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Otolith Age And Growth – Lab Report

Megan DeMatteo | Fisheries Science & Management

Stockton University| May 3, 2019


The main objective of this study was to obtain a better understanding of otoliths, understand their important role in conducting research, and understanding the relationships between their growth rings and fish size. In this laboratory experiment otoliths will be identified, removed, embedded, polished, and analyzed to properly read through several microscopic lenses.  The data collected was otolith length, width, and measurements were taken for each growth ring within their secondary growth center. When this data was compared to the overall fish size the results shown were a positive correlation between otolith growth and fish size. In simpler terms this means that as the fish grows so did its otolith rings. Another factor taken into consideration was although the data did show a positive relationship, once the fish reaches maturity and maximum size, the rings do come closer together because its growth is slowing down.


The target specimen of this lab experiment was Limanda ferruginea, more commonly known as the yellowtail flounder. Their geographic distribution is on the Atlantic coast ranging from Newfoundland to the Chesapeake Bay. Yellowtail flounder have a thin, and wide body shaped like an oval. This species of flounder has eyes on the right side of its body. The upper side of this flounder is brown with irregular red spots. One key distinguishable feature to this specimen is their yellow tail fin and edges of the two long fins, hence their name “yellow tail.” The bottom side of this specimen is white with exception to the caudal peduncle (body – tail region) having a yellowish tint.  They can live up to seventeen years growing up to twenty-two inches. Females reach sexual maturity at age 3 and have spawning seasons during spring and summer. Females deposit their eggs on the benthic floor, once the eggs fertilize, they float back to the surface and drift for up to two months until the eggs hatch. As the fish grows and reaches metamorphosis it settles down back to the benthic bottom as a juvenile. The food source of yellowtail flounder are worms and crustaceans and potential predators for this species are: dog fish, skate, and several larger fish such as cod and hakes. The yellowtail flounder is a protected species due to past conflicts of overfishing, strict regulations have been put in place to prevent any further damage to the yellowtail species.

Within the yellowtail flounder there are small white ear stones known as “otoliths” that provide the entire life story of that fish. As the otolith grows it collects information about the fish’s environment, food sources, pH of certain the water it interacted in at that time. Each growth ring within the otolith provide a record of age and growth both daily and annual depending on the fish’s life cycle stage. In this laboratory experiment otoliths will be identified, removed, embedded, polished, and analyzed to properly read their growth rings and determine the relationship between otolith growth and fish size.


The specimen collected were retrieved with a 2-meter beam in the middle portion of the continental shelf off Atlantic City, NJ with a water depth of 40-60 meters. The otolith analysis was broken down into a two-day process: one for removal and the other polishing.

Day 1: Before any removal was initiated the length and width of the yellowtail flounder fish sample had to be recorded. Then the specimen was placed on our slide and viewed under a microscope to begin otolith extraction. By using tweezers and fore septs the sagitta (largest otolith) was found with very fine and delicate stroking of the specimen. This technique helped peel away each skin lawyer of the specimen, thus unearthing the otolith. Once the otolith was discovered it was transported to another microscope slide that was then covered with ETOH. This part of the lab can be one of the most difficult aspects to otolith removal because students must have the precise technique to avoid potentially damaging/ cracking the otolith by squeezing to hard or even dropping it before it reaches the microscope slide. The slide was then set aside to harden while the alcohol evaporated, setting the otolith into place.    

Day 2: Now that the otolith is molded into place within the ETOH the next step was to polish the ETOH with different types of sand paper to unearth the otolith and a microscope with a ranging magnification of 40-500. The sandpaper with the roughest surface (220 grit) was used rapid polishing to sand through initial top layers of ETOH. The medium surfaced sand paper (400 grits) was used once the otolith reached the surface to begin initial otolith polishing until the core was reached. Lastly the fine sand paper (1200, 1500, 2000 grits) was used refine the otolith to get a clear picture for the core and secondary growth rings. The most difficult part of this procedure was applying the correct amount of pressure while sanding through the ETOH and polishing the otolith. This section of otolith processing has the tendency to be very tedious because the goal is to polish the otolith with as little pressure as possible, otherwise the slightest mishap in pressure can break the otolith or sand it too far past the point of a credible read.


A. The image shown above is otolith JA11.5mm at 100x magnification. Each line drawn on the otolith represents a certain part or size measurement used to depict information further mentioned throughout the report. Red: otolith length, Dark Blue: otolith width, Light blue: planktonic phase (Core), Yellow: benthic phase (secondary growth center).

B. Plot B shows a strong, positive consistency between otolith length(y-axis) and fish size (x-axis) based on the averages for the entire cohort. By looking at this graph it is likely to say that as the otolith grows, the fish itself is growing in size as well.

C. Plot C shows the positive relationship between the fish’s juvenile age (y-axis) and the fish size (x-axis) based on the averages of the entire cohort. By looking at this graph it can be said that as the fish continues to age, it will continue to grow in size.  

D. Plot D shows the relationship between the average increment otolith ring width (y axis) and juvenile age which is equivalent to the number of rings for each otolith(x-axis). The intent of this model is to show the increase in otolith ring width as the fish ages, however because of the incremental scale, the cohort exemplifies a small and steady in ring width after each additional day on the benthic sea floor. Some possible reasons why this trend is not as clearly seen unlike the previous models could be because of the sole fact that as the fish ages, it’s growth rate could slow down which would explain the decrease in ring width when the juvenile is older in age. Also, there is the possibility of students misreading ring width or measuring the otolith rings properly. On top of that, just by acknowledging the cohort, the sample size was relatively smaller than a typical otolith research study, which can account to why there was not definitive relationships shown.

E. Agreement

While processing the otoliths each some students analyzed the same ones, producing multiple reads for each otolith ring. This repetition of information can produce a percentage of agreement between each measurement recorded. Agreements also help determine which otoliths can be discarded if each read is a certain percentage apart. After applying a 10% agreement towards the otolith length data, otoliths: SV15 and NH13 can be discarded. After applying a 20% agreement towards juvenile age, otoliths: SV15, NH13, EW18, JA11.5, DS13, KH14, ER15.

Discussion / Conclusion

After overviewing all the otolith processed data, there is a strong, positive correlation between otolith ring width, juvenile age, and fish size. Hence, as the specimen spent more time on the bottom, day by day the rings kept growing, and the size of the species continued to increase.  As I briefly mentioned before some possible biases or potential sources of error could be found in misreading the otolith size or widths of any growth ring. Other students also could have had difficulties with sanding or polishing the otoliths where some may have been cracked from applying too much pressure, or not enough pressure was applied which produced a clouded otolith image causing possible misreads for otolith width. These factors are also held accountable as to why there was such a high discard rate in otolith removal.  Majority of the class, myself included had difficulties identifying the otolith, and polishing it to the preferred read. My personal issue was having the tendency of over sanding the otolith past the point of credible analysis. However, being that this was my first experience interacting with otoliths, I feel confident in knowing how to identify and extract them in future projects/endeavors. If I was presented with the opportunity to expand on this research I would see how certain environmental factors, if any at all, influenced otolith growth compared to fish size and study the relationships between fish size and otolith growth when compared to changes in the fish’s’ current living conditions (pH, salinity, water temperature, etc.).

Work Cited

NOAA. (n.d.). Yellowtail Flounder. Retrieved April, 2019, from

March 4thotilith manual

Otolith slide presentation from class