Geology topics

Benjamin H. Letcher

Publications and source records attributed to Benjamin H. Letcher.

23 records · Page 2Linked to original sources

USGS2013

Linking movement and reproductive history of brook trout to assess habitat connectivity in a heterogeneous stream network

1. Defining functional connectivity between habitats in spatially heterogeneous landscapes is a particular challenge for small-bodied aquatic species. Traditional approaches (e.g. mark–recapture studies) preclude an assessment of animal movement over the life cycle (birth to reproduction), and movement of individuals may not represent the degree of gene movement for fecund species. We investigated the degree of habitat connectivity (defined as the exchange of individuals and genes between mainstem and tributary habitats) in a stream brook trout (Salvelinus fontinalis) population using mark–recapture [passive integrated transponder (PIT) tags], stationary PIT-tag antennae and genetic pedigree data collected over 4 years (3425 marked individuals). We hypothesised that: (i) a combination of these data would reveal higher estimates of animal movement over the life cycle (within a generation), relative to more temporally confined approaches, and (ii) movement estimates of individuals within a generation would differ from between-generation movement of genes because of spatial variation in reproductive success associated with high fecundity of this species. Over half of PIT-tagged fish (juveniles and adults) were recaptured within 20 m during periodic sampling, indicating restricted movement. However, continuous monitoring with stationary PIT-tag antennae revealed distinct peaks in trout movements in June and October–November, and sibship data inferred post-emergence movements of young-of-year trout that were too small to be tagged physically. A combination of these methods showed that a moderate portion of individuals (28–33%) moved between mainstem and tributary habitats over their life cycle. Patterns of reproductive success varied spatially and temporally. The importance of tributaries as spawning habitat was discovered by accounting for reproductive history. When individuals born in the mainstem reproduced successfully, over 50% of their surviving offspring were inferred to have been born in tributaries. This high rate of gene movement to tributaries was cryptic, and it would have been missed by estimates based only on movement of individuals. This study highlighted the importance of characterising animal movement over the life cycle for inferring habitat connectivity accurately. Such movements of individuals can contribute to substantial gene movements in a fecund species characterised by high variation in reproductive success.

Freshwater Biology↗

USGS2010

Use of portable antennas to estimate abundance of PIT-tagged fish in small streams: Factors affecting detection probability

Portable passive integrated transponder (PIT) tag antenna systems can be valuable in providing reliable estimates of the abundance of tagged Atlantic salmon Salmo salar in small streams under a wide range of conditions. We developed and employed PIT tag antenna wand techniques in two controlled experiments and an additional case study to examine the factors that influenced our ability to estimate population size. We used Pollock's robust‐design capture–mark–recapture model to obtain estimates of the probability of first detection ( p ), the probability of redetection ( c ), and abundance ( N ) in the two controlled experiments. First, we conducted an experiment in which tags were hidden in fixed locations. Although p and c varied among the three observers and among the three passes that each observer conducted, the estimates of N were identical to the true values and did not vary among observers. In the second experiment using free‐swimming tagged fish, p and c varied among passes and time of day. Additionally, estimates of N varied between day and night and among age‐classes but were within 10% of the true population size. In the case study, we used the Cormack–Jolly–Seber model to examine the variation in p , and we compared counts of tagged fish found with the antenna wand with counts collected via electrofishing. In that study, we found that although p varied for age‐classes, sample dates, and time of day, antenna and electrofishing estimates of N were similar, indicating that population size can be reliably estimated via PIT tag antenna wands. However, factors such as the observer, time of day, age of fish, and stream discharge can influence the initial and subsequent detection probabilities.

North American Journal of Fisheries Management↗

USGS2010

Modeling migratory energetics of Connecticut River American shad ( Alosa sapidissima ): implications for the conservation of an iteroparous anadromous fish

We present a simulation model in which individual adult migrant American shad ( Alosa sapidissima ) ascend the Connecticut River and spawn, and survivors return to the marine environment. Our approach synthesizes bioenergetics, reproductive biology, and behavior to estimate the effects of migratory distance and delays incurred at dams on spawning success and survival. We quantified both the magnitude of effects and the consequences of uncertainty in the estimates of input variables. Behavior, physiology, and energetics strongly affected both the distribution of spawning effort and survival to the marine environment. Delays to both upstream and downstream movements had dramatic effects on spawning success, determining total fecundity and spatial extent of spawning. Delays, combined with cues for migratory reversal, also determined the likelihood of survival. Spawning was concentrated in the immediate vicinity of dams and increased with greater migratory distance and delays to downstream migration. More research is needed on reproductive biology, behavior, energetics, and barrier effects to adequately understand the interplay of the various components of this model; it does provide a framework, however, that suggests that provision of upstream passage at dams in the absence of expeditious downstream passage may increase spawning success — but at the expense of reduced iteroparity.

Connecticut, Massachusetts, New Hampshire, Vermont↗

USGS2008

Impacts of Low-Flow and Stream-Temperature Changes on Endangered Atlantic Salmon - Current Research

Recent climate studies in New England and the northeastern United States have shown evidence of physical changes over time, including trends toward earlier snowmelt runoff, decreasing river ice, and increasing spring water temperatures. A U.S. Geological Survey (USGS) study funded by the National Global Warming and Wildlife Science Center will be investigating changes in summer low streamflows and stream temperatures and the potential effects of those changes on endangered Atlantic salmon populations. The study also will evaluate management options that would be most likely to mitigate the effects of any changes in streamflow and temperature.

Fact Sheet↗

USGS1997

Prey patchiness and larval fish growth and survival: inferences from an individual-based model

We used an individual-based simulation model to evaluate how prey patchiness and fish swimming behavior affect larval fish survival and mortality source (predation or starvation). Simulations revealed that cohort survival increased linearly with greater average patch residence times and that patch residence times for individual fish with different fates (survived, starved, eaten) diverged substantially during the first few days of feeding. Further, by examining the interaction of patch spatial distribution (uniform random versus clumped) with three possible swimming behaviors, we found that swimming behavior, via its effect on prey encounter and feeding rates, affected both cohort survival rates and whether fish died from predation or starvation, but that the spatial distribution of patches (fine-scale (100s m)) only influenced whether fish died from predation or starvation. Within a particular patch spatial arrangement, however, patch intensity (division of food between patches and non-patches) had a major effect on survival. Except at high food levels, fish did not survive when there were no patches, suggesting that average, well-mixed prey densities will not support sufficiently rapid growth for survival. As patch intensity increased, survival increased to a maximum and then declined, with peak survival occurring at higher patch intensity as average food concentration declined. Finally, the degree of patchiness also determined the intensity of selection on growth rates. In patchier environments, there was stronger selection for fast growth rates leading to eight-fold differences in average cohort growth rates after only three days of growth. In general, survival was not directly related to fast cohort growth the best survival occurred with high average prey densities and weak patchiness. Prey patchiness, by influencing the average as well as the variance in individual growth rates, can have a substantial impact on survival rates of larval fish cohorts.

Ecological Modelling↗