#48: Part 7, Mechanisms of change in fish populations: changing food resources
Dan Isaak
Part 7, Mechanisms of change in fish populations: Changing food resources
Hi Everyone,
The first law of thermodynamics isn’t quite the same thing as Einstein’s famous mass–energy equivalence equation but it tells us some useful things about how fish populations will have to respond to climate change. The first law states that the total energy of a system is constant; it can be transformed from one form to another, but cannot be created or destroyed. In our stream world, therefore, a fish eating a bug is simply one way that food energy is transformed (some fly fishermen would say “blessed”) as it moves through the system. If the energy available from those food resources changes, fish populations will have to adjust accordingly (typically through adjustments in size, growth, & density; blog #44). The attached paper by Railsback & Rose does a nice job of describing bioenergetics in trout populations (graphic 1) & calibrating a model to rainbow trout population growth at several stream sites.
Fish bioenergetics has its complexities to be sure, but in the grand scheme of things, is relatively well understood given that many key parameters can be directly measured in the laboratory and/or small scale field studies. The bigger challenge, at least for an ignorant fish guy like me, is understanding and being able to predict how climate change will affect fish food in streams across a range of spatial and temporal scales relevant to research and management. There are a variety of tools, datasets, and models out there now to do this for hydrology (blogs #20 and #21), stream temperature (blogs #7 and #25), stream geomorphology (i.e., slope, size, elevation) and landuse/land cover (e.g., NHDPlus (http://www.horizon-systems.com/nhdplus/), but nothing tells me how much food is moving through river networks, now, or in the future. Granted, it is a complex process that involves many linkages between streams and terrestrial environments across multiple levels of biological organization (individual, population, community) and every part of the system is being affected by the environmental trends associated with climate change (Blogs 10, 11, 13, 15, 16, 17, 18, 22, 23), which propagate through foodwebs (graphic 2; good review by Woodward & colleagues hyperlinked here: http://izt.ciens.ucv.ve/ecologia/Archivos/ECO_POB%202010/ECOPO7_2010/Woodward%20et%20al%202010_II.pdf )….BUT…there have to be key indicators that can be measured efficiently to provide information about food resources in streams. As the seminal paper by Vannote & colleagues on the River Continuum Concept (hyperlinked here: www.limnoreferences.missouristate.edu/assets/limnoreferences/Vannote_et_al_1980_RCC.pdf) & the more recent paper by Wipfli & Baxter (.pdf attached) make apparent, the necessary conceptual foundation has been built. That foundation needs to be parameterized with data at high resolutions across large enough scales that it’s useful for real world applications (graphic 3; good review Naiman & colleagues hyperlinked here: www.researchgate.net/publication/233794482_Developing_a_broader_scientific_foundation_for_river_restoration_Columbia_River_food_webs/file/9fcfd50b8f106afed0.pdf ).
So my (admittedly naïve) proposal and/or sets of questions are these… What are the core set of parameters that convey the most relevant information about food resources? Do empirical datasets already exist that could be used to start getting a more precise handle on spatial patterns in food resources throughout at least some river networks? Do standardized techniques exist that make it easy to process measurements of key parameters on-site? If so, could intensive field campaigns be conducted to target interesting landscapes and/or ecoregions as a means of developing relatively dense datasets in some areas? Are any of the key parameters water quality attributes, or could these attributes be used as surrogate measures for key parameters? One attractive feature of many water quality attributes is that they’re cheap to measure and standardized protocols already exist. Moreover, as the recent paper by Olsen & Hawkins (attached) illustrates, large databases exist in some areas that might be compiled & mined to make initial maps and/or guide additional sampling surveys.
Once we have many samples of key parameters throughout river networks, it should be straightforward to use current geospatial technologies (e.g., remote sensing, GIS) and/or the spatial statistical network models (blogs #27, #28, #29) to interpolate among measurements and develop “smart maps” like those now being done routinely for stream temperatures at broad scales & high resolutions (blogs #26 and #40). Yes, some key food resource parameters will be too labor intensive or expensive to measure at very many sites (which is why we need surrogates), and yes, we’ll have to revisit a subset of sites to understand temporal variation in spatial patterns, but it all seems eminently doable.
Good stream fish food maps would not only provide important information for understanding the distribution & abundance of many fish species but would also create a bridge to the terrestrial realm. Continuous food maps would enable cross-referencing stream patterns with those in the surrounding environment so these linkages could be studied at a variety of spatial scales & proximities (e.g., riparian zone vs full watershed) to the channel network. Knowing these linkages intimately would yield a deeper understanding of how streams reflect their watersheds, which would ultimately enable better diagnosis and treatment of the ills that climate change may visit upon fish.
Until next time, best regards,
Dan