A Fry Trap Method for Estimating Salmonid Survival from Egg Deposition to Fry Emergence

Abstract

The method involves a trap of nylon netting placed over an individual redd with the trap's edges buried 15–20 cm in the gravel just outside the periphery of the redd. It has been used successfully on more than 70 coho salmon (Oncorhynchus kisutch) redds over the past 5 years, with as many as 2061 fry being captured from a single redd. The trap is relatively stable because it is flexible and conforms to the surface of the streambed, causing debris to float or roll over the surface. It can be used on individual redds; thus, emergent survival for separate parental combinations can be estimated. Field tests showed the efficiency of the trap approached 100%. Installation and presence of the trap had no significant effect on intragravel dissolved oxygen and gravel permeability. Mortality of fry in the traps averaged less than 1.5% when fry were removed at least three times a week. We concluded that the trap provides a more accurate estimate of survival from egg deposition through fry emergence than four other methods.