Karen A. Shearer

Development and Test of a Whole-Lifetime Foraging and Bioenergetics Growth Model for Drift-Feeding Brown Trout

Abstract We developed and tested a combined foraging and bioenergetics model for predicting growth over the lifetime of drift-feeding brown trout. The foraging component estimates gross energy intake within a fish- and prey size-dependent semicircular foraging area that is perpendicular to the flow, with options for fish feeding across velocity differentials. The bioenergetics component predicts how energy is allocated to growth, reproduction, foraging costs, and basal metabolism. The model can reveal the degree to which growth is limited by the density and size structure of invertebrate drift within the physiological constraints set by water temperature. We tested the model by predicting growth based on water temperature and on drift density and size structure data from postemergence to age 12, and we compared the predictions with observed size at age as determined from otoliths and scales for a New Zealand river brown trout population. The model produced realistically shaped growth curves in relation to...

Targeted gene enrichment and high‐throughput sequencing for environmental biomonitoring: a case study using freshwater macroinvertebrates

Recent studies have advocated biomonitoring using DNA techniques. In this study, two high‐throughput sequencing (HTS)‐based methods were evaluated: amplicon metabarcoding of the cytochrome C oxidase subunit I (COI) mitochondrial gene and gene enrichment using MYbaits (targeting nine different genes including COI). The gene‐enrichment method does not require PCR amplification and thus avoids biases associated with universal primers. Macroinvertebrate samples were collected from 12 New Zealand rivers. Macroinvertebrates were morphologically identified and enumerated, and their biomass determined. DNA was extracted from all macroinvertebrate samples and HTS undertaken using the illumina miseq platform. Macroinvertebrate communities were characterized from sequence data using either six genes (three of the original nine were not used) or just the COI gene in isolation. The gene‐enrichment method (all genes) detected the highest number of taxa and obtained the strongest Spearman rank correlations between the number of sequence reads, abundance and biomass in 67% of the samples. Median detection rates across rare (<1% of the total abundance or biomass), moderately abundant (1–5%) and highly abundant (>5%) taxa were highest using the gene‐enrichment method (all genes). Our data indicated primer biases occurred during amplicon metabarcoding with greater than 80% of sequence reads originating from one taxon in several samples. The accuracy and sensitivity of both HTS methods would be improved with more comprehensive reference sequence databases. The data from this study illustrate the challenges of using PCR amplification‐based methods for biomonitoring and highlight the potential benefits of using approaches, such as gene enrichment, which circumvent the need for an initial PCR step.

Temporal and spatial quantification of aquatic invertebrate drift in the Maruia River, South Island, New Zealand

Abstract We investigated temporal (day‐to‐day and season) and spatial (reach) variability of drift with the aim of guiding sampling protocol for quantifying drift at the whole river or reach scale. Overall, we found aquatic drift density and biomass varied considerably seasonally (CV = 72.9, 88.1) and to a lesser extent spatially (CV = 31.3, 30.7) and from day‐to‐day (CV = 45.2, 39.4). Although spatial and day‐to‐day variation in drift density and biomass were similar, sampling logistics suggest spatial sampling would be more cost‐effective and less time consuming. Drift density and biomass estimated from top samplers was often higher than estimates from samplers near the streambed or mid‐water column. A reliable estimate of mean densities and biomass at a site may require only two samplers— a top sampler and either a middle or bottom sampler. In our study, we calculated that sampling at four sites over 1 or 4 days at one site would be required to obtain a 95% CI within 50% of the mean drift density. Eight sites over 1 or 10 days at one site would be required to achieve a 95% CI within 25% of the mean drift density.

Relationships between drifting and benthic invertebrates in three New Zealand rivers: Implications for drift‐feeding fish

Abstract We assessed whether taxonomic structure and density of aquatic drift could be predicted from the benthos in three New Zealand rivers. The three main orders contributing to both the benthos and drift were Ephemeroptera, Diptera, and Trichoptera. Drift and benthic densities for all taxa and all rivers combined were not significantly correlated (adults inclusive and exclusive). There were significant positive correlations between benthic and drift densities for the three main drifting orders—Ephemeroptera, Diptera, and Trichoptera when data from all rivers were combined. However, these relationships were not always detected in individual rivers. The propensity for Deleatidium to drift was negatively related to chlorophyll a concentration; suggesting density‐dependent drift mediated by food limitation. Drift was reduced when periphyton chlorophyll a concentration was high in relation to benthic Deleatidium density. This highlights an unexpected effect of periphyton proliferation on invertebrate drift and drift‐feeding fishes. Despite finding some correlations between benthic and drifting communities, defining general relationships between benthic and drifting communities is challenging given the complexity of density‐dependent and density‐independent mechanisms that influence invertebrate drift.

Can Weighted Useable Area Predict Flow Requirements of Drift-Feeding Salmonids? Comparison with a Net Rate of Energy Intake Model Incorporating Drift–Flow Processes

AbstractWe compared a process-based invertebrate drift and drift-feeding net rate of energy intake (NREI) model and a traditional hydraulic-habitat model (using the RHYHABSIM [River Hydraulics and Habitat Simulation] software program) for predicting the flow requirements of 52-cm Brown Trout Salmo trutta in a New Zealand river. Brown Trout abundance predicted by the NREI model for the constant drift concentration–flow scenarios were asymptotic or linear, depending on drift concentration, increasing through the mean annual low flow (MALF; 17 m3/s). However, drift concentration increased with flow, consistent with passive entrainment. The predicted fish abundance–flow relationship based on flow-varying drift concentration increased logistically, and more steeply, with flow through the MALF and beyond. Predictions for the relationship between weighted useable area (WUA) and flow were made for three sets of drift-feeding habitat suitability criteria (HSC) developed on three midsized and one large New Zealand ...

Habitat suitability curves for benthic macroinvertebrates from a small New Zealand river

We developed habitat suitability curves (HSC) using generalised additive models (GAMs) for nine benthic macroinvertebrate taxa from a small New Zealand river for hydraulic-habitat modelling assessments of instream flow requirements. We included interaction terms between the primary variables (water depth, velocity, substrate) when significant, to address a longstanding criticism of univariate HSC. To date, only large-river univariate HSC have been available and these have been used in hydraulic-habitat applications on small rivers, despite doubt over the transferability of HSC between rivers of different size and type. We tested the outcome on the predicted abundance–flow relationship of applying the small-river habitat suitability GAMs versus large-river GAMs for two taxa on the same small river. We found the effects of flow allocation were overestimated by the large-river GAMs relative to the small-river GAMs. Further research to develop general HSC for categories of river size and type is needed to better inform hydraulic-habitat modelling applications.

Influences of geology and land use on macroinvertebrate communities across the Motueka River catchment, New Zealand

The influences of geology and land use on macroinvertebrate communities were investigated in the Motueka River catchment, New Zealand. Comparisons of functional feeding groups, multivariate community composition and biotic indices (MCI, QMCI, %EPT) were made between native forest stream reaches that differed in subcatchment geology (ultramafic, hard sedimentary, granite, gravel, karst) and between stream reaches surrounded by different land uses (native forest, exotic forest and pasture) within two of these geologies (granite, gravel). Differences in invertebrate community density and taxon richness were greater across the three land uses in the same geology than among the five geologies. Macroinvertebrate communities in streams flowing through native forest had greater numbers of shredders, and higher MCI scores than pastoral streams. Exotic forest macroinvertebrate communities were similar to the native forest communities in granite geology, but distinct from both native and pastoral communities in sites with underlying gravel geology. The results demonstrate that there is potential for underlying geology to influence the magnitude of land use impacts on invertebrate communities. The geologies within a catchment should be accounted for when considering the consequences of land development on river and stream communities.

Are aquatic invertebrate drift densities uniform? Implications for salmonid foraging models

Foraging models of drift-feeding fish usually assume that drift density is uniform at the reach or river scale (HUGHES & DILL 1990, HuGHES 1992a, b, HAYES et al. 2000). While this assumption simplifies model programming, if it is invalid, then substitution with defined relationships between drift densities and causal environmental variables may improve model performance and the accuracy of growth predictions. Such relationships may also benefit studies of recolonisation dynamics o f benthic invertebrates ( CoRKUM et al. 1977). SAGAR & GLOVA (1992) found up to 7-fold differences in drift densities in different-sized channels in the braided, gravel-bed Rakaia River in Canterbuty, South Island, New Zealand. If significant differences occur between adjacent channels, significant spatial variability in drift densities could also occur over the range of depths and velocities that exist within a single channel reach. The present research focuses on the development of spatially explicit trout distribution models covering a range of flows and extending the work of HUGHES (1992a,b), which assumed uniform drift densities and applied only to the flow at which measurements were taken. If drift densities do vary significantly within a reach, then improved accuracy in modelling positions of drift-feeding trout over a range of flows will require numerical definition of the underlying causes of spatial variability in drift densities/biomass. Many biotic and abiotic variables have been cited as factors influencing drift. These include current velocity or discharge, photoperiod, water chemistry, benthic densities, predators, and life-cycle stage (BRJTIAIN & En<ELAND 1988). The present study was designed primarily to collect data to develop and test spatially explicit drift dispersion models but it also allowed the examination of whether variability in drift densities or biomass was related to velocity or depth two easily measured variables that are key components of stream reach physical habitat characterisation, and particle transport in rivers. Methods