Spatial distributional shifts and associated body condition changes of haddock (Melanogrammus aeglefinus) following population expansion

Temperature fluctuations impact the distribution of various marine fish species and typically result in distributional changes, seeing either a geographical expansion or, possibly, a retraction. Survival and fitness in the new habitat depend, however, on the species' adaptability capacity. Here, we describe changes in geographical distribution of Icelandic haddock (Melanogrammus aeglefinus) based on annual survey data for a 38-year period, 1985-2022. Most of the stock is in the warm waters off the south and west coasts of Iceland (south area), but a small proportion in colder water masses off the north and east coasts (north area). Due to high recruitment, density increased greatly in both areas in the years 2001-2005. Haddock density was usually higher in the south area in 1985-2004, in the north area in 2005-2014, but similar in both areas in 2015-2022. Haddock condition was found to be influenced by both temperature and haddock density. Density-dependent factors influenced haddock mean weight, which dropped considerable during times of high haddock density. Capelin was of great importance as a diet, especially in the south, where stomach fullness decreased considerably when capelin was not a part of the diet. Food availability seems to be highly variable between the two areas but more stable in the north, where fluctuations in capelin fullness were small. Even though favourable environmental conditions may result in increased haddock density in the colder waters in the north, lower condition indicates that the northern area may be less suitable for this species.