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Age & Growth Program

Otolith Microchemical Fingerprinting: Assessing Juvenile Pacific Cod Habitat Utilization in the Gulf of Alaska

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Oct-Nov-Dec 2012
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Figure 7. Otolith collection sites in the eastern and Central Gulf of Alaska from the NPRB GOA-IERP project. Otoliths were collected during summer (July-August) and fall (September-October) 2011. Each site shown represents a nearshore bay (site) with replicate hauls performed in three site subareas.  

Otoliths act as data recorders of the environment a fish is exposed to during its lifetime. Chemical signatures within otoliths are often unique to a specific geographical area and period of time and may be used as natural tags to classify adult fish to their natal origins. The Gulf of Alaska (GOA) is a complex marine ecosystem that is spatially and temporally variable in its oceanography, physiochemistry, and primary production. It also provides critical nursery habitat for commercially important species of fish such as Pacific cod (Gadus macrocephalus).  The National Oceanic and Atmospheric Administration (NOAA) has in recent years supported a breath of habitat related studies as part of the Alaska Essential Fish Habitat Research Plan. 

The Age and Growth program recently received NOAA Essential Fish Habitat funding for a multi-year research project to conduct research to assess juvenile Pacific cod habitat utilization in the GOA. More specifically, otolith trace element composition may reflect differences in the environmental chemistry of nursery areas in the GOA, allowing discrimination of juvenile Pacific cod from different nearshore habitats. Age-0 fish were collected during the 2011 Middle Trophic Level component of the North Pacific Research Board GOA Integrated Ecosystem Research Project (Fig. 7).

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Figure 8. Trace element signatures of boron, barium, and magnesium  in an age-0 Pacific cod otolith.  

In summer 2013, fish from the same cohort (age-2) will be collected during the RACE GOA bottom trawl survey. Using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we will determine whether otoliths can be used to distinguish among age-0 fish from different nearshore nursery areas in the GOA and whether older individuals from the same cohort can be retrospectively linked to these nursery areas based on unique combinations of trace elements in their cores. Thus far, boron, strontium, magnesium, and barium have all displayed strong signals (Fig. 8) and may prove useful in discriminating between otoliths collected at different nursery sites. In particular, a large peak in 138Ba noted in some specimens may correspond to upwelling events or freshwater run-off into nursery bays. Analyzing the elemental signatures of the surviving 2011 cohort as they move from the nursery to offshore waters is the critical key to identifying and understanding the connectivity of habitat and recruitment to the offshore population. 

By Beth Matta, Thomas Helser, and Craig Kastelle

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