Capelin (Mallotus villosus (Müller, 1776)) is an important marine forage fish species that typically inhabits sub-Arctic and temperate circumpolar waters. Capelin have recently been reported in greater abundance in Arctic regions, and are considered an indicator for warming climate in the northern marine ecosystem. The goal of this thesis is to examine intrinsic factors among capelin populations, and whether niche overlap among sympatric capelin and Arctic cod (Boreogadus saida (Lepechin, 1774)) will affect the abundance and persistence of capelin in the Arctic as temperatures continue to rise. Life history comparisons of capelin from two Arctic regions (Western Beaufort Sea, Cumberland Sound) relative to a sub-Arctic population (Newfoundland) show that body size, body condition, growth rate and age-at-maturity vary among these regions. Life history characteristics that are adapted to northern environments and increasing temperatures will allow the presence and persistence of this species to increase in the Arctic. The consequence of increased abundance of capelin in the Arctic environment could be competition between capelin and Arctic cod, an Arctic species within a similar dietary niche. Stomach contents indicated that both species feed primarily on calanoid copepods and this result was corroborated with high dietary overlap in isotopic bivariate space (carbon and nitrogen stable isotopes). The occurrence of capelin is expected to increase in the Arctic with rising temperatures, thus adaptation in life history traits in capelin specific to each region may facilitate increased abundance and persistence, and possibly contribute to competitive pressure on Arctic cod. Potential competition will be particularly important in nearshore and shelf habitats where shifts in availability of intermediate trophic level taxa will influence diet and distribution of key predators such as beluga, sea birds, and anadromous fishes. As the Arctic environment changes with climate shifts, newly adapted or dispersed species from sub-Arctic systems are expected to drive shifts in ecosystem structure and function in the marine environment.