Ecology of stream-dwelling fishes in response to inter-annual variation in the abundance of spawning sockeye salmon
Bentley, Kale Tanner
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Each year, millions of salmon enter the final stage of their life-cycle and migrate back towards their natal watersheds to reproduce. After accumulating >95% of their adult body mass in marine environments, salmon spawning migrations generate massive fluxes of nutrients and energy to inland food webs that can exceed background levels of in situ productivity, and these resources are utilized by a wide range of taxa (Naiman et al. 2002, Gende et al. 2002, Schindler et al. 2003). However, one-half to three-fourths of all returning salmon are harvested by commercial fisheries in coastal oceans prior to reproducing, as salmon fisheries in Alaska are currently managed to maximize the long-term sustainable yield of salmon (Baker et al. 2009). Although this practice is widely touted as a fisheries management success story (Hilborn 2006), people are beginning to ask, what effect does removing the biomass of salmon prior to spawning have on freshwater and terrestrial ecosystems. Thus, there has been a call to shift the paradigm of fisheries management from one that focuses solely on maximizing the yield of single target species to a more holistic approach that accounts for other ecosystem processes (Pikitch et al. 2004, Crowder et al. 2008, Piccolo et al. 2009). In order to develop an ecosystem-based management approach, salmon managers need to be able to assess the trade-offs of different management scenarios that affect how many salmon are harvested versus released to the watershed (known as "escapement") to spawn and benefit inland ecosystems. Currently, assessing these trade-offs is difficult; while there is a well-established theory of how to optimize commercial catch based on stock-recruit relationships (Ricker 1954, Hilborn and Walters 1992, Quinn and Deriso 1999), we lack a quantitative understanding of how the number of salmon returning to spawn influences freshwater and terrestrial ecosystems. The focus of my thesis was to evaluate of the ecological response of Arctic grayling (Thymallus arcticus) and rainbow trout (Oncorhynchus mykiss), two species of resident fish that rely heavily on consumption of salmon resource subsidies (Scheuerell et al. 2007, Moore et al. 2008), to variation in the abundance of adult sockeye salmon (Oncorhynchus nerka) in the Wood River watershed, Bristol Bay, Alaska. The first chapter of my thesis assessed the ability of a body condition index to serve as a proxy for estimating individual instantaneous growth rates as obtaining direct measurements of growth can be time consuming, costly, and logistically impractical. We found that relative body condition of grayling and rainbow trout, as measured by the residuals around a length-mass regression, was strongly correlated with direct measures of individual instantaneous growth from recaptured tagged fish. Using the derived relationship between body condition and growth, we developed a model to estimate growth rates of individual fish based on their observed body condition. Chapter two evaluated the foraging and growth responses of grayling and rainbow trout in two streams that vary in in situ productivity to changes in the abundances of spawning sockeye salmon. Over 11 years, and across a greater than 10-fold variation in density of spawning sockeye salmon, both species of resident fish exhibited a relatively similar, but mechanistically different, saturating growth response to increasing salmon density. This growth response was driven by both an increase in consumption of salmon eggs and also a decrease in dietary overlap between the two species. However, the relative change in growth from low to high salmon densities was different between streams and depended on in situ stream productivity. In low salmon density years the growth of resident consumers fell 46-68% relative to high years in the low productivity stream, but only by 26-34% in the high productivity stream. Growth rates of both consumer species saturated in years when densities of sockeye salmon exceeded about 0.3 - 0.4 m2 on the spawning grounds. Chapter three evaluated the movement patterns of Arctic grayling and rainbow trout within and among streams, which offer patchily distributed foraging opportunities during the summer months. Across both years, approximately 50% of individual grayling and rainbow trout exhibited kilometer-scale movements among two or more streams across the river network within a single summer. Movements were concentrated in June and July, and subsided by early August, coincident with the arrival of spawning sockeye salmon (O. nerka). These inter-stream movements may represent prospecting behavior as individuals seek out the most profitable foraging opportunities. Thus, resident fishes in the Wood River system appear to use the broad network of habitat available to them across the riverscape, rather than depend on individual tributaries for achieving growth. Together the results of this thesis improve our understanding of how inland ecosystem respond to changes in salmon abundance. These results will be of use for resource managers interested in directly evaluating the socio-economic trade-offs of allocating salmon resources among user groups. The results of this work also highlight the importance of maintaining connectivity to enable movements of resident fish across river basins, the ecological consequences of which remain poorly understood.
- Fisheries