Age, not growth, explains larger body size of Pacific cod larvae during recent marine heatwaves.

Marine heatwaves (MHWs) are often associated with physiological changes throughout biological communities but can also result in biomass declines that correspond with shifts in phenology. We examined the response of larval Pacific cod (Gadus macrocephalus) to MHWs in the Gulf of Alaska across seven years to evaluate the effects of MHWs on hatch phenology, size-at-age, and daily growth and identify potential regulatory mechanisms. Hatch dates were, on average, 19 days earlier since the onset of MHWs, shifting a mean of 15 days earlier per 1 ℃ increase. Size-at-capture was larger during & between MHWs but, contrary to expectations, larvae grew slower and were smaller in size-at-age. The larger size during & between MHWs can be entirely explained by older ages due to earlier hatching. Daily growth variation was well-explained by an interaction among age, temperature, and hatch date. Under cool conditions, early growth was fastest for the latest hatchers. However, this variation converged at warmer temperatures, due to faster growth of earlier hatchers. Stage-specific growth did not vary with temperature, remaining relatively similar from 4 to 8 ℃. Temperature-related demographic changes were more predictable based on phenological shifts rather than changes in growth, which could affect population productivity after MHWs.