Do Rainbow Trout and Their Hybrids Outcompete Cutthroat Trout in a Lentic Ecosystem?

TL;DR

This study compares the physical condition of native cutthroat trout, hybrids, and introduced rainbow trout in a lentic ecosystem, finding that rainbow trout do not outcompete cutthroat trout in such environments.

Contribution

It provides new insights into competitive dynamics of trout species in lentic ecosystems, focusing on condition factors rather than just abundance or reproductive success.

Findings

Cutthroat trout are generally plumper than rainbow trout in lentic ecosystems.

Rainbow trout do not outcompete cutthroat trout in terms of physical condition.

Hybrids have intermediate condition factors, indicating mixed competitive effects.

Abstract

Much has been written about introduced rainbow trout interbreeding and outcompeting native cutthroat trout. However, specific mechanisms have not been thoroughly explored, and most data is limited to lotic ecosystems. Samples of Snake River cutthroat trout (Oncorhynchus clarkii bouvieri), the rainbow-cutthroat hybrid, the cutbow trout (Onchorhynchus mykiss x clarkii), and rainbow trout (Oncorhynchus mykiss), were obtained from a lentic ecosystem (Eleven Mile Reservoir, Colorado) by creel surveys conducted from May to October, 2012. The total length and weight of each fish was measured and the relative condition factor of each fish was computed using expected weight from weight-length relationships from the Colorado Division of Parks and Wildlife (CDPW). Data from the CDPW collected from 2003-2010 in the same lentic ecosystem were used to compute relative condition factors for additional comparison, as was independent creel survey data from 2011. Cutthroat trout were plump: the mean relative condition factor of the cutthroat trout was 112.0% (+/- 1.0%). Cutbow hybrid trout were close to the expected weights with a mean relative condition factor of 99.8% (+/- 0.6%). Rainbow trout were thinner with a mean relative condition factor of 96.4% (+/- 1.4%). Comparing mean relative condition factors of CDPW data from earlier years and plotting the 2012 data relative to percentile curves also shows the same trend of cutthroat trout being plumper than expected and rainbow trout being thinner than the cutthroat trout, with the hybrid cutbow trout in between. This data supports the hypothesis that rainbow trout do not outcompete cutthroat trout in lentic ecosystems. Comparison with data from three other Colorado reservoirs also shows that cutthroat trout tend to be more plump than rainbow trout and their hybrids in sympatric lentic ecosystems.