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.
Aging methods used at the IPHC include the following:
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.
This condition, which has been variously described as mushy or jelly-like, has become more frequently reported by recreational halibut fishers in certain parts of Southcentral Alaska, especially in 2011 and 2012. This is not a new phenomenon, with observations being noted as early as 1989. Recent analyses of flesh samples by the State of Alaska's Fish Pathology Lab noted that fish with the condition have large areas of body tissue which are flaccid or jelly-like. The fillets may ooze water and are mushy when cooked. While the cause of the condition is unknown, it is thought to be the result of nutritional deficiencies.
The IPHC is interested in learning about the geographical and seasonal occurrence of the mushy condition in Pacific halibut. To accomplish this, we'd like fishers to submit information on the following:
We plan to share information on the occurrence of mushy halibut with other agencies, but all personal information will be kept confidential.
Additional information can be found at http://www.adfg.alaska.gov/static/species/disease/pdfs/fishdiseases/mushy_halibut_syndrome.pdf
Thanks for your assistance!
and you've decided to collect the data yourself. There are a few of things we'd like you to do:
Don't lose the tag--it's the proof you'll need to get your reward.
Measure the length of the fish (preferably in centimeters) from the tip of its mouth (chin) to the fork of its tail. The proper measurement is a flat, horizontal distance rather than a measurement which follows the contour of the fish. Please make certain the fish is on a flat surface and that the mouth is closed.
The best way to make a measurement is to:
- Place the fish's chin against a stable vertical surface, such as a wall
- Put a marker at the fork of the tail
- Remove the fish
- Measure the distance from where the fish touched the vertical surface to the marker.
The otoliths, or earbones, are used to determine the age of the fish. There are 3 pairs of otoliths in each fish, though only 1 pair is easily visible. Of this visible pair, we want the one from the white side.
* Please don't attempt this if you're not good with knives. Minors (who are old enough to play with knives) should still have adult supervision. The Commission takes no responsibility for any possible injuries resulting from this procedure.
In addition to the otolith and fork length, we would like:
- Tag number: if the number appears illegible, don't throw the tag away. We can still read it in the lab.
- Recovery date: when the fish was caught
- Location: preferably latitude/longitude coordinates, though distance and bearing from a landmark would be acceptable (e.g.. 3 miles SW of Cape Barto)
- Depth: the depth at which you were fishing, in fathoms (1 fathom = 6 ft)
- Sex: male or female, if known
- Gear type: longline, troll, trawl, pot, or sport
- Landing port: where the fish was brought in
- Vessel name and number: if commercially caught
- Name and complete mailing address
- Reward type: baseball cap or $10.00
If more than one tag is recovered, please keep the information from each fish separate.
Return this information to:
Attn: Joan Forsberg
2320 W. Commodore Way, Suite 300
Seattle WA 98199-1287