Great Sculpin (Myoxocephalus Polyacanthocephalus) Age and Growth Research

The AFSC Age and Growth Program does not currently age great sculpins on a production basis. However, we have made a recent attempt to establish aging criteria and generate age estimates as part of a broader life history study of several large sculpin species. For this study, otoliths from 403 great sculpins were examined. An overall percent agreement of 61 percent (Table 1) and minimal bias between age readers indicates well-defined aging criteria for this species. At the time of this writing, the oldest specimen aged at the AFSC was a 16-year-old female (Fig. 2).

The first step in great sculpin age determination is to examine the whole otolith under 6x magnification using a dissecting microscope with reflected light from a fiber-optic light source. Typically, the distal surface is the preferred reading surface because the sulcus, located on the proximal surface, can obscure annual marks. On clear otoliths, age estimates of up to 4 years can be confidently assigned solely from examining the surface pattern.

On otoliths with faint growth patterns, measurements help identify the first three annual marks. The average anterior-posterior length of an otolith from a 3-year-old fish is 7 mm. The dorsal-ventral width of the first annual mark is typically around 1.2 mm, and widths of the second and third annual marks are approximately 2.2 mm and 3.8 mm, respectively (Fig. 3). While some great sculpin otoliths can be aged solely from the surface pattern, most specimens must be aged using the break-and-burn method. (Please see Goetz et al., 2012 for a more detailed description of standard AFSC otolith preparation methods).

The left sagittal otolith is usually selected for the break-and-burn method. The otolith is sectioned transversely through the core using a scalpel. Great sculpin otolith cross sections may be sanded to yield a smoother surface prior to burning. The otoliths of this species can crumble if burned too long. The otolith is examined with a dissecting microscope at magnifications ranging from 6–25x. The clearest patterns on great sculpin otolith cross sections are usually found from the core to the dorsal sulcus and from the ventral tip around the anti-sulcus to the dorsal tip (Fig. 4). Translucent growth zones that are complete concentric rings around the core are counted as annual marks.

Figure 2 shows an old specimen with a fairly clear pattern that still can be difficult to age. There are some checks and shadows within the first few annual marks. Measurements help identify the first three annual marks. There is a pre-annular check inside the third annual mark which is not considered a separate annual mark because of the spacing of the first 3 years. Additional pre-annular checks are present within the fourth and fifth annual marks. There appears to be a transition zone at the fifth annual mark, after which every translucent growth zone is counted as an annual mark. The estimated age for this otolith was 16 years.

Some great sculpin otolith patterns can have many checks, as observed in Figure 8. Measurements confirm the first three annual marks, indicating splitting has occurred within the first year. Numerous checks are found between the third and fifth annual marks along the dorsal sulcus and are not considered annual marks because of the relative intensity of the true annual marks, especially at the sulcus edge. The seventh and eighth translucent growth zones could be considered a single annual mark with doubling. However, both of these growth zones can be traced from the dorsal tip to the sulcus and therefore are separate annual marks.