Survival of Whirling‐Disease‐Resistant Rainbow Trout Fry in the Wild: A Comparison of Two Strains

Abstract

Introduced pathogens can affect fish populations, and three main factors affect disease occurrence: the environment, host, and pathogen. Manipulating at least one of these factors is necessary for controlling disease. Myxobolus cerebralis, the parasite responsible for salmonid whirling disease, became established in Colorado during the 1990s and caused significant declines in wild Rainbow Trout Oncorhynchus mykiss populations. Attempts to re-establish Rainbow Trout have focused on manipulating salmonid host resistance. A Rainbow Trout strain known as GR × CRR was developed for stocking in Colorado by crossing a whirling-disease-resistant strain known as the German Rainbow Trout (GR) with the Colorado River Rainbow Trout (CRR). The GR × CRR fish exhibit resistance similar to that shown by GR, and survival and reproduction were expected to be similar to those of CRR. One disadvantage of stocking GR × CRR is that outcrossing and backcrossing could decrease resistance, and laboratory studies have indicated that this can occur. A potential disadvantage of stocking pure GR is lower survival due to domestication. To compare fry survival between the strains, a field experiment was conducted in 1.6-km reaches of nine Colorado streams. Each stream was stocked in August 2014 with 5,000 GR × CRR and 5,000 GR individuals. In October 2014, April 2015, and August 2015, apparent survival was assessed. Two laboratory predation experiments were also conducted. The field experiment revealed that short-term apparent survival was influenced by stream, and growth rate was influenced by strain and stream. However, after 12 months, there was no difference in apparent survival or growth rate between the GR and GR × CRR strains. Laboratory experiments showed that survival did not differ between the strains when confronted with Brown Trout Salmo trutta predation. Our results indicate that the GR strain is a viable option for stocking in streams where M. cerebralis is enzootic. Further evaluation is needed to determine whether GR fish will survive to maturity and reproduce.

Dual resistance to Flavobacterium psychrophilum and Myxobolus cerebralis in rainbow trout (Oncorhynchus mykiss, Walbaum)

“…M. cerebralis was introduced to Colorado in the late 1980s, and later found in free‐ranging salmonid populations in 11 of the state's 15 major river drainages (Barney et al., 1988; Nehring & Thompson, 2003), resulting in the collapse of wild rainbow trout populations throughout Colorado (Nehring & Thompson, 2001). The state of Colorado has been using M. cerebralis ‐resistant rainbow trout to re‐establish populations in the presence of the parasite (Avila et al., 2018; Fetherman et al., 2014), and reproduction and recruitment are occurring (Fetherman et al., 2014). Stocking F. psychrophilum ‐resistant fish with no resistance to M. cerebralis could result in failure due to mortality associated with M. cerebralis exposure, as well as increased infection severity and loss of progress gained from M. cerebralis ‐resistant rainbow trout stocking efforts.…”

“…The state of Colorado has been using M. cerebralis-resistant rainbow trout to re-establish populations in the presence of the parasite (Avila et al, 2018;Fetherman et al, 2014), and reproduction and recruitment are occurring (Fetherman et al, 2014). Stocking F. psychrophilum-resistant fish with no resistance to M. cerebralis could result in failure due to mortality associated with M. cerebralis exposure, as well as increased infection severity and loss of progress gained from M. cerebralis-resistant rainbow trout stocking efforts.…”

Habitat associations of rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta fry

“…Avila et al . ( 2018 ) showed that the stream characteristics in systems in which M. cerebralis was present or absent affected the survival of two M. cerebralis– resistant strains of O. mykiss , stocked as fry, 2 months after stocking. The presence of other species may also affect stocking success.…”

“…The presence of other species may also affect stocking success. Previous work has shown that brown trout Salmo trutta L. competition with O. mykiss results in the exclusion of O. mykiss fry from desired habitats (Gatz et al ., 1987 ), and fry stocking success may be affected by predation from larger S. trutta (Avila et al ., 2018 ). Although mechanical removals may be an option for reducing competition and predation between stocked O. mykiss and S. trutta , removals can be both time intensive and expensive (Fetherman et al ., 2015 ).…”

“…Disease may be an especially important consideration in conjunction with habitat in systems where specific pathogens are established. Avila et al (2018) showed that the stream characteristics in systems in which M. cerebralis was present or absent affected the survival of two M. cerebralis-resistant strains of O. mykiss, stocked as fry, 2 months after stocking. The presence of other species may also affect stocking success.…”