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Fisheries Behavorial Ecology Program - Newport Laboratory

Applying Otolith Chemistry To Explore Dispersal and Mixing in Bering Sea Pacific Cod Populations

figure 8, see caption
Figure 8.  (a, top) Otolith of an 8-mm Pacific cod larva.  The otolith was polished with fine-grit sandpaper to reveal daily growth-increment formations that were used to age the fish.  (b, middle) A pair of otoliths of a 70-m juvenile Pacific cod.  The rectangular box indicates the thin cross-section of the otolith from which the chemical composition was analyzed.  (c, bottom) Cross-section of a juvenile Pacific cod otolith that has been processed for chemical composition.  A laser is used to ablate the otolith to collect material for chemical analysis.  The wide line is the path the laser took across the surface of the otolith from one edge, through the core to the opposite edge.

Population structure of Pacific cod (Gadus macrocephalus) in the eastern Bering Sea (EBS) remains unresolved, although genetic analyses currently indicate isolation-by-distance (i.e., that the closer two groups of fish are, the more genetically similar they appear). However, genetic-based approaches provide stock structure information on evolutionary timescales (thousands of years) whereas the effects of fishing and other management actions can influence populations on ecological timescales (within a few generations). Therefore, it is important to understand how much mixing and exchange occurs among geographically discrete groups of fish on ecological time frames.

In collaboration with Oregon State University, researchers from the AFSC's Newport Laboratory are exploring the variation in otolith chemistry of larval and juvenile Pacific cod in order to evaluate patterns of dispersal and mixing in the EBS. Researchers from Auke Bay Laboratories and the Recruitment Processes Program are also participating in this project supported by a grant from the North Pacific Research Board.

Otoliths are small bony structures composed of calcium carbonate in a gelatin matrix situated in the inner ear and used for fish movement and balance: as the fish moves or the head tilts, the otolith stimulates small hair cells, which send signals down sensory fibers to be interpreted by the brain. While otoliths are used for balance and orientation as well as sound reception, they also provide researchers with a tremendous amount of ecological information.

In most fishes, the otolith begins to grow during the egg stage, laying down one distinct increment every day and, eventually, one increment per year. Otoliths deposit layers of calcium carbonate throughout the life of a fish, which are analogous to tree rings. By counting the rings the age of a fish can be estimated in years (or in young fish, days; Fig. 8a). The otolith structure also records aspects of the chemical environment of a fish and, hence, can provide information on the environment in which individual fish lived, i.e., a "geochemical fingerprint."

In one component of the work, laboratory experiments were used to determine the influence of temperature and growth rate on elemental incorporation into larval otoliths. Larval cod were reared at 2°, 5°, and 8°C for up to 51 days, and the otoliths and rearing water were examined for chemical composition using an inductively coupled plasma mass spectrometer at Oregon State University's W.M. Keck Collaboratory for Plasma Spectrometry. The differences in elemental concentration between rearing waters and the fish otolith reflect physiological and chemical partitioning of elements in the deposition of the otolith matrix.

The effect of temperature on partition coefficients varied among elements: otolith incorporation of strontium and barium decreased with increased temperature, while incorporation of magnesium was unaffected by temperature. Incorporation of the three elements was not affected by variation in growth rate within temperature treatments.

This type of information is important for accurate interpretation of otolith chemical composition of fish collected in the EBS. We now know that it is unlikely that variation in growth will complicate any spatial patterns in otolith composition that we observe in the field.

Two sets of samples were used to evaluate the ability to use variation in otolith chemical composition to correctly identify fish to their collection sites. In one test, the elemental composition of otoliths from cod larvae (4-12 mm) collected on either side of the Alaska Peninsula in 2006 and 2008 were examined for spatial variation. In both years, fish could be classified to their basin of origin with over 70% accuracy based on standardized ratios of Ba, Mn, Mg, Sr, and Zn (referred to as the "elemental signature").

A second test examined spatial variation in elemental signatures of juvenile Pacific cod captured from six sites across the EBS in 2006 and 2008. Here, the chemical composition of a 200Ám transect (about 1/50th of an inch) along the otolith edge (corresponding to approximately the last 10 days of life prior to capture) was examined (Fig. 8c). Again, the majority of fish could be correctly assigned to their collection site. These two tests demonstrated that there is sufficient spatial variation in elemental signatures of both larval and juvenile Pacific cod otoliths to apply this technique in studies of population dispersal and movement.

A final component of the project is examining the elemental signatures of the otolith cores, which reflect the environment at the time of hatch of those Bering Sea juveniles (Fig. 9). Because variation in elemental signatures of otoliths appears spatially driven, the elemental signatures in the otolith that were deposited early in life can be used to determine patterns of movement and mixing prior to capture.

Through cluster analysis of elemental signatures in the otolith core (deposited during the early larval phase) the researchers hope to determine the number of distinct spawning regions contributing to the juvenile population in the SE Bering Sea. Although they caution that pinpointing the exact locations of the main spawning areas will not likely be possible yet. The degree to which fish with similar elemental signatures in the otolith core (presumed to represent distinct spawning areas) are found together as juveniles will provide evidence of the spatial scale of mixing during early life stages in this valuable resource species.

By Thomas Hurst

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