Are there any 'old fashioned' cod in captivity or maybe stored DNA samples? Maybe Collosal could splice the missing genes back in and bring them back into the gene pool.
If there is a survival advantage to larger cod (presumably there is, or they would not have developed) and if fishing is tightly regulated, they should return eventually.
Or, instead of wooly mammoths, bring back fishy ones.
This is about Eastern Balic cod, so theoretically genes from, for example, Norwegian cod could be spliced in. However body size is usually a polygenic trait with potentially hundreds of genes involved, so that's not possible.