Designing flow regimes to support entire river ecosystems

Overcoming the challenges of water scarcity will require creative approaches to flow management and modeling approaches that forecast the effects of management actions on multiple ecosystem components simultaneously. Using a mechanistic multispecies modeling approach, we investigated the cross-ecosystem effects of environmental flow regimes designed for specific ecosystem outcomes. We reveal tradeoffs associated with flow regimes targeting riparian vegetation, fishes, and invertebrates. The different frequencies associated with each flow regime in some cases caused non-target ecosystem components to become locally extirpated within short (decadal) timespans. By incorporating multiple flow frequencies (from intraannual-scale pulses to large decadal-scale floods), the natural flow regime enabled a balanced but sub-optimal response of the three ecosystem components (mean 72% of designer flow). Although returning to a natural flow regime may not be possible in highly managed rivers, novel flow regimes must incorporate diverse frequencies inherent to such a regime and accommodate the sometimes conflicting requirements of different taxa at different times.

Dynamic of the Flemish Cap commercial stocks: use of a Gadget multispecies model to determine the relevance and synergies among predation, recruitment, and fishing

Multispecies modeling is being increasingly accepted in stock assessment, especially in the context of an ecosystem approach to fisheries management (EAF). To achieve a future implementation of an EAF in the Flemish Cap, we present a multispecies model developed in Gadget, which covers the main commercial stocks over the period 1988–2012: cod (Gadus morhua), redfish (Sebastes spp.), and northern shrimp (Pandalus borealis). The model highlights the interdependent dynamic of these stocks and reveals strong interactions among recruitment, fishing, and predation (including cannibalism). These drivers have shown marked changes in their relative importance by species, age, and length over time, producing a transition from a traditional redfish- and cod-dominated system in the early 1990s to an intermediate shrimp and other fish species state by the late 1990s and in turn back to something close to the initial state by the late 2000s. The multispecies model developed in this paper shows that disregarding the species interactions would lead to serious underestimates of natural mortality and overestimations of the exploitable biomass and highlights the need to move beyond single-species management in this highly coupled ecosystem.