Age estimation, corroboration and back-calculation of four-spot megrim (Lepidorhombus boscii) on the Porcupine Bank using otoliths

Highlights

• Age and growth of four-spot megrim estimated for the first time at Porcupine Bank.

• Annual growth analysis is based on interpreting annuli of sagittal otoliths.

• Age estimation is corroborated by tracking year-class abundance in surveys.

• Back-calculation and annuli otolith distances demonstrate ageing consistency.

• Growth parameters from ageing and back-calculation available for stock assessment.

Abstract

Age and growth of four-spot megrim (Lepidorhombus boscii) at the Porcupine Bank, an important European demersal fisheries area, was estimated for the first time based on the analysis of 1334 otoliths. The state of the stock in the Celtic Seas (including the Porcupine Bank) and the northern Bay of Biscay (Div. 7.b-k and 8.a,b,d) has been recently assessed in ICES mainly based on survey abundance indices, but biological information for a forthcoming age-structured stock assessment process is required. In this study, age was estimated by counting the translucent bands (annuli) of the sagittal otoliths following internationally standardized protocols. Otoliths were obtained from specimens caught during five annual bottom trawl surveys (2008–2012) carried out at the Porcupine Bank. Consistency in age interpretation of four-spot megrim was demonstrated by analyzing the regularity of the distances of the annuli to the otolith's primordium and by the back-calculation of the annuli. The growth patterns obtained from direct otolith age estimation and from back-calculation were also compared. Age estimation was corroborated by tracking year-class abundance indices from the surveys. The strength of the year-classes was well tracked in abundant first age groups, highlighting the very abundant 2008 year-class. Furthermore, the von Bertalanffy growth parameters by sex and for combined sexes were also estimated and compared with those from other stocks. The results obtained for these parameters in the combined sexes direct age estimation, their back-calculation and the combination of both methods (L_∞: 50.093; k: 0.126; t₀: −0.296) will allow the upcoming age-structured assessment of this stock and thus a suitable population management. The strength of the cohorts here obtained was compared with those from a previous study in the Iberian Atlantic (8.c, 9.a) stock and based on age estimates calculated with the same ageing criterion. The differences in demographic structure and cohort dynamics found between stocks support the requirement that each stock continues to be assessed based on its respective age estimates and length distributions.

Introduction

The four-spot megrim (Lepidorhombus boscii) is a pleuronectiform fish distributed in the North-East Atlantic, from the Northern British Isles to Western Sahara (26° N), and in the Mediterranean, at depths up to 800 m (Whitehead et al., 1986). The four-spot megrim from Porcupine Bank (ICES Div. 7.b,c,k) belongs to the “Celtic Sea, west of Ireland, and northern Bay of Biscay stock” (Div. 7.b-k and 8.a,b,d) (ICES, 2018). The state of this stock has been preliminarily assessed in ICES (International Council for the Exploration of the Sea) mainly based on survey abundance indices, but no model assessment has been used. However, biological information, such as age structure and growth, is not available but is required for a forthcoming age-structured stock assessment process, aimed at gaining a better knowledge and management of this resource. L. boscii in 7.b-k, 8.a,b,d is caught in a mixed fishery, predominantly by Spanish, Irish, French and UK demersal fleets, with annual landings of around 1200 t (~12 M €) in 2017–18, more than 70% of which are Spanish (ICES, 2019).

Determining age and growth in fish is essential to understand their population life traits and assessing stock status using age-based models that include their population structure and mortality (Panfili et al., 2002). Otoliths, located in the inner ear of fish and composed of calcium carbonate and protein which is deposited at different growth rates throughout their lifetime, are one of the most commonly used calcified structures for age estimation (Casselman, 1987). The calcium carbonate-protein deposition process leaves periodical (annual or daily) opaque and hyaline zones in otoliths, mainly in temperate regions, which can be used to estimate annual age based on their seasonal formation.

Fish growth varies temporarily and geographically. Updated, accurate and precise age and growth information in each distribution area of a species is required to estimate catches at age and year required for stock assessment, thus allowing to obtain more accurate estimates of fishing mortality, biomass and recruitments. To avoid biased results in the stock assessment process, the information on age and population growth must be consistent and validated or corroborated by some alternative method to the direct age estimation of calcified structures.

Several studies have analyzed the age and growth of four-spot megrim, but mainly in areas further south (Iberian Atlantic stock and Mediterranean area) than the one studied here, where the species is most abundant (see review in Cengiz et al., 2013; Landa and Fontenla, 2016). We only have knowledge of three growth studies in the 7.b-k, 8.a,b,d stock: the preliminary work of Dawson (1991) in the Celtic Sea (ICES Div. 7.h,j), that of Robson et al. (2000) in west Ireland (Div. 7.b), and that of Landa et al. (2002) in the northern Bay of Biscay (Div. 8.a,b), showing growth parameters by sex in the first and last study, attending to its sexual dimorphism. Regarding the 7.b-k and 8.a,b,d four-spot megrim stock, robust and updated growth information in some areas is still required, since only the mentioned study of Robson et al. (2000) presented parameters of combined sexes that can be directly applied in age-structured stock assessments, and it was based on a reduced number of samples (150 otoliths) and data from the 90's. To our knowledge, there is no other study of four spot megrim growth on the Porcupine Bank, an over 40,000 km² relevant European fishing area, targeting mainly bottom species, located 150–200 km off the western Irish coastline.

The age and growth parameters of commercially important species contribute to understanding the biology of specific populations and their contribution to the ecosystem. Four-spot megrim stock biology at the Porcupine Bank has only been studied scarcely, but its state of exploitation is of great interest if the stock is to be assessed and managed properly. However current estimations considering only growth parameters in this stock, but lacking evidence of ageing validation or corroboration, are not appropriate if this biological information is to be provided with guarantees for an adequate and accurate stock assessment process. Thus, the present study is the first to: i) investigate growth patterns of four-spot megrim at the Porcupine Bank, based on the analysis of otolith (sagittae) annuli; ii) conduct a back-calculation of the annuli, showing consistency in age interpretation; iii) corroborate (or indirectly validate) growth patterns by tracking cohorts in the available time-series obtained in Porcupine Bank surveys. Furthermore, the growth parameters and performance indices obtained by direct age estimation and back-calculation were also compared with those shown by other authors in European waters, and are currently available for the forthcoming first age-structured assessment process of this stock.