​​​​Alexander Alexiades​

Alteration of stream nutrient cycles by an introduced species in the Ecuadorian Amazon

Astroblepus a genus of catfish (order Siluriformes) and the sole genus of the family Astroblepidae, were historically abundant in highland streams throughout the tropical Andes up to elevations of 3500m. However, since the introduction of rainbow trout (Onchorynchus mykiss) into the region, Astroblepus have been displaced to lower elevations and are rarely found in sympatry with rainbow trout. Beyond the direct effect of displacing Astroblepus, rainbow trout introductions in the tropical highlands also indirectly affect ecosystem function through altered nutrient storage and cycling. We examined the influence of introduced rainbow trout populations on nutrient storage and cycling relative to native Astroblepus vaillanti populations along an elevational gradient (2500m-1000m) in the Napo River Basin of Ecuador. Our preliminary results indicate that rainbow trout populations contribute a significantly higher nitrogen subsidy relative to Astroblepus populations through excretion, indicating potential alteration of instream nitrogen cycling. We did not find a significant difference for soluble reactive phosphorus subsidies between the two populations.

Environmental and Social Effects of Water Management and Climate Change in the Ecuadorian Andes 

Governments around the world are increasingly challenged to provide reliable and affordable drinking water and hydropower to rapidly growing populations while ensuring that water usage does not degrade freshwater ecosystems or disrupt other ecosystem services. Freshwater resources will become increasingly valuable as increased demands create tension with decreasing supply due to glacier recession, drought, and other climate change effects. Understanding how aquatic biodiversity and function in rivers are affected by development and climate change is a primary concern of ecology and conservation science. My dissertation research aims to predict and map vulnerability of aquatic organisms and ecosystem function to human-induced changes across a latitudinal gradient and how these alterations affect rural communities that depend on streams and rivers in temperate and tropical regions.

 

Nutrient subsidy synergies between agriculture and stocked trout in freshwater streams

Migratory fish contribute to the structure and function of freshwater ecosystems through material ecosystem subsidies. Material subsidies occur when resources, such as energy and nutrients, are transferred between ecosystems. Each year, the New York State Department of Environmental Conservation (NYSDEC) stocks approximately 3.6 million trout and salmon, many of which are non-native, into almost 10,000 km of freshwater streams. Nearly 700,000 of these trout are stocked into streams within the Adirondack Park, thereby increasing areal density of trout in stocked sections and potentially representing an ecosystem nutrient subsidy. I am working to quantify stocked trout nutrient loads by combining nutrient recycling rate estimates with population density and biomass estimates. I then couple the nutrient input model estimates with measures of areal nutrient uptake in order to better understand the relative contribution of stocked hatchery trout to stream nutrient dynamics in nine streams along a gradient of agricultural land use across New York State . I am also working on estimating net flux of SDN in recipient streams using a mass-balance approach. This project is primarily funded through the Cornell College of Agriculture and Life Sciences Kieckhefer Adirondack Fellowship.

 

Understanding the effects of supplemental fish stocking on stream biogeochemical nutrient cycles

Recent research has shown that fish play important roles in ecosystem function.13 Through the physiological processes of nutrient sequestration in body tissues and nutrient remineralization via excretion and egestion, fishes can influence freshwater nutrient cycles. In turn, these nutrient cycles affect nutrient availability and primary production and can constitute a large ecosystem-scale pool of nutrients.14 In this study we quantified the influence of supplemental nonnative fish stocking, a widespread recreational fishery management practice, on in-stream biogeochemical nutrient (N and P) storage and cycling in four freshwater streams. We found15 that when brown trout were added to these systems at density levels that were orders of magnitude higher than ambient native fish density, they provided a sizeable source of NH4+-N that could account for up to 85% of demand for that nutrient, providing a sizeable resource subsidy. This is a significant finding because stream fishery managers and ecologists often overlook the potential influence of stream fish on nutrient cycles. Though recreational angling provides economic and societal benefits, recreational fisheries that are supported through supplemental fish stocking must also be considered in terms of impacts to native species and ecosystem function.

 

Using broad-scale habitat classification variables to predict maximum local abundance 

In New York State, nonnative brown trout (Salmo trutta), which are large, predatory fish, are often stocked in sympatry with native brook trout (Salvelinus fontinalis). In order to minimize potential impacts of trout stocking programs, it is important to predict areas of brook trout suitability, occurrence, and thresholds for local abundance. Utilizing multiple-logistic and quantile regression, jackknife simulations, New York State’s long term fishery dataset, and a GIS, I used broad scale habitat factors to predict maximum abundance for brook trout and brown trout and to produce environmental suitability maps for 55,944 stream reaches in New York State.5 Managers can use these maps to predict trout occurrence in unsurveyed stream reaches, to prioritize conservation actions, to provide benchmarks of habitat potential for monitoring programs, and to identify threats to environmental suitability from anthropogenic sources

 

Modelling “true” survival: separating temporary and permanent emigration in estimates of surivorship

The focus of population ecology has largely shifted from estimating the size of a population to understanding the demographic mechanisms that control population size including reproduction, mortality, immigration, and emigration (Lebreton et al. 2009). Over the past few decades, advances in telemetry and tagging methods for studying fish movement in streams have shown that salmonids are much more mobile than previously thought, often showing a leptokurtic distribution of movement (Gowan et al. 1994). Separating emigration from estimates of survival rate is essential for understanding salmonid demographic processes in streams. The primary objective of this study is to use  multi-state mark recapture (MSMR) and robust design (RD) model frameworks to estimate “true” survival (i.e., not confounded with emigration and immigration) for three lotic salmonid species, native brook trout Salvelinus fontinalis and nonnative brown Salmo trutta and rainbow Onchorynchus mykiss trout. Furthermore, I will incorporate auxiliary habitat variables into our model, which will allow us to assess environmental factors that influence emigration and survival rates. Funding for this project is provided by the New York State Department of Environmental Conservation and the New York Cooperative Fish and Wildlife Research Unit.

 

 

Coupling physical and biological processes to understand carrying capacity in stream fisheries 

Stream fish-stocking programs assume adequate knowledge of factors limiting fish populations by exploiting the “unused” space between actual resident fish abundance and theoretical maximum abundance (i.e., carrying capacity). Effective management of a stocking program thus relies on a comprehensive understanding of the density dependent and independent forces acting on populations within managed stream sections. As both forces likely operate simultaneously on most populations, we tested for density dependence in native and stocked trout populations using stock-recruitment models then determined which density independent environmental factors are most limiting to stream trout populations using generalized additive models (GAMs). By coupling stock-recruitment models derived from longitudinal population data with multi-scale species habitat relationships, we provided stream managers with a mechanistic understanding of factors regulating populations of both wild and stocked fish.

 

Using model 2 regression to address measurement error in angler creel surveys 

Recent studies have demonstrated persistent bias when comparing the two estimators using catch data from incomplete and complete trips from the same sample of anglers and have promoted the use of linear regression models to correct for apparent bias in catch rates based on incomplete trips.  However, the reported bias in catch rate estimates may be an artifact of measurement error in incomplete trip angler surveys, rather than bias from the estimates themselves. Furthermore, we contend that ordinary least squares linear regression is inappropriate to correct for this apparent bias because measurement error is present in both the response (e.g., catch rate estimated from complete trips) and explanatory (e.g., catch rate estimated from incomplete trips) variables, leading to low estimates of the slope of the relationship. Alternatively, when both variables contain measurement error, model II regression methods provide less biased estimates. Using interview data (incomplete trips) from roving creel surveys and a catch card survey (completed trips) conducted on the same sample of anglers, we compared catch rates derived from both estimators. Our results show that linear regression underestimates the slope of the relationship and that model II regression reduces bias and provides a more accurate estimate.  

 

Effects of land use and stocked trout on stream invertebrate density and community diversity 

Rainbow (Onchorynchus mykiss) and brown (Salmo trutta) trout are commonly stocked in streams and rivers worldwide to enhance recreational fisheries but can have negative impacts on native species and ecosystem function. Detection of impacts resulting from trout stocking are difficult due to spatial and temporal variation and confounding with land use; therefore, effects in different systems vary considerably.To assess the impacts of trout stocking on invertebrate community structure and taxon level density in an agricultural and forested stream in central New York, we used a Before-After Control-Impact (BACI) design. Stocked trout typically have high mortality and harvest rates in streams in the northeast USA, therefore potential ecological impacts are likely to be pulsed (i.e., effects that may occur will take place within a short time span). Before implementing the BACI study, we first estimated stocked trout mortality using multiple pass depletions and angler harvest using creel surveys. We determined that 99.9% of stocked trout survived less than one year, with most mortality taking place within two months of stocking. Our results indicated that even during the period where trout stocking impacts are likely to be most pronounced, responses of invertebrate taxa were highly variable. By contrast, community level responses appeared to differ from taxon level responses. When compared to land use differences, trout stocking appears to have a lesser impact. Invertebrate communities in our study streams may have been more resilient to the effects of trout stocking than systems in other studies because they coevolved with a native salmonid predator (Salvelinus fontinalis) and a diverse piscivorous fish assemblage.