Below is a collection of underwater video footage taken by the IPHC showing halibut in the field. Additional footage courtesy of NOAA's Northwest Fisheries Science Center and the North Pacific Longline Association.


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


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.

Analyses Confirm that Alaska's Seafood is Safe from Radiation

(updated June 30, 2014)

The State of Alaska announced the results of radiation testing of seafood from the North Pacific and Alaska by the U.S. Food and Drug Administration in a press release and accompanying data charts. The testing confirmed information from federal, state and international agencies that seafood in the North Pacific and Alaska waters poses no radiation-related health concerns to those who consume it.


Fukushima radiation concerns and Pacific halibut caught in the eastern basin of the North Pacific Ocean

(Updated Jan. 30, 2014)

Recent reports about ongoing leaks of radioactive water and atmospheric exposure from the Fukushima nuclear plant accident have raised questions regarding the safety of seafood caught on the western coast of North America. At the IPHC we do not have personnel qualified to speak directly to the risks associated with radiation in seafood; this page is simply intended to provide background information to assist readers on this topic.

In the United States, the Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) have the responsibility for testing for contaminants in the environment and in food; EPA in a broad sense, and the FDA from a point of sale and food/consumer safety perspective. In Canada, Health Canada monitors environmental exposure, while the Canadian Food Inspection Agency (CFIA) has the responsibility for monitoring seafood safety. CFIA has published some early test results.

Radiation from Fukushima can arrive in the United States and Canada via water currents, atmospheric deposition, or migratory animals. (Tsunami debris is not generally considered a radiation concern [Boice 2012] because the nuclear plant melted down 24 hours after the debris was swept out to sea.) Most experts (Boice 2012, Buesseler et al. 2011) feel that a water-based route of Fukushima radiation to the North American coast is of a minimal concern due to the vast amount of dilution that would happen between there and here. That dilution is further assisted by the relatively short half-life (the unit of measurement for a radionuclide to decay) for most of the radioactive nuclides being released: (Iodine-131 [I-131], Cesium-134 [Cs-134]). Essentially those two nuclides would decay within days and never reach the waters on the eastern side of the Pacific Ocean (Fisher et al. 2013). The longer half-life radionuclide is Cesium-137 (Cs-137), and it has been closely monitored by both government bodies and universities. Monitoring of Cs-137 has been done with kelp (Manley and Lowe 2012, Chester et al. 2013), as it is suspected that deposition in rain water was the primary threat during the initial days when there was active dumping of water on the reactors to keep them cool, and radiation was getting into the more global air and moisture circulation. Some levels of Cs-137 were found in kelp along the west coast of North America (Chester et al. 2013) in the months following the disaster, which have since decreased after the nuclear plant capped the aerial emissions from the plant. Additionally, there is evidence of deer lichen (DOE 2013) in the Aleutians having elevated Cs-137 readings, presumably from atmospheric deposition. The same report found no increases in radiation levels in five Pacific halibut tested as part of the same study. Recent revelations by the Tokyo Electric Power Company (TEPCO 2013) and widely reported in the press indicate that there is likely ongoing groundwater radioactive contamination occurring at the nuclear plant. This could theoretically introduce increased Cs-137 exposure over time. However, due to the dilution effects, even that is expected to be at extremely low (though not undetectable) levels.

Another concern has been with more highly migratory animals, which is why analyses have been focused on species like tuna (Madigan et al. 2012) and salmon (Fisher et al. 2013). Generally, the primary species caught in the US and Canada do not get close enough to the Japanese coast to get significant exposure (DOE 2013, Madigan et al. 2012). Again, these studies have shown zero to trace levels of exposure. Trace levels are those where a highly sensitive machine is capable of registering exposure, but the levels are 1/1000th to 1/10,000th of the normal radiation (non-Fukushima source) which is naturally found in seawater. There have been reports of sand lance (Buesseler et al. 2012, Fisher et al. 2013) near the Fukushima site having Cs-137 levels at concentrations high enough to be of consumption concern, however those species are not part of the diet of adult migratory tuna or salmon. The southeastern-most documented occurrence of Pacific halibut is in the Sea of Japan off Hokkaido (Mecklenburg et. al., 2002), more than 300 nautical miles to the north and on the opposite side of the Japanese archipelago as Fukushima. As such, eastern Pacific halibut stocks are unlikely to show bioaccumulation from food sources experiencing localized exposure.

The IPHC has not been involved in radiologic sampling, but has been working with the Alaska Department of Environmental Conservation (ADEC), looking at both heavy metals (arsenic, selenium, lead, cadmium, nickel, mercury, and chromium) and some persistent organic pollutants (POPs) such as pesticides, PCB congeners, dioxins, furans etc. in Pacific halibut since 2002. The heavy metals have been well below listed levels of concern published by the FDA and the CFIA, with methyl mercury being the main focus of that work. For the POPs, all readings in Pacific halibut on those pollutants have been extremely low to undetectable, largely because the majority of those chemicals tend to be fat-soluble and halibut are low in fat.

We have found the following information on non-governmental radiological testing of some north Pacific specimens including halibut:

with their test results from September 2012 here:

and test results from March 2012 with some reading levels for halibut on Cs-134:

Some further sources of information can be found at the following Alaska state, and federal government websites:

Other websites to check:

A DFO-produced article on the arrival of the Fukushima radioactivity plume in North American continental watersnewmini

A report from Health Canada/DFO on radioactivity measurements of fish samples from the west coast of Canadanewmini

NOAA in collaboration with the FDA and EPA.

EPA RadNet site can be searched for data collected in Alaska regarding any atmospheric deposition or precipitation:

World Health Organization:

There has been some sampling and analysis performed by the Department of Energy out around Adak and Amchitka Islands in collaboration with the University of Alaska. The reports from the study can be found at:

The IPHC does not have plans for radiologic evaluations of fish, as this is largely outside our purview and expertise, but we continue to work with partner agencies on other fish health/pathology related topics. Please visit our Environmental Health and Halibut page.

If you have further questions, concerns, or information to share on this topic, please contact Claude Dykstra.


Boice, J. D. Jr. 2012. Radiation epidemiology: a perspective on Fukushima. J. Radiol. Prot. 32(1):N33-40. doi: 10.1088/0952-4746/32/1/N33.

Buesseler, K. O., Aoyama, M., and Fukasawa, M. 2011. Impacts of the Fukushima nuclear power plants on marine radioactivity. Environ Sci Technol 45:9931-9935.

Buesseler, K. O., Jayne, S. R., Fisher, N. S., Rypina, I. R., Baumann, H., Baumann, Z., Breier, C. F., Douglass, E. M., George, J., Macdonald, A. M., Miyamoto, H., Nishikawa, J., Pike, S. M., and Yoshida, S. 2012. Fukushima-derived radionuclides in the ocean and biota off Japan. Proc. Natl. Acad. Sci. U. S. A. 109 (16): 5984-5988.

Chester, A., Starosta, K., Andreoiu, C., Ashley, R., Barton, A., Brodovitch, J.-C., Brown, M., Domingo, T., Janusson, C., Kucera, H., Myrtle, K., Riddell, D., Scheel, K., Salomon, A., and Voss, P. 2013. Monitoring rainwater and seaweed reveals the presence of 133I in southwest and central British Columbia, Canada following the Fukushima nuclear accident in Japan. J. Environ. Radioact. 124: 205-213.

DOE (U.S. Department of Ecology), 2013. Amchitka Island, AK, Biological Monitoring Report, 2011 Sampling Results, LMS/AMC/S08833, Office of Legacy Management, Grand Junction, Colorado, September.

Fisher, N., Beaugelin-Seiller, K., Hinton, T. G., Baumann, Z., Madigan, D. J., and Garnier-Laplace, J. 2013. Evaluation of radiation doses and associated risk from the Fukushima nuclear accident to marine biota and human consumers of seafood. Proc. Natl. Acad. Sci. U. S. A. 110 (26): 10670-10675.

Madigan, D. J., Baumann, Z., and Fisher, N. 2012. Pacific Bluefin tuna transport Fukushima-derived radionuclides from Japan to California. Proc. Natl. Acad. Sci. U. S. A. 109 (24): 9483-9486.

Manley, S. L., and Lowe, C. G. 2012. Canopy-Forming Kelps as California's Coastal Dosimenter: 131I from Damaged Japanese Reactor Measured in Macrocystis pyrifera. Enivorn. Sci. Technol. 46(7): 3731-3736.

Mecklenburg, C. W., Mecklenburg, T. A., and Thorsteinson, L. K. 2002. Fishes of Alaska. Bethesda, Maryland, American Fisheries Society. 1037 p.

TEPCO (Tokyo Electric Power Company), 2013. Radioactive material (Total Beta) Density Increase at an Observation hole at Fukushima Daiichi NPS. Available from [accessed November 25, 2013].

Since 2002, the Commission has been working cooperatively with the Alaska Department of Environmental Conservation (ADEC) in a project monitoring environmental contaminants in Alaskan fish. The fish being studied include salmon (5 species), sheefish, pike, pollock, pacific cod, lingcod, black rockfish, sablefish, and halibut. The fish are analyzed for organochlorine pesticides, dioxins, furans, polybrominated diphenyl ethers, PCB congeners, methyl mercury and heavy metals (arsenic, selenium, lead, cadmium, nickel, and chromium). Results from these studies are used to identify ADEC's future research needs.

To date, 2,088 samples have been tested by ADEC. The mean level of total mercury for these samples has been 0.309 ppm (for comparison, the Food and Drug Administration (FDA) limit of concern is based on methyl mercury (~85% of total mercury) levels of 1.000 ppm, the Environmental Protection Agency (EPA) and Canadian Food Inspection Agency (CFIA) level of concern is 0.500 ppm) ranging from non-detectable to 2.000 ppm. Results from analysis of persistant organic pollutants (POP's - pesticides, selected PCB congeners, dioxins, and furans etc) found that in general these compounds are either undetectable in halibut or well below other marine fish species. This is a positive finding and is likely attributable to the lower fat content in halibut compared to these other species.

Analysis by the Alaska Department of Health and Social Services (DHSS) has found that most species of Alaska fish contain mercury levels that are too low to constitute a health risk. However, some Alaska fish species are consistently found to have elevated mercury levels; as such, consumption restrictions for these species are warranted for pregnant women, women of childbearing age that may become pregnant, nursing mothers, and children.

To find out more information on this topic, please follow one of the links listed below:

Specific contaminant level results and guidelines can be found on ADEC's Fish Monitoring Program website:

Fish Consumption Advice (AK Department of Health and Social Services):

State of Alaska Epidemiology Bulletin (July 21, 2014 release):new

IPHC report on total mercury in Pacific halibut:

Other health topics

We continue to work with partner agencies on other fish health/pathology related topics such as:

What is an otolith?

halibut otolith photo

Otoliths, also called ear-bones, are structures made mostly of calcium carbonate that are found in the head of most fish. Otoliths act as sound receptors and also play a role in balance and orientation. As the fish grows, so does the otolith, by deposition of concentric layers of material. Seasonal changes in the fish's growth rate are reflected in the otolith. A year's growth consists of a wider summer zone (reflecting faster growth) and a narrower winter zone (reflecting slower growth). Because halibut spawn in winter, the winter zones are counted to determine the age of the fish in years. These annual growth rings, or "annuli", are very similar in appearance to the growth rings of trees.

Ages are used for estimating growth and mortality rates as well as population age structure. Age data are incorporated into the IPHC's annual stock assessment. In the past, the IPHC also used otolith weight and length to estimate the size of halibut, though this later proved to be inaccurate.

Each year, alternating opaque (summer) and translucent (winter) rings are deposited on the otolith. The oldest age recorded for Pacific halibut is 55 years for a 118 cm male (~36 lbs, net) captured in 1992 in the Bering Sea on IPHC's setline survey. The oldest recorded age for a female is also 55 years. This female was 161 cm long (about 100 lbs, net) and was captured in the Bering Sea in June 2000, also on an IPHC survey. The mean age in years of the commercial catch has been 12-13 for the last several years.

Currently there are four staff members doing production aging of survey, commercial, and tag recovery halibut otolith samples. Sport-caught halibut otoliths from Alaska are also aged. Approximately 30,000 otoliths are read per year.

3D otolith drawing

Notes/guidelines for aging halibut

Aging methods used at the IPHC include the following:

  • Surface Ages: Otolith are read while on a piece of black cloth, immersed in water (to minimize glare from the light source and maximize contrast). The distal surface of the otolith is observed during surface reads; the proximal surface has a deep groove and annuli are obscured.
  • Break & Bake: Break and bake involves scoring the otolith surface through the nucleus with a razor blade, then snapping the otolith in two. We bake one of the halves and keep the other half unbaked so we can still do a surface reading. Baking the sections enhances the contrast between summer and winter zones (winter zones turn dark brown). Previously we heated the otolith sections one at a time over an alcohol flame (Break & burn); however, baking allows us to heat many sections at a time, saving time. We use metal trays for baking; they are divided into 50 indented cells, which keep the otoliths from getting mixed up. The baked sections are then mounted in plasticene and coated with mineral oil or glycerin solution before viewing under a dissecting microscope.

Through 2001, Pacific halibut otoliths were all surface aged at IPHC. The criteria used between 1992 and 2001 included performing break & burn or break & bake age determinations in cases where readers were not confident of the surface age, (e.g., thick/steep edge, opaque or cloudy surface, odd growth pattern, high surface age, etc.). The break & burn/bake method of age determination was validated by a bomb radiocarbon study and since 2002, all longline (survey and commercial) and sport-caught halibut are aged by break & bake technique. Since 2002, only otoliths from the trawl survey collections are surface aged. If the surface age is 5 or greater, the otolith is broken and baked. Trawl-caught otoliths that are obviously older than five are not surface-aged first.

We only collect and read the left or blind side sagittal otolith at IPHC. The right and left otoliths are not mirror images as they are in some species, and right otoliths are harder to read and give less accurate ages. We also do not age crystallized otoliths. Reasons for crystallization are unclear, but crystallization occurs in other fish species as well and one or both of the pair can be affected. The inorganic portion of crystallized otoliths is made up of calcium carbonate, just as in "normal" otoliths, but the crystalline structure is different and growth patterns are difficult to interpret. Total between-reader percent agreement of between 55 and 80% or agreement within one year for 80-95% of the readings is usual for halibut otoliths.

Otoliths contain other useful information besides age. They can be used to identify fish species in stomach contents of other fish or mammals, and have been used as biological "recorders" of environmental changes using growth patterns or trace elements within the structure of the otolith.