Abundance and population dynamics of invasive northern pike, Esox lucius, Yampa River, Colorado, 2004-2010

Authors: Koreen Zelasko, Kevin R. Bestgen, John Hawkins, Gary White

Year: 2014 (xlvi)

Abstract

Northern pike, Esox lucius, is one of the most problematic invasive aquatic species in the Yampa River basin and an impediment to recovery of endangered fishes in the upper Colorado River basin. A long-term mechanical removal program was implemented in 2004 to reduce negative effects of northern pike, but population abundance and trends and efficacy of removal are not well understood. We analyzed mark-recapture records of 8,929 individual northern pike, 2004-2010, from three reaches of the Yampa River: upstream reach Hayden to Craig, HC (designated as a ‘buffer zone’ between upstream pike production areas and downstream native fish critical habitat); middle reach South Beach-Little Yampa Canyon-Juniper, SLJ; and downstream reach Maybell-Sunbeam. Survival rate estimates for average-length northern pike were highest in downstream reach MS, averaging 0.54 (range: 0.36-0.71) across annual time intervals, followed by middle reach SLJ (average: 0.32, range: 0.18-0.48), and then upstream reach HC (average: 0.25, range: 0.12-0.38). Annual abundance estimates followed the opposite pattern: highest each year in upstream HC (range: 1192-3951), followed by SLJ (659-2446), and lowest in downstream MS (range: 233-645). Years with highest abundance included 2004 and 2009 for all reaches. Population increases due to combined effects of recruitment and immigration, calculated as the difference between predicted abundance at the end of a survival interval and the abundance estimate for each subsequent year, averaged 496% in reach HC, 491% in reach SLJ, and 144% in reach MS. Analysis of length-frequency histograms suggested immigration of young age-classes of northern pike. For example, few or no individuals from 2007 and 2008 cohorts were captured in sampling of reaches HC and SLJ those years, but were more abundant than expected in 2009. Hence, those young northern pike likely moved in from another reach or off-channel source, and likely from upstream. Most young northern pike captured in those reaches were produced in 2004 and 2009, years with markedly different spring discharge peaks and flows; thus, the relationship between Yampa River flow patterns and pike spawning success and recruitment is unclear. Mark-recapture records also showed a preponderance of downstream movement. For example, of 394 northern pike that changed reaches during their capture histories, 90% of movements were to a downstream reach, either within the Yampa River or to the Green River. Northern pike demographic data showed that current removal efforts are inadequate to permanently reduce pike abundance in the Yampa River, mainly because recruitment and immigration exceeded removal and mortality. This was especially true in higher-density upstream reaches, where control efforts such as northern pike source management and spawning disruption are needed, in addition to increased removal, to reduce negative effects of pike. This information should allow managers to better evaluate the role of northern pike removal in the ‘buffer zone’ on populations in downstream critical habitat and explore more effective means to reduce pike abundance river-wide to assist with recovery of native fishes.

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