My home river has a minimum size limit of 50 cm for keeping atlantic salmon, while I have seen online that some other places have a maximum size limit above which fish cannot be retained. Why is there such difference between fisheries? Which way is more efficient in terms of conservation? What are the arguments for one or another?
Generally, fisheries management is habitat and species specific requiring different tools depending on the outcome desired, i.e more "eating size" fish, more breeding size, more small fry, etc. Adjacent bodies of water may vary significantly in habitat due to fishing pressure, localized impacts (agriculture, human access, etc. This from Minnesota Dept. Natural Resources on walleye fishing might be useful:
Though the DNR is just beginning to experiment with special regulations for walleye, it appears likely they will become increasingly important. One kind of special regulation that has been used with other species and may be useful in walleye management is the "slot" limit, which protects a certain size fish. For example, anglers may be required to return all fish between 18 and 22 inches but can keep fish outside that slot. The slot limit would allow anglers to keep "eating-sized" fish as well as a few trophies. Medium-sized fish would be protected to make for better fishing (not to protect brood stock, which usually exists in sufficient numbers). The number of small fish would be reduced through angling pressure and cannibalism by large walleye. Often, growth rates improve as the small fish become fewer, and the number of large fish increases in this way as well. The secret lies in finding the proper slot for the productivity of the lake and the growth rates of the walleye in it. Success depends on good survey data. What about the familiar minimum-size limit? For example, a regulation may require that all fish under 12 inches be returned to the water. That way, the little ones will have a chance to grow to be big ones, right? Well, no. They'll have the chance to grow to about 113/4 inches and then likely will be yanked from the lake. A minimum-size limit will produce big walleye only if it is pushed up to lunker size--22 inches, for example. That kind of restriction is nearly catch-and-release requirement. Though size restrictions have been a hot topic, there are other ways to limit the kill and improve fishing.
see: Wiki link Fishing Slot Limits excerpt:
Generally, the purpose behind the implementation of protected slot limits is to improve the angling opportunities in a particular body or bodies of water. Protected slot limits are most often used to regulate the harvest from waters where natural reproduction of the concerned fish species occurs. The protected slot limit is set in such a way that it protects the size of those fish deemed most important to the species spawning success in that fishery. With the size of the most sexually productive fish protected from harvest it is likely that an increased number will spawn during a given year and hence lead to more naturally produced individuals. The effort to increase the number of fish through natural reproduction mitigates the need for artificial stocking of a species to provide for a viable recreational fishery.
A lower size limit is designed to allow a fish to reach breeding age before it can be taken.
An upper size limit is designed to prevent the most effective breeders from being taken. It is based on the fact that in many species a more mature adult will have a larger number of successful offspring, and fish continue to grow throughout their lives. Also, in some fisheries it has been found that using only lower size limits pushes fish to evolve to breed at a smaller size.
The choice of policy depends on what species are being regulated, the goals of the fishery management, and the history and politics of the regulating agency. Science is always considered when making these decisions, but sometimes politics ends up being a larger consideration.