Primary demographic attributes such as somatic growth, mortality, size- and age-at-maturity, fecundity, and recruitment drive the structure of fish populations, and these processes are also influenced by density-dependent and -independent processes. Here I study multiple populations of unexploited (i.e. assumed equilibria), lentic Brook Trout to describe variation in population structure. Studying unexploited populations gives fisheries managers and scientists an understanding of the intrinsic variation in systems absent from harvest pressure. First, I identify how much primary demographic attributes vary among populations, then attempt to attribute the observed variation to a suite of nested models containing terms for ecosystem productivity (total phosphorus concentration), climate (growing degree days or GDD), and density (fish biomass per hectare) in 9 lakes. I found that (1) populations varied substantially in somatic growth parameters (two-fold), natural mortality (three-fold), age-at-maturity (three-fold), length-at-maturity (two-fold) and recruitment (three-fold), (2) growth early in life was negatively correlated with density (r = -0.58), but maximum length was positively correlated with GDD (r = 0.61), and (3) spawning stock density was negatively correlated with recruitment (r = -0.57), but positively correlated with GDD (r = 0.55). I then experimentally harvested these previously unexploited populations with size selective gillnets by removing 30-70 % of the largest individuals from 5 of the 9 populations over 2 consecutive years. I tested the compensatory response to harvest with a BACI analysis, where I looked to see if absolute growth rate, size- and age-at-maturity, reproductive investment, and recruitment compensate for fisheries harvest. I found (1) strong evidence of recruitment compensation, (2) that overall (i.e. site-wide) stock-recruitment relationship was strongly density dependent and over-compensatory (i.e. a humped, Ricker type relationship), (3) positive but nonsignificant compensation in growth and age-at-maturity, and (4) no change in reproductive investment, but noted that populations may compensate for reproductive capacity in other ways (e.g. a combination of increased somatic growth and younger age-at-maturity). Comparing observed variation in unexploited populations’ demography with environmental variables helps fisheries managers and scientists understand intrinsic variability, and drivers of said variability; further exploiting these populations in an experimental fishery shows the initial mechanisms behind compensation. Examining both unexploited and responses to fisheries will help fisheries managers and scientists understand which populations can have their density reduced (via setting appropriate harvest rates), set realistic targets to recover populations, and increase understanding of the mechanisms that structure populations.