1. Brown Trout Growth: Growing, Growing, or Gone
Anthony R. Sindt and Joseph D. Conroy
Inland Fisheries Research Unit, Division of Wildlife, Ohio Department of Natural Resources
Introduction
Results
Each fall, the Ohio Division of Wildlife stocks approximately 25,000 brown trout Salmo trutta into three program streams (Clear Creek, Clear Fork of the Mohican, and Mad River) to provide put-grow-take fisheries. Brown trout spend about 315 days at London State Fish Hatchery and grow to an average total length of 190 mm prior to stocking; trout are not legal for harvest until reaching a length of 305 mm (12 inches). Differences in abiotic (e.g., temperature, refuge) and biotic (e.g., prey availability) conditions among program streams and years may result in variable growth rates which affect the duration stocked trout must survive before they reach harvestable size and, consequently, brown trout fisheries success.
Objectives
Compare stocked trout growth rates among streams and years.
Determine the number of days trout must survive to reach legal harvestable length (305 mm) in each program stream.
Quantify the proportion of each trout population larger than the legal harvestable length (305 mm) as an indicator of program success.
Figure 1. Mean length of brown trout stocked in four program stream reaches during October 2011, and mean lengths of the same cohorts collected during subsequent population assessments.
Figure 2. Mean length of brown trout stocked in four program stream reaches during October 2012, and mean lengths of the same cohorts collected during subsequent population assessments.
Figure 3. Mean length of brown trout stocked into Mad River during October 2011 and 2012 and the mean lengths of the same cohorts collected during subsequent population assessments.
Table 1. Mean length of brown trout at time of stocking (approximately 315 days old) in 2011, 2012, and 2013.
Table 2. Number of days after stocking when mean brown trout length reaches the 305mm minimum harvestable length for the 2011 and 2012 cohorts stocked in Mad River and Clear Fork above Pleasant Hill Reservoir. Number of days to 305 mm were calculated from linear regression models.
Year
Stream
2011
2012
2013
Mean
Mad River
199
198
193
196
Clear Creek
177
197
186
187
Clear Fork above
170
203
190
188
Clear Fork below
191
Days to 305 mm (years)
Stream
2011
2012
Mean
Mad River
614 (1.7)
498 (1.4)
556 (1.5)
Clear Fork above
841 (2.3)
557 (1.5)
669 (1.8)
Methods
Trout were tagged with magnetized coded-wire (NMT, Inc., Shaw Island, WA, USA) in specific body locations prior to stocking during fall 2011, 2012, and 2013 to distinguish cohorts and stocking locations. Subsamples of trout were measured for total length to provide mean length and length frequency distributions of trout prior to stocking into each program stream. During subsequent electrofishing assessments, trout ages were determined based on coded-wire tag location and measured for total length. Mean lengths of each cohort were monitored over time and used to calculate stream-specific and year-class-specific growth rates. Clear Fork of the Mohican River is divided into two very different reaches; one reach above and one reach below Pleasant Hill Reservoir. As such, growth was evaluated for each reach separately. At the time of this evaluation, the 2013 cohorts of trout had been stocked but subsequent capture events had only occurred in Mad River.
Stocked trout length averaged 190 mm during 2011–2013 but individual fish lengths varied 94–337 mm (Table 1; Fig. 4).
2011 trout cohorts did not survive to summer 2012 in Clear Creek or Clear Fork below Pleasant Hill Reservoir and thus were excluded from further comparisons (Fig. 1).
Trout stocked in fall 2012 survived to summer 2013 in all program stream reaches (Fig. 2).
Among streams, growth rates did not differ for trout stocked during fall 2011 in the Mad River and Clear Fork above Pleasant Hill Reservoir (Fig. 1; ANCOVA P = 0.48). Growth rates did not differ among all four stream reaches for trout stocked in fall 2012 (Fig. 2; ANCOVA P = 0.72).
Among years, growth rates did not differ between 2011 and 2012 cohorts in Mad River (Figure 3; ANCOVA P = 0.30) and Clear Fork above Pleasant Hill Reservoir (ANCOVA P = 0.28).
During this study, trout only survived long enough to achieve harvestable length in Mad River and Clear Fork above Pleasant Hill Reservoir. On average, trout needed a minimum of 498 days (the 2012 Mad River cohort) to a maximum of 841 days (the 2011 Clear Fork above cohort) post-stocking to reach harvestable length (Table 2).
Differences in survival among streams is evident by the fact that the average proportion of trout that are of harvestable length varies from 32.3% in Mad River to 0.3% in Clear Fork below Pleasant Hill Reservoir (Table 3).
Figure 4. Length frequency of sub-sampled brown trout (n = 7274) immediately prior to stocking into three program streams during October 2011, 2012, and 2013.
Tag
Stock
Evaluate
Table 3. Proportion of brown trout collected during winter and summer assessments (2011–2013) that were of harvestable length (≥305 mm) in each program stream. For this table, the Mad River is divided into an upper (upstream of rivermile 45) and lower (downstream of rivermile 45) reach to exhibit differences in brown trout population characteristics. *Includes spring assessments
2011
2012
2013
Stream 
Summer
Winter
Mean
Mad River
31%
23%
40%
27%
32%
Upper
11%
22%
19%
Lower
44%
47%
29%
54%
41%
Clear Fork 4%
15% 6% 7%
13%
Above 9%
19%*
16%
18%
Below —
0.0% 1%
0.3%
Clear Creek 3% 2%
Discussion
Individual trout lengths at time of stocking were variable and resulted in size differences, post-stocking, within individual cohorts. Trout rarely survive long enough in Clear Creek or Clear Fork below Pleasant Hill Reservoir to reach harvestable length. Despite abiotic and biotic differences among program streams, growth rates were similar and fit a linear model during the course of this study. In Mad River and Clear Fork above Pleasant Hill Reservoir, trout reach harvestable length within 2 years of being stocked on average. Based on these findings, recommendations will be made to eliminate the minimum length limit regulation since trout rarely survive to 305 mm in Clear Creek and Clear Fork below Pleasant Hill Reservoir.
Acknowledgements
Project funding came from the Federal Aid in Sportfish Restoration program, administered jointly by the United States Fish and Wildlife Service and the Ohio Department of Natural Resources, Division of Wildlife. Many Division of Wildlife Fish Section staff contributed to this effort.