The great sculpin (Myoxocephalus polyacanthocephalus), a member of the Cottidae family, is found over sand and mud bottoms in the North Pacific Ocean from the Washington coast to the Bering Sea at depths up to 240 m (Eschmeyer et al., 1983). There are currently no targeted fisheries for sculpins in Alaskan waters; however, they are often caught incidentally in trawl and longline fisheries, with Myoxocephalus species making up a large proportion of the total sculpin catch (Reuter et al., 2006; Reuter and TenBrink, 2007).
In the waters around Kamchatka, great sculpin spawning takes place in January and February on sandy and silty-sandy bottoms at depths of 120–210 m and water temperatures of 0.8-1.9°C (Tokranov, 1984). Absolute fecundity averages 185,000 (± 11,000) eggs in eastern Kamchatka waters and 225,000 (± 11,000) eggs in western Kamchatka waters; absolute fecundity depends primarily on weight and to a lesser extent on length and age (Tokranov, 1984). Kamchatka males begin to mature in their fifth year at lengths of 330–340 mm, with mass maturation taking place in the sixth year at lengths of 380–400 mm, while females begin to mature in their seventh year at lengths of 490–500 mm, with mass maturation taking place at the age of 8 years, or lengths of 540–580 mm (Tokranov, 1984).
According to Alaska Fisheries Science Center (AFSC) Age and Growth Program data, male and female great sculpins exhibit survey in 2005 were L∞=567.79 mm, k=0.21/yr, and t0=-0.46 yr (n=169) for males and L∞=836.51 mm, k=0.16/yr, and t0=0.51 yr (n=228) for females (Fig. 1).
Age Determination History
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).
Significant growth differences, with females reaching much larger sizes than males (Fig. 1). Von Bertalanffy growth parameters for great sculpin collected during a Bering Sea trawl.
Current Age Determination Methods
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).
Surfaces can indicate the approximate age of the fish, but the break-and-burn method generally results in greater contrast between translucent and opaque growth zones and in many cases reveals annual marks near the otolith margin that would not otherwise be visible.
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.
Great sculpin otoliths can be difficult to age because of checks, splitting, and pre-annular checks. For example, Figure 5 shows the surface and break-and-burn patterns of a 6-year-old specimen. The surface pattern is fairly clear but somewhat obscure along the otolith edge. The break-and-burn cross section has a check between the second and third annual marks, confirmed by measurements. Furthermore, preannular checks are present along the dorsal sulcus of the burn pattern. These checks are strong near the sulcus but cannot be followed throughout the otolith. Because of the confusion caused by pre-annular checks, the best way to estimate age for this specimen is using the dorsal and ventral tips or the anti-sulcus as reading axes and locating the early annual marks with measurements.
Figure 6 is another good example of how checks can complicate age determination. The surface pattern clearly is 3 years, but the break-and-burn pattern has checks that resemble annual marks. Measured distances on the break-and-burn cross section confirm that the otolith is indeed 3 years.
Figure 7 shows an otolith with a check near the sulcus between the sixth and seventh annual marks. Based on the growth pattern viewed along the tip, the otolith is clearly 8 years, but it appears to be 9 years reading down the sulcus. After careful examination, the translucent growth zone between the sixth and seventh annual marks is revealed to be a check because it cannot be followed throughout the otolith and it has a broader, weaker appearance.
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.
Otolith Edge Interpretation
To properly characterize edge type, edge growth needs to be examined from samples collected at various intervals throughout the year. To date, great sculpin otolith collections have been limited to early June through early July. Most otoliths collected in early to mid-June had either a fully formed opaque growth zone or a new translucent growth zone starting to form on the margin. Most specimens collected in late June or early July had a narrow opaque growth zone present on the margin. Therefore, we assume that translucent growth zone formation takes place in mid-to-late June.
- Eschmeyer, W. N., E. S. Herald, and H. Hammann. 1983. A field guide to Pacific coast fishes of North America. Houghton Mifflin Co., New York, 336 p.
- 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.
- Reuter, R. F., and T. T. TenBrink. 2007. 2007 BSAI Sculpins. 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 20:1117-1144.
- Reuter, R., T. TenBrink, S. Gaichas, and S. Lowe. 2006. 2006 BSAI Sculpins. In Stock assessment and fishery evaluation report for the groundfish resources of the Gulf of Alaska. North Pac. Fish. Mgmt. Council, Anchorage, AK, Section 20:431-448.
- Tokranov, A. M.. 1984. Reproduction of great sculpin, Myoxocephalus polyacanthocephalus (Cottidae), in Kamchatka waters. J. Ichthyol. 24(4):119-127.
- Age and Growth Research in Alaska
- Age Determination of Alaska Groundfish Species Manual
- Age and Growth Program Database
- Age Determination Equipment and Supplies
- Age Determination References
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