Challenges for Diadromous Fishes in a Dynamic Global Environment

Linkages with Ecosystem Energetics Preamble

Richard A. Cunjak

doi: https://doi.org/10.47886/9781934874080.ch24

The original intent of this session was to focus on ecosystem-level impacts of diadromous fishes, specifically the historic versus current extent of diadromy, linkages between diadromous fish and managed fisheries, the keystone role of some of these species, and the nutrient contributions of diadromous fish to freshwater and terrestrial ecosystems (Willson et al. 1998; Naiman and Latterell 2005). The six papers herein certainly demonstrate such breadth. Geographically, the work encompasses the rich foraging regions of the North Atlantic Ocean: freshwater systems of two continents (Europe and North America), rivers running through coastal rainforests, temperate and boreal zones, as well as northern (Arctic) ecosystems. Taxonomically, the papers focus on a variety of diadromous species that include salmonids, alosids, osmerids, and petromyzontids. In terms of temporal range, the research reflects trends from the past decade to millennial time scales.

Unlike the situation 20 years ago when the first diadromous symposium was held, scientists now commonly use novel technologies and approaches such as stable isotope analysis (SIA) to provide insight into the dynamics of marine-derived nutrients in freshwater systems (Wipfli et al. 2004; Hicks et al. 2005). This was obvious during our session where five of the six papers relied directly on stable isotope information to demonstrate the ecological processes central to their research objectives. For example, Sinnatamby et al. (2009, this volume) used SIA to determine if unsustainable exploitation by north Atlantic fisheries may have led to changes in marine food webs, forcing a trophic feeding shift in adult Atlantic salmon. Stable isotope signatures of the marine growth portion of scales showed no consistent patterns over time to support the hypothesis that the mean trophic level of adult Atlantic salmon Salmo salar has changed over the past several decades.

The specific mechanisms and pathways for ecosystem enrichment from marine-derived nutrients (MDNs) delivered by diadromous fishes remains a complex puzzle. The very thorough review by Naiman et al. (2009, this volume) clearly demonstrates the importance of MDNs to freshwater, estuarine, and riparian ecosystems in western North America. The influence of MDN on terrestrial plants and wildlife is especially fascinating. For example, the authors cite the work of Jauquet et al. (2003) who reported consumption of salmon by 10 bird species and four mammal species, including cougar Felis concolor, for a single stream in western Washington. For this reason, salmon have been described as keystone species significantly affecting coastal ecosystems (Willson et al. 1998).

The review paper by Gregory-Eaves et al (2009, this volume) is impressive in that diadromous fish (in this case, sockeye salmon Oncorhynchus nerka) population dynamics are reconstructed over millennial timescales using a multiproxy approach involving stable isotope analyses of lake sediments as well as paleolimnological indicators such as zooplankton and algal (mainly diatom) assemblages as proxies for sockeye salmon abundance in dated sediment cores. Such data have allowed paleolimnologists and fisheries scientists and resource managers to collaborate to test if climate-induced oceanographic changes may explain recent variation in salmon stock abundance by relating salmon historic salmon abundance to past climate regimes prior to anthropogenic impacts that would otherwise confound the interpretation.