| Studies preceding the stock depletions and collapses of the 1990s interpreted correlated recruitment among North Atlantic cod ( Gadus morhua) stocks, separated by hundreds of kilometers, as indicative that a proportion of cod pre-recruit mortality and the pattern of synchrony are forced by similarly or larger scaled abiotic conditions. This thesis documents that the characteristic length scale over which cod recruitment is synchronous declined concurrent with cod biomass. Results indicate local processes surpassed abiotic conditions in relevance for determining year-class strength, for the temporally-persistent large-scale signature of abiotic conditions was erased as synchrony patterns became increasingly localized. At finer spatial resolution, synchrony appears most prominently in the constriction of spawning and recruit populations to fewer and fewer geographic areas over time. Cluster analysis of survey abundance indices identified a number of subcomponents in each of four stock divisions, many of which eroded during the post-collapse period. Principal components analysis of recruitment revealed synchronous spatial contractions of recruit concentrations to only a few areas in all four management divisions. Erosion of the subcomponents and spatially restricted ranges for new recruits may prevent the collapsed stocks from recovery and place the remaining populations at higher risk for local extinction via stochastic conditions, failing inter-population connectivity, and depensatory dynamics driven by Allee effects. |
