Revisiting Maturity Data: Using Oocyte Diameter and Gonadosomatic Index to Retroactively Apply a New Maturity Scale to Greenland Halibut ( <i>Reinhardtius hippoglossoides</i> )

ABSTRACT Greenland halibut ( Reinhardtius hippoglossoides ) is a deep‐water flatfish which lives at temperatures of 1°C–4°C and produces large eggs (&gt; 3 mm). The combination of low temperatures and large eggs has resulted in an unusual ovary development cycle, with vitellogenesis taking more than 1 year. This means that fish with early vitellogenic oocytes (termed functionally immature) will not spawn during the next spawning season and should not be included in the Spawning Stock Biomass (SSB). During the Icelandic autumn groundfish survey, the ovary development of Greenland halibut has been evaluated using a 4‐stage maturity scale. The 4‐stage scale, unlike the 6‐stage scale developed specifically for Greenland halibut, does not separate fish in the first year and second year of vitellogenesis which can lead to inaccuracies in the estimation of SSB. The current study investigates whether the weight of the gonad, expressed as a percentage of total body weight, commonly known as gonadosomatic index (GSI), can be used to evaluate maturity for Greenland halibut. We show, from measurements of oocytes, for Greenland halibut collected in Iceland and East Greenland, 93% of Greenland halibut with a gonadosomatic index (GSI) &gt; 1.5% are in their second year of vitellogenesis and should be considered sexually mature. In addition, 97% of fish with a GSI &lt; 1.5% were considered sexually immature/functionally immature. Application of this GSI‐based approach to data collected during the Icelandic autumn groundfish survey demonstrates that it is a practical alternative to visually assessing maturity and can be retroactively applied to past data if gonad weight has been measured. Using GSI to discriminate sexually mature from sexually immature/functionally immature individuals resulted in a length at 50% maturity ( L 50 ) 2.4–8.8 cm higher in comparison to visually assessing maturity with the 4‐point scale. The GSI‐based approach indicated that fish visually classified as ‘spent’ were misidentified and are a mix of other maturity stages, this is supported by the spawning season being 7–9 months prior to the survey. Our data indicates that L 50 of Greenland halibut in Iceland increased in 2012, with L 50 in the period 1996–2010 approximately 73–77 cm whereas in the period 2012–2023 it was approximately 78–81 cm. While, for years 2014–2023, when age was estimated, age at 50% maturity ( A 50 ) was 18.2 years.