Abstract:
Understanding life stage connectivity is essential to define appropriate spatial scales for fisheries management and develop effective strategies to reduce undersized bycatch.
Despite many studies of population structure and connectivity in marine fish, most management units do not reflect biological populations and protection is rarely given to juvenile sources of the fished stock. 
Direct, quantitative estimates which link specific fishing grounds to the nursery areas which produced the caught fish are essential to meet these objectives. 
We used this dataset to develop a novel continuous-surface otolith microchemistry approach to geolocate whiting (Merlangius merlangus) and infer life stage connectivity to the west of Scotland and in the Irish Sea.
The element data was sampled using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and we showed significant connectivity across existing stock boundaries and identify the importance of the Firth of Clyde nursery area.
The approach we developed with these data offers fisheries managers the ability to account for the benefits of improved fishing yields derived from spatial protection while minimising revenue loss.

Methods:
Whiting were collected by bottom trawl from seven sample locations in October 2014 and ten sample locations in March 2015 from across the west coast of the UK.
200 individuals were collected from each sample site (n=100 > 20 cm and n=100 < 20 cm) and stored frozen at -20 °C. Total Length (TL, to nearest mm) was recorded, sex was determined during dissection and both sagittal otoliths removed using ceramic tipped forceps.
A TL stratified random sample (38%) of left sagittal otoliths was used for annual age estimation. The 2014 cohort was identified from annual increment counts and otoliths from age-0 (caught in October 2014) and age-1 (caught in March 2015) were used in the analysis.
Ceramic tipped forceps were used to handle right sagittae under laminar flow conditions and the otoliths were placed in Elga ultra-pure (>18 M·cm) water and decontaminated in a series of three sonification steps.
All equipment was washed in 10% nitric acid solution before use. The right otolith from each pair was mounted (in a random order to prevent sample batch bias) in Araldite resin (Araldite CY212, Agar Scientific) and sectioned axially to create wafer sections (300 µm) which included the cores.

Element composition was analysed using a PerkinElmer Elan DRCII+ ICP-MS (PerkinElmer, Buckinghamshire, UK) equipped with a New Wave Research UP213 laser ablation system (LA-ICP-MS), using helium gas (flow = 0.8 L/min) as the carrier and argon plasma (flow = 0.75 L/min).
The presence of 18 element isotopes (7Li, 23Na, 24Mg, 27Al, 31P, 44Ca, 45Sc, 47Ti, 52Cr, 55Mn, 60Ni, 65Cu, 66Zn, 85Rb, 88Sr, 89Y, 138Ba, 208Pb) were analysed. Pre-ablation transects (100 µm wide) were run in order to decontaminate the surface before a series of 55 µm diameter pits were sampled for a dwell time of 20 sec (~50 µm deep) from the core to the edge of each otolith (6 for age-0 and 12 for age-1).
The laser wavelength was 213 nm and pits were ablated at a shot frequency of 10 Hz. At the beginning of each experimental run and after each otolith was sampled, a helium gas blank, NIST and MACS (NIST 612; National Institute of Standards and Technology and MACS 3; United States Geological Survey) ablation were also taken for calibration and instrument drift correction.
Blank subtracted count data were gathered for each ablated pit and converted to element concentrations (ppm) by manual calculation using Ca as the internal standard.
The calculated ratios, using the internal standardisation equation, compensated for variation in analyte yield between samples and standards.
Mean percent relative standard deviations (% RSD) calculated for MACS 3 carbonate standard during the analyses were 8.11 (Li), 7.76 (Na), 25.46 (Mg), 21.40 (Al), 13.88 (P), 15.41 (Sc), 23.64 (Ti), 8.74 (Cr), 12.41 (Mn), 11.64 (Ni), 9.07 (Cu), 206.88 (Zn), 78.81 (Rb), 10.04 (Sr), 13.8 (Y), 18.61 (Ba), 25.7 (Pb).
Mean percent relative standard deviations (% RSD) calculated for NIST 612 standard glass were <0.001 (Li), 17.31 (Na), 7.94 (Mg), 12.06 (Al), 13.88 (P), 31.55 (Sc), 15.58 (Ti), 8.62 (Cr), 6.38 (Mn), 5.73 (Ni), 6.12 (Cu), 6.71 (Zn), 78.81 (Rb), 17.83 (Sr), 6.10 (Y), 12.15 (Ba), 8.47 (Pb). All elements are expressed as concentrations.