Workpackages

06.1 Otolith core microchemistry

Analyse the chemical composition of the core region of juvenile and adult herring otoliths and characterise the differences in otolith microchemistry between populations and the temporal stability of the discrimination.

Two analytical methods will be used to measure the chemical composition of the core region of herring otoliths. One of each otolith pair will have been embedded in resin and sectioned by grinding for microstructure analysis. A small training workshop will be run at the beginning of this project to ensure that the materials and methods used in the preparation of the otolith sections will use clean methods so that the same samples can be used for otolith microchemistry analysis with the minimum of contamination problems. The composition of the otolith core region will be measured with two surface analytical techniques: WDS for accurate, absolute concentrations of major elements, and by LA-ICP-MS for measurement of a wide range of trace and ultra-trace elements. Analytical results will be expressed as absolute concentrations (WDS), as elemental ratios (with respect to calcium, which comprises most of the otolith – LA-ICPMS), or as categorical presence/absence scoring for each element. Multivariate statistical analyses will be applied to the different data sets to optimise the number of elements used and the format of the data. Placement of individual fish based on a neural network approach (Thorrold et al. 1998) will also be evaluated.

06.2 Validation of spawning ground chemical signal

Validate the use of the otolith cores from adult herring caught on spawning grounds to represent the otolith chemical signature of larval herring hatched on the same grounds. If the chemical composition of the otolith cores from adult fish matches that of the larvae of a subsequent generation, then it is possible to extract stock identity information from the adult otolith without the need for larval surveys.

Samples of autumn-spawning herring will be collected on the Douglas Bank spawning ground SE of the Isle of Man in the Irish Sea. Plankton sampling cruises over the same grounds will commence 2 weeks after the capture of spawning fish, and continue as long as yolk-sac herring larvae are abundant. The otoliths of the adult herring (n=30) will be embedded in resin and sectioned through the core, using the clean methods suggested for otolith preparation for microchemistry analysis (de Pontual & Geffen 2000). Wavelength dispersive microanalysis (WDS) will be used to measure the absolute composition of the more abundant chemical constituents in the core region, such as calcium, strontium, magnesium, potassium, and sodium. After WDS analysis, the same samples will be analysed with laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The LA-ICPMS is able to detect a wide selection of trace elements in the otolith material, but only relative concentrations can be measured accurately. For otolith work it is usual to express the measured composition relative to the average calcium concentration. The otoliths the yolksac herring larvae will also be embedded, ground, and analysed with WDS and LA-ICPMS, using methods developed specifically for this purpose (Moksness et al., 2000). The composition of the larval otoliths will be compared with the composition of the otolith core region from the adult spawners collected on the same spawning grounds.

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