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.
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.
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.
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!
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