Resource Ecology & Fisheries Management: Research Feature

Age Validation of Pacific Cod Using Stable Oxygen Isotope (δ¹⁸O) in Otoliths (pg 1, 2, 3, 4 )

Figure 1.  Adult Pacific cod.

Pacific cod, Gadus macrocephalus, is the second largest groundfish fishery, next to walleye pollock, in Alaska
(Fig. 1). In 2013, the Gulf of Alaska, Aleutian Islands, and eastern Bering Sea total catch was 318,867 metric tons with a gross value of nearly $390 million. Management of this important resource is conducted through stock assessments using age-structured models in which fish length-at-age data are key to estimating mortality, recruitment, and recommended harvest levels. Hence, the accuracy of fish age determination of Pacific cod is critical.

The reliability of Pacific cod age determination, however, has recently been called into question. An inconsistency has been identified between mean length-at-age from age-length samples taken in scientific surveys and the yearly progression of modal lengths representing some dominant cohorts (from the 1996 and 1999 year classes for example), specifically for Pacific cod that are 2 to 5 years old.

Figure 2. Pacific cod otoliths showing paired calcium carbonate structures located in the fish's inner ear.

Otoliths, which are used to determine the ages of fish, are paired calcium carbonate structures located in a fish’s inner ear in which annual growth rings (growth zones) are deposited (Fig. 2). Counting otolith growth zones to estimate fish age is analogous to counting tree rings. Unfortunately, however, compared with more than 30 other species aged each year by the Alaska Fisheries Science Center’s (AFSC) Age and Growth Program, age determination of Pacific cod is relatively difficult and sometimes uncertain. For some Pacific cod otoliths, multiple interpretations of growth zone patterns are possible, resulting in more than one potential age estimate per specimen (Fig. 3). Further, the precision of repeated otolith age readings is lower for Pacific cod than many other species. Therefore, in such problematic situations, a second independent method of estimating fish age is necessary to validate or confirm the fish’s age. In other species a number of age validation methods have been used, such as bomb-produced radiocarbon, radiometric methods, or marginal increment analysis. But for a variety of reasons, these popular methods are not appropriate or have yet to be used for Pacific cod. 

The Age and Growth Program recently initiated a novel alternative strategy to validate age determination methods and estimate ageing inaccuracy for Pacific cod. This approach to age validation relies on a geochemical technique using stable oxygen isotope signatures in otoliths. In marine carbonates, the δ¹⁸O concentration is typically inversely related to water temperature. Therefore, δ¹⁸O measured from the center to the edge of an otolith represents temperature changes over the fish’s life history and inversely mirrors annual seasonal temperature cycles. Counting the number of peaks in δ¹⁸O cycles (maxima) provides an alternative method for determining the fish’s age. Our primary goal was to use this alternate method to validate traditional Pacific cod growth zone-based ages and to estimate the probability of inaccurately ageing specimens. In the simplest sense, we compared the number of counted growth zones to the number of δ¹⁸O maxima. If the number of counted growth zones agreed with the number of δ¹⁸O maxima, strong support was given to the age determination methods (interpretation of growth zones) and estimated ages. A secondary goal was to describe the relationship between otolith δ¹⁸O and water temperature.  

Figure 3. Transverse thin section of an otolith from a Pacific cod. The blue dots indicate translucent growth zones which were counted to estimate the fish’s age. The red dots indicate zones that were not counted, but represent alternative interpretations.