Over the past three decades, the waters of the Northeast Atlantic have warmed faster than the global average resulting in exaggerated changes in the distribution and abundance of fish species in this area. Climate change is expected to change productivity of fisheries in the future but ecosystem specifics are not well understood. Many populations, especially poorer populations, rely on marine fish as a main source of protein, making these populations especially vulnerable to the decline in the productivity of fisheries. Because of the complex impacts of climate change on marine ecosystems, it is challenging to predict responses at all ecological levels and spatial scales.
Climate change influences fishery production through effects on primary production, food web interactions, and the life history and distribution of target species. Changes in primary production are strongly influenced by changes in the physical and chemical environment, while changes in the food web are also influenced by primary production. Blanchard et al. (2012) combine physical-biogeochemical and size-structured community models with temperature effects to project future effects of climate change on fish biomass and production in 11 large regional domains which include most productive areas of the shelf seas. Changes in fish production are shown to be most strongly influenced by phytoplankton production. Blanchard et al. predict potential declines in fisheries production to be 30–60%, most notably in the Indo-Pacific, and the production of pelagic predators to increase by 28–89%.—Evelyn Byer.
Blanchard, J.L., Jennings, S., Holmes, R., Harle, J., Merino, G., Allen, J.I., Holt, J., Dulvy, N.K., Barange, M., 2012. Potential consequences of climate change for primary production and fish production in large marine ecosystems. Philosophical Transactions of the Royal Society B: Biological Sciences 367, 2979–2989.
Blanchard and colleagues from the United Kingdom coupled physical-biogeochemical models to predict climate change scenarios. Exclusive economic zones (EEZs) were deemed to be most relevant for predicting changes in fisheries, as the majority of the global fish catch is taken from EEZs and most of the marine primary production occurs in these areas. Forcing data from a global ocean assimilation, a re-analysis simulation, and an atmospheric re-analysis dataset enabled the outputs from the coupled model to be evaluated against oceanographic and fisheries data for the same period of time. Nutrient input from rivers was provided by the Global Nutrient Export from Water Sheds model. Data from the IPCC SRESAI1B ‘business as usual’ emissions simulation and a simulation forced with trace gases set to 1980 values from the Institut Pierre Simon Laplace Climate Model were also used.
The size-structured dynamics of marine animal communities were modeled using a previously published size-based model modified to incorporate a temperature effect on the feeding and intrinsic mortality rates of organisms. The model focuses on pelagic predators and benthic detrivores and is concerned with a continuous function that gives the density per unit mass per unit volume for organisms of mass m at time t. The feeding rate for the pelagic community is dependent on the preference for prey, the volume of water searched per unit time, and the amount of suitably sized food available. Contrastingly, feeding rate for benthic consumers is not dependent on prey size because the majority of benthic organisms feed on detritus. Instead, the benthic feeding rate is dependent on available biomass density of detritus. A temperature effect on feeding and intrinsic mortality rates was also incorporated into the model to enable the effects of changes in temperature to be assessed.
To validate the predictions of their models, Blanchard et al. used data from the Ocean and Atmospheric re-analysis datasets from 1992–2001. Fish production estimates were compared with national catch statistics from the United Nations Food and Agriculture Organization database.
The Blanchard et al. models predict both positive and negative responses in fish biomass density and production that mirror potential changes in primary production more strongly than changes in temperature. An advantage of this model over previous models is the inclusion of fishing effects, enabling the relative effects of climate change and fishing to be explored within and across size-structured ecosystems. Either low primary production or cold-water ecosystems conferred higher susceptibility to fishing effects, due to slow relative growth rates. Also, heavily fished ecosystems were less resilient to climate change compared to unexploited states because of reductions in size structure and higher induced growth rates. For 1992–2001 in analysis, the models generated catches and growth rates that were realistic, if it is taken into consideration that catch data can be misreported and true community-wide fishing mortality rates are not well known. A drawback of these models is that other sources of primary production besides phytoplankton are not incorporated. Blanchard et al. hope that these models will help assess the vulnerabilities of certain areas to climate change and that the models may be useful for establishing levels of threat and uncertainty in specific regions. The results most likely underestimate the effects of climate change on marine ecosystems and a greater understanding of the specific effects of climate change on the ocean will help to improve prediction models.