Alaska plaice (Pleuronectes quadrituberculatus) is a shallow-water flatfish generally found at depths less than 200 m. This species is easily identified by its yellow blind side, small mouth, and four prominent protuberances along the postocular ridge. In the eastern Pacific Ocean, Alaska plaice is found in the Chukchi Sea, Bering Sea, and northern Gulf of Alaska, predominantly on mixed sand and mud bottoms (Wolotira et al., 1993; McConnaughey and Smith, 2000).
This species is far more common in the eastern Bering Sea than the Gulf of Alaska, although its biomass appears to be increasing in the Gulf of Alaska (Turnock and Wilderbuer, 2007; Wilderbuer et al., 2007). Alaska plaice is not commercially targeted in Alaskan waters, and while it is caught incidentally in other groundfish fisheries it is seldom retained (Turnock and Wilderbuer, 2007; Wilderbuer et al., 2007).
Alaska plaice produce pelagic eggs and larvae that are dependent upon oceanic currents for transport to suitable nursery habitat areas, which are essential for recruitment success (Bailey et al., 2003). In the eastern Bering Sea, spawning occurs during the months of April through June over a wide area of the middle continental shelf (Zhang et al., 1998).
In females, 50% maturity occurs at approximately 310 mm, corresponding roughly to 6 or 7 years of age (Zhang et al., 1998). According to Alaska Fisheries Science Center (AFSC) Age and Growth Program data, females grow slightly slower and to larger sizes than males (Fig. 1). Von Bertalanffy growth parameters were L∞=392.05 mm, k=0.20/yr, and t0=1.50 yr for males (n=440) and L∞=503.49 mm, k=0.15/yr, and t0=1.55 yr for females (n=677) collected from the Bering Sea from 2005 through 2007.
Age Determination History
Over 4500 Alaska plaice otoliths have been aged by the AFSC Age and Growth Program since 1988.
At the time of this writing, the oldest Alaska plaice aged at the AFSC was a 37-year-old female (Table 1). To date, nearly all specimens have been aged using the break-and-burn method, although a small percentage has been aged by examination of surface patterns (Table 1). Alaska plaice otoliths are relatively easy to interpret, and inter-reader agreement tends to be high (Table 1). Age estimates have not yet been validated for this species.
Current Age Determination Methods
Alaska plaice is one of the easier species aged at the AFSC. In general, the otolith surface has a cloudy appearance; however, surface patterns may be used occasionally to determine age in young fish (Fig. 2). Even in young fish, seemingly clear surface patterns can sometimes be deceiving (Fig. 3). In these cases and for older fish, the break-and-burn method is more appropriate (Fig. 4). (Please see Goetz et al., 2012, for a more detailed description of standard AFSC otolith preparation methods.)
Alaska plaice otoliths are relatively large and can shatter easily if not sectioned properly. If a scalpel is used for cross-sectioning, it is best to score the surface prior to cutting in order to obtain an even break. A low-speed saw may be used to section the otolith instead of a scalpel. While this method is more time-consuming, it produces a smooth surface ideal for identifying annual marks. Topography on the break-andburn cross section can sometimes cause an age reader to mistake a “ridge” for an annual mark. On break-and-burn cross sections, translucent growth zones are very thin and almost appear etched across the reading surface. Adjusting the angle of the fiber optic light source can make these zones stand out.
Adequate burning produces higher contrast between translucent and opaque growth zones (Fig. 5), although care should be taken to avoid over-burning otoliths. Pre-annular checks are very common and are especially prevalent in lightly burned otoliths (Fig. 6). Longer burn times can be used to distinguish between preannular checks and annual marks, as checks typically become very faint and annual marks extend all the way around the otolith (Fig. 7).
The first annual mark is usually the most difficult one to identify in Alaska plaice otoliths; therefore viewing the surface pattern together with the break-and-burn pattern can be helpful.
The preferred reading axes in break-and-burn cross sections are from the core to the dorsal and ventral tips. The areas adjacent to the sulcus may also be used as reading axes, although it is often more difficult to distinguish between checks and annual marks. However, in certain cases the areas around the sulcus are clearest (Fig. 8). This is especially true in older otoliths, in which the dorsal and ventral tip axes may be compressed and contain many checks.
Irregular spacing of annual marks (Fig. 9) is encountered somewhat commonly in Alaska plaice otoliths, as are fast-growing (Fig. 10) and slow-growing (Fig. 11) growth patterns.
- Bailey, K. M., E. S. Brown, and J. T. Duffy-Anderson. 2003. Aspects of distribution, transport and recruitment of Alaska plaice (Pleuronectes quadrituberculatus) in the Gulf of Alaska and eastern Bering Sea: Comparison of marginal and central populations. J. Sea Res. 50:87-95.
- 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.
- McConnaughey, R. A., and K. R. Smith. 2000. Associations between flatfish abundance and surficial sediments in the eastern Bering Sea. Can. J. Fish. Aquat. Sci. 57:2410-2419.
- Turnock, B. J., and T. K. Wilderbuer. 2007. Gulf of Alaska shallow-water flatfish. In Stock assessment and fishery evaluation report for the groundfish resources of the Gulf of Alaska. North Pac. Fish. Mgmt. Council, Anchorage, AK, Section 4:313-338.
- Wilderbuer, T. K., D. G. Nichol, and P. D. Spencer. 2007. Alaska plaice. 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 9:955-998.
- Wolotira, R. J., Jr., T. M. Sample, S. F. Noel, and C. R. Iten. 1993. Geographic and bathymetric distributions for many commercially important fishes and shellfishes off the West Coast of North America, based on research survey and commercial catch data, 1912-84. U.S. Dep. Commer., NOAA Tech. Memo. NMFS-AFSC6, 184 p.
- Zhang, C. I., T. K. Wilderbuer, and G. E. Walters. 1998. Biological characteristics and fishery assessment of Alaska plaice, Pleuronectes quadrituberculatus, in the eastern Bering Sea. Mar. Fish. Rev. 60(4):16-27.
- 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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