The choice for the fish canner therefore becomes one of selecting the appropriate level of spore survival for each of the contaminating species. Thermophilic spores (those which germinate and outgrow in a temperature range of between 40ฐ and 70ฐC and have their optimum growth temperatures around 55ฐC) are more heat resistant, and therefore have higher D values, than spores which have mesophilic optimum growth temperatures (i.e., at 15ฐ to 40ฐC).