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How many bass are in lake Samsonvale?
  |  First Published: April 2016



If that sounds like the kind of philosophical question asked by fishers forever, you would be right. Most of the time it’s the question that gets asked when things aren’t going well and often with swear words. Still, it’s an important question when you are stocking fish.

This is a question that has vexed me ever since I first attended a meeting of the Pine Rivers Fish Management Association (PRFMA) to talk tagging data. At that first meeting, it was clear that the tagging data told a story without being clear on exactly what that story was. I felt as though we should be able to have a much better idea of what the state of the fishery was with the data we had.

It seemed like the kind of problem that would be particularly difficult to attack. That would prove to be the case, as a number of key stumbling blocks lay in the road, not least the lack of a method to assess the data. A bit over 12 months later and with a bit of creative thinking we came up with the method and once that hurdle was overcome we realised we were pretty close to be able to give a reasonable answer that will get better in the coming years.

More than that, we can predict what the effect will be of different stocking levels over time and even what the likely actual costs of stocked fish are. Before that we are best off starting at the beginning.

Background on Lake Samsonvale

Lake Samsonvale is located around an hour north of Brisbane, it’s around 22 square kilometers in surface area and has probably the most complicated access rules going. Fishing is generally restricted to the remnants of the North Pine River and access requires a permit from the Pine Rivers Fish Management Association (PRFMA). There are periodic monitoring events in the main impoundment as well and the PRFMA have conducted tagging over a long period of time.

Like many impoundments, the water levels have been variable over the past decade from lows in the mid 2000s to regularly near full in recent years. While most recaptured fish are recaptured in the impoundment, in 2010 the impoundment overtopped and for a time the fishing was pretty special in the North and South Pine rivers.

Catch rates in the impoundment are excellent, with taggers getting between 10-14 fish per day per person in the past three years. That is pretty consistent with the results reported by the general fishing community. Between 0.5-1 fish per fisher is kept, so the harvest rate is low, but still significant. Thanks to the PRFMA, the catch data in the impoundment is among the best in the state.

In September last year, there was a monitoring event sponsored by Insight Genesis. With nearly 1000 fish caught on the first day and less than 40 people fishing, it was pretty amazing stuff. It also shows that while the catch rate is great in the area available to fishers, there are still plenty more Australian Bass available in the main part of the impoundment.

How long do fish live in impoundments?

This might seem like an unconnected question, but as it turns out, it’s the figure that makes it possible to calculate how many fish are in the impoundment. One of the advantages of long term tagging programs is you can see things that happen in a fishery over a long period of time. When that is applied to bodies of water that are for the most part land locked you can get a sense of the dynamics of a population.

As it turns out, Lake Samsonvale is not ideal to determine this question, as tagging has been interrupted by drought and times where the water levels were too low to fish. There are three other impoundments for bass where we do have really good data.

As fish are not tagged when they are stocked (they are too small) we don’t have an actual age of fish but we can detect how long fish are in the system. We can still calculate an approximate upper age based on the data we have. While there are likely individuals that live longer than the standard age, they represent such a small percentage of the population as to be near enough to zero.

This opens up some interesting questions, such as should brood stock for the impoundment be taken from longer living fish?

How fast do Bass Grow?

This is not required to answer the question of how many fish are in the impoundment, but it does play a part in the story. What we find in Lake Samsonvale is that growth is highly variable. This could well be indicative of the variable conditions we see in terms of impoundment levels, but there could be other factors. Growth rates do affect fingerlings though, the faster they grow, the more likely they are to avoid predation. Equally, the faster they grow, the more likely they are to be able to compete for food.

There were 307 recaptured where the fish were out for 90 days or more and had positive growth. Growth appears to have been highly variable with a wide range of growth, however figure research indicates that most growth rates were between 0-60mm/year.

How does knowing how long fish live help?

In the process of working out how long fish live, we also calculated an annual mortality rate. This is the rate at which fish exit the system, through natural mortality, disease, predation or harvest. In the case of all three sites, this data was remarkably consistent and while we can’t say for sure just how many fish are harvested for example, we know as they age how many survive year on year from stocking events.

The rate of decline in Lake Moogerah and Lake Boondooma were around 10% per year higher than in Lake Somerset (20%) and the environment at Lake Samsonvale is closer to Lake Moogerah. As such the rate of decline in numbers is around 30% per year, that is each year 30% of the stocks for a given stocking year are lost to the system.

How many fish are stocked each year?

PRFMA keep really good records, so we know how many fish are stocked. The number stocked is variable depending on the amount of SIP funding available. This, however, is only part of the equation. Not all fish released survive. In fact, in wild populations, the survival rates on spawned fish can be less than one percent, which is why some species breed in huge numbers.

In the case of an impoundment, the fingerlings released have one big advantage over wild born bass in that they are grown out to 50mm at the hatchery. This means they can look after themselves to some degree. Having said that, the survival rate of bass on release plays a pretty big role. This is because the first year is the one where there is the highest rate of mortality.

Experimental work undertaken by the Department of Primary Industries and Fisheries estimated an adjusted mean recapture rate of 6% for bass released into a number of impoundments. The real world data in terms of catch rates, however, indicates that the survival rate may be higher in Lake Samsonvale.

Analysis of fishing records also shows a spike in the catch rates of bass in the region of release sites around release times, which has increased over time since 2010. This indicates that bass are not averse to snacking on the kids. Add to that other species such as spangled perch and even tilapia that are quite aggressive and found near sites where fingerlings will find shelter, and there are good reasons why the survival rate is not high.

Here is a simple scenario that demonstrates how many bass survive from a stocking using a middle of the road set of assumptions but also assuming no loss due to overtopping of the impoundment.

So how many Bass in Lake Samsonvale?

This is the first time an assessment has been made of the number of Bass in the lake based on stocking and tagging records. With each stocking, there were three estimates made of the survival rate of stocked bass.

In terms of the results from monitoring and events like the Insight Genesis 1000, we believe it’s likely the actual numbers of bass in Samsonvale are somewhere between middle and best case scenario. Further monitoring work will refine our understanding.

How much does a Bass Cost?

While we can’t publish the specifics of spend by PRFMA, we have done calculations based on their internal costs and given that fishers target and successfully catch mostly fish around the 5-year-old mark, that is the benchmark used. In this case, each bass that survive to that age work out at around $5-8.

To understand that calculation, using an assumed fingerling price of around $30c/fingerling and using the scenario from above, the cost for all fingerlings from the hatchery (100,000) = $30,000. The actual cost = 12% (survival rate year 1) = $2.5 per fish that survives. Year 2 (8,400 survive) = $3.57 per fish that survive. Year 5 (2,880 survive) = $10.41 per fish that survive.

In reality, that figure is high as these are just basic assumptions, but this makes the calculation easy enough to understand.

This process is in its early days and there is a bunch more work to do, but we have a lot of thanks to give to those groups that made this possible.

A big thank you to the PRFMA, whose efforts in collecting data have been second to none and they have been very willing to share data and. Lake Sommerset is a great place to fish and is very attractive as well. For more details on fishing Lake Samsonvale, go to the PRFMA website – www.prfma.com.au.

A big thank you to Insight Genesis as well who made the monitoring event in 2015 possible.

Thank you also to Hanwood Hatchery for providing a lot of insights on the process of supplying bass fingerlings. The people out there were very patient and put up with endless questions.

What do you think?

You can provide your thoughts on bass in Impoundments. Is there a question or issue that you would like us to look at? Send questions to --e-mail address hidden-- and we will do our best to provide an answer.

Data Sources Used in this Article

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

Fishing records provided courtesy of PRFMA.

“Impoundment stocking strategies for Australian native Australia: With an assessment of the value of scales as tags for stocked Barramundi” Michael Hutchison, Thomas Gallagher, Keith Chilcott, Robert Simpson, Glynn Aland and Michelle Sellin – FRDC Project No 98/221 (2006).

SURVIVAL ESTIMATION

IF 100,000 bass are stocked

YEAR 10 = 480

SCENARIOWORSTMIDDLEBEST
FINGERLING SURVIVAL YEAR 16%12%15%
ANNUAL REDUCTION IN TAGS50%30%20%
ESCAPEES (DAM OVERTOPPED)75%50%30%
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