A coupled stock-recruitment-age-structured model of the North Sea cod under the influence of depensation

Implications of Allee effects for fisheries management in a changing climate: evidence from Atlantic cod

There are concerns that increasing anthropogenic stressors can cause catastrophic transitions in ecosystems. Such shifts have large social, economic, and ecological consequences and therefore have important management implications. A potential mechanism behind these regime shifts is the Allee effect, which describes the decline in realized per capita growth rate at small population density. With an age-structured population model for Atlantic cod, Gadus morhua, we illustrate how interactions between human-induced stressors, such as fishing and climate change, can worsen the impact of an Allee effect on populations by promoting hysteresis. Therefore, the risk of population collapse and recovery failure is exacerbated and the success of preventing and reverting collapse depends on the climate regime. We find that, in presence of the Allee effect, a fishing moratorium is only sufficient for recovery when sea surface temperature rise remains within 2°C and fishing is restricted within 10 yrs. If sea surface temperature rises beyond 2°C, even immediate banning of fishing is not sufficient to guarantee recovery. If fishing is not fully banned and a residual fishing pressure remains, the probability of recovery is further decreased, also in the absence of an Allee effect. The results underscore the decisive role of Allee effects for the management of depleted populations in an increasingly human-dominated world. Once the population collapses and its growth rate is suppressed, rebuilding measures will be squandered and collapse will very likely be irreversible. We therefore emphasize the need for proactive management involving precautionary, adaptive measures and reference points. Our studies shows that climate change has the potential to strengthen Allee effects, which could increasingly challenge fisheries management.

Greenhouse Gas Mitigation in Chinese Eco-Industrial Parks by Targeting Energy Infrastructure: A Vintage Stock Model

Mitigating greenhouse gas (GHG) emissions in China's industrial sector is crucial for addressing climate change. We developed a vintage stock model to quantify the GHG mitigation potential and cost effectiveness in Chinese eco-industrial parks by targeting energy infrastructure with five key measures. The model, integrating energy efficiency assessments, GHG emission accounting, cost-effectiveness analyses, and scenario analyses, was applied to 548 units of energy infrastructure in 106 parks. The results indicate that two measures (shifting coal-fired boilers to natural gas-fired boilers and replacing coal-fired units with natural gas combined cycle units) present a substantial potential to mitigate GHGs (42%-46%) compared with the baseline scenario. The other three measures (installation of municipal solid waste-to-energy units, replacement of small-capacity coal-fired units with large units, and implementation of turbine retrofitting) present potential mitigation values of 6.7%, 0.3%, and 2.1%, respectively. In most cases, substantial economic benefits also can be achieved by GHG emission mitigation. An uncertainty analysis showed that enhancing the annual working time or serviceable lifetime levels could strengthen the GHG mitigation potential at a lower cost for all of the measures.

Spatio-temporal variability of the North Sea cod recruitment in relation to temperature and zooplankton

The North Sea cod (Gadus morhua, L.) stock has continuously declined over the past four decades linked with overfishing and climate change. Changes in stock structure due to overfishing have made the stock largely dependent on its recruitment success, which greatly relies on environmental conditions. Here we focus on the spatio-temporal variability of cod recruitment in an effort to detect changes during the critical early life stages. Using International Bottom Trawl Survey (IBTS) data from 1974 to 2011, a major spatio-temporal change in the distribution of cod recruits was identified in the late 1990s, characterized by a pronounced decrease in the central and southeastern North Sea stock. Other minor spatial changes were also recorded in the mid-1980s and early 1990s. We tested whether the observed changes in recruits distribution could be related with direct (i.e. temperature) and/or indirect (i.e. changes in the quantity and quality of zooplankton prey) effects of climate variability. The analyses were based on spatially-resolved time series, i.e. sea surface temperature (SST) from the Hadley Center and zooplankton records from the Continuous Plankton Recorder Survey. We showed that spring SST increase was the main driver for the most recent decrease in cod recruitment. The late 1990s were also characterized by relatively low total zooplankton biomass, particularly of energy-rich zooplankton such as the copepod Calanus finmarchicus, which have further contributed to the decline of North Sea cod recruitment. Long-term spatially-resolved observations were used to produce regional distribution models that could further be used to predict the abundance of North Sea cod recruits based on temperature and zooplankton food availability.