In many respects, population dynamics of actual stocks are more complex than those described by simple VPA or statistical catch-age models. For example, many stocks exhibit migratory patterns that vary from year to year. In some cases, the spatial scale of migration may be such that effects on population dynamics are negligible, and in other cases, migration is a valid consideration. Many migration models have been proposed; their implementation usually requires detailed information to characterize the migratory pattern, as migrations often depend on age, size, density, or environment.
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