The devil is in the details – the carbon footprint of a shrimp

www.frontiersinecology.org © The Ecological Society of America The devil is in the details – the carbon footprint of a shrimp Kauffman and colleagues (2017) reported new estimates of greenhousegas (GHG) emissions resulting from the conversion of mangrove forests into aquaculture ponds and concluded that 1603 kg of carbon dioxide equivalent (CO2e) are emitted for every kilogram of shrimp produced on lands formerly occupied by mangroves. The authors consequently argued for inclusion of land use and landuse change (LULUC) emissions in lifecycle assessments of shrimp. We share Kauffman et al.’s concern about mangrove forest loss, but we believe that their landuse carbon footprint for farmed shrimp has been overestimated. Two previous studies – conducted by various authors of this letter – found LULUCassociated GHG emissions from shrimp farming to be one to two orders of magnitude lower (Jonell and Henriksson 2015; Järviö et al. 2017) than that of the Kauffman et al. study. As explained by Kauffman et al. (2017), the carbon footprint of converting 1 ha of mangrove forest to 1 ha of aquaculture plot is dependent on data and assumptions with respect to several parameters. Although the three studies generated relatively similar estimates of carbon stocks (see next paragraph for details), model assumptions can substantially influence model output (Table 1). For instance, extensive shrimp farms generally coproduce several other valuable products, and the respective GHG emissions of those products should also be considered (ISO 2006). Jonell and Henriksson (2015) estimated a carbon stock of 406 metric tons of carbon per hectare (t C ha) down to 1m sediment depth based on a global estimate by Pendleton et al. (2012), and assumed 63% of that carbon to be oxidized into CO2 (with alternative values in the sensitivity analysis). Likewise, Järviö et al. (2017) concluded a total carbon stock of 724 t C ha down to 1.5 m depth based upon a review of geographically diverse sources from the literature, and assumed 55% of the belowground carbon to be oxidized (50% of sediments and 100% of roots). By way of comparison, Kauffman et al. (2017) measured carbon contents in mangrove forests in Mexico, Central America, and Indonesia, and reported values between 269 and 1663 t C ha down to 3m depth. They concluded a mean global carbon stock of 858 t C ha of mangrove forest, of which 91% and 54% of the aboveground and belowground carbon stocks, respectively, were assumed to react with oxygen during the conversion of mangroves to shrimp ponds. Extensive mangroveintegrated shrimp farms in Ca Mau, Mekong Delta (investigated by Jonell and Henriksson 2015 and Järviö et al. 2017) have been in operation since the early 1980s (Ha et al. 2012). These systems produce only 250–300 kg shrimp ha yr (Phan et al. 2011), resulting in large areas of land devoted to each kilogram of shrimp. However, besides the stocked Asian tiger shrimp (Penaeus monodon), large volumes of wild shrimp and crabs are also harvested (Jonell and Henriksson 2015). The lowest shrimp yield estimate cited by Kauffman et al. (45 kg shrimp ha; Bosma et al. 2012) was also from a system that coproduces other species, including milkfish (Chanos chanos; 375 kg), wild shrimp (Metapenaeus brevirostris; 160 kg), and crabs (mostly Scylla serrata; 11–80 kg), but Kauffman et al. did not account for such coproduction. In contrast, Jonell and Henriksson, as well as Järviö et al., resolved the coproduct issue using established allocation methods (ISO 2006). The assumed lifetime of shrimp ponds is important because emissions will be annualized or amortized over this time period (IPCC 2006). According to Kauffman et al., shrimp ponds are actively used only for between 5 and 10 years, with the final carbon footprint being amortized over 9 years. However, Jonell and Henriksson, as well as Järviö et al., both reported that farms could be used for at least 50 years. Interestingly, all three studies focused on “extensive” shrimp farming, systems that are less susceptible to disease outbreaks and therefore more resilient than “intensive” shrimp farming (Bush et al. 2010). The increased use of compound feeds, paddle wheels, alkalines, sediment drying/removal, probiotics, and improved water management has also helped enhance yields and prolong the longevity of shrimp farms (Lebel et al. 2010; Bosma and Verdegem 2011). Compound feeds WRITE BACK WRITE BACK WRITE BACK