Rates and Effects of Branding due to Electroshock Observed in Southern California Steelhead in Topanga Creek, California

Abstract

AbstractAs part of a 7‐year lifecycle monitoring study, electroshocking was used to capture and characterize a small population of federally listed endangered southern California steelhead Oncorhynchus mykiss in Topanga Creek, Los Angeles, California. Electroshocking is a tool widely used to study fish populations, despite its potential behavioral and physical effects on individuals. We examined rates of external hemorrhaging (i.e., branding) and its effect on growth rates and survival of recaptured individuals. Rates of branding were low (0–3%) during most November events from 2008 to 2014, but higher during all March events (8–23%) and during the November 2011 event (13%). Overall, 5% of the total captured individuals exhibited branding. Growth rates (mm/d) of individuals recaptured after branding were not significantly different, but on average were 9% lower than average daily growth rates for individuals in the same size‐class captured and recaptured in the same time periods. In general, larger fish were more likely to be branded than were smaller fish (P < 0.0001). Although 69% of brandings occurred in pool habitats, which tend to be larger and deeper than other habitats, branding was not significantly more likely to occur in any habitat type (P = 0.13) or in any substrate type (P = 0.16), and mean or maximum depth of habitat was not related to the rate of branding. The population‐level effects associated with branding remain unclear. Although important information has been obtained through the use of electroshocking, the cumulative effects of electroshocking‐induced injuries to this endangered species need to be considered.Received March 2, 2015; accepted March 27, 2016 Published online July 19, 2016