Redfish and Specks Benefit from Limited Weir Closings
 

Most know that over the past four years, colleagues and I have been weighing and measuring fish for a three week period in May/June for the purpose of computing relative condition factor (a measure of overall health/plumpness) and correlating with various environmental factors. Measurements of weight and length of over 1000 fish over 4 years allow us to test hypotheses and quantify which causal factors seem most closely related to fish plumpness.

After the Prien meeting in July, we contacted Leigh Anne Sharpe requesting the historical weir opening data for the purpose of trying to correlate weir openings and closings with fish plumpness. If closing the weirs was "choking the lake" then this would show up a positive correlation between weir openings and relative condition factor.

The results of the best fit trend lines are shown in the attachment for redfish and specks. The horizontal axis is the fraction of the total possible time and area of weirs being open the 90 days prior to a fish being measured. We picked a 90 day window, because fish plumpness tends to depend on food availability over the past three months and a 90 day window revealed the strongest correlations with all environmental factors (salinity, temperature, weir openings). The correlations were negative and significant for the available data between 40% and 80% weir openings. In other words, once the average area of the weir openings over the past 90 days is 40% of the total possible opening, keeping the weirs open for longer seems to decrease the plumpness of the fish.

Since this is a relatively recent and preliminary result, my colleagues and I have been hypothesizing why opening the weirs more is well correlated with the fish being thinner. Here are some ideas:

1. Specks and redfish are drawn to moving water. Perhaps the fish so strongly aggregate in the moving water of the weirs that the energy spent fighting the moving water and competing for the point source food supplies is less conducive to weight gain than foraging in other parts of the lake. It's not that the weirs don't add lots of food to the lake, but that the resulting competition and exertion promotes less plumpness than more sedentary feeding lifestyles throughout the lake.

2. Weir openings are a two way street. Bait flows back and forth. It may be that above 40% openings, the net flow of bait is out of the lake and back into the marsh, and that the marsh actually provides more challenging foraging conditions for the fish (or fewer fish make it back into the marsh).

3. In the case of specks, at least part of the negative correlation may be attributed to a separate causal factor: salinity and osmoregulation. Low salinity causes the weirs to be opened. Lower salinity also increases osmoregulation costs in specks (they prefer saltier water). However, if this were the whole story, relative condition factor in specks would be more strongly correlated (positively) to salinity than negatively correlated to weir openings. As it turns out relative condition factor in specks is more negatively correlated with weir openings than its (+ or -) correlation to any other factor studied.

4. Likewise, redfish have stronger negative correlations with weir openings than any other (+ or -) correlation studied. Unlike specks, redfish are very happy in the whole range of salinity found in Big Lake, and fresher water does not raise their osmoregulation costs.

We do intend to perform more detailed length class analyses to see of different length classes of each species respond more or less strongly to weir openings.

So lake fish tend to be fatter than river fish is (kind of over simplified) what this boils down?

The key experimental finding is that closing the weirs does hurt the redfish and specks, and it may even be helping them up to a point. The data shows that the fish have been thinner when the weirs are opened more.

The idea that needing to swim against the current to feed effectively at the weirs is a hypothesis to explain the unexpected experimental result. It is well known that (freshwater) trout in rivers and streams tend to be thinner than those in lakes, and the trout in streams compete with each other for the spots that allow them to rest out of the current but still ambush prey effectively. Trout in streams and rivers that fight the current most of the time quickly lose body condition, not from insufficient forage, but from the energy requirements of fighting the current.

I posted some of our hypotheses, not because we are highly confident of the explanations, but to stimulate discussion and get feedback on whether readers thought the hypotheses were right or wrong (and why) and also to see if readers here had any possible explanations we hand not thought of.

So you mean that opening the weirs actually makes for SMALLER FISH?!?!

GASP!!

And all this time we've been led by some to believe that the weirs should be open all the time!

Mathgeek, your hypothesis makes sense. However, I think it is flawed due to the assumption that every fish measured would have been caught near the wiers in strong current, which is obviously not the case.

Thanks for bringing this up.

There is no assumption about every fish. Just enough fish (a percentage of the fish sampled) spending time near the weirs over the few months prior to being caught to reduce the mean condition factor.

Think of an analogy with BMI in humans. Suppose 1/3 of the population caught the flu and lost 30% of their body weight. The average weight of the whole population would be reduced by something close to 10%, even though everyone did not catch the flu or lose weight. The average BMI would be lower, even though many humans lost no weight.

The average of a population can be decreased without every fish being decreased.

The opposite would also be true if opening the weirs significantly increased body condition. If only a percentage of the fish dined at the weir buffet and got a lot fatter, the result would be a smaller, but significant increase in the average body condition. Because our measurement methods can determine mean relative condition factors to an accuracy of 1% or so, we can detect relatively small changes in average body condition.

OK, I get what you mean by the average BMI. IMO the estuary is too large to make an assumption that even a large percentage of the fish caught and measured spent at least some point in their life near the wiers. You have measured some of my fish in the past, but I can't remember if you asked where the fish were caught. Maybe this would help develop your hypothesis, that is, in future samplings ask the angler where the fish were caught. Assuming your hypothesis is correct, it may be useful to break down the data into groups, say fish caught at wiers, near the wiers on east bank, west cove, north end, etc... Then compare the data to see if indeed fish near the wiers have a lower body mass.

saying its too large? have you seen the area behind the weirs? how far back that ecosystem goes? I cant imagine how many hundreds of thousands of acres are hidden from just the push of a button. theres also no proof that if someone catches a fish in turners that it couldnt have been attempting to feed at the jetties 2 days before that. the fish move!

great graph mathgeek and i think until there is a chip in every single fish that there is no better way to tell when the fish are being starved.

Yes, I do know how expansive that marsh is. I used to collect data for the coastal restoration projects and ran that marsh for 5 years. You are correct, the fish do move. I still think breaking the data into groups would give a more accurate representation for MG's hypothesis. Furthermore, in order to develop his theory, he would also have to take into account salinity, temperature, turbidity, etc...

true true

but im sure if he was getting paid to do these observations he would have so much info we wouldnt even wanna see it. this is all done because he wants to. i cant imagine if we paid him for the info!!

I have my own science !! It's called years and years of fishing this lake and when the weirs are closed for a long period of time , it effects the lake !

It effects everything , the natural flow of water , shrimp migration etc

No doubt. I agree, however the wiers are there for a good reason, and that is to save the marsh behind the wiers. It is a damn shame it is so poorly managed, that is the opening and closing of the wiers.

Most available data show that over a three month period, fish are all over the estuary, and the conditions (food availability, salinity, temperature, exertion) over the 90 day window before fish are caught have a significant impact on body condition. Knowing where a fish was caught, just tells us where it was that day. Knowing where a fish was for the 90 days before it was caught would require outfitting a bunch of fish with acoustic telemetry and then having enough of those fish caught and returned to measure their weight. But then one needs to assume that adding the acoustic device did not impact the condition of the fish (unlikely).

Since spatially resolving where a specific fish has been over the past 90 days is a very tricky deal, one generally needs to develop methods for testing hypotheses which does not depend on the locations of each specific fish over the past 90 days. Perhaps a more approachable method to test this hypothesis would be in a laboratory where groups of fish are exposed to the different levels of current with the same amount of food. This would enable quantifying how much fighting a given current impacts body condition. These kinds of studies have been done in freshwater trout. But the lab study would only show that the hypothesis is plausible. A sonar study could estimate the biomass of fish in different current zones of the weirs at different times. If a significant fraction of the speck and redfish biomass were coming to the weirs and in locations where they were fighting the current, that would be much more compelling support for the hypothesis than angler reports regarding where specific fish were caught.

We are also computing correlations with every available environmental metric. So far, the biggest factors are the WEIRS and the OYSTER REEFS.

The effects of salinity and temperature are predictable. Fish that prefer saltier water (specks) have a slight positive correlation between salinity and body condition. Fish that prefer fresher water (puppy drum) have a slight negative correlation between salinity and body condition, etc.

But in most cases, whether the weirs are open is a bigger effect, and for benthic feeders, the health of the oyster reefs is a bigger effect.

Grouping the fish by where they were caught is not only problematic because of the degree of fish movement over several weeks, but also because anglers are notoriously inaccurate when reporting where fish were caught.

Exactly my point, it is indeed very tricky to resolve where a fish has spent its time over the previous 90 days. Without out more tagging data, your hypothesis is difficult to develop. However, I would think that a fish caught in the Haymark or Prien is much less likely to have been spending time at the wiers than say a fish caught on the east bank. Last year I tagged approximately 75 fish, and the majority of the tags that are retrieved indicate the fish have moved less than 1/2 mile from where I tagged them.

Weirs should never be closed 4 days before full moon and 4 days after
Same with new moon

So they should be open at least 16 days a month at bare minimum...

The should also operate on tide changes of the marsh if salty with low water and readings are higher in marsh than in lake like last year , you need fill it back with lower salinity water even if it's over the 5ppt or what ever it is


They do it all for duck grass

No doubt!! All the fish I catch are from Sabine!

You need to realize that the weirs are never completely closed. Even when all of the mechanical gates and flaps are closed, over 10% of the total area coupling the marsh and lake remain open. And the operating procedures for opening the mechanical gates are such that, over most 90 day operating periods, the average coupling between the marsh and lake is at least 40% of the possible total.

So, you may be right that extended closures (90+ days) of all the gates so that only 10% remained open might negatively impact the fish. However, the historical operating data shows that the gates are never all closed for more than a couple weeks at a stretch, and that this only occurs during high salinity periods.

You cannot judge the openings from the boat bay or from the surface gates. There are numerous gates and flaps and slats below water level that allow bait to move back and forth even when the boat bay and surface gates are closed.

BTW, kudo's to you MG for your efforts in gathering more data on the health of BL

Which is why I believe the oyster dredging has more of a detrimental effect on the health and quality of the lake than wier management.