Quantifying the Net Benefits of Suppression: Truncated Size Structure and Consumption of Native Salmonids by Invasive Northern Pike in an Alaska Lake

Abstract

Predation by nonnative Northern Pike Esox lucius is implicated in the decline of native salmonid Oncorhynchus spp. populations in south-central Alaska. In response to the ongoing invasion of Northern Pike, eradication and suppression programs are under way in several severely impacted watersheds throughout the region. As previous research indicates that owing to the particular composition of their diet small Northern Pike may have disproportionate impacts on juvenile Pacific salmon Oncorhynchus spp. (hereafter, “salmon”) populations, there is a potentially important trade-off between overall predator reduction and truncation of the size structure of the population. We examined this trade-off using bioenergetics models and field data collected during a 5-year Northern Pike suppression program in Shell Lake, a heavily impacted Sockeye Salmon O. nerka nursery lake in the Susitna River drainage of south-central Alaska. Our results confirm that Northern Pike of all sizes consumed salmon and that on average salmon made up a smaller proportion of the diets of large Northern Pike. However, large Northern Pike consumed more salmon biomass per capita than smaller Northern Pike, according to the bioenergetics model incorporating size-dependent metabolic needs. Compared with the period prior to initiation of suppression efforts in 2013, Northern Pike consumed 68% less salmon in 2014, 87% less in 2015, and 88% less in 2016. Despite the fact that the catches are now dominated by smaller and younger Northern Pike, the suppression program has been highly effective at reducing the consumption of salmon in Shell Lake, with stark reductions in the catch of all size- and age-classes. Importantly, the reduction in the abundance of the oldest (age 5) Northern Pike to near zero was an key element in this reduction in predator consumption. Therefore, we conclude that suppression strategies targeting a broad range of size-classes of Northern Pike are more likely to be effective at reducing the consumption of salmon than strategies targeting only smaller size-classes.