Redfish and Specks Benefit from Limited Weir Closings

[MathGeek]

Originally Posted by dmtfish

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.

[dmtfish]

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.

[MathGeek]

Originally Posted by dmtfish

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.

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.

[dmtfish]

Originally Posted by MathGeek

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.

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.