Climate change and shrinking fishes
Anthropogenic global warming is everywhere. Nearly every week, new research with impacts of a warming Earth gets published. Global climate change is recognized as an important determining factor for the future distributions of marine organisms, notably fishes and invertebrates. It had already been suggested that the most prominent biological responses are changes in distribution, phenology, and productivity.
More specifically, using temperature data since 1960, last year an international team of researchers found that life under water must migrate several hundreds of kilometers to keep up with the optimal temperature, resource necessities and seasonal conditions (1). The waters around the equator are warming faster than elsewhere, initiating marine organisms to migrate toward cooler areas.
Marine refugees from warmer regions overwhelm the polar seas; scientists do not know whether these migrating organisms from around the equator will be replaced and, if so, by which new life forms. A study by William Cheung of the University of British Columbia and colleagues, recently published in Nature Climate Change (2), adds data to this finding: for the first time a speed is proposed for the poleward shift of fish populations: ~ 30 km/decade by 2050.
The study also reports the effect of climate change on the body size of aquatic animals. This feature is most strongly affected by temperature, oxygen levels, and resource availability. Using the Dynamic Bioclimate Envelope Model (DBEM), the biological responses of 610 fish species were studied, one of which is the effect on body size.
The first step in this model is to infer, for a given species, a bioclimate envelope based on its current distribution. Bioclimate envelopes are characterized by seawater temperature, underwater depth, habitat, and distance from sea ice. Secondly, the DBEM predicts the shifting of the bioclimate envelope for the current species, based on changes in climate variables.
By specifically focusing on the maximum body weight, which is limited by the balance between energy demand and supply (net growth = 0), several observations and assumptions have been made. Over 75% of the studied populations are expected to experience a 5-39% reduction of their maximum body weight. This reduction will be larger for fishes in the Pacific and Southern oceans, as result of the higher rates of warming and reduction in oxygen content.
A study like this, with its simplifications and shortcomings, does not provide the exact truth for what is going on below the ocean’s surface. However, it may function as the foundation for future work – as the ultimate concern is to understand these complex biological systems and to evaluate the effects of climate change upon marine populations of fish and invertebrates.
A 2008 laboratory research pointed out that cumulative effects of multiple stressors are synergistic (3). Generally it is true that the more stressors present, the more synergistic the cumulative effects will be – suggesting that synergies are common in natural settings where more than two stressors almost always coexist. Consequently, other human impacts, like the overexploitation of marine stocks (as we are ‘fishing down the food web’, 4) and pollution with persistent chemical contaminants, are likely to further intensify the reduction in body size.
(1) Burrows MT, Schoeman DS, Buckley LB, Moore P, Poloczanska ES, Brander KM, Brown C, Bruno JF, Duarte CM, Halpern BS, Holding J, Kappel CV, Kiessling W, O’Connor MI, Pandolfi JM, Parmesan C, Schwing FB, Sydeman WJ, Richardson AJ (2011) The Pace of Shifting Climate in Marine and Terrestrial Ecosystems. Science 334 (6056): 652-655.
(2) Cheung WWL, Sarmiento JL, Dunne J, Fröhlicher TL, Lam VWY, Deng Palomares ML, Watson R, Pauly D (2013) Shrinking of fishes exacerbates impacts of global ocean changes on marine ecosystems. Nature Climate Change 3: 254-258.
(3) Crain CM, Kroeker K, Halpern BS (2008) Interactive and cumulative effects of multiple human stressors in marine systems. Ecology Letters 11 (12): 1304-1315.
(4) Pauly D, Christensen V, Dalsgaard J, Froese R, Torres F (1998) Fishing down marine food webs. Science 279: 860-863.