Spring Thermal Fronts and Salmonine Distributions in Lake Ontario

Abstract

The hypothesis that salmonine catches in Lake Ontario are greater at thermal fronts in spring and early summer was tested in 1990 by comparing catches in non-frontal water and three types of fronts (thermal bar, 4 °C; spring thermocline, 6-8 °C; thermal break, ? 9 °C). A thermal front in the spring on Lake Ontario is a rapid temperature cline across the surface of the lake (in this study defined as ? 0.15 °C/min at the standard 3.2 - 4.8 km/h trolling speed) parallel to shore that extends obliquely from the surface toward shore and the bottom. Surface temperature was recorded every 2 min during 45 hours of fishing. Only 20% of the time was spent fishing in thermal fronts where 35% of the 88 strikes occurred. Catch per unit effort (CPUE) for salmonines at thermal fronts was significantly greater than non-frontal CPUE on each of the 11 sampling dates (P < 0.001). Catches were better in thermal breaks (P < 0.002), spring thermocline (P < 0.01) and thermal bar (P < 0.05) than in non-frontal waters. The data support the hypothesis that there is a relationship between salmonine susceptibility to capture and thermal fronts. Relative to non-frontal water, coho salmon (Oncorhynchus kisutch) CPUE was greater in the spring thermocline (P < 0.01); rainbow/steelhead trout CPUE was greater in thermal breaks (P < 0.05), spring thermocline (P < 0.05) and thermal bar (P < 0.002). It appears that anglers can effectively catch specific salmonine species by fishing specific thermal structures. These results likely are applicable to other pelagic habitats utilized by salmonines.