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Age & Growth Program

Age and Growth Program Uses Otolith Shape to Identify Blackspotted and Rougheye Rockfish 

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Summer 2014
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Figure 1. Plots showing several otolith metrics with respect to increasing age. Black circles represent blackspotted rockfish otoliths & red circles represent rougheye rockfish otoliths.

Black spotted (Sebastes melanostictus) and rougheye rockfish (Sebastes aleutianus) are large, red, viviparous rockfish found in the North Pacific Ocean with a contiguous distribution along the west coast from California up around the Gulf of Alaska and across the Aleutian Islands and eastern Bering sea. They were considered to be one species until recently when they were genetically identified as two.

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Figure 2. Graph showing estimates for each otolith. Otoliths with a probability less than .5 are assigned to blackspotted and those above .5 are identified as rougheye. If the 95% confidence interval around an estimate crosses the .5 threshold, then the otolith is classified as uncertain. The true genetic identity of each specimen is shown on the horizontal axis, with blackspotted rockfish to the left of the vertical line, and the identity predicted by the model is shown on the vertical axis.
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Figure 3. The same procedure from figure 2 is applied to a new data set to independently validate the results.

In 2009 and 2013 NOAA scientists collected tissue samples (i.e., fin clips) and otoliths from field-identified blackspotted and rougheye rockfish. The tissue samples were genetically analyzed to test the accuracy of field identification. The otoliths were used by Age and Growth scientists to determine if otolith shape could separate the two species.

A logistic regression model was developed to discriminate the two species based on six otolith shape measurements and estimated age. The measurements were length, width, major axis, minor axis, perimeter, and area. The otoliths were aged as part of the Age and Growth program’s regular production ageing process. It was noticed that the size-at-age of the two species differed, and this was incorporated into the model.

The effect of age was found to be a significant factor in accurately identifying each species as seen in Figure 1. Using these parameters, a statistical model assigned a probability to each otolith that was used to determine whether it was a rougheye or blackspotted rockfish (Fig. 2). The model was then applied to a separate dataset to test its efficacy, and was found to still yield accurate results (Fig. 3).

The Age and Growth program will implement otolith shape distinction for the following situations: to use as quality control with ongoing NOAA otolith collections; to separate the species from AFSC fishery observer collections; and to separate archived collections when the species were identified as one.

By Charles Hutchinson and Jeremy Harris

 


 


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