if you have one of those new fish from a big sea he may not be very valuable unless his one of the best raised fish from a big sea , in that case he becomes more valuable to a larger amount of fishermen that want a local fish.a new fish in a small sea however starts more valuable due to the scarcity of fish in that sea and while he will become more valuable then 90% of the fish in the big sea aslong as all fish skills are equal he will never surpass the fish from the bigger sea in price.
your hinting on something alot of managers have not clued in on yet :)