A Useful Little Bone

Post and photos by Christine Kircun, NOAA/NEFSC

As we near the end of the 2017 spring bottom trawl survey, we are about 650 miles north of our southernmost station near Cape Hatteras.  With that said, it wouldn’t be surprising to know that the fish are very different in the northern part of the survey.

Here we’ll find haddock, cod, pollock, and American plaice to name a few.  And just like down south, while some fish look very different there are a few that are tricky to differentiate, especially for someone’s first time out.  While external features are used to quickly identify a fish, there is another important structure that can also identify a fish: the otolith.

The otolith is an earbone that we primarily use to figure out the age of a fish, although it can also be used to identify fish.  Otolith shapes vary widely, but the more closely species are related, the more similar they look.  I’ve chosen three fish to highlight this.


This picture appeared in one of my earlier blogs, but I wanted to bring it back because it’s a great example of the otoliths being easier to tell apart than the fish itself.  Silver hake is slightly more silver in color and has 16-20 gill rakers while offshore hake tend to be bluer and have 8-11 gill rakers.


pic 2a_opt

The top fish is a silver hake and the bottom is offshore hake.  Otoliths from these fish are at left

pic 2b_opt


Here’s a close-up of those otoliths. The  offshore hake otolith (top) isn’t as long and slender as the silver hake otolith, even from fish of approximately the same size.  The offshore hake otolith in general is much wider, and its tips are more rounded than the silver hake otolith.




pic 3_opt

Atlantic herring (top) vs Alewife (bottom)

Above are two fish we’re currently catching in almost every tow: Atlantic herring and alewife, a river herring.  While these two fish may seem very different in this picture, it’s not uncommon to toss a couple from the sorting belt into the wrong bucket.  The quickest way to tell the difference by looking at the outside is to run your finger along its top side from tail to head.   It will be smooth on the herring, but not so with the alewife as you’ll get caught on its scutes — spiny, rough scales.  Their otoliths are very small in comparison to their overall body size. You can barely see them in this photo. The Atlantic herring otos are placed in the vertical black band on the measuring board, just below the herring’s jaw. The alewife’s are even smaller, placed in the black tag on the measuring board, just to the right of the number “4”,  above the fish’s eye.

pic 4_opt

Here’s a close-up of those otoliths.  They are very similar in shape, with the rostrum (top portion) being about the same length relative to the total length of the otolith.  The width is where you’ll find the difference.  The alewife otolith is wider relative to its height than is the slightly more slender herring otolith.

pic 5_opt

White hake (top) and red hake (bottom)

Here’s another confusing pair.  Until you get the eye for making distinctions, these two fish are very easy to confuse.  A quick way to tell the difference by just looking at these two is the length of the pelvic fin, that reddish looking string you can see running from near the throat and down the belly.  It extends further along the body of the red hake.  If the pelvic fin — also called a filament — is broken, the silvery color or smaller scales of the white hake are other quick identifying characteristics.  But let’s go to the otoliths!

pic 6_opt

These otoliths (right) came from two very different sized fish, but the size isn’t necessarily what’s going to differentiate these two fish. Look at the rostrum, the pointed top end.  Red hake have a smooth rostrum (left) while white hake’s rostrum (right) is bumpy.  Also, the red hake otolith tends to have a little edge right before the rostrum begins.

I find myself using this skill the most when examining stomach samples to find out what a fish has been eating.  If I am trying to ID a partially digested fish, the otolith is often remains more intact than the rest of the animal.  It can also be used to identify a fish  down to genus or species.  This is just another way we rely on this useful little bone!

Christine Kircun, biologist

Aboard the NOAA Ship Henry Bigelow

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