Rock of Ages

The stench was overwhelming. I recall struggling to keep my lunch down, but I was focused on my prize. This massive black drum, almost as big as I was at 8 years old, had definitely been afloat for a while, but I was determined to add the "rocks" from this specimen to my collection. As a kid, I went to great lengths and faced down many a foul-smelling fish carcass to obtain the precious gems hidden in their skulls. At the time, I had no idea what the "rocks" (or otoliths as I now know them) really were, or what they were for. Fast forward a few years, a college degree in fisheries management, and tens of thousands of hours of fisheries field work, and I find myself in a lab with a table of finfish and a fillet knife looking for otoliths. I'm a little older, yes; I'd like to think that I'm a little wiser too. Yet I find myself drawing on the same skills I mastered as a youngster.

Commonly referred to as "ear stones" or "ear bones", otoliths are hard calcium carbonate structures formed by the accretion of aragonite (also found in coral skeleton and bivalve shells) and the protein otolin. Rings are created daily on the otolith as new material is added. Otolith growth can be influenced by salinity, water chemistry, stress, reproductive state, and a number of other factors. Seasonal changes in water temperature and food availability often have the greatest influence on the deposition rate of otolith-building material. Periods of faster growth are characterized by more translucent zones visible in the cross section of an otolith. During periods of slower growth, the rings form densely packed, opaque zones called annuli. Biologists use annuli to estimate fish age, much like counting the growth rings of a tree.

All teleost (bony) fish have three pairs of otoliths: asteriscii, lapilli, and saggitae. These "float" beneath the brain in a fluid-filled membranous sack. The asteriscii are involved in the detection of sound and hearing. The lapilli detect sound and gravitational force, helping fish orient themselves in the water column. The saggitae, often the largest and most studied pair of otoliths, aid in hearing by converting sound waves into electrical signals.

Historically, biologists have used techniques such as scale analysis and mark-recapture studies to determine fish age. Due to the proportional relationship of fish growth to otolith accretion, otolith analysis has been determined to be the most accurate method for aging fish. This is critical to assessing populations and developing management strategies for sustainable fisheries. Underestimating age can result in an overestimation of population production and can lead to a management strategy that appears sustainable but actually allows overfishing.

At the Perry R. Bass Marine Fisheries Research Station (PRB) in Palacios, otoliths removed from fish collected during Texas Parks and Wildlife Department (TPWD) routine monitoring are analyzed. The otoliths are embedded in resin and cut into thin sections using a high-speed diamond blade saw. Using a microscope and an optical imaging system, the distances from the center of the otolith to each age ring, or annulus, are measured and recorded. The distance from the last annulus to the edge of the otolith section (the marginal increment) is measured and plotted over a period of time to confirm that annuli are formed once a year. Using otolith analyses, the staff at PRB examine life history parameters for important populations of sport fish and develop age-length keys for individual species. These keys are used in population assessments.

Figure 2 shows how likely a female spotted seatrout (SST) of a determined age is to reach a certain length. You can see that a female SST has a very low probability of reaching legal size until her second year of growth. Taking into account that most female SST are sexually mature at 1-1.5 years of age, TPWD has set length limits to allow a majority the chance to spawn before they are harvested.

What else can we learn by studying its otoliths? How about historical water temperatures? Migration history? Because otoliths are metabolically inert, they do not lose the materials they accrete. By using certain chemical tracers, biologists are able to determine a wide variety of indicators for past feeding history, metabolic rates, water temperatures, salinity, pH, and many other environmental factors.

Otoliths also have a very distinct shape that is characteristic of individual fish species (Figure 3). By examining otoliths contained in the stomach contents and even the droppings from terrestrial animals, one can determine what fish species comprise the predator's primary diet. Archaeologists have even examined otoliths found in Native American shell middens to characterize the diets of native peoples, and determine seasonal use of fisheries, past climate and environmental conditions, and historic fish population distributions.

Obviously otoliths are not just precious gems for ambitious, young beachcombers. They provide fisheries managers a wealth of information including the age structure of fish populations, life history traits, growth rates, and habitat and diet preferences. Taken together, all of this information can allow managers to build a more complete picture of fish populations, which can lead to improved management for better fisheries.