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Research on Fish Otoliths Yields Key Environmental Clues

Marine scientists are making great headway in understanding fish survival by studying small bones called “otoliths” in most fish

Small bony structures ranging in size from micrometers to centimeters (roughly from one-tenth of an inch to one inch long), otoliths are found in the heads of all bony fishes. Housed in three separate fluid-filled chambers within the inner ear, otoliths help fish sense up from down and have a role in hearing also. They have become important research tools for understanding the life of fish and fish populations, information essential to sustain the nation’s $50 billion recreational and commercial fishing industry.

Research on otoliths is also improving scientists’ understanding of coastal and marine ecology – helping managers make informed issues such as conservation of coral reefs and nursery habitats, management of fish stocks, and siting of marine protected areas.

Otoliths are the first calcified structures that appear during early fish development. They grow incrementally through differential deposition of calcium carbonate (usually argonite) and protein that generally occurs on a daily cycle.

Thus, like trees’ annual concentric growth rings, the number of otolith increments can be used to age fish in days. In addition to the daily patterns in increment deposition, an annual pattern also is evident. Fish and otolith growth is slower at some times of the year than at others (typically slow during winter), leading to daily increments that are closer together. This seasonal pattern in growth results in annual growth rings in otoliths, allowing determinations of the age of the fish in years.

When viewed through a microscope, daily growth rings from the first year of life reveal detailed information about age-related growth patterns of larvae and juvenile fish. Distinctive patterns can also be observed at life history stage transitions, with the first increment formation occurring at hatching in some species and at yolk absorption and first feeding in other species. A distinctive pattern, or settlement mark, commonly occurs in the otoliths of many fish species at the time of the larval/juvenile transition, for example when free-floating larvae settle to the ocean floor.

Together, this information on age and growth rates can help explain selective processes that determine why some individual fish survive while others do not – key processes thought to be at the core of much fish ecology. Jonathan A. Hare of the National Centers for Coastal Ocean Science and Simon R. Thorrold, of Woods Hole Oceanographic Institution, say otoliths are key to understanding whether mortality is dependent on the size at which a fish settles. Or whether faster growing fish have a higher probability of survival. Or whether certain larval stage characteristics convey a “survival advantage” to post-settlement individuals.

“All these questions can be examined through analysis of otoliths microstructure,” the two scientists have written.

In addition to providing information on fish age and growth, otoliths also record information on the environment in which fish live. As the otolith grows, trace elements are incorporated into the calcium carbonate matrix. Daily changes in the ambient environment of individual fish can be observed with microchemistry analysis of the otolith. One particular characteristic that makes otoliths ideal for chemical analysis is that, unlike the other calcified structures in fish skeletons, they are chemically inert. Material laid down at one point does not get reworked or absorbed later on.

Another characteristic that makes otoliths ideal for chemical analysis is that more than 90 percent of the otolith is composed of calcium carbonate and trace elements derived from the ambient water, as modified by temperature. Sophisticated chemical techniques, using state-of-the-art instrumentation, therefore can construct an “elemental fingerprint” of the chemistry from wherever a fish happened to be on a given day

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