I dont have much faith that "a scientific study" would be much use at all really - if it was the typical long grass review based on years of monitoring data on selected representative rivers around the country where trialling was being done as part of the study - there would simply be far too many variables to ever prove anything.
Scientific opinion, however, is most certainly required to inform any decision. So how can an opinion be drawn without any evidence?. Seems to me that the first thing that needs to be established are answers to the question "What is the Closed Season For?". Once a list of objectives for the closed season have been drawn up, it should be straightforward to seek the opinion of relevant scientists on how best to pursue those objectives - closed season or not or anything in between.
I suspect that it boils down to ethical questions concerning catching gravid fish or disturbing spawning, or just giving fish a break to act naturally in a less pressurised environment during their spawning period. I would doubt that many fish populations could ever be impacted by fishing - certainly not as much as by habitat alteration, pollution, or random good or bad spawning years caused by natural events. Only exceptions might be Salmon or others that are taken for food and congregate in small areas with potential to be over exploited.