It’s a gut feeling- Lessons learnt from fish autopsies

by Lee Brake on 20 May 2013 This week, Lee Brake has a look at an underutilised avenue of deciphering fish feeding patterns, the autopsy. If the idea of Rex Hunt meets CSI appeals to you, then read on!

Alright, so some of you are going to cringe a little at this, but others, like myself, will find it rather interesting and even educational. Yes, today ladies and gentlemen, I am going to be talking to you about guts, more specifically, the interesting things you can find in amongst the contents of a fish’s stomach!

Sounds morbid right, maybe even a little Dexter Morganish… well, it’s not, and it’s something that I highly recommend every fillet flayer does, especially on bigger fish. Firstly, you see some really cool stuff, and secondly, you can learn a lot about a fish’s feeding preferences. As a word of advice though, make sure you limit nostril directed breathing; the mouth is where it’s at while eviscerating a cod’s stomach lining!

I first became intrigued with fish autopsies around half a decade ago when I was crouched over the old esky we used as a filleting table doing the honours to a nice plump coral trout that we’d taken flicking a rocky headland. As I got up near the stomach, I noticed a really firm area and upon closer consideration I decided to see just what had this fish’s guts so hard. Well, as I slit open the stomach a whole greenback herring came out. Cool right? It gets better! The herring had a set of three ganged 4/0’s still firmly imbedded in the bait! There was not a touch of rust in sight and judging by the state of the herring, I’d have to say that the trout had probably eaten the bait the very same day. This was not only very cool, it also made me think. This tout had a gut loaded with fish and metal and yet it had still smashed our lure! Obviously, when they’re feeding, they’re really feeding!

While on the topic of autopsy discoveries, I can rattle off a whole list of lessons learnt. For example, cobia we’ve caught in the Shipping Channel have had stomachs full of mantis shrimp, fingermark jigged from the deep offshore have had stomachs filled almost 80 percent with crabs rather than baitfish (and one prawn), grunter will almost always have more than one of your baits in their stomach after you’ve caught them and, lastly, dear old cod eat just about any old crap (thus the stench); from baby magpies (the feathers were still intact) to mudcrabs around the size of your hand and bigger!

However, from these findings one really can make plenty of educated assumptions, from bait/lure choices to feeding times and behaviours. Bait and lure choice requires little out-of-the-box thinking. Trout eat herring – use a silver chrome lure! No brainer there, but what about these pelagics and offshore trophy predators with guts full of crabs and crustaceans? Is it worth putting on a crab imitation or something shrimpy (like an Atomic Prong or Zman ShrimpZ) and letting it twitch slowly along the bottom? Interestingly, with the big fingermark we’ve been getting, not a single one has contained a squid… Maybe the whole live squid at night could be substituted for crab at night? And while on the subject of fingermark, all the fish we’ve been getting of late have been around the hour of sun-up, and the fish caught before sun-up have had fairly empty stomachs while the fish caught after sun-up have been bulging (mostly with crabs). Makes me think that they hunt hard on dawn and probably through dusk but feed sparingly through the early hours of the morning.

The crabs in cod is an interesting one as well. I know plenty of crabbers who believe that cod frames are no good in crab pots, because big cod are a natural predator of the mudcrab and one look at a cod’s head will see a crab scampering off quick smart. Well, having seen some pretty big muddies in cods' stomachs, I’d have to say it’s possible, because they definitely love a feed of crab! Oh, and flying foxes. The guys up north reckon they camp out under the flying foxes at night and chomp them as the babies fall off their mangrove perches, thus explaining why you often find baby birds and bats in their stomachs. This goes for big barra too!

Stomach contents can tell you a range of things and can answer some of the big questions. For instance, ever wondered if your burley trail is attracting fish or just getting swept away? Well, check the fish’s guts. If they’re finding it, it’ll be in there. Also, if they’re jam packed with it – you’re probably burlying too heavily!

On a trip where you’re struggling to get the barra to bite? Well then, check the guts of any you did catch that day and return the following day with lures that match the contents (by that I mean what it looked like before the stomach juices got to it). Often you’ll find them full of small prawns which will tell you that a substantial lure downsize is in order!

All in all, filleting doesn’t have to be a mundane job and can be a very educational experience if you take the time to do a bit of an autopsy. I enjoy it, so sue me! As a tip, if you have a queasy stomach, then a little Vicks up the snoz will keep the stench at bay.

Oh, and while I think of it, have any of you ever noticed the little weird black things you get in coral trout? They must be a parasite of some kind, and they seem very prolific on trout taken from the reefs. I remember one fish we took near Buggati Reef that had this mottled black mark on its cheek. While I was filleting it I must have hit something, because next thing I know there’s black inky stuff squirting out like a squid’s jet! This was around the rib cage too, not the black cheek! Weird stuff, and I’ve seen a similar black fluid in trout since, but never to the same extent.

Obviously, what I’ve mentioned is just a scratch on the surface, so I’d be really interested to hear from you, the reader, as to what interesting things you’ve found while filleting.