Fish stranding in freshwater systems: Sources, consequences, and mitigation

Abstract

Fish can become stranded when water levels decrease, often rapidly, as a result of anthropogenic (e.g., canal drawdown, hydropeaking, vessel wakes) and natural (e.g., floods, drought, winter ice dynamics) events. We summarize existing research on stranding of fish in freshwater, discuss the sources, consequences, and mitigation options for stranding, and report current knowledge gaps. Our literature review revealed that ∼65.5% of relevant peer-reviewed articles were found to focus on stranding associated with hydropower operations and irrigation projects. In fact, anthropogenic sources of fish stranding represented 81.8% of available literature compared to only 19.9% attributed to natural fish stranding events. While fish mortality as a result of stranding is well documented, our analysis revealed that little is known about the sublethal and long-term consequences of stranding on growth and population dynamics. Furthermore, the contribution of stranding to annual mortality rates is poorly understood as are the potential ecosystem-scale impacts. Mitigation strategies available to deal with stranding include fish salvage, ramping rate limitations, and physical habitat works (e.g., to contour substrate to minimize stranding). However, a greater knowledge of the factors that cause fish stranding would promote the development and refinement of mitigation strategies that are economically and ecologically sustainable.

Introduction

Fish stranding is any event in which fish are restricted to poor habitat as a consequence of physical separation from a main body of water. This phenomenon can occur in both lentic and lotic environments and is caused by natural and anthropogenic processes that generally result in rapidly falling water levels. Arguably, the majority of stranding research to date has emanated from hydropower studies that have typically focused on quantifying and reducing mortality of salmonids during hydropeaking operations (Cushman, 1985; Fig. 1). Comparatively little is known about non-salmonid species or in other contexts, and in general little is known about the factors that are associated with stranding, making it difficult to develop mitigation strategies. Nonetheless, there are other examples of stranding studies in the literature from freshwater systems around the globe (e.g., billabongs in Australia [Ward, 1998], ship wakes in navigation canals in Europe [Wolter et al., 2004]).

With increasing levels of aquatic habitat alteration (Richter et al., 1997) and increased management of flows and water levels in freshwater systems (e.g., hydropower development [Bunn and Arthington, 2002, Nilsson et al., 2005], irrigation [Haag et al., 2010]), there is a need to understand the extent of stranding, the factors that contribute to stranding, and the consequences of stranding at various biological levels. Because fish stranding is a natural phenomenon in some systems (e.g., flood pulse concept; Junk et al., 1989), it may also play an important role in structuring aquatic systems (Junk et al., 1989) or even provide important sources of fish protein in developing countries (Martin et al., 2011). Despite the fact that fish stranding may be a significant issue, there is currently no synthesis of knowledge related to stranding in freshwater systems which makes it difficult to assess its relative threat to biodiversity or determine the need for mitigation strategies.

The objective of this paper is to generate a synthesis of knowledge related to the topic of fish stranding with a focus on freshwater systems. Specifically, we will: 1) characterize the literature on fish stranding using a quantitative literature review; 2) describe potential and documented sources of stranding; 3) summarize the factors affecting stranding rates; 4) discuss possible effects of stranding at organismal, population, community and ecosystem (including socio-economics) levels; 5) consider mitigation strategies that have been proposed and tested, and (6) identify knowledge gaps and suggest possible future research directions.