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At least 379 records · Page 21Linked to original sources
USGS2023

Salvage using electrofishing methods caused minimal mortality of burrowed and emerged larval lampreys in dewatered habitats

Objective Human-induced dewatering of freshwater habitats causes mortality of larval lampreys (family Petromyzontidae). Salvage by electrofishing at dewatering events is assumed to reduce this mortality, but to our knowledge this assumption remains unassessed. Methods We estimated mortality of salvaged larval lampreys ( Lampetra spp. and Pacific Lamprey Entosphenus tridentatus ) within 24 h following collection at field dewatering events in July and October. To assess when salvage may reduce mortality, we compared mortality of salvaged individuals from field dewatering events to mortality of burrowed and emerged individuals in dewatered habitats in the laboratory. Salvage protocols included electrofishing and foot pressure from walking in test enclosures before and after dewatering. Electrofishing after dewatering (“dry shocking”) involves positioning probes on moist sediment to entice burrowed larval lampreys to emerge. Result During the July salvage, air temperature averaged 36°C, bottom water temperature averaged 20°C, and many emerged larval lampreys were dead on the sediment surface. During two October events, air temperatures averaged 18 and 11°C, bottom water temperatures averaged 12 and 7°C, and only one dead emerged larval lamprey was observed. Estimated mortality of salvaged larval lampreys was 0.20 (90% credible interval = 0.09–0.37) in July and 0.00 (90% credible interval = 0.00–0.06) and 0.06 (90% credible interval = 0.01–0.18) in October. All larval lampreys that remained burrowed and were excavated from enclosures after salvage were dead in July but alive in October. Logistic regression suggested that mortality declined with increasing larval length. Mortality of salvaged 80-mm larval lampreys in October was lower than that of 80-mm individuals emerged for 1 h or burrowed for 8 h at similar water temperatures (8–10°C) in the laboratory. Conclusion In this study, electrofishing for salvage caused minimal mortality of burrowed and emerged larval lampreys in dewatered habitats. Thus, salvage using electrofishing methods could aid conservation of native lampreys by reducing mortality associated with human-induced dewatering events, especially when temperatures are elevated.

Washington↗
USGS2023

Biotic and abiotic factors affecting short-term survival of two age-0 Rainbow Trout strains in Colorado streams

Both biotic and abiotic factors can influence the survival and growth of age-0 salmonids. Diseases, such as whirling disease, can also affect salmonid demographics and population dynamics. Here, we conducted a supplementary analysis and evaluated specific stream characteristics that may have been responsible for the differences in growth and survival of two whirling disease resistant Rainbow Trout Oncorhynchus mykiss strains observed by Avila et al. (2018). We used regression modeling to analyze the influence of the biotic and abiotic characteristics of nine streams on the short-term apparent survival and growth of two Rainbow Trout strains, 5,000 German Rainbow Trout and 5,000 German Rainbow Trout × Colorado River Rainbow Trout in each stream. Akaike's information criterion (AIC c ) model selection was used to identify the factors that most affected short-term survival and growth. Average stream temperature had the largest (positive) effect, β temp = 0.060, on short-term survival. Rainbow Trout strain, average stream temperature (β temp = 1.55), competitor biomass (β competitor biomass = −0.002), and predator number (β predator number = 0.01) additively affected short-term growth. Our results indicate that both biotic and abiotic factors are important short-term determinants of Rainbow Trout poststocking performance and may account for the differences in survival and growth that we observed among stocking locations.

North American Journal of Fisheries Management↗
USGS2023

Combining expert knowledge of a threatened trout distribution with sparse occupancy data for climate-related projection

Objective To evaluate the vulnerability of Bull Trout Salvelinus confluentus to potential climate changes across its range in Oregon, we compiled disparate expert knowledge of the distribution of spawning and rearing and combined these probabilistic statements as data along with documented records of breeding and rearing in a joint occupancy model. Methods The joint expert knowledge–occupancy model, which was based on discrete patches of cold water (≤13°C) suitable for spawning and rearing, permitted the association of true occupancy with climate and other explanatory variables while accounting for variation in detection probability. We then applied estimated relationships of patch occupancy with explanatory variables to projected coldwater patch configurations in the years 2040 and 2080. Result Projections of the kilometers of occupied coldwater patch in future decades suggest precipitous declines if current relationships of occupancy with environmental variables are maintained. Impacts of climate changes in future decades manifest directly through the outright loss of coldwater patches and increases in winter high flows but also indirectly by increased isolation. Conclusion Combining probabilistic statements of species distributions from knowledgeable experts with sparse occupancy data may be a robust and timely alternative when large numbers of repeated occupancy surveys are infeasible.

Oregon↗
USGS2023

Lake Trout population dynamics in the Northern Refuge of Lake Michigan: Importance of stocking rate

Objective The Northern Refuge of Lake Michigan was established under the auspices of the Great Lakes Fishery Commission in 1985 as an additional step toward restoring a self‐sustaining Lake Trout Salvelinus namaycush population to the lake. The overall goal of our study was to assess progress toward Lake Trout rehabilitation in the Northern Refuge through 2021. Methods We conducted annual (1992–2021) gill‐net surveys in the fall to assess the adult population and beam trawl surveys in the spring to assess naturally reproduced age‐0 Lake Trout. Result Spawner abundance averaged 45 fish • 305 m of gill net −1 • day −1 during 1992–1999, just 4 fish • 305 m of gill net −1 • day −1 during 2000–2014, and then 67 fish • 305 m of gill net −1 • day −1 during 2018–2021. Sea Lamprey Petromyzon marinus wounding rate decreased by nearly 90% between the 2000–2014 and 2015–2021 periods. The average annual mortality rate of adult Lake Trout decreased from 43% during the 1990s and 2000s to 22% during 2015–2021. All of these population dynamics were partly driven by changes in Lake Trout stocking rates, which declined by roughly 50% during 1994–1996 but then increased roughly twofold during 2006–2010 and remained at this higher level during 2010–2018. In 2021, early signs of natural recruitment of Lake Trout became evident. Beam trawling during June 2021 yielded a wild age‐0 Lake Trout, the first ever caught since the surveys began in 1992. In addition, 15 (4% of total catch) of the Lake Trout caught in the fall 2021 gill‐net survey were unclipped (wild) fish. Estimated ages of these wild fish indicated that natural recruitment had begun sometime around 2015. Conclusion With Alewife Alosa pseudoharengus abundance in Lake Michigan at record low levels, Sea Lamprey wounding rate greatly reduced, and Lake Trout spawner abundance at relatively high levels, the outlook for Lake Trout rehabilitation in the Northern Refuge holds promise.

Northern Refuge of Lake Michigan↗
USGS2023

Estimates of adult Lake Trout mortality from coded wire tags in a population with developing natural reproduction in southern Lake Michigan

Objective Overfishing and Sea Lamprey Petromyzon marinus predation led to extirpation of Lake Trout Salvelinus namaycush from Lake Michigan in the 1950s. Large populations of hatchery‐reared fish were developed by the 1970s, but natural reproduction was limited until the early 2000s when it began to increase in the southern main basin. Hypothesizing that the relatively low mortality of spawning‐aged fish contributed to this reproductive success, we estimated the total annual mortality rate for this population. Methods We used catch curves to estimate the total instantaneous mortality rate Z using coded wire tags, which provided definitive ages. We made separate estimates from fish collected in three on‐going surveys: a spring gill‐net survey, a fall gill‐net spawning survey, and a sport fishery survey. Result Our estimates of Z ± SE were 0.297 ± 0.019, 0.239 ± 0.009, and 0.205 ± 0.007 for the spring, spawning, and sport fishery surveys, respectively. We suggest that the mean Z ± SE of all survey estimates of 0.247 ± 0.027 would be a reasonable estimate for this population, which equates to a total annual mortality of 22 ± 3%. This estimate is in the low range of rates reported for the species and is in the same range as other populations in the Great Lakes with well‐established natural reproduction. Conclusion We concluded that these low total mortality rates contributed to the reproductive success in southern Lake Michigan through increasing spawning stock density and age structure and that previous estimates of another important population parameter, the instantaneous natural mortality rate M , were too high. Estimates of M ranged from 0.210 to 0.240 and were based on the Pauly equation, a growth‐ and temperature‐based estimator. We suggest maximum‐age‐based estimators of M are more appropriate for Lake Trout. Several alternative maximum‐age‐based estimators produced estimates for M of 0.132–0.058, all of which are more compatible with our estimate of Z .

Lake Michigan↗
USGS2023

Adult Sea Lamprey approach and passage at the Milford Dam fishway, Penobscot River, Maine, United States

Objective Sea Lamprey Petromyzon marinus provide important ecological services within their native range, such as nutrient cycling, and can also act as a prey source for other species. Adult Sea Lamprey must access freshwater rivers to spawn, and because of this they are susceptible to changes in river connectivity. Human-made structures, such as dams, can exclude them from usable habitat. Sea Lamprey dam passage has not been extensively studied in Maine, despite Maine being within the native range of this species. The goals of this study were to evaluate upstream passage efficiency at the Milford Dam on the Penobscot River, Maine, and to provide comprehensive information about adult Sea Lamprey passage at five other dams throughout the Penobscot River watershed. Methods In 2020–2021 we captured and tagged 150 Sea Lamprey at the Milford Dam, the lowest dam in the Penobscot River, Maine, and displaced them downstream to assess passage efficiency at this dam and five upstream dams. In 2020, 50 Sea Lamprey were released on the east shore of the river downstream of Milford Dam; in 2021, the east shore release was repeated with an additional 50 fish and another 50 fish were released on the west shore. Result Between 70–82% of Sea Lamprey were observed passing Milford Dam again after mean delay times of 9–11 days. The release location did not affect dam passage success or the amount of time that was required to locate and use the passage structures. Sea Lampreys from both release groups were equally likely to approach the entrance to the fishway upon returning to Milford Dam, despite the fishway being located against the eastern shore of the river. However, high flows shortly after release may have resulted in higher attraction to the fishway in 2020. Passage success at dams upstream of Milford was highly variable. All Sea Lamprey were able to successfully navigate past West Enfield Dam (100% passage, n = 63), whereas Brownsmill Dam apparently acted as a complete barrier to further migration (0% passage, n = 7). Fish from all years and release groups together had a median upstream migration distance of 38.8 km after fish had passed Milford Dam, and a maximum observed upstream travel distance of approximately 100 km, indicating that most tagged Sea Lamprey ended their migration in the vicinity of a dam. Conclusion The results of this study indicate that Sea Lamprey have high passage efficiency at the Milford Dam and highlight areas within the Penobscot River basin—such as the Brownsmill Dam—where passage facilities are currently inadequate for Sea Lamprey.

Maine↗
USGS2023

Growth performance of Rainbow Trout in reservoir tributaries and implications for steelhead growth potential above Skagit River dams

Objective In the Pacific Northwest (USA), Pacific salmon Oncorhynchus spp. populations have been declining significantly for decades, prompting stakeholders to respond with a variety of conservation and restoration measures. One such measure being considered in the Skagit River basin (Washington, USA) is the introduction of steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) above the impassable Gorge, Diablo, and Ross dams to bolster their populations. Because freshwater growth is key to survival at subsequent life stages, we evaluated current trends in size and growth of Rainbow Trout among key tributaries to Gorge, Diablo, and Ross reservoirs using empirical data collection and bioenergetics modeling. Methods For nine candidate streams, a bioenergetics model was used to assess how temperature and prey consumption affected growth performance of Rainbow Trout between annuli 1 and 2, and 2 and 3. Thermal scenarios were created to evaluate how fish growth responded to temperature variability while total annual consumption was constrained within empirical growth estimates. We then compared these results to back-calculated size thresholds established by size-at-age observed in wild steelhead adults that returned to the Skagit River below the dams. Result Of the streams proposed for introductions, there was one instance (McMillan Creek) in the nominal simulations where growth met or exceeded the size at annulus 2 or 3 of a returning adult steelhead (24.9 g at annulus 2 and 50.3 g at annulus 3). Modeled growth under different thermal scenarios showed that colder temperatures (0.1–10.7°C, Canyon Creek) produced higher growth than under the nominal or warm scenarios (2.0–15.3°C, Canyon Creek), as well as one additional tributary where size at annulus 2 or 3 (±2 SE) was comparable to the threshold established by adult steelhead below the dams (Big Beaver Creek, annulus 3). Conclusion These results suggest Rainbow Trout growth is most limited by prey availability in the examined upper Skagit tributaries.

Washington↗
USGS2023

Spatial and temporal overlap between hatchery- and natural-origin steelhead and Chinook salmon during spawning in the Klickitat River, Washington, USA

A goal of many segregated salmonid hatchery programs is to minimize potential interbreeding between hatchery- and natural-origin fish. To assess this on the Klickitat River, Washington, USA, we used radiotelemetry during 2009–2014 to evaluate spatiotemporal spawning overlap between hatchery-origin and natural-origin steelhead Oncorhynchus mykiss and spring Chinook Salmon O. tshawytscha. We estimated percentages of tagged fish that spawned naturally in the Klickitat River subbasin, emigrated from the Klickitat River, or died before spawning. For steelhead, 12% of hatchery-origin and 50% of natural-origin fish spawned naturally. For spring Chinook Salmon, 18% of hatchery-origin and 44% of natural-origin fish spawned naturally. Tag loss may result in underestimates in these percentages. Most hatchery-origin steelhead (90%) spawned downstream of rkm 32 and 75% spawned from November to mid-March, while the majority of natural-origin steelhead (64%) spawned upstream of rkm 32 and 75% spawned from mid-March to late May. Spawn timing of hatchery-origin Chinook Salmon (early August to mid-September) overlapped with that of natural-origin Chinook Salmon (late July to late September), and fish of both origins spawned in the same 30-km reach of the river. We estimated the percent of hatchery-origin spawners on the natural spawning grounds (pHOS) to be 12% for steelhead and 40% for spring Chinook Salmon across all study years. A kernel density analysis was used to estimate probability of spatiotemporal overlap between hatchery- and natural-origin spawners. For steelhead, we estimated this overlap to be 25% (95% CI 22.5–28%). For spring Chinook Salmon, tight spatial clustering of hatchery-origin fish resulted in a lower overlap estimate of 21% (13%-31%). We suggest adjusting pHOS estimates using these overlap estimates or similar spatiotemporal data on actual spawner proximity and possible interactions, and that these types of analyses be used in conjunction with gene flow analysis to accurately evaluate effects of individual hatchery programs.

Washington↗
USGS2023

Evaluation of shoreline rotenone application to control Largemouth Bass recruitment in small impoundments

Objective Reducing Largemouth Bass Micropterus salmoides recruitment and therefore population density could benefit recreational fisheries in small impoundments by improving individual growth rates and increasing the average size and condition of Largemouth Bass. To achieve these effects, methods of controlling Largemouth Bass recruitment should avoid reducing the productivity of their primary prey species, the Bluegill Lepomis macrochirus . Methods We tested this hypothesis by evaluating the effects of shoreline rotenone application on the density of Bluegill and the density, growth, and survival of age-0 and age-1 Largemouth Bass in 15 Alabama small impoundments. Result After treatment, Largemouth Bass age-0 densities declined and mean age-1 length increased, whereas Bluegill populations were not significantly reduced. Conclusion Our study indicates that shoreline rotenone application may be a valuable method for reducing Largemouth Bass recruitment and increasing the growth of age-1 Largemouth Bass in small impoundments. However, further research is needed to understand the effects of treatment on nontarget fishes and to better assess the effects of factors such as impoundment surface area and treatment frequency and duration on the ultimate utility of the approach.

Alabama↗
USGS2023

Validating morphometrics as a nonlethal tool to determine Arctic Grayling sex

Objective Some graylings Thymallus spp. possess an elongated dorsal fin and other morphological traits that can be sexually dimorphic, as demonstrated in the European Grayling T. thymallu s. North American Arctic Grayling T. arcticus are assumed to follow these trends, but decisive evidence is lacking. This study aimed to determine whether sexually dimorphic characteristics, including posterior dorsal height, can be used to accurately predict the sex of Arctic Grayling in Interior Alaska. Methods We used computer imaging software to measure 22 morphometrics on 97 Arctic Grayling of known sex from streams in Interior Alaska, and we developed a set of binomial models to evaluate the validity of morphometrics as predictors of Arctic Grayling sex. Result Posterior dorsal height was a reasonably accurate predictor of sex (~90% accurate at fork lengths ≥300 mm), although models containing additional morphometrics were more accurate (100% accuracy at fork lengths ≥250 mm). Conclusion This study presents an affordable, noninvasive, and replicable method for nonlethal determination of Arctic Grayling sex by using digital images from the field, with potential application to other salmonids.

North American Journal of Fisheries Management↗
USGS2024

Encounter rates and catch-and-release mortality of steelhead in the Snake River basin

Objective The potential influence (i.e., impact rate) of catch-and-release fisheries on wild steelhead Oncorhynchus mykiss is poorly understood and is a function of the abundance of wild fish, how many fish are encountered by anglers (i.e., encounter rate), and the mortality of fish that are caught and released. In Idaho, estimates of wild steelhead encounter rates have been derived using the number of wild and hatchery steelhead passing Lower Granite Dam, the number of hatchery steelhead harvested, and the number of hatchery steelhead caught and released. The method includes assumptions that hatchery and wild steelhead have equal encounter rates and catch-and-release mortality is 5% for wild steelhead. Here, we investigated wild and hatchery steelhead encounter rates by anglers, estimated catch-and-release mortality, and concatenated both aspects to examine how existing recreational steelhead fisheries influence wild steelhead mortality. Methods We sampled, tagged, and released 1,251 spawn-year 2020 (SY2020) and 1,956 spawn-year 2021 (SY2021) adult steelhead at Lower Granite Dam with T-bar anchor tags and passive integrated transponder (PIT) tags to estimate steelhead encounter rates and catch-and-release mortality. Differences in survival of caught steelhead and those not reported as caught were evaluated using detections at various locations (e.g., PIT arrays, weirs). Result Estimated encounter rates were 43.7% (95% credible interval; 28.2%, 100.0%) for wild fish and 46.7% (29.6%, 100.0%) for adipose-clipped fish in SY2020. In SY2021, encounter rates were 47.2% (32.4%, 100.0%) for wild fish and 52.3% (37.1%, 100.0%) for adipose-clipped fish. Based on detections of caught fish and those not reported as caught, catch-and-release mortality of wild steelhead was estimated to be 1.6% (0.0%, 5.2%). Wild steelhead impact rates were 0.7% (0.0%, 2.7%) in SY2020 and 0.7% (0.0%, 2.8%) in SY2021. Conclusion Estimated rates of impact on wild steelhead were consistent and low across years despite major differences in the structure of the fisheries. Our results suggest assuming that encounter rates are equal between hatchery and wild steelhead, and that steelhead catch-and-release mortality is 5%, will likely lead to a conservative estimate of the wild steelhead impact occurring from catch-and-release fisheries.

Idaho, Oregon, Washington↗
USGS2023

Quantifying the contributions of tributaries to large-river fish populations through mark-recapture modeling

Tributaries may play a vital role in maintaining populations of large river fishes, although the specific contributions of tributaries toward recruitment of river-wide populations are not often understood. Tributaries may experience fewer cumulative anthropogenic impacts relative to mainstem rivers and may offer more natural conditions supportive of native fish populations, which may provide opportunities for fish population restoration. Thus, an improved understanding of tributary-mainstem population dynamics may inform targeted conservation actions for spatially structured populations of large-river fishes. Colorado River tributaries in the Grand Canyon, Arizona, USA are a focus of imperiled Humpback Chub Gila cypha conservation, which includes translocations to enhance population redundancy and to expand the overall population. However, the fate of fish dispersed to the mainstem has not been thoroughly quantified. Using open population mark-recapture models, we quantified the relative contribution of three groups of Humpback Chub, including fish of confirmed tributary origin that were either translocated or produced in situ , and others presumed to be Colorado River mainstem origin fish, to three mainstem populations. Our specific study objectives were to 1) estimate Colorado River abundances of tributary and mainstem-origin fish over time, 2) compare relative group-specific contributions to three mainstem populations, and 3) compare group-specific survival rates of Humpback Chub in the Colorado River and in a tributary where a recent translocation has occurred. Tributaries contributed 26% and 43% of the overall abundance in two tributary inflow reach populations, and zero in a third, which we attributed to uncharacteristically low tributary survival immediately following translocation. In the mainstem, survival of tributary-origin fish was higher compared to mainstem-origin fish, suggesting an advantage of tributary residence. Our contrasting results from three different tributary inflow populations highlight the potential role for tributaries in sustaining large-river fish populations, which may have important implications for long-term maintenance of river metapopulations.

North American Journal of Fisheries Management↗
USGS2024

Estimating internal transmitter and external tag retention by Walleye in the Laurentian Great Lakes over multiple years

Objective Both electronic tags (e.g., acoustic and radio transmitters) and conventional external tags are used to evaluate movement and population dynamics of fish. External tags are also sometimes used to facilitate the recovery of internal electronic tags or other instrumentation because healing can make it difficult to identify fish with internal tags based on appearance alone. With both tag types, tag shedding and failure of electronic tags can affect accuracy and precision of study results. Methods We used a decade (2011–2021) of recapture data for Walleye Sander vitreus tagged in the Laurentian Great Lakes, where fish were double- or triple-tagged with external tags (T-bar, loop, or internal anchor tags) and internal acoustic transmitters, to quantify external tag and internal transmitter shedding and transmitter failure rates. Result In total, 1125 (33%) Walleye were recovered that had retained at least one external tag or internal transmitter. No confirmed cases of transmitter shedding were observed; 15 of 899 transmitters (2%) that were checked for functionality failed prior to the expected battery expiration. The retention of external T-bar tags 1 year after release differed depending on whether the tag was placed anterior or posterior to the secondary dorsal fin (anterior, fish length = 420 mm: 73% retention; anterior, fish length = 700 mm: 73%, posterior: 63%) but was <26% after 4 years for both tag positions and fish sizes. Internal anchor tags had an 88% 1-year retention probability and 81% 4-year retention probability. Loop tags had the highest 1-year retention (89%) but after 4 years retention (28–34% depending on agency) was comparable to that of T-bar tags. Conclusion Better understanding of tag retention characteristics through long-term tagging studies such as this can inform study design, be considered in model design, and ultimately improve inferences from mark–recapture studies.

North American Journal of Fisheries Management↗
USGS2024

Precision of four calcified structures for age estimation of Black Carp

Objective Black Carp Mylopharyngodon piceus is an emerging invasive species in North America with an expanding population in the Mississippi River basin. Current aging methods use a suite of structures for age estimation, and a single structure is needed to minimize processing time, to maximize consistency of age and growth measurements, and to allow for back-calculation of individual fish length at age. Methods In total, 236 Black Carp were collected throughout the year from 2017 to 2019 through incidental captures by commercial fishers and biologists. In a subsample, 119 Black Carp from 429 to 1268 mm total length were compared for the precision of annuli counts by two experienced readers for the two anterior pectoral fin rays, the anterior dorsal fin ray, sectioned vertebrae, and lapilli otoliths based on percent exact agreement (PA), percent agreement within 1 year (PA ± 1), coefficient of variation (CV), and bubble plots. Consensus annuli counts were compared by PA and CV between structures. We applied annuli counts, as opposed to age, as fish were collected during all seasons and structures were examined independently by readers without knowledge of capture date. Result Vertebrae and dorsal rays had the highest PA between readers. Dorsal ray had the lowest CV and higher PA ± 1. Pectoral rays and lapilli otoliths had the lowest PA. Between-structure consensus annuli counts indicated that pectoral rays and the dorsal ray annuli counts were the most similar (PA >70% and CV <12) and lapilli otoliths were the most dissimilar (PA >35% and CV >24). Reader-reported confidence was highest for vertebrae, then in decreasing order, dorsal ray, pectoral rays, and lapilli otoliths. Lumens were present in all fin rays with only minor effects observed on annuli counts, aside from the anterior-most pectoral fin ray, in which lumens obscured the presence of one or more potential annuli in 61% of structures. Conclusion The dorsal ray had the highest agreement between readers, with agreement with ±1 year at 98%; the structure can be efficiently extracted and processed, and the structure's morphology and growth pattern are conducive to aging older fish. Vertebrae had high agreement between readers, but a systematic pattern of double bands in vertebrae increased the between-reader variation in older fish as the interannual growth pattern compressed the viewable area; this was identified by readers based on aging criteria and highlights the need for training novice readers. Validation of annual growth in the tested suite of structures is still needed.

Mississippi River↗
USGS2023

Tagger effects in aquatic telemetry: Short-term and delayed impacts of surgery in Atlantic salmon smolts

Objective An assumption of biotelemetry is that animal performance is unaffected by the tagging process and tag burden, yet this assumption is often untested or not thoroughly explored. Our objective was to explore how transmitter implantation procedures influenced Atlantic Salmon Salmo salar smolt survival and migratory performance. Methods We monitored radio-tagged smolts, first in the hatchery and then in a river with a receiver array. We assessed survival and in-river performance in relation to surgeon, surgery duration, processing order, and fish size. Results Mortality was 13.3% during an 8-day hatchery-observation but was higher for fish processed by one of two experienced surgeons (25% vs. 2%). Mortality peaked three days post-surgery and was higher for smaller fish and fish tagged during morning tagging sessions (compared to afternoons). The size effect changed over time, being greatest during the first two days post-surgery, and continuing thereafter at a diminished level. Fish performance once released into a river also differed between surgeons (migration initiation 66% vs. 82%; and to-lake migration success 22% vs 43%), and consistent with hatchery observations, fish tagged in the morning by one surgeon performed poorly once released. Conclusion We highlight immediate and lingering effects of surgical procedures on smolt survival that, if not accounted for, could bias inferences about the study population. Researchers should anticipate tagger effects during study design to ensure potential tagger effects (i.e., surgeon, order tagged, conditions during tagging) are balanced across study groups of interest. Testing for a fixed tagger effect in analyses may not always be adequate because a tagger effect may covary with processing order and fish size and may change over time.

North American Journal of Fisheries Management↗
USGS2024

Fish size structure analysis via ordination: A visualization aid

Objective Visual aids like length-frequency histograms are widely used to examine fish population status and trends; however, comparing multiple histograms simultaneously becomes cumbersome and inefficient. Complicating matters further, overlaying covariates on histograms to highlight connections with length frequencies can be challenging. An alternative, and the subject of this Perspective, is to display length distributions as an ordination using similarity indexes; in many cases, this allows for improved visual organization and representation of relationships with covariates. Methods I review the application of ordination methods for analysis of size structures using alternative visualizations that may facilitate the identification of connections that are concealed when analyzing a series of histograms. After a brief introduction to similarity indexes, types of ordinations, and sample sizes, I examine four case studies to illustrate size structure analysis via similarity indices: (1) unconstrained ordination to identify “bass-crowded” populations in a set of 34 small fishing lakes, (2) unconstrained ordination to evaluate the effect of three consecutive length limits on a Largemouth Bass Micropterus nigricans population over a span of 28 years, (3) constrained ordination to assess the relationships between fish community size structure and in-lake and off-lake environmental descriptors in 30 oxbow lakes, and (4) constrained ordination to identify what aspects of Largemouth Bass size structure were related to six types of reservoir habitats. Result Size structure analysis via similarity indexes enabled the exploration of extensive length-frequency data. It is important to acknowledge that ordinations serve solely as a visual aid for assessing size structure—no statistical testing is involved. Conclusion Ordination techniques and software are advancing at a quick pace, holding great promise for the future of size structure analysis via similarity indices.

North American Journal of Fisheries Management↗
USGS2024

Using environmental DNA to assess the response of steelhead/Rainbow Trout and Coastrange Sculpin populations to postfire debris flows in coastal streams of Big Sur, California

Objective Debris flows are among the most extreme disturbances to streams and are predicted to become more frequent under climate change. We assessed the response of steelhead Oncorhynchus mykiss (anadromous Rainbow Trout)/Rainbow Trout (hereafter, collectively referred to as O. mykiss ) and Coastrange Sculpin Cottus aleuticus populations to major postfire debris flows in two small coastal basins of California using noninvasive environmental DNA (eDNA) sampling. Methods We analyzed water samples from disturbed reaches for eDNA in the first two summers after debris flows. In Big Creek, all Coastrange Sculpin habitat was disturbed by debris flows, but a major tributary occupied by O. mykiss was not affected. In Mill Creek, all available habitat for both species was disturbed. We also sampled two unburned basins as undisturbed reference streams. Result In Big Creek, O. mykiss eDNA was detected in all water samples during both years, and concentrations in some samples approached the lowest concentrations in the reference streams. Coastrange Sculpin eDNA was detected only in a single water sample in the first year and was not detected in the second year. In Mill Creek, neither species was detected in the first year, but during the second year, O. mykiss eDNA was detected at very low concentrations in 40% of samples and Coastrange Sculpin eDNA was detected in a single sample. Conclusion Our results indicate that O. mykiss and Coastrange Sculpins either survived in or quickly reoccupied the disturbed reaches from within the basins. However, the detection rates for cases in which all habitat for a species in a basin was affected by debris flows indicate that abundances were very low, suggesting that persistence may be uncertain unless there is successful reproduction or immigration. Finally, eDNA appears to be effective for monitoring sensitive fish populations after a major disturbance; however, detection rates in individual samples may be low and require appropriate sampling designs to achieve the desired detection probability.

California↗