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.

I believe the data. Fish condition correlates positively with oyster stock assessments for benthic feeding fish. Fish condition correlates negatively with weir openings.

We were certainly open to the idea of the weirs being the dominant effect on fish condition in the lake, which is why we did so much work acquiring and analyzing the weir data.

Other than the effects of oyster reefs on fish condition, most of what we are seeing relates fish condition negatively to factors that increase metabolic costs (salinity, temperature, etc.) rather than positively to factors that increase food supply. Slowing down currents and providing fish places to rest out of the current may be a more important factor for increasing fish condition than increasing the food supply. We may not need to feed them more so much as work them less.

bounce da hen 2030 will not exist!! cant wait!!

I havent fished the lake for near as long as some ppl on here. But from i think their are many many factors that effect the fish. IMO one of the biggest factors is Pressure/ IE boat traffic! SOOO much easier to find fish on a monday than a saturday..... ppl dont realize how many fish they run over on the way to the "hot" spot...

If I'm reading MG's statements correctly, then you obviously aren't. He clearly states that when the weirs are between 40-80% open over a particular period of time there is a NEGATIVE correlation between body condition and weir openings.

This clearly contradicts your hypothesis.

MG, feel free to correct me if I'm reading that wrong, but I definitely see a chart that shows at 80%, the Condition is below 100, whereas around 40%, the Condition is above 100.

1st off let me make this statement

Trout weight diffrent every month ... So you can not go by body weight or fatness

Here is why , when fish are schooling and gorging on shrimp and pogy they burn as much as they eat !! Have you ever followed a school of fish ? They can move over 12 Miles a day !!

Colder month fish are not as mobile and eat slower and bigger meals and do not burn the enegry

Also fish sitting closer to the weirs over a givin time will likely be fatter depending on there movement compared to a trou up north ?


So saying fish are fatter or better health with weirs open or closed is not telling me anything

Unless you sit and catch every fish at weir !!


Trout move miles everyday

I can show you this summer when I caught 45 trout all soild 2-4lb trout and next day 45 dinks non over 3lbs on same spot same tide ?

The numbers are from 3 week periods of may-june over a 4 yr time frame.

Exacty you can not tell the health of the estuary with that data


Now don't get me wrong MG you are an asset to this fisheries I wish they would find you to do all our research

That's right. One should be careful not to extrapolate the line below 40% where there is no available data. Shutting the weirs up to a 10% opening may well negatively impact the fish. But once they are open 40%, opening them up more DOES NOT FATTEN the fish.

Sure you can. The "expected weight" is determined from state wide data from the same time window. We are not comparing our data from a three week window to data from throughout the year. We are comparing our data with statewide data from the same time of year. Seasonal variations are thus accounted for.

The data are from a three week window each year, so they indicate the health of the estuary during that time window and things that impact fish condition when assessed at that time of year.

It is possible that fish condition is somehow negatively correlated with weir openings when assessed in late May and early June, but that there is some mysterious factor that makes the relationship so strongly opposite the rest of the year that the data we are missing the rest of the year would overwhelm our seasonal snapshot?

Yes, it is possible. But it isn't likely.

By what mechanism would fish condition be negatively correlated with weir openings in May and June, and then suddenly switch to a strong positive correlation the rest of the year?

But it is a point worthy of some consideration and debate.

So Could it be that when the weirs are open fish are more active and gorging ( regurgitating) due to the amout of bait and tidle flow?

When weirs are open we have move tide and bait on east side as to none when close ? So could it just change there feeding habits as when you have affective tides and bait trout on east side gorge more ?

Let's take last summer for prime example , weirs were closed all summer long and trout fishing was horrible. East side and south end were not very productive as it has been this year due to them being open all summer long

Redfish and Specks Benefit from Limited Weir Closings
 

Who decides when to open or close the weirs?


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You have some interesting ideas here, to be sure. Regurgitating would likely decrease feed efficiency and increase energy costs.

Question: specks are well known for their rapid gorging and regurgitating under certain conditions. This may account for their negative correlation with weir openings in the spring months. What do you know about redfish gorging and regurgitating? Sure, any fish can vomit in the boat or in the ice chests, but do you know of redfish gorging and regurgitating like specks do? Or is this a unique behavior to specks and more closely related species?

The weirs concentrate the fish in a predictable location so they are easy to catch. The fish may be responding more to the moving water than actual feeding efficiency. "Productive" from an angling viewpoint means hungry (easy to catch) and concentrated fish. "Productive" from a fish condition viewpoint means well fed and spread evenly through areas of the estuary with sufficient forage.

In 2013, the weirs were mostly open through May and June, and the mostly closed situation began July 10, 2013 and persisted until early October when fresher water allowed switching to a mostly open operational scheme. In 2014, the weirs were mostly closed (30% or so open) from April 15, closed even tighter in early May, and remained closed tightly (10% or so open) until mid July when heavy rains dropped the salinity and allowed going back to a mostly open condition.

There were similar length weir "closings" in both 2013 and 2014, but it occurred earlier in 2014. The greater productivity of the estuary in 2014 was not due to the weir openings after mid-July. The productivity was obvious back in May when we were doing our measurements. Lots of anglers were complaining that the fish were hard to find, but the fish were fat and happy and chasing abundant bait fish (menhaden, croaker, mullet) in schools scattered throughout the estuary rather than stacking up at the weirs sharing a meager influx of shrimp (came on stronger later in the summer).

More specifically keeping the weirs open more than 40% of the total gates is correlated with THINNER fish.

In the long run, protecting the marsh is in the best interests of the entire fishery.

MG,
I am a pro-weir big lake guy. So I appreciate your thoughts and research.

Do you or anyone have an knowledge if the weirs affect the oysters any? Not sure if weirs being closed for a long period hurt the oysters, which would indirectly negatively affect the fish.

Just wanted to get your 2 cents on that.

I have been looking into the whole oyster issue for a while now. It appears that the main problem with the oysters is that the salinity levels in the lake are too high. There is data to back this up. The upcoming salinity control projects would be good for the oysters and hopefully would possibly allow the weirs to be open almost all the time.


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What are they planning to do that would lower the salinity?



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Miami corp duck hunters

There is a group I have been in contact with That is planing to install a rock or levee system along ship channel to lower salinity by 6-8 ppt

That's a great idea worthy of consideration.

Other than overharvesting and dredging, the biggest enemies of oysters are the oyster drill, the black drum, and overly low and high salinities.

Is it possible that opening or closing the weirs somehow impacts (for better or worse) one of the key predators so that predation pressure is increased or decreased? Sure, but this is likely a second order effect that may vary depending on the time scale. For example, suppose opening the weirs pulls a lot of the drum off of the oyster reefs to feed at the weirs, thus reducing predation pressure. This is good in the short term, but could also have the effect of increasing the drum population in the longer term.

One big issue for the oysters at the S end of Big Lake (east side) is the additional stress from high salinities. The weirs may have a small effect here, but in the long run, improvement requires smaller coupling between the lake and the Gulf.

There is a lot of truth here, in that the high salinities are the main problem in recovery of the oysters after the major overharvesting event on the E side in 2010.

Reducing salinity is one key issue to the future of the oyster reefs in the estuary.

But it was over harvesting rather than salinity that created the original problem back in 2010. Oyster harvesting was closed in most of the state due to the oil spill, and tremendous harvest pressure was concentrated on Big Lake.

Addressing the salinity issue will likely allow the oysters to return to historical levels.

But keeping harvests at a modest and sustainable level is also key to long term productivity of the system.

There is little doubt that this would benefit the oyster reefs (lower salinity), the marsh (lower salinity), and the lake (weirs open more, lower volume tidal flow).

Getting the funds and approvals is a non-trivial challenge.

I think things are going in the right direction with the harvesting issues. Side note, LWF built a reef south of the old jetties that had never been harvested (15 acres) it basically died last year. No dredging had been done. I'm sure you have looked at the assessments the sample stations in the lake show zero oysters on them. It seems it would have to be the salinity levels stopping oysters from coming back not dredging. FYI i would like to see the oyster dredging stopped.

Yes, salinity levels at the S end of the lake are high enough to cause a problem with no harvesting. The reefs at the N end of the lake are in better shape. Most of the LDWF sampling stations for the stock assessments are near the S end of the lake, because this is where most of the commercial harvesting occurs. There is a line running east-west near the middle of the lake N of which harvesting is closed even when the S end of the lake is open. Because the N end is closed to harvest, LDWF has less interest in rebuilding or assessing stocks in this area.

Right. Since there has been no dredging on the E side since 2010, dredging on the E side is not the current problem.

But note that LDWF is more interested in rebuilding reefs S of the line where oystering is allowed, rather than N of the line where reefs would be protected. Since salinity levels are lower the further N you go, it would make more sense to restore oysters N of the line. Restoring reefs S of the line in the higher salinity areas suggests that the purpose of reef restorations is focused on harvest, not habitat and ecosystem services. See the attached figure.

I'm sorry, but LDWF is going to build a 50 acre reef next year and it will more than likely be North of the line. They do not want to build it south of the line. LDWF has to monitor the harvest area to regulate oyster havests, so i guess it may appear that they don't care.

A 50 acre reef N of the line would be awesome.

It's too bad they waited so long.

Both salinity and harvest prohibition favor long term success for reefs N of the line. Let's keep the pressure up to make sure this happens. I could see them caving to the oyster lobby and moving it S.

Very Very Interesting !!!

There's a bunch of greedy Lawyers out there that rationalize they're just being kind and supportive to the Cajun Culture. Yet most charge hourly and a % of the take/reward/compensation for their "expenses". And the Oyster lobby has political clout, the American Way.

Oyster fisherman have the strongest hand on the lake by far

Almost untouchable


Don't understand how they control so much power with the state but they do. !

I will sum it up, as it's those who want "Immediate Gratification" , that often do long term damage. EXAMPLE.....disappearance of SE LA Coastline in order to maintain salinity levels. SE LA also raises cane at the suggestion that Diversions would be beneficial. Face it people....every wave takes some coastal mud with it, with nothing to replace it. Can't have your cake and eat it too. Need to adjust priorities....

For Example Atchafalaya Delta, Wax lake Outlet...both are low on Specs most of the time BUT...it is doable to catch them, just requires more effort. AND...that place is both gaining acreage, though much in the form of "Spoil Banks", and it is a Waterfowl Mecca.