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Home > Fisheries FAQs > Red Snapper FAQs > Stock Assessment

Red Snapper FAQs: Stock Assessment

Please send your questions and feedback to redsnapper@lsu.edu

What kinds of data go into a stock assessment?

The NMFS has gathered red snapper harvest data for the recreational fishery since 1981 and for the commercial fishery since 1950. These data are available at the following links: Recreational Fishery Statistics Catch Time Series Query and NMFS Commercial Fishery Landings Data. In addition to this fishery dependent (FD, data from the commercial and recreational fisheries, including shrimp trawling) information, fishery independent (FI, data not from the directed commercial and recreational fisheries – this usually means scientific survey efforts) information of several sorts are also gathered. The FI sources are quite varied and include such things as trawl surveys for juvenile red snappers and estimates of the abundances of red snapper larvae through time. Both the FD and the FI data are converted to various indices of abundance such as “catch per unit effort” (CPUE) over time and geography. These indices of abundance are used to “tune” the assessment so that the computer models both provide the best fit to the landings data and produce trends in time that are consistent with the FI and FD data. The effects of weather and market forces on red snapper populations are more difficult to deal with and are not significant components in the assessment models.

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How are management alternatives identified and applied?

There is natural variation in most, if not all, aspects of red snapper life history, even among the things fishery scientists and managers know well, that can generate statistical uncertainty in assessment outcomes. For example, the assessment models are very sensitive to age at harvest. Even though individual red snapper can be aged with great accuracy, it is impossible to age every fish that is brought to the dock. However, a great many harvested red snapper can be measured for length in a relatively short period of time. Fish biologists have developed age at length relationships (also called age/length “keys”) based on thousands of otolith-aged red snapper that, based on probability, assign ages to harvested red snapper from their lengths. For example, let’s say that based on the age at length relationship, a 24 inch long red snapper has a 10% probability of being age 3 years, a 20% probability of being age 4, a 40% probability of being age 5, a 20% probability of being age 6, and a 10% probability of being age 7. Among 1000 red snappers for which we only have a 24 inch length, 100 would be assigned an age of 3 years, 200 would be 4 years, 400 would be 5 years, 200 would be 6 years, and 100 would be 7 years. This is, in all likelihood, is not the true distribution of ages within our sample of 1000 red snappers of 24 inches length, but it is likely very close.

Because of the wide variation in red snapper length at age, this assigning of ages from age/length keys generates statistical uncertainty within the computer model. To overcome this problem, the computer models allow the life history variables of interest, such as age at harvest, to fluctuate over ranges normally observed in nature. In addition, the models are run over and over again, usually as many as 500-1000 times (called bootstrapping), until a set of models emerge that provide the best fit to the life history data, the historical landings, and the FI and FD indices of abundance. Most of the time spent on assessments is devoted to this first step, since every subsequent calculation hinges on these calculations. This is the base model and it is used to predict allowable biological catches (ABCs) over the next couple of years, which is all that stock assessment models are intended to do.

Predictions of long-term management objectives (stock recovery) use results from the assessment above and project them forward in time based on options for management alternatives (season length, minimum sizes, total allowable catch, etc.) that the Gulf Council wishes to consider. For example, a run of the model might be made with a 40 percent reduction in bycatch, two fish bag limit for recreational fishery, 15 inch size minimum for the recreational fishery, 13 inch minimum for the commercial fishery, seasonal closures, and each fishery with its specific regulatory discard rate and discard mortality. The next run could be a 50 percent reduction in bycatch, three fish bag limit and 20 inch minimum size for the recreational fishery, 14 inch minimum for commercial sector, slight changes in discards, shorter fishing season, etc. These are projected forward in time, perhaps 1000 times under different assessment uncertainties, both to estimate a harvest rate and to determine if the management goal can be reached by the year at which recovery is to occur, i.e. 2032 in the case of red snapper. A realistic estimate of the probability that management objectives can be achieved under a certain set of management criteria is generated from the uncertainties both in the assessment estimates and in future levels of recruitment. Thus, one set of management criteria may produce only a 50% statistical probability of successfully achieving the management objectives while another set may have a 90% probability of success.

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What does the Gulf Council do?

The Gulf Council must choose a total allowable catch (TAC) from the range of ABCs for red snapper for the next fishing year (and sometimes for several years) as the final step of the stock assessment process. The chosen TAC should have a high probability (see above) of ensuring that the SFA mandated stock rebuilding schedule will be achieved. By law, the Gulf Council must pick an annual TAC from among those sets of management alternatives that have a 50% or higher probability of success in achieving stock recovery. For the last 15 years, the Gulf Council has picked TAC from the low probability end (closer to a 50% chance than to a 100% chance of rebuilding the stock by 2032) of the allowable biological catch (ABC) range. These alternatives all were based on an anticipated, but never achieved, significant reduction in shrimp bycatch mortality. Each year that shrimp bycatch remained high, the less likely it became that a 9.12 million pound combined harvest levels could be sustained. Because it was possible, albeit improbable, that recovery could occur with the lenient regulations, the hard decisions were postponed until the shrimp bycatch numbers had been fully resolved. When it became overwhelmingly obvious that a technological solution to decreasing shrimp bycatch of red snapper was not possible, no options other than major catch restrictions were feasible.

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More information on this topic is available at:

Gulf of Mexico Fisheries Management Council
www.gulfcouncil.org

SouthEast Data, Assessment, and Review
www.sefsc.noaa.gov/sedar

NOAA FishWatch – Red snapper
http://nmfs.noaa.gov/fishwatch/species/red_snapper.htm

NOAA Office of Science and Technology
http://st.nmfs.noaa.gov/index.htm

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Please send your questions and feedback to redsnapper@lsu.edu


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