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NCCOS 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 NCCOS's Center for Coastal Fisheries and Habitat Research, 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… and a virtual "diary" of its early life.
This technique works best for fish that move between or inhabit very different environments with respect to nearness to land and elemental composition of water. Estuarine and near-coastal waters tend to have more pronounced differences in water chemistry. Otolith chemistry has been used successfully to determine whether certain reef species are using estuarine nursery areas, and may soon provide a method to determine the specific nursery estuary of origin.
Elemental fingerprinting becomes more difficult in the open ocean, and where differences among sites of interest are subtle or non-existent. Many tropical reef systems, for example, lack sufficient runoff from adjacent continental or island land masses to generate ecologically significant water chemistry differences.
How an otolith is prepared for study depends on what information is sought. Dissolving a whole otolith provides a record over a fish's entire life history. Sampling a particular location on the otolith's growth rings yields information about an individual fish's physiology and ambient environment from a particular time in its development.
Essential habitat determination
Otolith research is helping to quantify the link between coastal habitat and marine fisheries production. Near-shore marshes, seagrass beds, and mangroves are highly productive nursery habitats for many fish species, including many species found on reefs. These same habitats are coming under increasing pressure from development, pollution, and other human-induced environmental stresses.
A 1996 study of Australian blue groper otoliths contradicted the conventional wisdom that seagrass beds serve an important role in the early life history stages of that reef species. In that research, the scientists found that new additions to the offshore adult population of Australian blue groper came primarily from young that had settled on offshore rocky reefs.
According to Cynthia Jones and her colleagues at the Center for Quantitative Fisheries Ecology at Virginia's Old Dominion University, the absence of reliable information on fish life history has resulted in over-use of the term "essential" to encompass most coastal waters and the continental shelf.
"When essential habitat is ubiquitous, the concept loses its impact and ability to motivate conservation," Jones has cautioned. Her research examines otoliths to track fish growth and survival in various habitats during vulnerable early life stages. This method has advantages over physically tagging fish, which Jones says can be labor-intensive and can pose risks in particular to the smallest and youngest fish. Her otolith research findings may have important implications for determining what environmental changes a species can tolerate, and what habitats are essential to protect key fish species.
Reef ecology applications
In 2002, NCCOS's Hare and Woods Hole's Thorrold surveyed otolith applications in reef fish ecology. They found that studies of annual and daily otolith increments, and application of otolith chemistry more generally, have contributed significantly to scientists' understanding of the ecology of reef fishes. They cite several pivotal studies.
One 1995 study examined annual increments in otoliths of coral trout to investigate the effectiveness of marine protected areas on the Great Barrier Reef. Although the scientists had found no significant differences in size or sex ratios between protected and unprotected reefs, they did find differences in age structures consistent with the effects of protection from fishing pressures. They concluded that for coral trout, and presumably other long-lived species, age structure was more sensitive than size structure to the effects of fishing.
According to Thorrold and Hare, application of otolith chemistry is improving understanding of the exchange of individuals among geographically separated populations, and making it possible to quantify the proportion of larvae that are locally retained around natal coral reefs. They acknowledge that study results from using natural tags such as geochemical signatures in otoliths are unlikely to be as definitive as those in which a fish is physically tagged, released, and then recaptured.
"However," they have written, "the power of natural tags is that every fish from an area is invisibly tagged, and capture of a single fish spawned in that area represents a recovery." As a result, "small uncertainties in the classification of an individual fish may be overcome by much larger sample sizes ('recaptures') than can typically be achieved in conventional tagging studies."
According to Thorrold and Hare, "the prospects of tracing larval dispersal from spawning to settlement are especially intriguing given the importance of such information in the design and implementation of marine protected areas." For a protected area to contribute to fish populations in other areas, the scientists explain, it must supply fish to these other areas. A major mechanism of supply is through the dispersive larval phase.
Most marine fish have complex life histories. Each female will spawn thousands to millions of eggs during her lifetime. These eggs are typically small (~1mm) and larvae are planktonic, which means they drift with ocean currents.
After this planktonic larval stage, fish settle to juvenile habitats and change into recognizable forms. Spawning in a protected area has the potential to supply juveniles to a wide area through the transport of planktonic larvae by ocean currents. The ability to follow larvae from known spawning sites to successful juvenile habitats would greatly enhance the management and conservation of marine fisheries, Hare and Thorrold emphasize.
Clues about ancient climates
Otoliths tell us much about the daily and yearly life of fishes, but otoliths can also provide important insights into ancient climates in which the fish lived. Strontium thermometry uses the strontium/calcium ratios in calcium carbonate to reconstruct modern and ancient temperature histories. In research reported in the Feb. 22, 2002, issue of Science, Fred T. Andrus and colleagues examined the oxygen isotope profiles in otoliths from 6,000-year-old sea catfish at two Peruvian archeological sites, finding temperatures three to four degrees warmer than they are today.
These findings suggest that the modern El Nino Southern Oscillation (ENSO) pattern began after the mid-Holocene sea epoch, creating a very productive fishery that may have contributed to the growth in human population and cultural complexity that occurred in Peru roughly 5,000 years ago. The pattern Andrus observed from fish otoliths has been corroborated by archeological data on the kinds of species excavated, providing further evidence, in his view, that climate change is occurs throughout the history of the Earth.
Insights unavailable through other research techniques
Otolith microstructure and microchemistry provide insights not available through other research techniques.
"The daily rings in the otoliths of juvenile bluefish, for example, contain a geochemical signal that could be used to map their migration pathways," says Thorrold. "Some fish may be making migrations as amazing as the monarch butterfly, but currently we have no way of knowing."
In 2001, Thorrold and several colleagues used otolith microchemistry to assess the connectivity rates and spawning site fidelity of weakfish, Cynoscion regalis.
Spawning site fidelity, they concluded, ranged from 60 to 81 percent, a rate comparable to the natal homing estimates for birds and anadromous fish. This contrasted with earlier genetic analysis methods that found no genetic evidence to differentiate among sites. This discovery of separate metapopulations has significant implications for the management of weakfish, which is currently considered one coastwide stock.
Also, says Thorrold, the results have considerable implications for the design of marine protected areas along the U.S. coast. As he wrote in Science magazine in 2001, "Weakfish subpopulations with high levels of natal homing will be significantly more vulnerable to fishing activity than would be predicted on the basis of current stock models."
Annual rings in otoliths have been used to age fish for more than 100 years. The finding of daily increments in the 1970s opened a new door into understanding the dynamics and ecology of the pre-recruitment stages of fish. Advances in otolith microchemistry make it possible to examine the environment experienced by fishes. The techniques and applications of the study of otoliths are still developing, but the research continues to offer important insights on fish ecology and continue to improve the scientific bases of fisheries management and conservation.
