Why It Is Time to Put PHABSIM Out to Pasture

Abstract

The Physical Habitat Simulation System (PHABSIM) was developed in the 1970s to fill an important void in instream flow assessment. Although considerable progress has been made in ecological modeling since the 1970s, there has been little change in instream flow assessment. PHABSIM has two general problems. First, PHABSIM is a habitat selection model (HSM)—but not a good one: it no longer conforms to standard practices in the wider fields of ecological and wildlife modeling, especially by using inappropriate spatial scales and outdated methods for modeling habitat preference and by producing output that lacks clear meaning. Second, HSMs, in general, are not well suited for many instream flow decisions. HSMs cannot consider variation in flow over time, whereas dynamic flow regimes are now considered essential, and HSMs do not make testable predictions of fish population responses. Alternatives to PHABSIM include analyses based on explicit understanding of species ecology, individual‐based models, and more powerful modern habitat selection modeling methods.

Environmental flows all at sea? Charting a new course through choppy waters

“…In an article published in Fisheries entitled, "Why it is time to put PHABSIM out to pasture", Steven Railsback (2016) criticizes the use of a tool widely adopted to help assign environmental flows and presents a case for why it is not fit for purpose in the current era of environmental modelling and management. Those that are familiar with PHABSIM (Physical Habitat Simulation) will be aware that it has been used by regulatory agencies for decades, particularly in the United States, to manage flows in modified fluvial systems, and today continues to be adopted in many other regions.…”

“…However, there are strong limitations to scientifically establishing thresholds which may be inherently stressful rather than fish habituation and compensation. Railsback (2016) also highlights the use of inappropriate scales, driven more by hydraulic modelling convenience than a justification based on an understanding of the ecology of the target species (note that the focus is on target species rather than communities may itself be criticized). Decisions on what scale to model at should depend on the appropriateness to the species and the purpose of the model.…”

“…So let us focus on the big ones. Railsback (2016) highlights the lack of accommodation of discharge variability, while inclusion of dynamic regimes is now considered an essential component of modern flow modelling. Indeed, a bias towards considerations of ecological response as a function of flow magnitude, rather than dynamics, is a common problem in the environmental flows literature more generally.…”

“…Those that are familiar with PHABSIM (Physical Habitat Simulation) will be aware that it has been used by regulatory agencies for decades, particularly in the United States, to manage flows in modified fluvial systems, and today continues to be adopted in many other regions. The title of this editorial, combining and extending archaic nautical phrases, is based on an assumption that Railsback (2016) is correct, and that the field of environmental flows is in an uncertain position and at risk of becoming, or has for a long time been, lost in the face of troublesome and difficult times ahead. Here we summarize some of the concerns raised by Railsback (2016) and others and highlight potential alternative approaches that may represent a positive change in direction.…”

“…The title of this editorial, combining and extending archaic nautical phrases, is based on an assumption that Railsback (2016) is correct, and that the field of environmental flows is in an uncertain position and at risk of becoming, or has for a long time been, lost in the face of troublesome and difficult times ahead. Here we summarize some of the concerns raised by Railsback (2016) and others and highlight potential alternative approaches that may represent a positive change in direction. Our vision is that the Journal of Ecohydraulics will play an important role in facilitating the debate in this area and publishing the evidence needed to better develop policy and navigate a route through the challenges of environmental flow management.…”