Observed controls on resilience of groundwater to climate variability in sub-Saharan Africa

Groundwater in sub-Saharan Africa supports livelihoods and poverty alleviation1,2, maintains vital ecosystems, and strongly influences terrestrial water and energy budgets. Yet the hydrological processes that govern groundwater recharge and sustainability—and their sensitivity to climatic variability—are poorly constrained4. Given the absence of firm observational constraints, it remains to be seen whether model-based projections of decreased water resources in dry parts of the region4 are justified. Here we show, through analysis of multidecadal groundwater hydrographs across sub-Saharan Africa, that levels of aridity dictate the predominant recharge processes, whereas local hydrogeology influences the type and sensitivity of precipitation–recharge relationships. Recharge in some humid locations varies by as little as five per cent (by coefficient of variation) across a wide range of annual precipitation values. Other regions, by contrast, show roughly linear precipitation–recharge relationships, with precipitation thresholds (of roughly ten millimetres or less per day) governing the initiation of recharge. These thresholds tend to rise as aridity increases, and recharge in drylands is more episodic and increasingly dominated by focused recharge through losses from ephemeral overland flows. Extreme annual recharge is commonly associated with intense rainfall and flooding events, themselves often driven by large-scale climate controls. Intense precipitation, even during years of lower overall precipitation, produces some of the largest years of recharge in some dry subtropical locations. Our results therefore challenge the ‘high certainty’ consensus regarding decreasing water resources in such regions of sub-Saharan Africa. The potential resilience of groundwater to climate variability in many areas that is revealed by these precipitation–recharge relationships is essential for informing reliable predictions of climate-change impacts and adaptation strategies

State of the climate in 2013

In 2013, the vast majority of the monitored climate variables reported here maintained trends established in recent decades. ENSO was in a neutral state during the entire year, remaining mostly on the cool side of neutral with modest impacts on regional weather patterns around the world. This follows several years dominated by the effects of either La Niña or El Niño events. According to several independent analyses, 2013 was again among the 10 warmest years on record at the global scale, both at the Earths surface and through the troposphere. Some regions in the Southern Hemisphere had record or near-record high temperatures for the year. Australia observed its hottest year on record, while Argentina and New Zealand reported their second and third hottest years, respectively. In Antarctica, Amundsen-Scott South Pole Station reported its highest annual temperature since records began in 1957. At the opposite pole, the Arctic observed its seventh warmest year since records began in the early 20th century. At 20-m depth, record high temperatures were measured at some permafrost stations on the North Slope of Alaska and in the Brooks Range. In the Northern Hemisphere extratropics, anomalous meridional atmospheric circulation occurred throughout much of the year, leading to marked regional extremes of both temperature and precipitation. Cold temperature anomalies during winter across Eurasia were followed by warm spring temperature anomalies, which were linked to a new record low Eurasian snow cover extent in May. Minimum sea ice extent in the Arctic was the sixth lowest since satellite observations began in 1979. Including 2013, all seven lowest extents on record have occurred in the past seven years. Antarctica, on the other hand, had above-average sea ice extent throughout 2013, with 116 days of new daily high extent records, including a new daily maximum sea ice area of 19.57 million km2 reached on 1 October. ENSO-neutral conditions in the eastern central Pacific Ocean and a negative Pacific decadal oscillation pattern in the North Pacific had the largest impacts on the global sea surface temperature in 2013. The North Pacific reached a historic high temperature in 2013 and on balance the globally-averaged sea surface temperature was among the 10 highest on record. Overall, the salt content in nearsurface ocean waters increased while in intermediate waters it decreased. Global mean sea level continued to rise during 2013, on pace with a trend of 3.2 mm yr-1 over the past two decades. A portion of this trend (0.5 mm yr-1) has been attributed to natural variability associated with the Pacific decadal oscillation as well as to ongoing contributions from the melting of glaciers and ice sheets and ocean warming. Global tropical cyclone frequency during 2013 was slightly above average with a total of 94 storms, although the North Atlantic Basin had its quietest hurricane season since 1994. In the Western North Pacific Basin, Super Typhoon Haiyan, the deadliest tropical cyclone of 2013, had 1-minute sustained winds estimated to be 170 kt (87.5 m s-1) on 7 November, the highest wind speed ever assigned to a tropical cyclone. High storm surge was also associated with Haiyan as it made landfall over the central Philippines, an area where sea level is currently at historic highs, increasing by 200 mm since 1970. In the atmosphere, carbon dioxide, methane, and nitrous oxide all continued to increase in 2013. As in previous years, each of these major greenhouse gases once again reached historic high concentrations. In the Arctic, carbon dioxide and methane increased at the same rate as the global increase. These increases are likely due to export from lower latitudes rather than a consequence of increases in Arctic sources, such as thawing permafrost. At Mauna Loa, Hawaii, for the first time since measurements began in 1958, the daily average mixing ratio of carbon dioxide exceeded 400 ppm on 9 May. The state of these variables, along with dozens of others, and the 2013 climate conditions of regions around the world are discussed in further detail in this 24th edition of the State of the Climate series. © 2014, American Meteorological Society. All rights reserved

Changes in population structure of the white grouper Epinephelus aeneus as a result of long-term overexploitation in Senegalese waters

In Senegal, a significant decrease in catches indicates that many demersal fish stocks are being overexploited. The white grouper Epinephelus aeneus, locally known as the 'thiof', is exploited by both small-scale and industrial fisheries. A 28-year database of E. aeneus catches along the Senegalese coast provided by the Centre for Oceanographic Research of Dakar-Thiaroye, and size at maturity measured in Dakar (Senegal) from monthly samples in 2010, were used to analyse changes in population structure in the area over the past 37 years. Catches from the northern fishing areas were lower than those from the southern fishing areas, and decreased steadily during the period (Kolmogorov-Smirnov test, D = 0.243, p = 0.0002). The individual mean weight of catches decreased from 1974 to 2010 (linear regression, r(2) = 0.40, n = 37) and only 60% of the individuals were mature. The calculated sizes at maturity were 49 cm total length (TL) for females and 55 cm for males, and the optimal length of capture for a sustainable fishery was 96 cm, but only 0.03% of E. aeneus caught reached this length. Most of the catch consisted of juveniles; the larger reproductive individuals had disappeared. The number of individuals caught decreased significantly between 1974 and 2010 (1974-1983, r(2) = 0.98, n = 74 674; 1984-1993, r(2) = 0.95, n = 96 696; 1994-2003, r(2) = 0.93, n = 12 619; 2004-2010, r(2) = 0.91, n = 12 887), whereas the length range remained the same (10-110 cm TL). Biological indicators clearly showed that E. aeneus stocks in Senegal are overexploited and the species is now endangered. Immediate active management of fishing pressure is needed, therefore, to maintain E. aeneus populations in the area. Our results suggest a minimum size of <50 cm should be introduced and that fishing effort should be reduced

Dynamics of Ethmalosa fimbriata in Southern Senegal [résumé]

ICAWA : International Conference AWA, Dakar, SEN, 13-/12/2016 - 15/12/2016The Bonga Ethmalosa fimbriata (Bowdich, 1825) is the most third small pelagic exploited in the Senegalese waters and is commonly consumed locally. Studies were conducted on the dynamics population of E. fimbriata. Monthly catches by landing sites of the Senegalese artisanal fisheries from 1996 to 2013 are provided by the CRODT. They are structured according to the fishing gear, per month and landing sites along the South Senegalese coast (between 14°36N to 13°36N and of 13°40N to 12°20 N). Growth parameters were determined using the length-frequency distribution per month over the period 2014-2015: the asymptotic length "Linfini"=38.2 cm total length, the asymptotic body weight Winfini= 956 g, the rate by which Linfini is approached K=0.48 and the theoretical age at zero length t0= -0.3; which are used to estimate the optimum length (Lopt) of a cohort and its fecundity are maximum at 28.9 cm for an age topt=2.8 years. We estimate the total mortality, Z=1.8 per year, natural mortality, M=0.4 per year and fishing mortality, F=1.4. The current rate of exploitation (E) was found at 0.32 and the yield per recruit (Y/R) was estimated at 0.32, indicating that E. fimbriata stock is not fully exploited. We report a seasonal variability in the recruitment and biomass of the Senegalese part of the stock, with a clear trend downward in the analysis period. The seasonal signal shows three phases: between 1996 and 2001, recruitments are higher (50%) and took place from November to March. Over the period 2002 to 2008 (27%), the maximum values are observed May to September; and finally between 2009 and 2013 (23%), the recruitment of E. fimbriata peak is observed in the same time of the first phase (November to March). E. fimbriata is not fully exploited, that suggest a reallocation of the fishing effort on this species to decrease fishing effort on other species highly exploited as Sardinella aurita and so contribute to a better fisheries management of small scale activities in this area. Moreover that will allow to fit to the theoretical maximum yield per recruit (Ypot) for different species targeted by the same fishermen can prevent stock collapse

Dynamics of Ethmalosa fimbriata in Southern Senegal [résumé]

ICAWA : International Conference AWA, Dakar, SEN, 13-/12/2016 - 15/12/2016The Bonga Ethmalosa fimbriata (Bowdich, 1825) is the most third small pelagic exploited in the Senegalese waters and is commonly consumed locally. Studies were conducted on the dynamics population of E. fimbriata. Monthly catches by landing sites of the Senegalese artisanal fisheries from 1996 to 2013 are provided by the CRODT. They are structured according to the fishing gear, per month and landing sites along the South Senegalese coast (between 14°36N to 13°36N and of 13°40N to 12°20 N). Growth parameters were determined using the length-frequency distribution per month over the period 2014-2015: the asymptotic length "Linfini"=38.2 cm total length, the asymptotic body weight Winfini= 956 g, the rate by which Linfini is approached K=0.48 and the theoretical age at zero length t0= -0.3; which are used to estimate the optimum length (Lopt) of a cohort and its fecundity are maximum at 28.9 cm for an age topt=2.8 years. We estimate the total mortality, Z=1.8 per year, natural mortality, M=0.4 per year and fishing mortality, F=1.4. The current rate of exploitation (E) was found at 0.32 and the yield per recruit (Y/R) was estimated at 0.32, indicating that E. fimbriata stock is not fully exploited. We report a seasonal variability in the recruitment and biomass of the Senegalese part of the stock, with a clear trend downward in the analysis period. The seasonal signal shows three phases: between 1996 and 2001, recruitments are higher (50%) and took place from November to March. Over the period 2002 to 2008 (27%), the maximum values are observed May to September; and finally between 2009 and 2013 (23%), the recruitment of E. fimbriata peak is observed in the same time of the first phase (November to March). E. fimbriata is not fully exploited, that suggest a reallocation of the fishing effort on this species to decrease fishing effort on other species highly exploited as Sardinella aurita and so contribute to a better fisheries management of small scale activities in this area. Moreover that will allow to fit to the theoretical maximum yield per recruit (Ypot) for different species targeted by the same fishermen can prevent stock collapse

International conference ICAWA 2015 : extended book of abstract : the AWA project : ecosystem approach to the management of fisheries and the marine environment in West African waters

International Conference AWA (ICAWA), Dakar, SEN, 17-/11/2015 - 19/11/201

International conference ICAWA 2015 : extended book of abstract : the AWA project : ecosystem approach to the management of fisheries and the marine environment in West African waters

International Conference AWA (ICAWA), Dakar, SEN, 17-/11/2015 - 19/11/201