Growing degree-day influences growth rate and length of maturity of Northwest Atlantic halibut (Hippoglossus hippoglussus L.) across the southern stock domain

The NAFO divisions 3NOPs4VWX5Zc Atlantic halibut (Hippoglossus hippoglussus L.) stock is managed over a large spatial scale, spanning over 20° of longitude and 6° of latitude. Previous studies have shown that female halibut in the warmer southern area mature at sizes 10–20 cm smaller than female halibut in the colder north. Our goal was to examine the role of growing degree-day (GDD, the number of days X temperature °C above a minimum threshold) on growth rate and length at maturity (LM50%). Two separate datasets, associated with the stock-wide DFO-Industry Halibut Longline Survey, were used to estimate these life history traits. GDD had a significant effect on both growth rates and probability of maturity. Females grew faster and matured at a larger size than males. Female growth rate at a reference length of 90 cm corresponding to the southerly NAFO 4X was ~11.5 cm/ year, significantly faster than in the northerly NAFO division 3N (8 cm/year). Male growth rate at 90 cm corresponding to NAFO divisions 4X and 4W were 7.2 and 7.5 cm/year, significantly faster than in NAFO divisions 3N and 3Ps (5.6 and 5.8 cm/year). Both sexes matured at smaller sizes in the southern areas with higher GDD. Females in the warmer southern divisions were more likely to mature ~10–28cm less than the more northerly divisions. However, the corresponding prediction intervals for NAFO divisions were very wide. On average, fish mature at a smaller size in 4WX, but we cannot predict length at maturity for any new observations in any division. We have identified large variation in life history traits across the stock domain associated with the thermal regime. Such variation in life history traits could be used to improve the assessment models and may also be indicative of stock structure that could be eroded if not addressed in the management of the fishery.