Report of the Workshop on Age Determination of Redfish (WKADR)
Since 1995, the year of the previous workshop, considerable sampling effort has been directed towards collecting redfish otoliths, more than 300 000 from three major areas: Northeast Arctic, Iceland-Greenland-Irminger Sea, and Newfoundland-Flemish Cap. This effort reflects the fisheries and scientific interest in the species. However, in total only 22% of the otoliths collected have been read, reflecting the low capacity available to participate in age determination, especially for some stocks. This is partially due to the lack of trained technicians and the lack of standardized application of existing accepted and recommended aging criteria. Currently, six laboratories from five different countries determine the age of redfish on a routine basis, although with varying rates of production, with two countries only reading them occasionally. There is a certain degree of heterogeneity among laboratories regarding the methodology used. Otoliths are read across laboratories using three different cross-section methods: broken and burnt, thin sections and broken and bake. Although there are some optical differences in how the annual growth patterns are revealed, the patterns themselves are predetermined and the same basic criteria is used to differentiate annuli from checks for all three methods. The technical pros and cons of each were discussed during the meeting and a comparative analyses of age readings was done during the workshop regarding precision and accuracy.
Clearly, species and/or stocks yielded different biases and variation among readers. The bias varied considerably for Iceland S. marinus between readers and a relatively high variation in age estimates was observed for all readers. On average, the broken-and-burnt otoliths were aged 3-4 years older than the broken-and-baked otoliths. This was similar for Irminger Sea S. mentella where between-reader bias and high variation in age estimates was evident in all comparisons. The variations among readers were high enough to prevent a proper comparison among methods. No defined trend was detected, even when readings from the same reader using different otolith preparation techniques were analyzed. As an example, the thin sectioned otoliths sometimes delivered higher age estimates, sometimes lower. The overall bias was comparably low for the northeast Arctic S. mentella stock and although still a relatively high variation in age estimates was observed in some readers, in general, the readers produced similar ages.
It is recognized that among readers random differences with respect to interpretations and age estimate errors will always exist. The occurrence of such differences may only be reduced through frequent otolith exchanges and comparative readings. The most serious systematic error or bias discovered during the workshop was that some participants were not taking the thickness of the otolith cross-section into consideration when ageing and therefore did not count along growth axes that included the proximal side of the section. Rather, they were assessing age along a distal (nucleus to dorsal/ventral) axis where not only is it difficult to differentiate checks from annuli but annual growth zones cease to form after about 15–20 years. This resulted in under-ageing. It was also discovered that some readers who counted only along the distal dorsal axis tended to mis-interpret checks as annuli (over-ageing) and thus by chance got the same age as if they had counted on a proximal axis. Recommended and documented criteria indicates that a growth zone should not be identified as an annulus unless it can be followed/traced over a certain distance, preferably to the dorsal tip of the section, from the dorsal area to the sulcus area. An often difficult task is the correct identification of the first few “juvenile” annuli that frequently form in association with prominent checks. Some of the age differences originated from this problem. Measurements of the location of the first few annuli on otoliths from known-age fish or on very clear otoliths have the potential to minimize over-ageing due to counting checks formed in the during the first years. The measurements could serve as a guideline in all routine readings for the same stock.
During the workshop, it was pointed out that there is a scarcity of validation research and/or publications for redfish. Validation due to following strong cohorts, as those conducted in Flemish Cap, can be a great help confirming interpretation of the juvenile portion of the otolith growth pattern where many checks are observed. Although allowing rough validation of older ages, published radiometric research inferred a slight tendency towards underestimation of age by traditional annulus counts.
Apart from the Fish Aging Lab at Pacific Biological Station, Canada, only Norway has implemented a full Quality Assurance system for production redfish age determination. It was agreed that each laboratory should implement a confidence index (readability) for assigning a quality level to each age reading. For circulated otolith material, the different labs are requested to include their quality assignment as a parameter. In addition it is recommended that reference material, of past read otoliths, should always be at the readers’ side when reading new otoliths. This will help to avoid drifting away from the standards of criteria application when reading.
During the workshop the information available on redfish growth studies was presented. The calculated growth parameters varied considerably between readers and only slightly between ageing methods. Results showed, however, that the growth curves produced by the thin sections and broken and burn methods did not differ significantly. The group noted that, only in three cases, the data was divided into sexes. Since it is known that males and females show different growth trajectories in redfish, combining sexes prevented conducting correct analyses of the growth. Thus, it was agreed that from 2007 onwards age information will be separated by sex and original data will be fit to growth curves using the same procedure and curves compared by a statistical test. It was not possible to produce specific guidelines for the interpretation of growth structures in otoliths given the lack of common criteria on age reading. However, it was acknowledged that, based on the different life history and biological experiences, differences in growth pattern and hence in its interpretation among species and stocks may exist. In general, there was the perception that readers should know about the biology of fish to interpret properly the otolith growth pattern. It was agreed that considerably more effort and research is needed in this direction in particular for measuring growth increment pattern in the otolith. This technique has been proved in Sebastes to be useful to assist in identifying growth patterns related to the biology of the species/stock, as well with environmental features.
Only a few of the redfish stocks defined in the North Atlantic are assessed analytically. The high bias and low precision observed in age determination of redfish have prevented the use of age data for other redfish stocks. The effects of age reading error on the assessment have not been tested thoroughly yet. The workshop recommends that all labs providing age data for assessments for a certain stock should investigate uncertainties in assessments due to age readings in redfish. Within the next two years, these analyses should be performed on those stocks that are currently assessed analytically (Icelandic S. marinus, Northeast Arctic S. marinus).
The studies conducted since 1995 to combine age readings based on scales and otoliths yielded poor results and virtually no possible conversion factor was obtained. In spite of 1995 workshop recommendations, Russia has continued to read scales of S. mentella in the Irminger Sea, but has also collected several thousand scales and otoliths from the same fish in the period 1999-2005. This collection is a great opportunity for further research supporting standardizing redfish ageing methodology as the Russian readers work to adjust application of criteria as recommended during the workshop. That is, considering the proximal zone of the otolith sections and then conducting calibration exchanges where sub-sets of these otoliths are sent to other age reading labs for comparative reading.
The workshop agreed on several sets of exchange samples for the purpose of inter-calibration between ageing labs within the next two years. The results of this exchange should be analyzed during a workshop to be held in 2008.