Letter 1: From Brian J. Rothschild to Dr. William Karp and John Bullard
August 16, 2013
Hi Bill and John,
Thank you for holding the yellowtail meeting in the library on Monday, August 12. As you know, I asked a few questions, and I am not sure that I fully understood the answers.
1. The natural mortality of cod appeared to quadruple. I asked how this affected management regulations. Normally, an increasing natural mortality would suggest a smaller optimal mesh size and higher fishing mortality reference points. How is the quadrupling of natural mortality taken into account in management?
2. Alternative models. I asked why alternative models were not considered. I think the answer was that many alternative models were considered and lead to the same catch advice for yellowtail. We wrote a paper comparing standard production models to age structured production models. We found that standard production models gave a much more favorable view of the status of the stock than the age structured production models that are presently used. When Steve Cadrin showed his results from multiple model applications, these showed that the standard production model gave a better perception of the status of the stock than the age structured model. My conclusion is that the models give different results, and this is supported by earlier calculations and simulations….
To read to remainder of Dr. Rothschild’s letter please click below.
Letter 2: From Dr. William Karp to Brian J. Rothschild
August 30, 2013
Thank you for your detailed letter of August 19, 2013 regarding our August 9 presentations on TRAC species status. In response to your questions, we have provided detailed answers.
Q1: The natural mortality of cod appeared to quadruple. I asked how this affected management regulations. Normally, an increasing natural mortality would suggest a smaller optimal mesh size and higher fishing mortality reference points. How is the quadrupling of natural mortality taken into account in management?
A1: A natural mortality rate (M) of 0.8 per year was applied to Eastern Georges bank (EGB) cod VPA model for ages 6 and older from 1994 onward. A value of M=0.2 was prior to 1994. Thus, the quadrupling of natural mortality is only for older fish in recent years. In contrast, the Georges Bank (GB) cod assessment vetted at the December 2012 Stock Assessment Workshop (SAW) is based on an ASAP model using M=0.2 for all age groups in all years.
In the EGB cod VPA, the range of years and ages for M=0.8 was primarily determined by model fit rather than by evidence of a mechanism for such high mortality on the older ages. Seals are present on Georges Bank during the spawning season as indicated by satellite tagging; however, the residence time on the Bank of these animals is unknown. Moreover, there is no quantitative information regarding the amount and frequency of seal predation on cod on the Bank that would justify such a high M based on predator/prey dynamics…..
To read the remainder of Dr. Karp’s letter please click below.
Letter 3: From Brian J. Rothschild to Dr. William Karp
September 16, 2013
Many thanks for your constructive and informative September 3 response to my August 16 e-mail on the “yellowtail” meeting.
I hope our dialogue is continued in a broader context. Your idea of a blue-ribbon panel makes sense, particularly inasmuch as detailed discussion is difficult to cover by correspondence alone.
Based on our correspondence, I wanted to call your attention to three issues: 1) production models, 2) alternative values of x, and 3) scientific and legal overreach.
Production Models—The response to Q2 says, “surplus production models have much less realism, and much stronger assumptions about the dynamics of the population”.
This response is not in a direction that I would expect. It is not clear at all that the F40% (and its attendant VPA) is more realistic or based on stronger assumptions than the production model. What is the metric for realism? What is the metric for “strength of an assumption?” Our concern should not focus on good or bad properties of a surplus production model, but how a surplus production model in a particular assessment setting compares with an alternative model….
To read the remainder of Dr. Rothschild’s letter please click below.