Evaluation of micromilling/conventional isotope ratio mass spectrometry and secondary ion mass spectrometry of δ18 O values in fish otoliths for sclerochronology.

RATIONALE: Stable oxygen isotope ratios (δ18 O values) measured in fish otoliths can provide valuable detailed information on fish life history, fish age determination, and ocean thermography. Traditionally, otoliths are sampled by micromilling followed by isotope ratio mass spectrometry (IRMS), but direct analysis by secondary ion mass spectrometry (SIMS) is becoming more common. However, these two methods have not been compared to determine which, if either, is better for fish age validation studies. Hence, the goals were to: (1) determine if the δ18 O signatures from the two different methods are similar, (2) determine which method is better for fish age validation studies, and (3) examine biogeographic and migration history. METHODS: Both analytical techniques, micromilling/IRMS and SIMS, were used to measure δ18 O values in six Pacific cod (Gadus macrocephalus) otoliths. A series of measurements was made from the center of each otolith to its edge to develop a life-history δ18 O signature for each fish. RESULTS: The sampling resolution of SIMS analyses was 2-3 times greater than that obtained by micromilling/IRMS. We found an offset between SIMS and micromilling/IRMS δ18 O values, about 0.5‰ on average, with SIMS yielding lower values. However, the δ18 O patterns from both methods (i.e., the number of δ18 O maxima) correspond to the estimated age determined by otolith growth-zone counts, validating fish age determination methods. CONCLUSIONS: Both techniques resolved δ18 O life-history signatures and showed patterns consistent with seasonal variation in temperatures and changes due to fish migration. When otoliths are large, micromilling/IRMS can provide adequate resolution for fish age validation. However, SIMS is the better option if greater sampling resolution is required, such as when otoliths are small or specimens are longer lived and have compact growth zones.

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.

Genetic structure and dispersal in peripheral populations of a marine fish (Pacific cod, Gadus macrocephalus) and their importance for adaptation to climate change.

Small and isolated peripheral populations, which are often remnants of glacial refugia, offer an opportunity to determine the magnitude and direction of fine-scale connectivity in high gene flow marine species. When located at the equatorial edge of a species' range, these populations may also harbor genetic diversity related to survival and reproduction at higher temperatures, a critical resource for marine species facing warming ocean temperatures. Pacific cod (Gadus macrocephalus), a marine fish in the North Pacific, has already experienced major shifts in biomass and distribution linked to climate change. We estimated the magnitude and direction of connectivity between peripheral populations of Pacific cod at the southern edge of the species' range, by conducting restriction site-associated DNA (RAD) sequencing and individual assignment on fish collected around the Korean Peninsula during the spawning season. Three populations on the western, eastern, and southern Korean coasts were highly differentiated (FST  = 0.025-0.042) and relatively small (Ne  = 433-1,777). Ten putative dispersers and estimates of contemporary migration rates revealed asymmetrical, west-to-east movement around the Korean Peninsula, at a higher rate than predicted by indirect estimates of connectivity (FST ). Allele frequencies at 87 RAD loci were decisively correlated with strong marine temperature gradients between the warmer southern coast and the cooler waters of the eastern and western coasts. Despite relatively small sample sizes, our data suggest asymmetrical dispersal and gene flow, potentially involving adaptive alleles, between peripheral populations inhabiting markedly different thermal regimes. Our study emphasizes the conservation value of peripheral populations in high gene flow marine fish species.

In situ reproduction, abundance, and growth of young-of-year and adult largemouth bass in a population exposed to polychlorinated biphenyls.

We conducted a two-year field study (2000-2001) in the Housatonic River, Massachusetts (USA) to determine if we could detect in situ population-level effects on largemouth bass (Micropterus salmoides) exposed to elevated levels of polychlorinated biphenyls (PCBs). Calculated whole-body PCB concentrations in adult bass in 2002 averaged 121 mg/kg (range = 34-556 mg/kg). Polychlorinated biphenyl concentrations in young-of-year (YOY) composites in 2000 and 2002 averaged 28 mg/kg (range = 21-41 mg/kg) and 19 mg/kg (range = 16-24 mg/kg), respectively. Laboratory studies of fish have reported PCB toxicity at exposure levels below and within the range of those found in the Housatonic River. We evaluated five field-derived metrics: reproductive activity, relative abundance of YOY, YOY growth rates, adult growth, and adult condition to determine whether we could detect effects of PCBs in the largemouth bass population. These computed metrics, when compared with data sets assembled for numerous largemouth bass populations in North America, provided no evidence of population-level impairment. Results of this study suggest that PCB tissue concentrations associated with effects in laboratory studies do not necessarily translate to detectable effects on largemouth bass populations in their natural environment.