Stream length is measured for many fisheries management applications. Characteristics of populations and habitats measured at field sites are commonly generalized to unsampled areas using estimates of stream length or stream network length. There are many ways to measure stream length, but map-based stream length measurements are commonly used in fisheries applications even though they are known to be biased. We evaluated how length of headwater streams in Arizona may be underestimated by the National Hydrography Dataset and how that bias influences streamwide abundance estimates for adult Apache Trout Oncorhynchus apache. As expected, stream lengths measured using National Hydrography Dataset flowlines underestimated true length revealed by National Agricultural Imagery Program imagery on average 11.1% (SD = 4.1%), and this bias was higher in meadow versus forested habitats. The observed bias led to streamwide estimates of adult Apache Trout abundance that were only 88% on average (SD = 5%) of those made with more realistic imagery-based stream measurements. As we have shown, high-resolution imagery, now widely available, can be used to assess and quantify stream length bias, and we conclude that it is important to assess whether this bias has the potential to negatively impact important fishery management decisions.
Obtaining reliable information on the age structure of fish populations is important for making conservation and management decisions. We sought to evaluate precision and reader confidence in age estimates from scales (two body locations), sectioned fin rays (pectoral, pelvic, anal), and sectioned sagittal otoliths from Apache Trout Oncorhynchus apache (n = 78 fish) sampled from the East Fork White River, Arizona, in 2017. Structures were aged independently by two experienced readers without prior knowledge of fish length. Each reader provided a confidence rating of zero (no confidence) to three (completely confident) as a measure of readability. Both readers were unable to estimate age from scales collected from the area just posterior to the insertion of the pectoral fin. Scales removed from an area just dorsal to the lateral line and posterior to the dorsal fin were used in all analyses. Percent exact agreement between readers was highest for scales and otoliths (> 72.0%) and lowest for fin rays (31.8–58.1%). Average confidence rating was highest for sectioned otoliths (mean ± SE; 2.1 ± 0.07), and lowest for anal fin rays (0.3 ± 0.06) and scales (0.7 ± 0.05). Consensus ages from otoliths were compared to the other structures. Percent exact agreement with otolith age was low and varied from 21.6–35.7% among structures. Similarly, percent agreement within one year was also low among structures (58.0–70.2%). Scales consistently underestimated age of age-4 and older fish (based on otolith age); whereas fin rays tended to overestimate age of younger fish and underestimate age of older Apache Trout. Although sectioned otoliths require lethal sampling, they produced the most precise age estimates for Apache Trout with the highest reader confidence. Dorsal scales may be a suitable non-lethal alternative to otoliths if ages for only young fish (age-3 and younger) meet study objectives.
Objective The Southwest has the hottest and driest climate in the United States, and projections show that it will only get hotter and drier into the 2100s. The Apache Trout Oncorhynchus apache is native to the Southwest and is currently listed as threatened under the U.S. Endangered Species Act. Our goals were to understand how climate factors influence the distribution of juvenile Apache Trout (<125 mm TL) and how climate change will influence the suitability of Apache Trout habitat into the 2080s. MethodsWe used a species distribution model to evaluate how climatic and other factors influence the distribution of juvenile Apache Trout. We used predictions from the model to evaluate how climate change might impact the suitability of streams designated for recovery of the species into the 2080s. Result Juvenile Apache Trout occurrence was predicted well by mean July stream temperature (°C), mean annual precipitation (dm), stream slope (%), and the presence of nonnative trout (area under the receiver operating characteristic curve = 0.85). Standardized parameter estimates showed that Rainbow Trout O. mykiss presence and annual precipitation influenced occupancy the most. Model predictions for the 2080s showed suitable habitat (occurrence probability ≥ 0.25) to increase for 11 (of 45) Apache Trout streams in the increased temperature (+3°C) only scenario (scenario 1), as headwater reaches that are currently too cold warmed to become more suitable. When we also included projected declines in annual precipitation (−5%) for the 2080s (scenario 2), the amount of suitable habitat decreased for eight Apache Trout streams and remained unchanged in all other streams. Conclusion Most Apache Trout populations are isolated upstream of barriers to nonnative trout in stream reaches that are currently thermally suitable with respect to mean July temperatures and would remain suitable into the 2080s. Cold headwater reaches are projected to warm, becoming more suitable in the 2080s. Thus, intentional isolation and the resultant truncated downstream distributions of Apache Trout populations in headwater streams explain the nominal effect of projected temperature increases due to climate change on this cold‐adapted salmonid. Standardized model parameters suggest that future declines in precipitation, manifested through reduced snowpack and its influence on streamflows, will play a larger role than temperature in the suitability—and, thus, resiliency—of Apache Trout habitats at least into the 2080s.
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