Otolith growth increment studies

Otolith Increment Research

Figure 1: Otolith increment measurements of 15-year old Pacific halibut from the Gulf of Alaska, collected summer 2007

Besides age information, it is possible to gather additional information from an otolith by studying the physical properties of its annual rings, or otolith increments.  The winter (translucent) zone and summer (opaque) zone make up an annual increment on an otolith.  Researchers at the IPHC, and at other organizations, have investigated whether otolith increments may correlate with environmental conditions and physiological processes, such as growth. 

A study conducted in 1991 found a strong linear relationship between sea surface temperature and Pacific halibut otolith increments between the ages 0 and 2 years.  The authors found that this relationship did not persist beyond age 2, suggesting that sea surface temperature is not the only important variable that regulates juvenile Pacific halibut growth.  However, there appeared to be a slight positive correlation between otolith growth at ages 1 and 2 years and estimated year class abundance.

Most recently, the IPHC conducted a study to compare the width of otolith increments and somatic growth of adult Pacific halibut in the Gulf of Alaska.  The Gulf of Alaska is the site of a significant decline in Pacific halibut length-at-age over the last four decades. The goal of this study was to determine if there was a proportional decline in otolith growth during this time period. By examining the relationship between otolith growth increments and changes in somatic growth of Pacific halibut from four year classes spanning the period of decline, it was observed that otolith growth and Pacific halibut length-at-age were decoupled.  In other words, the decrease in the width of the otolith increments was not proportional to the decline in observed halibut length-at-age in this area.  Otolith increments could therefore not be used as a proxy for somatic growth.

Additionally, a preliminary study in 2009 described depositional events in the otolith, non-annual marks also known as “checks”, with the hope that they could be used to determine the general geographic nursery origins of adult halibut.  These techniques are borrowed from dendrochronology, the scientific method of dating tree rings.  Although this study was not completed, this may be an area of research that the IPHC explores in the future.

Figure 2: On the left, an otolith without a non-annual check from a three year old Pacific halibut; on the right, an otolith with a non-annual check from a two year old Pacific halibut

Reference

Hagen, P.T. and Quinn, T.J., II, 1991. Long-term growth dynamics of young Pacific halibut: evidence of temperature-induced variation. Fish. Res., 11:283-306