Redfish and Specks Benefit from Limited Weir Closings

[MathGeek]

Originally Posted by Duck Butter

On what planet does it make sense that if more food is available (weirs open) would fish be less fit? None.

Plumpness in animals depends not only on food intake, but on the balance sheet between food intake and energy expenditures.

It is very common for freshwater trout in mountain reservoirs to lose body condition over the spring and summer months (when most food is available) because their energy requirements are also a lot higher in the summer months. Conversely, it is common for freshwater trout in mountain reservoirs to gain body condition over the winter (when food is scarce), because their energy requirements are much lower.

Similarly, stream trout can lose body condition under high current conditions because the additional energy expenditures exceed the additional caloric intake.

With brackish species, osmoregulatory costs also factor in: salinity much higher or lower than the preferred range of a species significantly drives up metabolic costs.

A human counterexample would be an athlete losing BMI with the same caloric intake on which most office types would quickly gain BMI.

Originally Posted by Duck Butter

On what planet does it make sense that if more food is available (weirs open) would fish be less fit? None.

In the case of the weirs, the planet is planet earth. For additional weir openings to increase fish condition, the additional food needs to exceed the additional energy requirements of the change.

Opening the weirs in addition to the baseline opening may not provide a net gain in additional food at all if the net change in forage flow is negative.

Opening the weirs in addition to the baseline opening may provide additional food, but it may increase the metabolic costs by a larger amount. This seems more likely.

It's like giving away Big Macs on the top of a mountain with the parking lot at the bottom. The people eating the extra burgers would probably lose BMI.

[Duck Butter]

Originally Posted by MathGeek

Plumpness in animals depends not only on food intake, but on the balance sheet between food intake and energy expenditures.

It is very common for freshwater trout in mountain reservoirs to lose body condition over the spring and summer months (when most food is available) because their energy requirements are also a lot higher in the summer months. Conversely, it is common for freshwater trout in mountain reservoirs to gain body condition over the winter (when food is scarce), because their energy requirements are much lower.

Similarly, stream trout can lose body condition under high current conditions because the additional energy expenditures exceed the additional caloric intake.

With brackish species, osmoregulatory costs also factor in: salinity much higher or lower than the preferred range of a species significantly drives up metabolic costs.

A human counterexample would be an athlete losing BMI with the same caloric intake on which most office types would quickly gain BMI.



In the case of the weirs, the planet is planet earth. For additional weir openings to increase fish condition, the additional food needs to exceed the additional energy requirements of the change.

Opening the weirs in addition to the baseline opening may not provide a net gain in additional food at all if the net change in forage flow is negative.

Opening the weirs in addition to the baseline opening may provide additional food, but it may increase the metabolic costs by a larger amount. This seems more likely.

It's like giving away Big Macs on the top of a mountain with the parking lot at the bottom. The people eating the extra burgers would probably lose BMI.

seatrout (drum/apples) freshwater trout (salmonids/oranges) but anyhoo

[MathGeek]

Originally Posted by Duck Butter

seatrout (drum/apples) freshwater trout (salmonids/oranges) but anyhoo

Your statement about more food always leading to plumper fish seemed quite global:

Originally Posted by Duck Butter

On what planet does it make sense that if more food is available (weirs open) would fish be less fit? None.

Since there are well-known counter examples on planet earth, it seemed worthy to point them out. Why would spotted seatrout not be subject to the same balance sheet considerations of every other fish species? To gain weight, the caloric intake must exceed the calorie requirements. More food is not enough if the energy requirements increase by more than the energy intake. This would be true on every planet, and for every fish species on planet earth.

We've been looking through the data and analyzing in more detail. One notable fact is that there are almost always high flow conditions (full moon, new moon, high water behind weirs) when more than 40% of the gate area is opened. It would be useful to know what current speeds are present at the weirs under these conditions, and how these current speeds compare with the naturally occurring current speeds in the estuaries over the past few thousand years.

If the freshwater trout energy expenditures can be strongly impacted by the same current speeds they have seen continuously for thousands of years, it would stand to reason that current speeds much higher than Gulf inshore species have seen for thousands of years could also have a big impact on energy expenditures. Natural selection has done its job preparing freshwater trout for stream current conditions, but natural selection may not have made inshore species well adapted to the current conditions present at the weirs.

[Duck Butter]

Originally Posted by MathGeek

Your statement about more food always leading to plumper fish seemed quite global:



Since there are well-known counter examples on planet earth, it seemed worthy to point them out. Why would spotted seatrout not be subject to the same balance sheet considerations of every other fish species? To gain weight, the caloric intake must exceed the calorie requirements. More food is not enough if the energy requirements increase by more than the energy intake. This would be true on every planet, and for every fish species on planet earth.

We've been looking through the data and analyzing in more detail. One notable fact is that there are almost always high flow conditions (full moon, new moon, high water behind weirs) when more than 40% of the gate area is opened. It would be useful to know what current speeds are present at the weirs under these conditions, and how these current speeds compare with the naturally occurring current speeds in the estuaries over the past few thousand years.

If the freshwater trout energy expenditures can be strongly impacted by the same current speeds they have seen continuously for thousands of years, it would stand to reason that current speeds much higher than Gulf inshore species have seen for thousands of years could also have a big impact on energy expenditures. Natural selection has done its job preparing freshwater trout for stream current conditions, but natural selection may not have made inshore species well adapted to the current conditions present at the weirs.

i hear you and my original comments were just in response to you asking for some hypotheses to why which I think are very plausible. I think the weir openings correlating to thinner fish in a system that is almost 80 square miles is pretty unplausible especially when the weirs are not stopping all bait from getting through.

There are likely several factors if not 10 or 20 that are contributing to thinner fish. A correlation does not = causation.

[MathGeek]

Originally Posted by Duck Butter

There are likely several factors if not 10 or 20 that are contributing to thinner fish. A correlation does not = causation.

Right. But given 10-20 factors, when a single factor has a correlation coefficient close to -0.5 and a P value less than 0.001, the possibilities of actual causality and the physical mechanisms that might relate to causality are probably due careful consideration.

Ultimately, hypothetical causal mechanisms are supported or rejected with data. Correlation never proves causation, but it can support causation. However, correlation can disprove causation.

In this case, the claim that closing the weirs is choking the lake or starving the fish is completely unsupported in the available data.

The case that fully opening the weirs would harm the fish is only mildly supported.

One feature of Louisiana law is that wildlife management may proceed with the best available science, even if that science is imperfect and hypotheses are only supported by the data and not convincingly proven.

The assertion that current weir operations according to the established plan are not harming the fish and are benefitting the ecosystem as a whole is well supported by the data.

Fish and the fishery are not being compromised by closing the weirs as necessary to protect the marsh.