A Deeper Look into Snook

Written by Linda Roberts with Ron Taylor FWRI | 15 October 2009

In the X Ray image above you can see the Otolith highlighted in the otic capsule near the brain in a spotted sea trout. Did you know? Everyday, a specially trained team of FWRI Scientists study the 'growth rings' in the ear bones of sampled specimens of 12 species of FL fish, racking up detailed data on 7-12,000 individual fish per year

Another angle on aging

What's the most important part of the fish? Your focus as an angler might be on the jaws, or the pinched tail; as a consumer, it might be the filet, or the protein and oil content. But Ron Taylor has a different take: "Otoliths (ear bones) are the most important material we collect from fish", reports the head Snook Research Coordinator for FWRI.

The cornerstone for determining Snook age is the sagittal otolith. Being able to determine the ages of snook in sample populations lets biologists predict the species' ability to sustain itself under the given conditions of mortality and reproductive output. Ultimately, it's what the otoloiths reveal that help set slot limits and to make the decision to limit the take of fish within a certain size-age range in order to assure the species remains in a robust condition.

Otoliths are found in all bony fishes, near the brain, and are thought to assist fish with sound detection, attitude, depth location and maybe even balance. Most otoliths are the size of your thumbnail, and as thin as a pencil, but all have rings on them, that can be used to determine the age of a fish, much like ageing a tree by counting the rings on a section. When FWRI scientists ask you to save fish carcasses, this is one of the things they are looking for.

In most Common Snook, an annulus, or growth ring, is laid down once a year, between March and May, The annulus is the dark part of the ring marked with a white dot in the image above - the otololith of a 13 year old snook.

Otoliths taken from random samples of sacrificed snook are prepared for study in an elaborate process.

Examination of sectioned otoliths is the most accurate way to 'age' fish which means a certain number of snook must be sacrificed each year. This is why biologists utilize snook that have been caught and harvested by anglers - to refrain from having to kill additional snook for aging purposes.

To date, the oldest snook aged was estimated to be 21 years old, and FWRI snook biologists are continually looking for a specimen that is that old or older which will indicate the slot and bag limits are indeed working.

How did FWRI scientists determine this?

Counting anything, you have to first determine what equals 'one'. For counting fish age, you have to know when the first orbital ring, or annulus, is laid down. Each species is different. Studies have determined that for common snook, this occurs in the spring of the snook's first year, between March and May. Since snook have a long spawning period, from spring-early fall, the first annulus is laid down when the snook is 6-12 months old.

In contrast, Redfish have a spawning period of August-October, and their first orbital ring is formed in March/April, when they are approximately 6-9months. old.

For Snook, the first annulus is counted as year 1; for Redfish, scientists start with the second annulus.

Otlolith showing daily growth rings measured in the 456 days of this snook's life.

How Can you tell how old a fish is?

The process of discovery begins with one fact, the date a fish is harvested, and the ability to see daily growth rings using high power magnification. Knowing what day a fish died, Ron is able to count daily growth rings, backwards to the first annulus. Remember, the annulus or orbital ring is laid down in common snook, between March and May each year.

Checking the photo above, 290 daily rings are counted from the margin to the first annulus in this fish harvested on January 15th. Subtracting 290 days from January 15th, Ron concludes the first annulus was laid down in late March - the 31st to be exact.

From there, an additional 166 daily rings can be counted back to the core of the otolith, indicating the snook was 166 days old on March 31st. To establish the fish's birthday from here requires deducting 166 days from March 31st, giving us October 16th.

Here's a handy calculator if you want to check the math! Date Calculator

In older fish, additional adjustments must be made to place sampled fish into the correct age groups or cohort. It is impossible to calculate mortality rates of a fish that is a partial year old. Not all fish of the same species form the last growth ring at the same exact time. For instance, the 5th annulus for a snook born June 1st may be laid down in March even through the fish has not reached a full 5 years of age. In such a case the fish would be considered one year less old even though the growth ring for his 5th year is complete.

By standardizing the procedure to assign an age to a total year’s cohort, scientists may group all the snook that were spawned in a single year into a common age. This procedure is necessary in order to calculate the number of snook that survive from one year to the next- the rate of total mortality.

How can you tell how old a fish is? It's not as easy as it looks, says Ron Taylor. He should know, having studied and written about snook during most of a long and fruitful career at FWRI.

"Aging fish is an art and a science," says Ron Taylor, senior research scientist with FRWI. "You have to prepare the otolith, section the otolith, mount the section on a slide, count the rings, make the adjustments, and then determine maximum age, age at harvest and what percent is in the slot, determine the spread of ages, and compute mortality rates for total, fishing, catch and release, and natural mortalities. And we do that for more than a dozen species, some times annually."

For more information on this topic:
Introduction to Aging Fish: What Are Otoloths
(http://research.myfwc.com/features/view_article.asp?id=21978)

• Thanks to Ron Taylor, FWRI, for patiently and generously sharing his insights.