Pressure indicators

Illegal harvesting

Illegal harvesting of biodiversity includes poaching, capture, and the unauthorized collection, transportation and trade of wildlife specimens (Semarnap, 1997). For marine turtles, the most common illegal harvesting practice is the capture of breeding adults and egg plundering at nesting beaches (CITES, 2001; Traffic, 2004; Bräutigam et al., 2006). Adults are also exploited for their leather to be used in the manufacture of boots and wallets; the tortoise shell is used for manufacturing ornamental items; and their eggs and meat are eaten either as delicacies or because of their attributed medicinal properties (CITES, 2001; Traffic, 2004). It has been shown that these activities have decimated many sea turtles populations around the world (Conabio, 1995; Traffic, 2004). Although the impacts on the populations are not fully understood, it is recognized that illegal harvesting promotes the reduction of the gene pool, altered sex ratios and, overall, reduces the long-term population variability; this is exacerbated by the marine turtle’s life cycle characteristics, such as slow growth, late sexual maturity and low reproductive rates. The indicator Number of marine turtle specimens and eggs seized denotes the minimum detectable pressure that illegal harvesting imposes on wild marine turtle populations in Mexico.

Bycatch

The poor selectivity of traditional fishing gear leads to the incidental capture of many species that bear little or no commercial value. In addition to a wide range of fish and invertebrates, threatened species including whales, sharks and marine turtles are caught (Alverson et al., 1994; Lewison et al., 2004; IATTC, 2007; FAO, 2009). As for marine turtles, they are incidentally caught in a variety of fisheries using a diversity of fishing gear: tuna fishing; swordfish and shark fishing with longlines and gillnets; and shrimp trawls (Manjarrés et al., 2008) in addition to pitfall traps used for crab and lobster fishing (Brady and Boreman, 1997; García and Hall, 1997; Lewison et al., 2004; AIDA, 2007). Marine turtle mortality caused by these activities is enormous: it has been estimated that the world’s longline fleet killed around 200 000 loggerhead and 50 000 leatherback turtles in the year 2000 alone (Lewison et al., 2004). In this sense, bycatch can cause highly significant reductions in the populations of marine turtles. The indicator National catch of tuna, shrimp, shark and lobster indirectly denotes the pressure that bycatch might impose on marine turtle populations.

Coastal Development

Population growth in coastal zones impacts marine turtles through the degradation of water quality and their nesting areas. Water quality degradation occurs as the volume of municipal waste being discharged into the water increases; in addition to containing an excess of nutrients and pollutants, municipal waste may include discarded polythene and other plastics that, upon ingestion, disrupt the digestive system and might cause death (NOAA, 2008). Habitat quality of nesting beaches is affected by infrastructure construction and maintenance (e. g., works to protect the beach from wave erosion), transit of heavy vehicles, introduction of exotic vegetation (e. g., trees and shrubs to provide shade in the beach) and light pollution from artificial lighting (NOAA, 2008). These pressures might lead to a poor reproductive success, mainly through failures in nesting and egg hatching or difficulties for hatchlings to return to the sea (NOAA, 2008). The indicator Population growth in coastal zones harboring marine turtle nesting beaches indirectly denotes the level of pressure that population growth might impose on marine turtle populations nesting on Mexican beaches. However, as the data available lack the detail necessary to estimate population growth rates specific for the country’s coastal zones where marine turtles nest, this indicator is not included in this report. Population growth rate is one of the pressure indicators included in the UN’s (ONU, 2007) and OECD’s (OCDE, 2008) lists of Sustainable Development Indicators; here it is focused on the country’s coastal zones.