Halibut hooking behavior - or - how halibut find and attack baited hooks, and often get hooked in the process

Introduction

We have studied baited hooks in particular at the Halibut Commission.  Our primary stock resource tool is our annual survey, which causes to be set over a million circle hooks off the shores of California, Oregon, Washington, British Columbia, and both the Gulf of Alaska and the Bering Sea areas of Alaska.  For the past couple of decades these have been size 16/0 circle hooks, baited with ¼ to 1/3 pound pieces of food grade chum salmon.  The hooks are fished on setlines, 500 or more hooks evenly spaced every 18 feet on a line that can be miles in length, and set under very stringent and therefor somewhat repeatable conditions.  The Halibut Commission purchases over 350,000 pounds of chum salmon every year for our surveys.  Predictably, a fair amount of our research is directed towards a better understanding of those factors which influence the catches of halibut and other fishes on those baited hooks. Unless we are actually looking at the effect of hook size, we are using a 16/0, or Number 3, circle hook with chum salmon.

Sources of hooking behavior information

There are two main sources of catch and sometimes behavior information about halibut.  The first of these are experiments where we fish thousands of hooks rigged one way against thousands of hooks rigged another way and then compare the results.  We might vary the hook type or size, hook spacing, bait size or type, or any of a myriad of conditions under which the gear is set.  By looking at the catch by size in either numbers of total weight, species composition, or size composition of the catch, we can infer the performance of the gear, and draw conclusions based on those comparisons.  For example, longer spacings between hooks results in more pounds per hook, smaller hooks catch more small and other less large fish, and larger baits catch more pounds of fish.  This is not to say that you would see these differences every time you fished one way against the other, but when you fish thousands of hooks one way against thousands the other way, statistically you can describe a difference. 

The second source of particularly behavior information is direct observation, actually looking at a baited hook lying on the seafloor (or drifting above it) and observing and recording what occurs.  It is this type of observations that I will spend most of the rest of this page discussing.

Typical halibut behavior around a baited hook

Hook approach and attack

In 1997 and 1998 we used an at the type state of the art underwater camera to observe behavior around baited hooks.  The camera was capable of monochrome images in very low light, and by deploying it when there were few plankton blooms we were able  to get good images at depths of over 200 feet.  The camera was mounted in a trim/tilt unit which was then put into a steel and aluminum quadra-pod that stood over ten feet tall and had a base with 10-foot sides.  This frame was deployed onto the seafloor with a special winch and via a 12 conductor cable which doubled as both a power and data cable as well as a strength member for deploying and retrieving the gear.  A piece of 5/16 inch groundline was suspended inside the base of the frame by bungie cords.  Hooks subsequently snapped onto this groundline could have some 'give' which simulated the action of a long setline on the seafloor.


Fig 1. Frame for deploying underwater camera in 1997 and 1998.

With this system, and by attaching a piece of surveyors tape to a leg so we could see current flow, we observed 100 halibut approaches and 50 hook attacks by halibut.  This first video is really one of the nicest we have collected. I would suggest viewing it, then reading the following, and then viewing it again


While the seafloor looks extremely bright, we are not using any lights.  This is just the reflection of surface light off the seafloor with the enhancement of the ultra-low light camera circuitry.  The initial view is down-current (see the surveyor's tape fluttering near the bottom of the support leg).  In many areas the water just above the seafloor is rarely still, tidal and other influences have currents almost constantly flowing over the substrate, carrying among other things scents of any potential foods that might be lying on the bottom.  The halibut is approaching directly up the scent trail.  Studies have shown that depending on their hunger state, fish will follow a scent for a great distance, perhaps even a mile or more, to find a food source. You can see that the halibut swims past the baits, and only after exiting the scent trail, turns back to find the scent again.  I have seen this many times.  Once back in the scent the halibut locates and orients onto the scent source, in this case, the baited hook.  Here, it is common for the halibut to lie just below the bait, within an inch or so, and just down current for 10 or 20 or 30 seconds"¦.likely reinforcing that this is the source of the scent.  When ready to take the bait, the fish gives a flip of the tail, and as it is moving forward, 'inhales' the bait (and hook).  This is suction feeding, flaring the opercular plates while keeping the gill cover closed (via the accordion-like pleats on the edges of the plates).  This creates a much larger space within the buccal cavity, creating a strong suction to draw the baited hook into the mouth and gullet.  Think of watching a goldfish suck in the food you have just sprinkled into the fishbowl.  Over 95% of bony fish are suction feeders.

Now watch the video again and notice the various behaviors leading up to the hook attack.  As an interesting aside, look at this figure.

Fig 2. Approach direction and resultant biting action during 1997 and 1998 experiment.

 Of the 111 halibut I observed approaching the baits, 69 approached up-current (presumably following the scent trail).  Another 8 came from the side, 16 approached downstream, and 18 during slack current.  Even more interestingly, almost half of the halibut which approached up-current made a bait attack, a far higher proportion that any of the other categories.

Side bar:  Suction feeding is the most widely used mechanism of prey capture in ray finned fishes and in many other groups of aquatic fish.  The fish expands the buccal cavity by expanding the opercular flaps while sealing the posterior edge of the flaps with accordion pleats, thereby creating a flow of water that engages the prey, and draws it into the mouth.

Hooking success


The hook is in the mouth, the fish is making a run, and you've caught a fish, right?  Not quite so fast.  If we make a chart of the hooking success (how many attacks result in a hookup) against fish length, an interesting relationship emerges.  

           

Fig 3. Hooking success from 1997 and 1998 experiment and as estimated from stock assessment survey data.

In the figure on the left, fish length is across the bottom and hooking success across the left side, increasing from the bottom to the top. First, note the solid line.  That is the estimated hooking success from the IPHC stock assessment program.  This is a calculated curve, but if our assumptions in the stock assessment are correct, then this should be pretty accurate.  Now look at the heavy dotted line.  This is built from the actual observations of the 50 fish that made hook attacks.  We didn't have many larger fish, so the line is only predictive over the smaller sizes, and then starts to wander. But the observed line does seem to validate the relationship predicted by the stock assessment program.  What does this mean for someone fishing for these animals?  Now look at the second chart, the one on the right.  The blue lines coming from the left axis represent 50% and 100% hooking success. The 50% line intersects our graph at about the 74cm mark, which would be about a 10 pound Sportfish.  The 100% line intersects at about 91 cm, or 20 pounds.  Only half of the hook attacks by ten pound fish result in a hookup.  Almost all the attacks by 20 pound and larger fish result in captures. 

Why does fish size affect hooking success?

While hook attack is a function of many things, including the size and quality of the bait and the hunger state of the fish, once the bait and hook are inside the mouth, hooking success would appear to be a very predictable mechanical process, driven by the size of the mouth and the size of the hook.  


Figure 4. Hook gape, jaw thickness, and jaw cross-section showing how circle hook 'snaps' around jaw bone.

Consider  this figure.  On the lower right is a cross section of how fish are most often caught on a circle hook.  The hook actually circles the jaw bone.  Once in that position, it is very difficult for a fish to 'shake' itself loose from the hook.  To get to this point, the fish first inhaled the bait and hook.  As the fish swam away, first the gangion and then the hook slid out around the corner of the jaw.  This is also shown in this video.


    

Most halibut caught on circle hooks are hooked in the corner of the jaw.  Most of these halibut are hooked on the white side.  This is the result of taking the bait and then swimming up and away.  The gangion pulls along the corner of the white-side jaw, and the fish is hooked (or sometimes not).  Sometimes halibut are hooked in the corner of the dark-side jaw.  This is the result of the bait being suspended or for some reason not lying on the bottom. When the halibut takes this hook, it then swims down and away, with the gangion pulling out of the dark-side corner of the jaw, resulting in a dark-side hooking.

The actual hooking occurs when the point of the circle hook catches on the flesh on the inside of the jaw and with increasing pull, penetrates the flesh and 'snaps' around the jawbone.  Operative factors here are the thickness of the jaw hinge and the size and particularly the gap of the hook.  Later, I will describe the effect of hook size on hooking success, and this will become more clear.  For now, a good illustration of the importance of this hook orientation to hooking success is the following.


Figure 5. Illustration of front and rear 'threading' of gangion on circle hook.

An IPHC study showed that by the simple expedient of putting the gangion loop through the front of the hook eye, a fisher could increase their catch by over 50%.  This front hook threading presents the hook in a slightly different orientation and allows a deeper catch on the inside of the jaw.

How does hook size affect hooking success?

The previous observations on hooking success were based on observations of 50 hook attacks.  In 2007 and 2008 we had the opportunity to observe hundreds of hook attacks using an underwater sonar.  Being based on sonar, rather than video capture, we were not restricted by the availability of natural or artificial light.  


Fig. 6. Deployment frame for using the DIDSON acoustic camera in 2007 and 2008 to observe fish behavior around baited hooks.  

We deployed the sonar on a frame with the sonar looking forward, to usually three baited circle hooks.  This frame was set like a crab pot, and retrieved after an hour or so.  While we had no real time view of what was occurring, the sonar did record everything it 'saw'.  Later review of these digital files gave us over 500 hook attacks to analyze.  As well, the sonar came with software that allowed very accurate estimation of the length of fish that attacked but were not hooked.  We had so many observations with the 16/0 hooks that we were able to also collect over 200 observations with the smaller 14/0 hooks.  This allowed a very instructive comparison.




Figure 7. Comparison of fitted hooking success curves for 16/0 and 14/0 hooks for Pacific halibut from 2007 and 2008 Didson experiment.

For either hook size, the hooking success increased from almost zero to near 100% over a length range of about 50 cm.  Fish under 50 cm are seldom caught by either size hook. The presumption here would be that the fish are too small to take the hook in their mouths. I have seen these small halibut attack a bait, but they end up with the hook sideways in their mouth, their jaws clamping on the sides of the hook.  After a bit of a struggle, they open their mouth and the hook falls out. What is most interesting is that the curves for both sizes of hook appear to be the same is shape, but with a shift to larger fish for the larger hook.  Within the range up to around 120 cm, larger fish don't catch a given size of fish as well as the smaller hooks.  Once up around 100 or 120 cm, it would appear that both sizes of hook catch the larger fish well.  At some size, we would expect the smaller hooks to catch less large fish, but we didn't catch enough large fish in this study to make that prediction.

So, do any other studies back this up?

Actually watching a fish approach and attack a hook is great, but we're only talking about six or seven hundred observed events.  Do other studies support what we've seen directly?  The short answer is yes.  For many years, the IPHC has conducted gear research in carefully designed studies where thousands of hooks fished in one manner are compared to thousands fished in a slightly different manner.  The differences can be in hook size or type, hook spacing, bait size or type, to name a few.  What have these studies shown?

Hook type

Back in the mid-80s, the circle hook was introduced to the northwest Pacific and it was so superior to the J hook that within two years literally all the thousands of fishers had switched from J to circle.  Why so quick?  Because our studies, as well as the practical experience of fishers, showed that circle hooks caught over twice the pounds of legal sized (over 32") halibut.  This was from a combination of better hooking, and less loss after hooking. Another advantage of the circle hook is that fish are very seldom deep hooked, making release of unwanted fish easier and less damaging to the fish.

Hook size

Many studies have been conducted looking at hook size.  In two typical studies, smaller hooks in general catch more small fish and fewer large fish.  It would appear that fish size can be targeted by a careful selection of hook size.


           

Figure 8.  Two hook size studies, comparing catches on 16/0 hooks with the smaller 14/0 or 13/0 hooks.

Where can I go for more information?


The IPHC website had all or our research information going back more than two decades, and most of our reports back to the early part of the 20th century.  It is also searchable.  On the topics in this page, try searching 'hook size' or 'bait type' to get started.