2017 Spring Bottom Trawl Survey

Leg I: Sometimes It’s All About the Little Things

All photos in this post by NOAA/NEFSC/Christine Kirkun

Fish identification is an important part of the job on the bottom trawl survey.  When the catch comes down the sorting belt, it is our job to separate them into baskets, buckets and pails.  Some fish look extremely different from each other.  Others may appear almost exactly the same, especially if it’s your first time looking at them.  The sorting belt is constantly moving so even the most seasoned scientist may get their eyes crossed as they pick through the unsorted catch.

The people at the top of the line typically pick up the large animals, such as dogfish, skates, rays, and goosefish, or a single fish, if the catch has a lot of a particular species, such as haddock, Acadian redfish, or silver hake.  This makes it easier for the people at the bottom of the line to pick up the smaller animals, such as juvenile fishes, squid and crabs, and the more difficult fish to pick up, such and windowpane and fourspot flounders.  Ultimately, if there are two species that look similar, and you’re unsure, you can ask someone or, if it’s busy, place them both in a bucket to be sorted separately at the end.

pic 1_spiny dog back-smooth dog front_opt

A spiny dogfish and a smooth dogfish

Dogfish have been everywhere so far, and we’ve caught a lot of them.  Spiny dogfish is our most commonly caught shark, but there are areas where the smooth dogfish is abundant as well.  At first glance, these two sharks may seem identical, but on close inspection, it is quite clear that they are very distinct from each other.  But remember, we’re not looking at them, perfectly lined up and still.  They are moving down the sorting belt all mixed together with the rest of the catch.  In this case, the sorter looks specifically for the presence or absence or the dorsal spines, although on closer inspection, it is clear the shape of the head, eyes, and caudal tail are very different.

pic 3_2nd dorsal fin_spiny back-smooth front_opt

The telltale spine

The two main sea robins we’ll find are striped and northern.  While both species can reach into the 20cm range for length, the northern sea robin is usually smaller.  Much about them looks very similar except for the prominent dark lateral stripe of the, you guessed it, striped sea robin.

pic 4_striped back-northern front_opt

A striped and a northern sea robin

Squid is also a very common species caught along the entire survey track.  Loligo, or longfin squid, is most dominantly in our survey, although it’s not uncommon to have a mixed catch with Illex, the shortfin squid.  Right away, there is an obvious color difference, with the Illex being more golden/orange and the Loligo a deep red/maroon or light purple, depending on its mood.  The fin length on the mantle is another identifier.  Notice the fin on the Illex is about one-third of the mantle length, while on the Loligo, it’s about half the length of its mantle.

pic 5_illex top-loligo bottom_opt

Shortfin and longfin squid

Silver hake is another fish we catch throughout the entire survey, but when trawling offshore, we keep our eyes out for the very similar looking offshore hake.  The slight color difference is what pops out on the sorting belt.  Silver hake reflect a golden color along their dorsal side while offshore hake show more of a bluish hue.  The best way to tell these two apart is to count the gill rakers—the stiff filaments on the gill arch used to filter solids away from the gills.  Silver hake have 16-20 and offshore have 8-11.

pic 6_silver top-offshore bottom_opt

The nearly identical hakes

Christine Kirkun, fishery biologist

Aboard the NOAA Ship Henry B.  Bigelow

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