A successful discrimination of fish populations is essential for sustainable management and assessment. Otolith shape analysis has been used on several species to reveal their population structure. Our aim was to use the otolith shape of European sprat (Sprattus sprattus) to investigate large- and small-scale geographical variability across the Greater North Sea ecoregion. The otolith shape was extracted from digitalised images and transformed into Wavelet coefficients to be analysed with multivariate statistics. Otolith shape was observed to follow the genetic population structure recently defined for the region, supporting the latest revision of the stock boundaries. Four main groups were identified based on phenotypic variability: (i) Norwegian fjords; (ii) North Sea and offshore Skagerrak–Kattegat; (iii) coastal Skagerrak–Kattegat; and (iv) Uddevalla fjord. However, 4-fold cross-validations based on Linear Discriminant Analysis resulted in low accuracy limiting at the moment the ability to use otolith shape analysis for population identification at an operational basis. Our results show the importance of coastal areas, which might be inhabited by distinct populations of sprat that are currently not acknowledged in the management and assessment.

Peer-reviewed. This is the pre-peer reviewed version of the following article: Coad, J. O., Hussy, K., Farrell, E. D. and Clarke, M. W. (2014), The recent population expansion of boarfish, Capros aper (Linnaeus, 1758): interactions of climate, growth and recruitment. Journal of Applied Ichthyology, 30: 463–471. doi: 10.1111/jai.12412, which has been published in final form at DOI: 10.1111/jai.12412

The boarfish (Capros aper) is a pelagic shoaling species widely distributed along the Northeast Atlantic continental shelf. In recent years, this species has experienced a dramatic boom in abundance in the Bay of Biscay and Celtic Sea. This study aims at resolving the mechanisms responsible for this increase in stock size. Based on annual otolith growth increments, we developed a growth chronology as a proxy for stock fecundity. Growth patterns were similar between geographically separate areas west and south of Ireland, with distinct years of good and bad growth. In both areas growth was significantly influenced by summer temperatures and autumn food abundances. These months are the primary growing season of the boarfish. Recruitment was not correlated with growth in the same year; it was however significantly correlated with adult growth the previous year, together with temperature during the months following spawning. The age structure shows that this species is very long lived (> 30 years), but that a considerable proportion of fish are only aged 4-6 years. These age classes correspond to the year with exceptionally high recruitment. This study has demonstrated that both adult growth as a proxy for reproductive potential and environmental conditions favouring early life stage survival may be the cause for the boom in boarfish abundance.

This study presents the first detailed investigation of the oocyte development and maturity classification of boarfish, Capros aper, which has recently become the target of an industrial fishery in the Northeast Atlantic. A total of 2014 boarfish were collected from January to December 2010. Mature male and female boarfish were sexually dimorphic and could be readily identified based on external characteristics. A comprehensive maturity scale was developed, which indicated that the length at 50% maturity for males and females was 9.7 cm total length. Female boarfish were observed to spawn in Irish waters in June and July. Once spawning ceased the remaining mature oocytes were resorbed. Preliminary analysis of reproductive strategy indicates that the boarfish is likely an asynchronous batch spawner with indeterminate fecundity.

The boarfish (Capros aper) is a pelagic species of recent interest to the fishing industry, with landings increasing by .500% over the past 3 years. The objective of the study was to provide a method for age determination based on whole sagittal otoliths, with the results to be used in stock assessment. Translucent zones laid down at regular intervals are identified by marginal increment analysis as seasonally recurring. Translucent zones are formed between September/October and March/April, regardless of fish age. The occurrence of the first annulus is validated by analysis of presumed daily growth increments. Subsequent annulus deposition is homogenous between individuals and allows general guidelines to be derived for interpreting the age of boarfish using their otoliths.