Northwest Fisheries Science Center

Display All Information

Document Type: Journal Article
Center: NWFSC
Document ID: 8762
Title: Assessing the assumptions of geographic closure in mark-recapture surveys using passive integrated transponder (PIT) antennas as virtual block nets
Author: Gus Wathen, Nicholas Weber, Stephen Bennett, Nicolaas Bouwes, Chris E. Jordan
Publication Year: 2017
Journal: North American Journal of Fisheries Management
Keywords: mark recapture,block nets,PIT tag

Fisheries managers typically use block nets to ensure the assumption of geographic closure in mark-recapture surveys is met. However, block nets are ill suited for use in some systems as they are labor intensive to install and maintain, and prone to failure.  Our goal was to assess the extent of and factors affecting emigration between mark and recapture events across multiple seasons and watersheds and to provide recommendations on minimizing the bias associated with violating the geographic closure assumption.  We quantified emigration rates of juvenile Steelhead Trout (Oncorhynchus mykiss) in 61 mark-recapture surveys utilizing PIT antennas to act as virtual block nets.  Average emigration rate was low across all sites and seasons (5.1%), with higher emigration rates correlating with larger fish, shorter site length, smaller substrate, shallower pools, and season.  We conclude that the bias associated with violating the geographic closure assumption can be minimized by employing alternative estimation techniques based on the type of emigration affecting the sample, sampling longer sites, and exercising diligence while introducing fish back into the stream after mark events.  Furthermore, we demonstrate that virtual block nets can be a valuable tool for testing assumptions and increasing the accuracy of abundance estimate.

Theme: Recovery and rebuilding of marine and coastal species
Foci: Characterize the population biology of species, and develop and improve methods for predicting the status of populations.
Evaluate the effects of artificial propagation on recovery, rebuilding and sustainability of marine and anadromous species.