Dispersal of Atlantic salmon fry from a natural redd: evidence for undergravel movements?

Abstract

In this paper we describe a modified version of a box trap used for bank-to-bank trapping during a study of dispersal of Atlantic salmon fry (Salmo salar L.). Two such traps were positioned 2 m upstream and downstream of a single isolated natural redd and a third was placed 20 m downstream. All fry captured in each trap were marked and released beyond the trap. Of the fish caught in the second downstream trap, 64% were unmarked. The seasonal patterns of trapping for marked and unmarked fish were identical, but the unmarked fish were significantly smaller than their marked peers. We argue that these unmarked captures represent fish that evaded capture in the first downstream trap, either by dispersing from the redd deep within the gravel or by leaving the water column and burrowing into the gravel on encountering the trap. Implications for the interpretation of trapping data on newly emerged salmonids are discussed.