snapper since 2006 and in the past decade the Suntag detected level of undersized snapper has been well under the long term average.

The LTMP detected a 34 year old snapper (75cm) in 2012 and a pair of snapper at 26 years (71cm and 90cm), which highlights that snapper have very variable growth rates. The combination of variable recruitment and variable growth does make them somewhat susceptible to harvest pressure relative to yellowfin bream and dusky flathead, which both seem to have consistent replenishment of the stocks relative to harvest.

What do you think?

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Data Sources Used in this Article

Tag and catch records from http://qld.info-fish.net 1985 to 2015.

DAF Monitoring for snapper, yellowfin bream, and dusky flathead can be found through the Queensland Fisheries website. Follow the links through ‘Monitoring our fisheries’ – ‘species specific programs’ for more information. Other sources include,

• The annual spawning aggregation of dusky flathead platycephalus fuscus at jumpinpin, Queensland Pollock, b.r. 2015.

• http://fish.gov.au/reports/finfish/flatheads/Pages/dusky_flathead.aspx.

• Migratory dynamics and recruitment of snapper, Pagrus auratus, in Victorian waters Paul A. Hamer and Gregory P. Jenkins

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