This may give you the real reason for the closed season,contrary to popular myth it was coarse anglers who were the actual instigators, and I quote.........
"It was the London anglers and the Sheffield anglers who jointly agreed on fixing a fence around the breeding seasons to save gravid fish from being killed thus at least saving the eggs.
The Sheffield anglers, about 7,000 of them, wanted no dace to be taken in the months of March and April, no gudgeon, ruff, pope, pike or perch taken during March, April or May, and no barbel, carp, bream, roach, tench, rudd or bleak taken during May and June. Which was complicated to say the least. The London anglers, headed by the Piscatorial Society, simply wanted the months of April, May and June to be fenced off. Ultimately, the Londoners struck a deal with the Sheffielders and split the months of March and June, hence we have a closed season from 15th March to 15th June inclusive.
Eventually the Freshwater Fisheries Act came into being in 1878 and the very first case of fishing during a closed season was brought at Newark in 1879. The two defendants were fined five shillings (25p) each upon payment of which they were discharged. The London anglers again wanted a fish size limit to be introduced into the act, but this was opposed by the Sheffield anglers saying it was an issue that should be decided locally.
Without going into the why's and wherefore's of it all, that's how we arrived at the present closed season."
Hope this helps.