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Resource Ecology & Ecosystem Modeling Program

Octopus Catch Limits and Beak-Size to Body-Mass Relationship

figure 1, see caption
Figure 1.  The relationship between beak-size and body-mass for Enteroctopus dofleini.  Although each sex is distinguished in the plot, the regression is independent of sex because this can rarely be determined from the remains in the predator stomachs.
 
 

This year, food habits data were used as the basis for calculating predation-based estimates of octopus natural mortality to define the allowable biological catch (ABC) and overfishing limit (OFL) for the Bering Sea/Aleutian Island octopus stock (2,590 t and 3,450 t, respectively). The method combined groundfish ration, diet, and biomass estimates from 25 years of surveys to calculate both these estimates, their confidence limits, and range of interannual variation.

In discussion of this new methodology, questions were raised about the size distribution of the consumed octopus. To answer these questions, AFSC staff will be estimating the size of octopus prey from the size of octopus beaks found in the stomach contents of groundfish.

A reference set of beaks, stylets, and statoliths from octopus of known size was collected as part of North Pacific Research Board (NPRB) Project 906, Field Studies in Support of Stock Assessment for the Giant Pacific Octopus Enteroctopus dofleini (PIs E. Conners, R. Brewer, and C. Conrath).

We took several standard measurements from both the upper and lower halves of these beaks to assess the relationship of beak size to the known body mass of E. dofleini specimens and to assess the ease and consistency of taking the measurements. We found hood length (for both upper and lower beak) has advantages over several other beak measurements: endpoints are clearly defined so consistent measurements among analysts is possible, and these endpoints are relatively easy to access when the beak remains encased in the buccal muscle mass, so handling-time is minimized during stomach content analysis.

Based on preliminary data (Fig. 1), a power function of hood length to predict body mass produces an R2 of 0.86 for the lower beak and 0.85 for the upper beak. The reference collection of E. dofleini hard parts is being used to examine possible methods for ageing octopus by Lisa Kautzi from the Age and Growth Program.

By Troy Buckley, Kerim Aydin, Sean Rohan, Christina Conrath, Elizabeth Conners and Lisa Kautzi


Fish Stomach Collection and Lab Analysis

During the fourth quarter of 2011, Resource Ecology and Ecosystem Modeling (REEM) program staff focused their stomach contents analysis effort on samples from the Aleutian Islands and Bering Sea.

The contents of 1,206 stomach samples (38 species of fish and squid) were analyzed from the Aleutian Islands, and 4,345 stomach samples (28 fish species) were analyzed from the Bering Sea. REEM staff also analyzed 61 stomach samples from the Gulf of Alaska. In total, 24,244 records were added to the REEM food habits database.

In preparation for stable isotope analysis, tissue samples of muscle and liver from arrowtooth flounder, Pacific cod, and walleye pollock have been dried, ground, and tinned (400, 284, and 423, respectively).

By Troy Buckley, Geoff Lang, Mei-Sun Yang, Richard Hibpshman, Kimberly Sawyer, Caroline Robinson, and Sean Rohan
 

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