Flathead sole (Hippoglossoides elassodon) is a right-eyed flatfish commonly encountered in the eastern Bering Sea (EBS) and the Gulf of Alaska (GOA). Flathead sole is similar in appearance to a less common, closely related species, Bering flounder (H. robustus). Despite differences in biology and spatial distribution, H. elassodon and H. robustus are managed together as a two-species complex in the Bering Sea/Aleutian Islands (BSAI) assessment area where their ranges overlap (Stockhausen et al., 2008).
Flathead sole are typically found on silty or muddy bottoms at depths from shallow nearshore waters to 1,050 m, although they most often occur at depths less than 366 m (Mecklenberg et al., 2002). Flathead sole spend the winter along the outer shelf, moving to shallower waters in the spring (Witherell, 2000). Stock biomass in the BSAI increased dramatically during the 1980s and peaked in the mid-1990s but has since decreased to a level of around 800,000 metric tons (Stockhausen et al., 2008). There are directed trawl fisheries for flathead sole in the BSAI and the GOA, with fishery recruitment beginning around age three (Stockhausen et al., 2007; Stockhausen et al., 2008). Flathead sole are also caught incidentally in target fisheries for Pacific cod (Gadus macrocephalus), walleye pollock (Theragra chalcogramma), and other flatfish (Stockhausen et al., 2007; Stockhausen et al., 2008).
Flathead sole are spring batch spawners (Stark, 2004). In the GOA, spawning begins in April and peaks in May or June (Stark, 2004; Porter, 2005). Females attain 50 percent maturity around 320–330 mm, or 8.7–9.7 years (Stark, 2004). Females reach a larger maximum size than males (Fig. 1). Von Bertalanffy growth parameters for flathead sole collected during an eastern Bering Sea trawl survey in 2007 were L∞=381.08 mm, k=0.20/yr, and t0=0.19 yr for males and L∞=467.80 mm, k=0.15/yr, and t0=0.09 yr for females (Fig. 1). Males grow faster in the EBS than in the GOA, although females exhibit no such regional differences in growth (Stark, 2004).
Age Determination History
From 1985 to 2008, the Alaska Fisheries Science Center (AFSC) has collected and determined ages from over 13,000 flathead sole otoliths. Age estimates range from 0 (young-of-year) to 33 years (Table 1). Flathead sole age determination is fairly difficult, with an average inter-reader CV of 7 percent and percent agreement of approximately 42 percent (Table 1).
The surfaces of flathead sole otoliths tend to be cloudy, and therefore surface examination is the primary age determination method only for relatively young or clear otoliths. The break-and-burn technique was favored until the break-and-bake method was introduced in 2003. Baking is now generally the preferred method for flathead sole otoliths because it usually produces more even contrast between translucent and opaque growth zones than burning. However, for some otoliths, the break-and-burn method may result in clearer growth patterns. It is up to the discretion of the age reader as to which method is chosen, although it is often helpful to periodically test both methods on either half of the same otolith. (Please see Goetz et al., 2012 for a more detailed description of standard AFSC otolith preparation methods.)
Current Age Determination Methods
Flathead sole otoliths are relatively difficult to interpret, with growth patterns comparable to those of rex sole (Glyptocephalus zachirus).
Generally, it is best to closely examine the surface pattern of the eyed-side otolith first and then compare it to either the break-and-burn pattern (Fig. 2) or the break-and-bake pattern (Fig. 3). When clear, the surface pattern is generally reliable up to about 8 years old (Figs. 4, 5), although clear surface patterns are observed in older fish as well (Fig. 6).
In flathead sole otolith cross sections, the areas on either side of the sulcus are generally the preferred reading axes. However, in some instances the dorsal and ventral tips may be clearer than the sulcus (Fig. 7). Whenever possible, the age reader should attempt to follow each translucent growth zone from the sulcus to either the dorsal or ventral tip. A translucent growth zone that can be followed around the circumference of an otolith is much less likely to be a check, especially if it is strong.
Occasionally, some opaque growth zones may be relatively wide or narrow compared to other opaque zones within the same otolith. In these cases, the comparison of surface and breakand-bake patterns is especially useful in distinguishing between checks and annual marks (Fig. 8). The unburned cross section may also be helpful for comparison, particularly in younger fish (Fig. 9).
Another problem typical of flathead sole age estimation is identification of the first annual mark (Fig. 10). Again, viewing the surface pattern in combination with the baked pattern is often necessary to arrive at a final age estimate. Additional examples of flathead sole otolith growth pattern interpretation are shown in Figures 11 through 14.
- Goetz, B. J., C. E. Piston, C. E. Hutchinson, C. G. Johnston, and M. E. Matta. 2012. Collection and preparation of otoliths for age determination. In Age determination manual of the Alaska Fisheries Science Center Age and Growth Program (M. E. Matta and D. K. Kimura, eds.), Chapter 3. NOAA Professional Paper NMFS 13.
- Mecklenberg, C. W., T. A. Mecklenberg, and L. K. Thorsteinson. 2002. Fishes of Alaska, 1037 p. American Fisheries Society, Bethesda, MD.
- Porter, S. M.. 2005. Temporal and spatial distribution and abundance of flathead sole (Hippoglossoides elassodon) eggs and larvae in the western Gulf of Alaska. Fish. Bull. 103:648-658.
- Stark, J. W.. 2004. A comparison of the maturation and growth of female flathead sole in the central Gulf of Alaska and south-eastern Bering Sea. J. Fish Biol. 64:876-889.
- Stockhausen, W. T., M. E. Wilkins, and M. H. Martin. 2007. Gulf of Alaska flathead sole stock assessment. In Stock assessment and fishery evaluation report for the groundfish resources of the Gulf of Alaska. North Pac. Fish. Mgmt. Council, Anchorage, AK, Section 8:505-562.
- Stockhausen, W. T., P. D. Spencer, and D. Nichol. 2008. Flathead sole. In Stock assessment and fishery evaluation report for the groundfish resources of the Bering Sea/Aleutian Islands regions. North Pac. Fish. Mgmt. Council, Anchorage, AK, Section 8:777-864.
- Witherell, D.. 2000. Groundfish of the Bering Sea and Aleutian Islands area: Species profiles 2001. North Pac. Fish. Mgmt. Council, Anchorage, AK.
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