Pre-programmed or flexible? Lab and field studies don't add up and suggest strong individual influence on spawning time in Atlantic cod

The Hjort-Cushing hypotheses imply that timing of spawning is a key parental trait for achieving recruitment success. North Sea Atlantic cod (Gadus morhua) spawn ca 40 days earlier than the Northeast Arctic (NEA) stock in the colder Barents Sea. A difference of this magnitude is seen in lab experiments, resulting in a predictive equation for oocyte size based on time of year and experienced temperature. Assuming temperature from DST tags, the relationship fits almost perfectly with observations from NEA cod caught near the spawning grounds. However, the sampled fish likely belong to a late-spawning component, suggesting that individuals in the population at large vary more than the lab relationship suggests. Further, differences in basin-level temperature across years has no effect on observed oocyte size. This seeming paradox may logically be explained by processes at the individual level: 1) by selection of habitats with specific temperatures, or by physiologically varying 2) onset of oocyte development or 3) oocyte developmental rate. DST tags and lab experiments show significant individual variation through all three mechanisms. Here we reanalyze past lab and field data for individual variation and its environmental and adaptive causes.