Mary Elizabeth Matta

Otolith oxygen isotopes measured by high‐precision secondary ion mass spectrometry reflect life history of a yellowfin sole (Limanda aspera)

RATIONALE The oxygen isotope ratio (δ(18)O value) of aragonite fish otoliths is dependent on the temperature and the δ(18)O value of the ambient water and can thus reflect the environmental history of a fish. Secondary ion mass spectrometry (SIMS) offers a spatial-resolution advantage over conventional acid-digestion techniques for stable isotope analysis of otoliths, especially given their compact nature. METHODS High-precision otolith δ(18)O analysis was conducted with an IMS-1280 ion microprobe to investigate the life history of a yellowfin sole (Limanda aspera), a Bering Sea species known to migrate ontogenetically. The otolith was cut transversely through its core and one half was roasted to eliminate organic contaminants. Values of δ(18)O were measured in 10-µm spots along three transects (two in the roasted half, one in the unroasted half) from the core toward the edge. Otolith annual growth zones were dated using the dendrochronology technique of crossdating. RESULTS Measured values of δ(18)O ranged from 29.0 to 34.1‰ (relative to Vienna Standard Mean Ocean Water). Ontogenetic migration from shallow to deeper waters was reflected in generally increasing δ(18)O values from age-0 to approximately age-7 and subsequent stabilization after the expected onset of maturity at age-7. Cyclical variations of δ(18)O values within juvenile otolith growth zones, up to 3.9‰ in magnitude, were caused by a combination of seasonal changes in the temperature and the δ(18)O value of the ambient water. CONCLUSIONS The ion microprobe produced a high-precision and high-resolution record of the relative environmental conditions experienced by a yellowfin sole that was consistent with population-level studies of ontogeny. Furthermore, this study represents the first time that crossdating has been used to ensure the dating accuracy of δ(18)O measurements in otoliths.

Age and Growth of Elasmobranchs and Applications to Fisheries Management and Conservation in the Northeast Pacific Ocean

In addition to being an academic endeavour, the practical purpose of conducting age and growth studies on fishes is to provide biological data to stock assessment scientists and fisheries managers so they may better understand population demographics and manage exploitation rates. Age and size data are used to build growth models, which are a critical component of stock assessments. Though age determination of elasmobranchs in the northeast Pacific Ocean (NEP) began in the 1930s, the field has evolved substantially in recent years, allowing scientists to incorporate age data into assessments for more species than ever before. Owing to the highly diverse biology of this group of fishes, each species has its own set of challenges with regard to age determination. Age determination methods typically rely on semicalcified hard structures that form regular growth patterns; however, the structure selected and preparation method used is often species specific. New staining techniques have improved the ability to assess age and improve ageing precision for some species, and advances in microchemical methods have allowed for independent means of estimating age and validating age determination accuracy. Here we describe current age determination methods for NEP elasmobranchs. While the library of available techniques is increasing, there are still some NEP species for which reliable ageing methods have yet to be defined; we discuss these challenges and potential avenues of future research. Finally, we conclude by describing how age estimates are used in growth models and subsequently in stock assessments of selected NEP elasmobranchs.

Are Pacific spiny dogfish lying about their age? A comparison of ageing structures for Squalus suckleyi

Historically, Pacific spiny dogfish (Squalus suckleyi) have been aged using dorsal fin spines, a method that was validated through bomb radiocarbon analysis and oxytetracycline tagging. However, ages generated using this method generally have poor precision and require estimation of missing growth bands in eroded spines, prompting a search for improved age determination methods. In the present study, spiny dogfish were aged using the historical spine method and a new method involving stained thin sections of vertebral centra. Results of an inter-laboratory exchange demonstrated the need for readers to calibrate ageing criteria with a reference collection before reading structures, a practice that yielded significant improvements in between-reader precision of spine band pair counts. After calibration, the primary readers examined the full sample set. The two structures yielded similar age estimates for younger animals, but centrum estimates were consistently younger than spine estimates after age-10. Although further work is necessary to fully explore potential reasons for the observed bias, such as centrum size and location within the vertebral column, at the present time centra are not a suitable alternative to dorsal fin spines for age determination of Pacific spiny dogfish >10 years of age.

Intrinsic and environmental drivers of growth in an Alaskan rockfish: an otolith biochronology approach

Otolith growth-increment chronologies provide an approach for evaluating the impacts of both high-frequency (e.g., interannual) and low-frequency (e.g., interdecadal) climate variability on fish growth. A growth-increment biochronology spanning six decades, spanning several warm and cold climate regime periods, was developed for a commercially important species of rockfish, Sebastes polyspinis, in the Gulf of Alaska. To confirm that all increments were correctly identified and placed in time, we borrowed the technique of crossdating from the tree-ring science of dendrochronology, which ensured high data quality. We then used a mixed effects model to partition variance in otolith growth-increment width among intrinsic (e.g., age-related) and extrinsic (e.g., climate-related) factors. This biochronology was contrasted with one recently developed for S. alutus, a closely-related species which exhibited a significant change in growth following the late 1970s North Pacific climate regime shift. Both species generally showed positive relationships between warm climate conditions and growth, though S. polyspinis experienced a relatively smaller step-increase in growth following the regime shift. The new S. polyspinis otolith biochronology represents a long-term record of growth that extends well before biological specimens were first collected in the Gulf of Alaska, providing a potential tool for fisheries managers to evaluate the effects of climate variability on growth and biological reference points.