Climate variability affects fish in a variety of ways and throughout its life cycle. Sea temperature is the variable that has received the most attention from researchers in terms of its effects on both pelagic and demersal fish. Successful individual growth often occurs within a limited thermal range that differs among species and even between developmental stages within the same species. Generally, fish in colder waters tend to exhibit slower individual growth than those in warmer waters (Godø, 2003).
Faster growth results in reduced susceptibility to predation due to shorter durations during early development stages, thereby affecting mortality rates. Temperature also affects swimming speed and activity rate (Fuiman et al., 2005, 2006), which in turn affects both feeding success and anti-predator behaviour through changes in encounter rates with prey and predators, respectively. Temperature can affect gonadal development resulting in spawning times generally occurring earlier under warmer-than-normal conditions (Hutchings and Myers, 1994). The age-of-maturity in different stocks of Atlantic cod, including the Northeast Arctic cod, varies with temperature (Drinkwater, 2000) and is believed to be caused by faster growth rates for those cod stocks inhabiting warmer waters. Recruitment varies with the temperature experienced during the first years of life with higher recruitment generally occurring during periods of higher temperatures for both cod (Sætersdal and Loeng, 1987; Ellertsen et al., 1989; Ottersen and Sundby, 1995; Ottersen and Stenseth, 2001) and herring (Toresen and Østvedt, 2000). Earlier seasonal warming leads to earlier migratory movements, e.g. for capelin in the Barents Sea (Ozhigin and Luka, 1985, Tjelmeland, 1987). Persistent warming has resulted in northward expansion of several species in the Barents Sea, including Atlantic cod and Atlantic herring (Drinkwater, 2006) and capelin (Vilhjálmsson, 1997). In addition, Atlantic cod has been shown to favour more northern spawning during warm conditions and more southern under cold conditions (Sundby and Nakken, 2008).
Other abiotic factors than temperature affect fish. Dispersion of fish eggs and larvae from their spawning ground is considered a key aspect of recruitment success as currents may carry them into or away from favourable nursery areas. Numerical models are well suited for the study of transport of fish larvae, e.g. for Northeast Arctic cod (Vikebø et al., 2005). These studies indicate that where the larvae settle and recruitment success has a strong dependency on wind-dependent drift. Turbulence levels can also be important as these affect the contact rate between larval fish and their prey (Rothschild and Osborn, 1988), which in turn can affect their feeding rates (Sundby et al., 1994).
