Identification of spawning areas of two forms of Atlantic salmon ( Salmo salar L.) inhabiting the same watershed

Abstract

This study examined ecological and behavioral differences between migratory and resident forms of Atlantic salmon occurring in a watershed of southeastern Newfoundland, Canada. Lake-dwelling, resident forms of Atlantic salmon ( Salmo salar), in which males and females mature at a small size (14 and 20 cm, respectively), were captured in two separate areas of a watershed, equipped with sonic tags, and released from a third site 1 week prior to the anticipated spawning time. The location of these 10 fish was determined for 15 consecutive days, and 8 of the 10 fish converged at their initial recapture site near the outlet of one of the lakes and were found in this area for the same 5 consecutive days. Eggs were later found scattered between crevices of rocks and boulders (5–30 cm diameter) at the confluence of the outlet stream and lake in water 1–1.5 m deep. No signs of excavated redds were found in this area. At a distance of 8 km downstream from this spawning area and 2–3 km above head of tide, much larger anadromous salmon (40–60 cm) were observed by divers to be inhabiting pools in the main river for a period of 2 weeks prior to spawning. Excavated redds were found in adjacent gravel areas that contained eggs that were much more brightly pigmented than those recovered from near the lake outlet. Because there are no barriers preventing access between these two spawning sites within this system, we suggest that large anadromous females and small resident females select different areas and substrates for spawning. The possibility of genetic difference between the two forms, arising from such reproductive segregation, is considered.