Summary of Meeting to Discuss the Planned Simulation Studies of Alternative Stock 

Assessment Approaches for the Little Colorado River Population of Humpback Chub 

Venue:

Conference Call – April 26, 2005, 3:00 -5:00 p.m. 

Attending:

Lew Coggins - GCMRC 

Barbara Ralston - GCMRC 

Bill Persons – AZGFD 

Rich Valdez – Interested Party 

Mark Brouder – FWS 

Glenn Knowles – FWS 

Dave Otis – Iowa Co-op 

Purpose:

1) To brief interested parties on the background, methods, and objectives of the planned 

simulations to investigate the performance of alternate humpback chub stock assessment options. 

2) To solicit suggestions and possible modifications to the study objectives from interested 

parties. 

Agenda:

3:00 pm Introductions and review of study motivation, background, and 

meeting materials Coggins 

3:15 pm Overview of planned simulations Otis 

3:45 pm Discussion of objectives All 

4:45 pm Wrap up and recount any study modifications Coggins 

5:00 pm Meeting Ends 

Summary:

The participants generally expressed that they had a much better understanding of this study as a 

result of the meeting. Much discussion centered on: 1) the scope of the simulation effort relative 

to both the population that was to be modeled and the types of models and assessments to be 

compared; 2) the utility of the effort relative to providing guidance and clarification to managers 

on uncertainties associated with stock assessment results; 3) how parameters would be 

incorporated to best simulate the important characteristics of the LCR population such as 
movement and skip spawning, 4) how parameters would be incorporated to best simulate 

alternative sampling designs, 5) recommendations for modifications to study objectives. 

Revised Study Objectives:

1) Estimate the magnitude of bias in abundance estimates produced by each of the 3 ASMR 

models, Jolly-Seber models, closed models, and Robust Design models, when there is: i) error is 

assigning age at first capture, ii) various types of heterogeneity in capture probability not related 

to age, iii) annual variation in adult mortality rate, iv) various assumptions in movement patterns 

between the Little Colorado River (LCR) and mainstem Colorado River. Each of these scenarios 

will be evaluated under various designs that involve multiple sampling occasions in the LCR 

only or in both the LCR and mainstem Colorado River. . 

2) Compare the bias and precision of abundance estimates and the power to detect population 

trends, from ASMR, Jolly-Seber, closed and Robust Design models when sampling is conducted 

every year, every other year, every third year. 

3) Compare statistical precision of population growth rate estimates from Pradel (1996) models 

to growth rate estimates derived from population abundance estimates, in the context of the 

listing status decision process. 

4) Advise on the potential utility of model selection procedures for ASMR models. 

5) Use simulation results to make recommendations on the design of future HBC stock 

assessment programs in Grand Canyon. 

6) Explicitly evaluate concurrent sampling approaches during Spring months compared to 

Fall months incorporating differences in proportions of fish inhabiting either the LCR or 

the LCR Inflow reach of the Colorado River, and differences in sampling efficiencies in 

each of those locations. 

7) If possible, compare the bias and precision of recruitment estimates and the power to 

detect recruitment trends among various models, sampling designs, and assumption 

violations as specified in 1). 

8) Discuss all findings, particularly the statistical power and robustness of trend, adult 

abundance, and recruitment assessments, in the context of utility to assessing humpback 

chub recovery goals. 

Bolded objectives were identified during the course of the meeting. 

Timing of Next Meeting:

A subsequent meeting of this group and interested parties within Region 6 of the USFWS will be 

scheduled prior to the August AMWG meeting in order to brief participants on study progress 

and possibly refine simulation structure and input parameters. 
Minutes:

Lew Coggins stated the overall purpose of the meeting is to brief on the objectives and overall 

tact of the simulation study that Dave Otis has been contracted to provide us. Also, to solicit 

suggestions from everyone relative to those objectives and the overall study to make sure any 

kind of items that has been missed that anyone would like to see addressed and for us to look at 

to see if they can be addressed and, therefore, modify the overall objectives before Dave 

launches off on this project. 

Lew Coggins gave some background on why the simulations are being conducted. Over the last 

few years there has been a lot of discussion about the adequacy of the current chub monitoring in 

Grand Canyon, particularly for assessing Fish and Wildlife Service (FWS) recovery goals. 

Several years ago, about 1 1⁄2 years ago, Adaptive Management Program commissioned a review 

of Humpback Chub (HBC) stock assessment programs, specifically for the Grand Canyon, but 

also this independent panel looked at stock assessments programs in the upper basin. In the 

March 2004 meeting of the Adaptive Management Work Group (AMWG), the results of that 

panel’s findings were provided by Dave Otis, who was on that panel, to the AMWG and their 

final report was made available to that panel. Also at that meeting, a letter to the chairman of the 

AMWG was available from both Regional supervisor from Region 6 and Region 2 FWS and the 

Director of the Arizona Game and Fish Department (AZGFD) was also available. A copy of that 

letter is available upon request. Basically, the AMWG passed a motion at that meeting to fund 

concurrent abundance estimates for the LCR population of HBC. The next thing that happened 

was a minority report was sent to the Secretary from some of the members of the AMWG that 

recommended against that recommendation. Subsequent to that, the Secretary asked the DOI 

AMWG members for recommendations, essentially what the Secretary should do since she was 

confronted with competing recommendations. He also has a copy of the minority report if 

anyone needs one, it will be available. The AMWG members then drafted this letter, which he 

sent to everyone, suggesting or recommending rather than initially funding the concurrent 

abundance estimates that these simulations be conducted and that filtered down to him to make 

sure that it happened. He (Lew) has been in contact with Dave Otis and, after much discussion, 

he has developed this simulation overview, which everyone has copies of, and one thing he 

(Lew) points out is on the final page of the simulation overview is a timeline for this project. At 

this point, the timeline suggests a first progress report in July 05 and what that is meant to 

coincide is the summer AMWG meeting, which he (Lew) now understands is going to be in 

August, not July, so the hope is that Dave will be able to provide a progress report to the AMWG 

at their summer meeting and that there will be subsequent progress and final reports in December 

05. That is the overall timeline and what got us here. 

The next thing Lew hopes to do was to have Dave Otis essentially walk through his simulation 

overview as well as get into some specifics of the objectives and how to achieve them. Then we 

will have a discussion among everyone to look at the objectives and what Dave has provided and 

talk about how they do or did not meet some of the groups expectations. 

Dave Otis stated he became involved in this about 1 1⁄2 years ago. He started out by talking about 

the simulation approach in general in a sequential way. When the committee wrote the report 

that Lew mentioned, one of their recommendations was to do some simulation. It really wasn’t 
in the context of this concurrent sampling issue. That was discussed at that meeting in 

California, but we (the committee) were really more focused at that point of the conversation 

about ASMR models and closed models vs open models and that issue as opposed to the 

concurrent sampling issue. So we had proposed that some simulation be done to see and dig into 

the operating characteristics, so to speak, of the ASMR model and to validate it, if you will, or 

whatever term you want to use. That is a fairly standard approach that statisticians often times 

use to try to look at things like bias, precision, and variance and those kinds of things statisticians 

worry about in a situation where we know what the right answer is. Of course, that is the big 

advantage of doing all the simulation modeling if you know what the right answer is so you can 

calculate things like bias and in the field, of course, we never know what the right answer is. 

The other part of that was not only to evaluate bias precision, but to evaluate how that model and 

other models broke down when some of these assumptions that you all are concerned about may 

be violated so that gets into this business about heterogeneity in capture probabilities and what 

happens if fish move from place to another and become basically invisible to the capture and 

those kinds of issues. What kinds of trouble we get into if we use one method or another and 

which is the best one that might give us the best insurance, so to speak, against those kinds of 

breakdowns. That is kind of why we had suggested simulation in the first place. 

Then the concurrent sampling issue came up and the notion of doing the field survey and try to 

compare the estimates that resulted from two field surveys to, and in some sense validate ASMR 

model or try to decide the best way to go. His thought about that has been that is not necessarily 

a bad thing to do, but to him, that you would do that within the context of knowing something 

about the statistical properties of these techniques, in a perfect world admittedly. The downside 

is that these models are based on assumptions, they are not real like the field is, and we all have 

to recognize that. But it places results that you get from the field can help place those results in 

context so they are more interpretable in terms of what do these things really mean, given what 

we know about how these different models work. Again, simulations seemed like a valuable sort 

of tool in this context as well as the initial one about evaluation of the ASMR model. This gave 

us a way to directly compare the strengths and weaknesses of all these potential competing 

models because we can basically can simulate data under a given set of assumptions and we can 

feed that data into several different models and estimators and see what happens and do this over 

and over again and try to get a picture about how it performs on the average and how much 

variation there is in that performance. We can look at breakdowns and assumptions on a level 

playing field; we can also play games with looking at different sampling designs. There are 

issues like aside of all the capture probabilities issues, how many passes should we make and 

what do we gain if we go down 3 times instead of 4 times, etc., we will be able to look how those 

basic sampling dimensions effect the precision and the power to detect trend over annual 

timeframes so we can simulate this over multiple years, whatever makes sense to you guys in 

terms of what those timeframes would be. I am sure the recovery goals are relevant in that 

context and are really probably the most powerful tool to do that sort of thing. 

Admittedly, as I said before, these models are based on assumptions and so how good these 

models and how good the simulation is obviously depends on what kind of parameter values we 

are able to feed the models and, fortunately, you guys know a lot about the system and are able 

to translate that into reasonable specifications, so to speak, about what values some of these 

things we worry about might take on, how much do capture probabilities vary, how much 
movement might be among these fish, what are the different population size ranges we want to 

look at, all those kinds of things that normally have to have a number assigned to them. We are 

informed about what those things might be so that is obviously a huge advantage over some 

place that does not have a very rich data history that can be mined to give us those kinds of 

things as opposed to having start from scratch. That is sort of the philosophy I guess of the 

simulation approach toward the general. As he said before, no one pretends that it is real, 

because it is not and that is a downside. On the other side of the coin, in terms of a single sort of 

year of concurrent sampling, statisticians would react and say “well, that’s ok, but that is only 

sort of a single realization of a very complex and variable process and what can we learn from a 

single realization.” Again, we can learn some things but there are a lot of things we can’t learn 

about a sample size of one, so to speak, in terms of that kind of comparison. But together, I think 

with simulation again put in context in a single realization of unfortunately what is going to be a 

very complex process is very informative. Of course, in the real world we don’t know what the 

right answer is, so there is this context of bias and that is what we can really get out of the 

simulation. 

So, a few words about the assumptions that I am making with my limited knowledge about the 

canyon and the fish is that it will build basically two subpopulations, for the lack of a better term, 

or maybe it is a single population that occupies two different places, however we want to frame 

that, but there is the issue of the LCR and the issue of the mainstem near the confluence and, of 

course, that relates back to this movement back and forth between the two, but we can build 

populations in both of those places and assign them different capture probabilities, which I 

understand in probably appropriate since again we know you guys are not as successful catching 

them in the mainstem as you are in the LCR, we can attach to them some movement rates and 

that sort of thing and allow there to exchange between these two places based on the guidance 

that you guys have in biology. We can have different capture probability structures in those two 

places, we can make different assumptions on how fish react to capture in those two places, 

perhaps depending to what gear types are used in different places, maybe it makes sense to not 

assume the fish are not going to react in one place the same they will react in another place. 

Again, to build that population up in the two places and then go sampling as you all plan to do 

with concurrent sampling, with multiple passes presumably through each subpopulation and 

again sort of allowing things to be going on during that sampling, most notably I suppose 

movement, and then under all these various scenarios, sort of subject that data to any one of a 

number of different ways of handling the data. Which might range from using the outline that 

Lew referred to from the Secretary to look at closed population estimators, perhaps 

independently in the two places, do a closed population in the mainstem and a closed population 

in the LCR, and add them together. There are other ways to doing that, you could sort of pull all 

the data together and treat it as one big single concurrent sample, if you will. There is interest 

then, of course, if we only sample in the LCR and use only data captured from the LCR either in 

a closed population or in an ASMR model, there is the ability to then do the sort of validation we 

talked about in the original Kitchell Report, which was ok, lets see if the ASMR model does 

anything different than the Jolly Seber model does. I guess which it probably doesn’t, and then 

also look at the possibility of using what is sometimes called the Robust Design, which I think is 

also mentioned in the Secretary’s letter and is mentioned in our report as sort of a way to 

combine the best of both worlds for those models and is really appropriate for a situation you all 
might get into were you have multiple passes in one or more of these places, you already have 

this kind data actually, you have multiple passes within a year and you have multiple years. 

Again, there is no reason not to try to utilize that information among years and treat year 

independently and, in a perfect world, there is no reason if you can do it not to pool all the data 

in a given year because pooling usually loses information, and try to squeeze the most out of 

each individual run down the river that you can, that sort of notion that the Robust model uses 

and see how that might work. There are still lots that he (Dave) has to learn. He doesn’t 

necessarily know what the statistical objectives are of the whole project in general. That goes 

back to the recovery goals, in terms of how precise we want these estimates to be, he knows that 

the issue of power have been mentioned and what we would have to do to be able to detect such 

and such a decline and such and such an increase, which always begs the question, well how 

much of an increase or decrease we want to detect and all those nitty gritty statistical details that 

we still need to work out and obviously get feedback from you all on. That is sort of my 

thumbnail sketch that maybe helps a little bit more than did the original 3-4 pager, because I am 

trying to work my way through this as well and trying to make sure at the end of the day it 

accomplishes what you guys need and it is certainly worthwhile trying to decide that before 

launching off. 

Lew Coggins opened up the discussion relative to what is outlined in this document and what 

Dave just talked about and how that may be different than what some of you had in mind. 

Maybe that is one way to phrase the question in order to get some questions and kick this off a 

bit. 

Rich Valdez thanked Dave for providing that overview and for what he provided over the phone 

was very helpful. Dave laid it out very well and this addressed some of the major issues with 

regard to precision and some of the questions regarding the statistical aspects. I think the 

fundamental question that brought this about was how many HBC are there in the Grand Canyon 

population and he thinks another thing we need to keep in mind, as we go through this exercise, 

is to try to see if once we get beyond the evaluation of the precision and the sensitivity of the 

estimates, is it in fact possible to come with some kind of a real estimate, if you want to call it 

that. He asked Dave what his take was on that. Is it going to be primarily statistical exercise or 

can we come back to the managers with some kind of real numbers? 

Dave asked if he meant to come back to the managers at what stage, after the simulation or after 

we do concurrent sampling has been done for while? 

Rich answered after the simulation. 

Dave stated that one way to think about that you could start by laying out the optimum case, 

perhaps, and work back from that. He thinks, for example, that if it turns out that we decided 

what would happen if we made four passes (arbitrarily picked numbers) down each of these 

places in a year, which is probably exceeding the limits, the first question would be “how good is 

that?” under all the potential of the worse case or best case scenarios, the absolute maximum sort 

of effort case, you can be assured as much as we ever are as field ecologists that you would 

likely get pretty close to the answer that is dependable statistically and that will satisfy what you 
need to do to manage the species. As you back away from that, here is what you give up and 

here is how much more uncertainty you are left to grapple with, so to speak, in knowing how 

many fish there are and what are the trade offs. I don’t know if that is the sort of thing, Rich, 

that you think will resonant with the managers or decision makers. What do we do, will we be 

able to make an informed decision if we do this, our expectations is we can get this out of it, if 

we only do this we get this much less. We never know the truth, but I don’t think anybody 

expects us to know the truth, we just have to provide information to them so they can make the 

most informed decision there are about much uncertainty is acceptable. 

Rich responded that he agreed and thinks that is a realistic answer and thinks it is important that 

the people understand. Managers always want a number, but Dave is right, with that comes a 

certain tradeoff and understanding that with that number comes a certain amount of precision and 

confidence and they are either accepted or like Dave said, make a 4th pass if they have the 

money to do it. He asked Dave if he would populate the models with prior data with real data 

that has been collected. How is he going to handle the simulation? 

Dave said initially he thought they would parameterize the models based upon previous analyses 

of real data. So again, if we are going to try to simulate the probability of a capture of an age 

structure, or of such and such age old fish that pass through the river or through the reach, or 

whatever, we have a reasonable guess from the past analyses what that might be, is it 10 %, 20%, 

or 50%. These models are going to have a fair number of those kinds of parameters in them that 

are going to be required to build sort of make believe populations, but again, as he said, before 

we could make pretty good guesses what is realistic and what is not and we know the differences 

and things about the mainstem vs the LCR. I guess that is the way he envisions using the past 

analyses is to extract not raw data per se, but through parameter estimates that come out of those 

things to be able to parameterize the simulations 

Mark Brouder added onto Rich’s first question from the service perspective and what he has 

heard from the short time he has been in the group is there is strong interest on the part of 

everyone that we want to know when can we get an accurate population estimate of the Grand 

Canyon population of the HBC and start the clock toward trying to meet some of those recovery 

goals, which is where you were kind of leading in the first place in regard to numbers, right now 

we have estimates of the LCR HBC with concern that there may be a significant component of 

the mainstem that goes unaccounted for during spring and fall estimates in the LCR. Will the 

stimulation modeling get us to the point where we know if there are unaccounted for fish in the 

mainstem? Mark had some other comments that were impossible for the transcriber to 

understand because he was speaking too low. 

Dave responded he had a couple of thoughts. We are going to have to make some assumptions 

about and play out some scenarios about how many fish might there really be in the mainstem 

while we are doing this concurrent sampling and how much movement might there really be 

between the LCR and the mainstem, I guess that is the skip spawning issue, all those sorts of 

things that can screw us up and give us only an estimate of part of the population. Again, under 

the assumption of so many fish in the mainstem and whether they move or not, we can make 

some informed guess about how much we might be underestimated the total population if, for 

example, we only sample in the LCR. Of course, that average is going to be different depending 
on assumptions about how many fish are in the mainstem and whether they move or not. So 

ultimately you can think about that as a circular argument. If there is a bunch of fish in the 

mainstem and they don’t move and we sample in the LCR, of course we are going to 

underestimate things. But there is no way, if that is the case, you are going to get to that problem 

unless you sample in the mainstem. In order to be perhaps more confident or the most confident 

about it in the long term relative to getting this clock started, the only way you are going to know 

that is to actually go out and do the simultaneous concurrent sampling. 

Lew added that we need to think about the broader structure of the HBC in the Grand Canyon, 

the distributional structure and, in his mind, none of the work these simulations are going to 

address are going to, for instance, try to account for fish that all of our marking data suggest 

don’t frequent the area of the Little Colorado or the LCR inflow area. Like Randy’s Rock or 

Pumpkin Springs. Relative to an overall Grand Canyon estimate of HBC, I think we are just 

talking about the LCR aggregation in terms of how he thought of these simulations. That gets 

into some areas the recovery goals that he doesn’t understand real well in terms of do we 

consider fish as contributing to the recovery goals that we really don’t have any information to 

suggest that they contribute to a spawning population of fish or contributing to the recruitment 

dynamics of fish, of chubs, in the canyon. This is part of Mark’s question. The other part is that 

we can formulate some simulations in terms of movement dynamics of the LCR population of 

fish, some of which occupy the mainstem and the Little Colorado River, that includes a 

component of the population that never moves out of the mainstem Colorado. We can easily 

simulate that as a potential population migration structure. It will become clear that if we have a 

monitoring program that only includes sampling in the Little Colorado, those fish that never 

move into the LCR and, in fact, likely don’t contribute to the recruitment dynamics are invisible 

to that monitoring program. We would also simulate a monitoring program that includes 

sampling in both areas that will likely have the ability to monitor those fish. That also begs the 

question of whether or not it is important to count towards recovery those fish that likely never 

contribute to the recruitment dynamics. However, that is a question for the FWS and will not be 

answered through these simulations. Ultimately relative to having some understanding of what 

the overall population estimate is, if it turns out that when we simulate a sampling program that 

basically looks like past sampling that has occurred in the canyon and if the simulations uncover 

that we are likely to have underestimated or have negative bias in our estimates by some amount, 

it is that information that we would potentially use and that managers would potentially consider 

relative to the current abundance estimates that we have. How they may be biased in the context 

of an overall abundance estimate. Additionally, this exercise is not meant to reanalyze the data 

that we have in some fashion that gives us a less biased estimate than our models currently 

provide. It is meant to look at different sampling options including the ones we currently utilize 

and given certain population characteristics in the form of differences in capture probabilities by 

location, movement dynamics, and then to look at how badly our current estimates or those from 

alternative sampling designs and models may be biased. 

Rich stated that helped at lot and he was hoping Lew would bring forth some of that explanation. 

He gathered we should have enough information regarding the movement characteristics of that 

group of fish to and from the mainstem, to have a pretty good idea of the numbers of fish that 

come in and if there is skip spawning and so on going on. With regard to the first issue brought 

up, he addressed as when the recovery goals were drafted, the idea was that the what was 
considered to be the population or recovery in the Grand Canyon, that population was considered 

to be those fish that are centered around the Little Colorado River and reproduce in the Little 

Colorado River because that was the existing condition at the time, and still is, of the recovery 

goals were developed. Lew was right about that. Assessment of the recovery goals will not 

include fish in Middle Granite Gorge or Pumpkin Springs or any of those other populations, 30-

mile or any place else in the canyon, it is only going to include this group of fish that are tied to 

the LCR. There may be, and he doesn’t know how large it is, a component of fish that remain in 

the mainstem at the mouth of the LCR and don’t enter into that population in which case you are 

probably right, they won’t enter into overall contribution to recruitment and reproduction. He 

thinks that at this point in time, the universe of what we are defining as the population that is 

going to be examined or is in this exercise is probably pretty much what will be assessed in the 

recovery goals. It is those fish that contribute to reproduction in the LCR. The reason that was 

done in the recovery goals was that the anticipation was that if something changes in Grand 

Canyon, for example the temperature control device, and if there were to be mainstem 

reproduction, then it would change down the road the need for the way we monitor that 

population. He just threw that in for now as a thought for everyone to keep in mind, he was not 

suggesting we change the protocol or design at this time, just kind of a thought for the future 

possibilities. 

Lew said one could also make the argument that even if these other downstream aggregations 

currently are not contributing to recruitment or if they never contribute to recruitment because of 

changes in the mainstem that need to happen for that to occur never occur, they do also 

potentially serve as reservoirs of founders in the event of a drastic collapse of our reproducing 

population. So he didn’t mean to suggest they were useless, but he was glad the recovery goals 

were clarified and how they look at those aggregations. 

Rich agreed that we don’t know what might happen in the future but for right now we need to 

focus on those fish that are reproducing in the LCR. 

Rich asked Lew if he was finding evidence of skip spawning and does he thinks they have a 

fairly good handle of the movement of mainstem fish to and from the LCR in order to at least 

portray some realistic characterization of what those numbers of individuals might be. 

Lew responded that what we can tell from the analyses that we have done thus far is that the 

vulnerability or capture probability to fish as function of age is really dynamic over a course of a 

year. You look at this dynamic of, for instance looking at some of the oldest fish in the 

population, the largest fish in the population, and you look at how their capture probability 

changes over a course of a year. You see time periods where they are much vulnerable to 

capture, particularly in the spring, right when you expect them to be based on most successful 

sampling occurring in the Little Colorado and then it falls off in other months of the year. And 

so, the problem is that there are two ways to interpret that pattern, you can either interpret that 

given most of the sampling is in the Little Colorado, as these fish move in and out of the Little 

Colorado or you can interpret that as these fish remain in the Little Colorado and, for whatever 

reason, they become much less vulnerable to sampling, so a behavioral issue, perhaps. We do 

not have the data to resolve those two competing hypotheses. As far as evidence of skip 

spawning, all you really end up with if you think about the way the data have been collected is 
you see for example a fish over the course of say 12+ years. Now that fish may have been 

captured 10 times and sometimes its capture is in the mainstem and sometimes its capture is in 

the Little Colorado, so you can definitely say this fish has moved between these two places, but 

when it wasn’t captured in the Little Colorado in a particular year, you don’t know it wasn’t 

captured because it simply because sampling is a stochastic process of whether a fish gets 

captured or not, or whether it wasn’t captured because it didn’t come into the Little Colorado. 

He thinks that relative to capture probabilities, we can make parameterize the sampling process 

in a fairly informed way. But relative to the movement, we are basically going to have to specify 

a range of movement probabilities that operate on various proportions of the populations, a range 

of proportions and look at how the estimates associated with these various sampling regimes and 

various models then become biased because of that range. Without additional information that 

can be gleamed through telemetry or, for instance the remote PIT-tag antennae that Bill has just 

got done messing with and has been pioneering for the last 2 years, it is those type of data that 

are going to allow us to get estimates on rates of skip spawning and so far. In other words, we 

can’t simulate or model our way out of this. All we can do is look at what kind of biases may be 

evident in a simulation under some various movement rates and proportions of the population 

that may exhibit skip spawning. 

Rich asked Dave if that age specific capture probability is capured in your Objective 1. 

Dave said yes, well, for example the open ASMR model and Jolly Sever model can be stratified 

by age to account for those by different age and different capture probabilities. He was thinking 

of things more that are akin to the kinds of issues the closed population models try to account 

for, like this behavioral response, that kind of thing that Lew referred for whatever reason some 

gear might cause subsequent changes in capture probability, so after you are captured once, there 

is no way you are going to get captured again, you change your behavior for whatever reason. 

That is a different issue. And maybe there is heterogeneity, he doesn’t know, maybe there is 

heterogeneity caused by the gear. These are things that may violate that sort of critical 

assumption that everybody has the same capture probability that may be operating in addition to 

age and the behavioral one is potentially the most serious and maybe it is the least likely, he 

doesn’t know. You can get in some real trouble if you have some serious response on the part of 

the fish or the animal to that initial capture. He doesn’t know if we have any evidence of that, 

we probably don’t have very good data from adequate passes in a given year to get a peek at that. 

Lew said Rich is the person who has tried to utilize those models the most and his read on that 

work is because of a lot of those passes basically occurred over a long enough period of time, 

that you started running into other types of assumption violation with close populations models 

like closure because to have to deal with movement, relative to sampling in the mainstem, and 

you also have to worry about, over a period of time, some demographic closure problems with 

mortality and recruitment. As he recalled, Rich’s best model was the “T” model that looked at 

heterogeneity in capture over sampling occasions, but that result seems to be prevalent to work 

being done in the upper basin and is likely due to sparse data sets for the most part because 

getting a good handle on that behavior requires a lot of recaptures and those models are data 

hungry to be able to estimate some of those other ways that we can be introducing bias as a result 

of behavior and time. 
Rich said he thought that was an accurate assessment. It is a problematic issue in the mainstem 

because the fish tends to occur in quite dramatic clumps and at specific times of the year and 

aggregate in large eddy complexes. You have large numbers of them if you drop a net in the 

right place, you catch a lot of them, but if you put the net over 30-40 ft and put it in the wrong 

direction, you won’t get too many. 

Dave said he thought it was that kind of phenomena that could potentially cause problems with 

closed models. We have groups of animals that maybe on an individual by individual basis have 

similar capture probability, but we have different groups of animals that in effect have very 

different capture problems. It doesn’t have anything to do with age. That is the kind of 

phenomena he was thinking of, but needs guidance on what sort of realistic kinds of things Rich 

just said to know how to model that kind of thing. 

Dave asked when this concurrent sampling is envisioned, would you think of it as you were 

making multiple passes down in both places, that those passes would be occurring roughly 

simultaneously or alternatively or what? How might that stack up against what sort of 

movement rates—how we model movement rates. He asked if anyone could give him some feel 

for how to think about that in practical terms, could this be viewed simultaneously to speak or 

would that be totally naïve. 

Lew responded there has never really been a fleshed out proposal, but he thinks the way we have 

all envisioned it is that sampling would occur in the Little Colorado and in the mainstem at the 

same time. Based on past efforts, we spend about 12 days in the Little Colorado in order to 

sample the Little Colorado and we spend about 12 days to sample that LCR inflow in the 

mainstem and these distances are large enough, you are not necessarily sampling every area of 

the LCR inflow all at the same time nor are you sampling every area of the Little Colorado all at 

the same time. Presumably you start at one of the LCR inflow reach in the mainstem which is 

miles away from the confluence and over the course of 12 days, about 1⁄2 way though you are 

sampling real close to the confluence and towards the end of the 12 days you are perhaps 

downstream of the Little Colorado and so there does exist opportunity for there to be movement 

within the events. 

Rich said we have never come up with a formalized proposal for a concurrent estimate, but the 

other consideration to try to minimize that effect was to conduct a concurrent estimate at a time 

when there was not as much movement in and out of the LCR that is outside of the spawning 

period. The tradeoff of course there is you may have lower capture efficiencies in the LCR and 

the question becomes how much do you sacrifice there for that part of the estimate. So there are 

tradeoffs, no matter which way you approach it. 

Bill Persons said that is where some simulation modeling might be real helpful in how we design 

a concurrent estimate, do we do it when most of the fish are in the LCR when we think we have a 

higher capture probability, do we do it when most of the fish are in the mainstem when we think 

we have lower capture probability. It sounds like you can run some capture modeling that might 

give you some guidance there, which he thinks would be pretty useful. 

Dave said Bill was suggesting a mix of a fall parameterized scenario and a spring one. 
Bill Persons said it was always tough in the spring because the fish don’t have calendars and we 

schedule our work for Apr 7-19 in the hopes that the majority of the spawners will be in the LCR 

but they don’t always cooperate. He doesn’t know any way around that, that is just the noise we 

have to live with. That is a tough one, he was hoping someone from FWS upper basin would 

participate here and maybe give us a little more guidance on what an acceptable population 

estimate might be for them, but that is probably a different call. 

Rich said Dave characterized quite well when he asked that first question, and that is you do the 

best you can with the available data and you understand the physical parameters of that effort 

that hopefully reflect the population and then you recognize that there is a certain amount of 

precision and managers have to make the decision as to whether that is acceptable or not. 

Lew told Dave that he thinks that basically when we look at the range of movement probabilities 

and look at differences in what proportion of the population occurs in one these strata or the 

other, we are basically looking at the alternatives of sampling in a time period of low movement 

fall vs higher movement spring and differences of which strata the majority of the population 

occupies. 

Dave said and perhaps higher or lower, whichever, average capture probabilities as well. He 

was trying to get an image or feel about how to think about that concurrent sampling. 

Rich asked Dave if he was talking about making some year to year comparisons using pradel’s 

lambda value. 

Dave said he hadn’t thought much about that, so far just sort of focusing on Capital N(?) and 

probably that is sort of an issue that would be helpful to get some feedback from the group, 

because to be honest he doesn’t know from the services standpoint or whomever—in the big 

scheme of things he was thinking—he didn’t know whether to put it on an even priority with 

Capital N or to think of it as more of a secondary thing or what. He doesn’t think it is that big of 

deal to throw some of those pradel models into the mix or some of the Jolly Seber things that can 

be reparameterized in terms of rate of change of Cap N, that kind of stuff. He knows there was 

some very specific recovery goals in terms of how many fish and that sort of thing, but he don’t 

know if there are similar goals in terms of trends over a specific time periods---- 

Bill said there is a specific recovery goal of not having a downward trend. 

Lew said this was one of the items in his original list of objectives that he hoped we could look at 

and a big part of why it was included it was because it was one of the recommendations from the 

panel. In part, he sees it as another metric that managers might consider if they thought it was a 

reliable way to look of trend and it could also be compared in terms of robustness with 

abundance estimates. In his mind it was value added thing to get someone like Dave to do this 

because it is another way to assess trend that in the literature is portrayed as more robust and 

more reliable than abundance estimates. 
Rich said he thought that was true, we are interested in both the point estimators as well as a 

trend that is probably over less than a 10 year period. Along with that too, something he thinks 

would be helpful to the managers would be an understanding of the risks that are involved in 

power if we call trend a non-significant decline and it is. I think that some kinds of things we 

need to understand is what would it take to be reconciled through sample size or whatever, what 

would it take to minimize that risk. 

Dave said he agreed, especially if it is another goal that has to be achieved, so to speak, of no 

negative trend, no significant negative trend. Then of course that begs of statistical questions, 

one which he just mentioned, do we think of that trend in a sort of statistical terms or do we think 

of it more in biological terms. That gets into the power issue, is it any deviance away from 

significant at all. 

Rich said it should be a full power issue, a full power trend analysis. 

Dave agreed. It is similar to the Capital N business, the decision makers should understand what 

the relative cost is of messing around with and trying to decide if it worse to miss a trend that is 

that is there or declare a trend that isn’t there. Of course, in this situation, that is a pretty big 

decision. 

Lew said that thus far he hadn’t heard any specific recommendations on modifying objectives 

and asked if he was missing something. 

Rich said that one is the one that Bill Persons brought up and he would concur with that and that 

is to at least look at the two population segments and sampling those in the spring and fall under 

different assumptions of totals numbers present and the associated performance. 

Persons said he could see that in #5 and we may come up with other specific questions that we 

might want to have answered once we see what you can do with this simulation model. 

Dave responsed that as Lew said, we can come with a combination of parameters in terms of 

proportional residence and movement that portray fall sampling as opposed to spring sampling. 

Rich said there is another aspect and that he thinks we are talking about adults, but the recovery 

goals call for estimates of both adults and juveniles. Lets look at juveniles to try to get some 

ideas for recruitment. The ASMR model approached that in a more comprehensive manner. He 

asked Lew if he had idea as to whether we need to address recruitment in this respect or does he 

think the ASMR pretty much covers that. 

Lew said that the ASMR and the age-structured Jolly can deal with that relative to once you 

define the criteria age at which a fish is called an adult and also recognizing the problems with 

assigning age is a function of length, which is in the first objective. Right now we basically 

don’t plan on trying to deal with fish that were too small to PIT-tag so we are dealing with 

basically 2-year old fish that are usually through their first year based on the growth curves, and 

about half of them are large enough to PIT-tag at the end of their second year. So the way we 

have dealt in the ASMR is that we are estimating an abundance of 2-year old fish and estimating 
a vulnerability parameter that is reflecting that not all that age class is vulnerable to tagging. 

Certainly and with regard to closed estimators, we are in about the same boat. We can certainly 

stratify the abundance by size, the estimate by size, at 200 millimeters, which is what we have 

settled on kind of universally, and we can do that as well. He hadn’t talked specifically with 

Dave about this, so we can get his reaction, in achieving this #1 objective, because of the way we 

assigned age to fish, we have to basically grow these in our population as well and assign them 

age and that doesn’t seem to him a difficult thing to do and that allows us to stratify closed 

population estimates by size as well as look at issues associated with mis-aging fish because of 

variability in the growth function. That is how he had conceived this and he asked Dave if this is 

what he was thinking. 

Dave answered yes and to be honest, he had thought a little bit about trend and hadn’t thought at 

all about recruitment. You can get estimates certainly out of the open model directly and 

indirectly, but any way, in comparison to closed models, he doesn’t know that there would be 

extra to learn about that. It is just a matter of—he thinks what we learn about the operating 

characteristics assuming that these are all adults, is that going to be much different than what we 

would learn by parameterizing a little bit differently if they were juveniles? 

Lew said with regard to closed models, he doesn’t think so. But Jolly and ASMR deal with 

recruitment estimates in different ways. That is probably the biggest difference between the two 

models. He had in mind, particularly when we want to look at change detection under a 

population that is either growing or shrinking or has some pulses of recruitment, that we can in 

particular utilize the ASMR model but also the Jolly Sever to look at detecting changes in 

recruitment. He has actually done some of it with the ASMR model, what magnitude of change 

of recruitment is likely to be visible. But of course, that is assuming some of the other 

assumption violations we are going to look at. 

Rich asked if Lew is using something more kin to a robust design 

Lew said in a sense we kind of do have a robust design now, but he wasn’t trying to examine the 

detection of a recruitment signal with a closed population model, it was the ASMR model. What 

amount of recruitment above a kind of a baseline would it take before we said a big spike in age 

2 and 3 abundance. That is something that he and Dave can talk about some more, but is that a 

specific objective that Rich would see as far trying to characterize just what Lew said, what 

magnitude of recruitment signal are we likely to detect under various sampling designs. 

Rich said he thought that was an important parameter. 

Lew said then the question is does Dave think we can do it. 

Dave said he needs to think about that. He is not quite sure of what Lew was trying to say. 

Lew said basically what he did was he simulated the data set relative to capture probabilities in 

the most recent years since we are basically doing closed populations in the Little Colorado and 

with those capture probabilities in hand, he then assumed that the overall recruitment had 

increased by an order of 25%, 50%, 75%, or 100% for a 2-year block of time and he sampled 
that simulated population and saw that in a Monte Carlo setting, it was unlikely we would see a 

significant spike in abundance of 2-year old fish unless we at least a 50% increase in recruitment 

and a 100% is when it really showed up well. 

Rich said the reason he brought it up was related to the recovery goals and said Dave would have 

to decide if it could done and again to focus in on the recovery goals. 

Dave asked if the recruitment goals were in terms of trend, absolute, or-- he can’t remember 

what that parameter is. 

Rich said it would be an average annual recruitment that would equal or exceed average annual 

mortality, very generally population stability or growth. 

Lew said as he understood it, the upper basin program is based on a closed estimate type of 

analysis, at least currently, and recruitment strength is inferred only indirectly. That you basically 

make the assumption that if the adult population falls, then recruitment was less than adult 

mortality, right? 

Rich said yes, but with HBC we are having difficulty coming up with that parameter, obviously, 

because the fish are difficult to capture and we have small recapture rate. 

Dave said that part of his hesitation initially is that population trend is probably the easiest thing 

to estimate, perhaps as we mentioned earlier, at least the most robust, then comes the population 

size and then comes recruitment. In his experience, trying to estimate recruitment rates, they are 

just pathological sometimes, they are just very difficult to estimate. He doesn’t know about the 

ASMR model, but he does know it about the Jolly Sever model. To be honest, the right 

parameters are just often not very good and it could sort of muddy the water, so to speak, in 

terms of trying to make decisions about which way to go. But those tradeoffs, again, should be 

known. We can certainly say things about making recommendations about decisions on how 

these models perform with respect bias or whichever one makes no sense, we are just sort of 

driving the bus here, and so secondary issue, here is what you can expect in terms of recruitment 

but not base decisions on sampling designs and things on how well we are estimating 

recruitment. 

Lew said we could need huge changes in recruitment to have a good chance of detection, which 

is another way to look at it.. 

Rich said that some treaties about this subject would be appropriate for this exercise and he 

would leave that up to Lew and Dave to decide about how much they would want to go into 

actual analysis, certainly this information should be provided to the managers. 

Bill Persons said it was a good idea and to let them know there are different estimators that you 

can get a trend estimate, he doesn’t want to call it easy, but it is probably the easier of those 

three, the abundance, which is still tough, and recruitment, which may not be possible. As long 

as they understand that. 
Dave said that expectation may be more realistic. 

Glenn asked to the extent that they could, he asked that they could comment on that relative to 

the recovery goals in their report, that would be really helpful to the managers too. He doesn’t 

know if that was the goal necessarily to this project, or how easy it would be for us to do, but it 

would be really helpful if we could do that. 

Lew asked for verification of what Glenn was asking – for an assessment of the robustness of 

those three estimators relative to reliability about making decisions about recovery- the 

robustness of them? 

Glen said he just meant specifically what the recovery goals say are good estimators to use, not 

necessarily good estimators to use, but the language in recovery goals to speak to what they are 

saying in regards to what these simulations tell us about those estimators. 

Dave said to make recommendations in the context in being able to evaluate things relative to 

those recovery goals, specifically consider recovery goals—put in context of what the recovery 

goals are. 

Lew asked if there are any other suggestions/discussions. So the ones he got are explicitly are: 

looking at spring vs fall designs; recruitment trend detection; overall robustness of trend 

estimates; and kind of a relative comparison of trend estimators, adult abundance estimators, 

recruitment estimators, in the context of the recovery goals 

Lew asked if anyone would like to discuss anything else about their studies. 

Rich asked Dave how they are going to do this-- You are going to start cranking away on this? 

He asked if we will see anything before July or are there scheduled periodic conference calls or 

what? 

Dave said he would defer to GCMRC folks. He and Lew had discussed getting together as soon 

as school is out and he gets loosened up a bit, to discuss some nitty gritty details and start 

plunking away. He defers to Lew. 

Lew said it was to be determined, but he had in mind that at this point, with the exception of him 

and Dave getting together early next month, they would turn Dave lose and see what he came up 

with for the AMWG meeting as far as a progress report. That is what he had in mind. 

Rich asked if a conference all at the end of June or first of July after Dave had a chance to look at 

some of this would be appropriate as kind of to touch base on how you are doing and if you have 

specific questions perhaps we can assist at time. 

Dave said that was fine with him. Maybe we can agree to that but leave it a little bit lose, maybe 

for a 2-3 weeks period, sort of if we are going to finish something up at a certain time. Maybe 

revisit that in a month or two, certainly before the AMWG meeting. 
Lew said we are going to have our annual stock assessment meeting the later part of July, we 

could also get some other involvement in conference call and you could even consider coming 

out here then, but if nothing else, we would have Carl here as well as Steve Martell and our other 

fish cooperators. Stock assessment will be the topic of the week. We will all have our pencils 

about as sharp as they get right during that week. 

Rich suggested however we accommodate that, we have the opportunity to meet. This has been 

very helpful for him and however, that is done, on a conference call or physically at the Stock 

assessment workshop, however that works. 

Lew said we would make it happen, we will figure out the right time over the next month and 

keep everyone informed. 

Rich said at that point in time we will hopefully be able to bring the Region 6 folks in from the 

service and apprise them of what is going on. 

Lew said that would be great. He will send a meeting summary to each of the participants and 

rely on Mark to forward the summary to region 6 staff. 

 

Mark agreed. 

Rich asked Mark to forward that at least to Bob Muth and Tom Czapla. 

Conference call ended.