Modeling growth of larval cod (Gadus morhua) in large-scale seasonal and latitudinal environmental gradients

The spawning strategy of cod has evolved through natural selection to give larvae a good start in life. Therefore, larval drift, growth, and survival are key processes to understand spawning strategies. Spawning of Northeast Arctic (NA) cod stretches from late February to early May over 1500 km along the Norwegian coast. Hatching occurs from late March to late May, a period when the number of daylight hours increases from 11 to 17. Larval feeding opportunities are constrained by prey abundance and environmental variables such as light, while temperature determines the maximum growth potential. Here, we model seasonal and latitudinal constraints on larval cod growth by combining predictions from a bio-physically coupled model providing input on nauplii production and development (Calanus finmarchicus), a 3D physical model (ROMS) providing flow- and temperature-fields, and an individual-based model (IBM) of larval cod physiology and feeding processes. Our aim is to investigate the relative significance of temperature, turbulence, light, and prey density on growth of larval cod by integrative modeling. The models suggest that larval cod experience lower growth if hatched early in the season (prior to mid-April) when the foraging hours are few. Larval cod hatched in early May experience higher temperatures, better growth conditions, and are less susceptible to prey limitation due to increased day-length. We also suggest that increased prey abundance is more valuable early in the spawning season compared to later, when larvae have better feeding conditions. The model quantifies the strong relationship between larval feeding and growth in relation to day-length, time of the season, and water temperature, and the seasonal and spatial appearance of prey.