Trade-offs in the response of mayflies to low oxygen and fish predation

Predator or prey? The dive response to aerial and aquatic predators of Arafura filesnakes

In air-breathing aquatic animals, predation is a strong selection pressure that may be reduced by modification to diving patterns. The risk of predation increases with time spent at the surface, thus shorter, more frequent surfacing events or longer, less frequent surfacing events would decrease predation risk. A reduction in time spent on the surface can be achieved by use of bimodal respiration, which is an ability to extend dive duration using dissolved oxygen to supplement aerially acquired oxygen. Air is a more efficient respiratory medium; however, under predation pressure, the cost of surfacing increases and the reliance on aquatic gas exchange should therefore increase. We tested whether the bimodally respiring filesnake (Acrochordus arafurae) changed its diving behaviour under simulated aerial (model bird) and aquatic (large fish) predation. Aerial predation did not alter dive or surface duration, percentage time surfacing or activity. However, a greater number of longer dives were observed with fewer long surface intervals, suggesting an increase in the use of aquatic gas exchange. The diel diving patterns (short night dives, long day dives) may provide an in-built antipredatory response to aerial predation. The threat of aquatic predation produced atypical antipredator behaviour, with longer surface intervals, shorter dives and increased activity, indicating that piscivorous filesnakes may have identified the predatory fish as prey rather than a predator.

Management issues in aquatic macrophyte ecology: a Canadian perspective

During the past few decades, attempts to manage excessive abundance of freshwater macrophytes, decreasing abundance of marine eelgrass (Zostera marina L.), and abundance and diversity of seaweeds have yielded modest successes, some short-term recoveries, and other complete failures. As many broad principles apply to the management of all aquatic plants, the aim of this paper was to synthesize the issues and provide direction for the management of freshwater rooted macrophytes, eelgrass, and marine macrophytic algae and place them in a Canadian context. Specifically, we examine biomass and landscape objectives for macrophyte management, assess the role of environmental manipulation for the management of macrophyte assemblages, and provide direction for managing macrophyte assemblages within the context of ecosystem sustainability. Finally, we advocate that, given the unexpected outcomes that may arise from uncontrolled events (e.g., weather, inherent variability in life history characteristics), management interventions should be designed in such a way that it is possible to make valid conclusions about the outcome of any given management action (i.e., design the management action to be a scientific experiment).Key words: macrophyte, algae, eelgrass, herbivores, nutrients, landscape ecology.