*More information needed on cod migration/spawning in New England!
Migration and Spawning Locations Around Greenland
http://journal.nafo.int/J16/ratz.pdf
Basic Cod Spawning Times/Locations:
Northern Cod--A Matter of Survival
http://www.cdli.ca/cod/history4.htm
1. Labrador and northern Newfoundland spawn from March to May along the outer slopes of the continental shelf in depths from 200 to 600 m
2. Grand Banks: spawning April-June
3. South coast of Newfoundland: spawning May
4. Nova Scotia banks: spawning March-April
The Grand Banks contains significant populations of Cod (population range outlined below). Also, the region of sea directly northeast of Newfoundland has a separate population (see second map below for general population area). This region is a combination of the Belle Island and Hamilton banks. 
In general, most Cod overwinter on the eastern edge of their boundary - on the edge of the Continental shelf. During the summer, they come inland (western edge of the boundary) to
spawn.
The cod along the northern shore of Newfoundland - the Belle Island and Hamilton Bank population - tend to move southward and inland between April and June. From June to September, they spawn inshore; most Cod return to around the same location each year -within a 100 mile range. However, to find this range, they evidently must travel up and down the shore a considerable amount (between 49 and 55 degrees latitude). See below for visualization of specific range, with actual data points of tagged, tracked fish population.
After spawning, the population moves back northward and eastward in September.
The cod along the southern shore of Newfoundland - the Northern Grand Banks area - tend to follow a similar spawning/overwintering cycle. However, they tend to fluctuate more in location - both latitudinally and longitudinally - than their northern cousins. What does this mean for creating closed areas?
(1) Cod populations are relatively constrained to certain areas; the differences between these two populations of study were determined genetically as well as spatially, meaning that separate populations of the same species can be maintained independently. Closed areas can created within each population of cod.
(2) Each cod returns to around the same area each summer and most fishing occurs during the summer, especially around Newfoundland; perhaps rotating small closed areas should be created near shore so that the population that spawns in a particular position along shore isn't destroyed. Source:
http://www.phys.ocean.dal.ca/~taggart/Publications/Taggart_NAFOSCS-29_1997.pdf