Why bother to get involved in such a seemingly endless round of arguments on the issue of old age for Hoplostethus atlanticus? The reason is simple. Allen Andrews wrote a letter to robertgauldie.com to tell me that he had validated old ages for H. atlanticus; and that I should do an about-face and fall into line with the old age consensus for the age of H. atlanticus that Andrews had validated using the 226Ra: 210Pb ratio method; or explain why not. Here is Andrews’ letter:

Dear Bob,

I encourage you and your readers to more thoroughly consider lead-radium dating as an indication that many fishes do live for many decades. Our most recent paper on orange roughy provided a validation of age exceeding 93 years (Andrews et al. 2009). The method works and much of the work published to discredit the method did not properly consider the results or alternate interpretations. It is the kind of misinformation presented above that continues to exacerbate the situation, rather than helping with making responsible decisions in fisheries management. If “the idea of old fish” is “easy to disprove…” then please help me/us with understanding why there are so many indications that some fishes do get to be rather old. I challenge you to turn this around and more thoroughly investigate your position, which seems to exemplify the “the political analysis of fish age and growth” that you lament.

Allen

My response was to read the paper by Andrews et al. (2009) and have a look at the basis for Andrews’ validation argument for the old ages of Hoplostethus atlanticus. After discussion with my colleague Ian West, we decided that the validation proposed by Andrews et al. (2009) was another variant of the same arguments that we believed that we had dealt with previously in West and Gauldie (1994: Can. J. Fish. Aquat. Sci. 52: 2333-2340).

We contacted Dr Don Jackson the Editor of the Canadian Journal of Fisheries and Aquatic Sciences (CJFAS) on 1/8/09, shortly after the publication of Andrews et al. (2009) and informed him that we had problems with Andrews et al. (2009) and that we would write a rebuttal. And so we did, submitting our rebuttal on 24/8/09 titled: Radiometric Data Published by Andrews et al. (2009) Do Not Support Their Claims That They Have Validated “Centenarian” Ages For Orange Roughy, Hoplostethus atlanticus. The paper by Andrews et al. (2009) was quite long, about 10,500 words. Our rebuttal was also fairly long, about 6,300 words; but not excessive for a rebuttal of a long paper. To our surprise Dr Jackson informed us that our rebuttal was too long and that it was editorial policy at CJFAS to only accept rebuttals that were 1000 words or less. It was difficult to compress our 6,300 words into 1000 words, but we did so. Two reviewers for CJFAS considered our abbreviated rebuttal to be insufficient to publish in CJFAS. The Associate Editor and Editor of CJFAS agreed with the reviewers and we were informed on 24/3/10 that our criticisms would not be published in CJFAS. Our original long rebuttal of the claims of Andrews et al. (2009), the subsequent shortened rebuttal and the comments of the two reviewers are posted as a single pdf file (300 Kb) that can be downloaded by clicking on the link as indicated.

What, then, were the problems with Andrews et al. (2009) that we thought were so important?

The amounts of 226Ra that Andrews et al. (2009) measured were very small. Our long rebuttal spells out the problems caused by the low levels of radioactive material. Basically, the 226Ra content of most of the samples of Andrews et al. (2009) were similar, if not statistically the same. The basic model of the 226Ra: 210Pb age estimation method requires that as 226Ra decreases, so do the amount of daughter product 210Pb increase accordingly. If there are large differences in 210Pb between samples and low differences in 226Ra between the same samples, then the basic model cannot be applied because the 210Pb must have come from somewhere else, not from the decay of the parent 226Ra. Or, certain special conditions applied to the amounts of 226Ra that were taken-up in the otoliths of the samples. These special conditions are that all of the samples must have started with different amounts of 226Ra such that when they decayed over different time periods it just so happened to be at similar amounts when measured, although they actually came from fish with much different ages. For this to happen, the young samples would need just enough 226Ra to appear young and the old fish would need just enough 226Ra to appear old.

Our long rebuttal takes the commonsense view that the reason why the fish appear so old is because the 210Pb does not come from decay of the parent 226Ra but from elsewhere. Is there an “elsewhere” from which the 210Pb in the otoliths of Hoplostethus atlanticus could have come? Yes. For H. atlanticus that “elsewhere” is all around the fish. In our long rebuttal we showed the published, experimental evidence that that H. atlanticus swims around in a veritable soup of 210Po and 210Pb. We think that it would be surprising if the ambient 210Po and 210Pb did not leak in the otolith of H. atlanticus. After all, if the ambient 210Po and 210Pb cannot leak into the otolith of a fish, then where do the heavy elements in otoliths that figure so prominently in the studies of the elemental composition of otoliths actually come from?

It is worthwhile to have a look at the radioactive isotopes from the 238U decay series that are actually involved in the discussion about using 226Ra:210Pb ratios to estimate ages in fish otoliths. The radioactive decay series with the half-lives of the isotopes and the emitted particles at decay are shown in the graph in Figure 1: Isotopes, emitted particles and half-lives in the decay series from 226Ra through 210Pb to 206Pb. All of the elements in the decay series from 226Ra to 206Pb are soluble, particularly in chloride-rich solutions such as the endolymph. In addition, alpha particles are effectively an emitted proton and cause significant local damage to the aragonite mineral structure nearby (i.e. in the range < 40nm), thereby further increasing the chances for diffusion out of the otolith of the resultant soluble isotope. One must assume, therefore, that some, at least, of all of the isotopes in the decay series from 226Ra through to 206Pb are diffused out of the otolith; and that some, at least, of all of the isotopes in the decay series from 226Ra through to 206Pb are diffused into the otolith. If not, then how did the 226Ra get into the otolith in the first place?

If one were to mix some 226Ra into some radionuclide-free aragonite and then hold the mixture for 20 years in a radionuclide-free laboratory, then the true age of the mixture could be estimated from the 226Ra:210Pb ratio of the mixture. Such an estimate would be inaccurate in comparison to using a clock to do the same thing. But floating the aragonite mixture around in the radionuclide-rich environment of the deep ocean would change any expectation of obtaining a true age from the 226Ra and 210Pb in the aragonite mixture. It is sometimes easy to forget that fish are, indeed, fish; and that fish rarely behave as simply and predictably as laboratory chemical mixtures.

The age estimates for Hoplostethus atlanticus are used to manage the fishery for H. atlanticus that is worth hundreds of millions of dollars to the New Zealand economy. With so much at stake, it is unfortunate that the critical assumptions about isotope diffusion in, and out, of the otolith have never been tested at depth under in situ conditions typical of H. atlanticus. Without the experimental data, the critical assumptions will always be glossed over with statistical analyses based on yet further assumptions.

There is nothing in Andrews et al. (2009) that allows the reader to discriminate between the two alternative situations over the life of Hoplostethus atlanticus. Firstly, that the otolith is a completely open system in which all of the otolith 226Ra and 210Pb accumulated over the life of the fish was of exogenous origin; or, secondly, that the otolith is a selectively closed system in which all of the 210Pb accumulated in the otolith over the life of the fish was of endogenous origin, but all of the 226Ra accumulated over the life of the fish was of exogenous origin.

Read the long rebuttal and you will see why we are disbelievers of old ages for Hoplostethus atlanticus that were obtained by radiometric age estimation based on the 226Ra: 210Pb ratio method.

Responses and Comments

If there are any responses or comments about radiometric age estimation, or even comments on comments, then they will be posted in the order received. The first comment will be at the top etc.

9 April 2010: Comment form Allen Andrews

Dear Bob and Ian,
Thank you for the interesting read. I’d like to address a couple of issues right up front in a friendly manner. To clarify my letter to you, I did not imply that you should fall in line as though you needed to be obedient in some way. I was asking that you reconsider your perspective in light of the recent information we came up with. A cordial and respectful conversation and exchange of information is what I was after with you to better understand why you maintain your position, but instead you chose to close the door on talking with me and stated that all correspondence needed to go through the editor of CJFAS. So, here we are doing just what I was hoping to do in a more circuitous manner.

There are some fundamental problems with what you and Ian have stated in this rebuttal and while I understand your concerns fully, there are numerous reasons and scientifically robust data published in pier-reviewed journals that indicate the method works. There are direct and indirect measures that answer your questions about the validity of the approach.

First and foremost, I think your rebuttal would have been better if the details of our experimental design were properly considered. The most significant error in your criticism is with regard to measurement of radium-226. We did not measure the activity of radium-226 via direct counting of radioactive decay. The assays were made via mass-spectrometry, a high precision process that is used in numerous applications that has greatly reduced detection limits. This improvement is why we can now apply the method to individual groups of fish; a major improvement from previous applications that had to assume radium-226 uptake was consistent (see Andrews et al. 1999). I could go on, but I have thoroughly discussed this and other issues in a number of previously published papers; others have as well.

The arguments in the rebuttal go down hill from here with discussions of the decay of radium-226, uptake of radium-226 and other isotopes, measuring lead-210 by polonium-210 proxy, etc. Radium-226 does not decay significantly in 100 years (half-life is 1600 yr), let alone in ages that you stand behind as longevity for orange roughy. This factor is not even a concern in this approach and there are numerous published works that provide this kind of information (see Smith et al. 1991). With regard to measuring lead-210 via polonium-210, this is a well known process that dates back more than 50 years. Polonium-210 has a half-life of 138 days; hence, any measure of polonium-210 from an otolith collected more than 2 years ago is going to be from the decay of lead-210. Younger fish have had less and older fish have had more, given a relatively consistent uptake of radium-226 in the core material, which is an indication that time is passing and ingrowth is happening. Yet you dismiss this fact as happenstance with no data to support this perspective.

I could go on, but I don’t see a point in doing so because most of what is stated is speculation. Please help us with understanding your perceptions with factual evidence in the form of rigorous science. I would be very much interested in hearing any suggestions on what we might do to test any of the problems you and Ian perceive. This I think would be a better approach for all involved, as opposed to bantering over assumptions.

I close with stating that there is nothing presented in your rebuttal that could not be interpreted in another way. Attempting to pick apart my/our work is annoying and not constructive. Please notice that all of what you presented in your rebuttal is focused on discrediting the technique and supporting preconceived notions of age, and not on finding truth. Please see Karl Popper (1935), The Logic of Scientific Discovery. There is good information there about how to approach problems from an unbiased position.

Thank you for contacting me about your rebuttal and for the opportunity to respond.

Sincerely,
Allen

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