Natural Hazards, Stock Depletion, and Stock Management in the Southern Gulf of Mexico Pink Shrimp Fishery

Abstract

The southern Gulf of México has historically sustained important fisheries, particularly shrimp. From the mid-I950s to earl 1970s, annual yields of shrimp averaged about 27,000 rnetric tons (rnt), of which the pink shrirnp Farfantepenaeus duorarum contributed more than 80%. At that time, three fleets, from the United States, Cuba, and Mexico, exploited the stock. Pinj shrim captures have declined from the mid-1970s to the present level of about 1,000 mt per year, indicating severe stock depletion. A monotonically decreasing recluitment rate, beginning in the early 1970s, was identified through reconstruction of the stock, based on age-structured analysis. At the beginning of the 1980s, total fishing effort decreased more than 40% because U.S. and Cuban fleets retired from this fishing ground, and about 50% of the Mexican fleet stopped operations because boat ownership was transferred from the private sector to fishing cooperatives. During this time, recluitment continued decreasing but at a slowe rate. Because of the high yields obtained during the 1950s and 1960s, depletion was interpreted as caused by overfishing, especially since juveniles and been intensively exploited in coastal areas, another hypothesis to explain the drop in recluitment was the sart of oil industry opeations in the 1980s; this theory was subsequently discarded because the recruitment rate reduction starded in the early 1970s. In addition to he Ixtoc I oil spill in 1979, several natural events during the 1980s and 1990s coincided with the decrease in recruitment rate, including three high-impact hurricanes. Also, during this time, ash ejected by the Chichonal volcano covered large expanses of coastal sea grasses, which sever as shimp nursey areas. Using trends in recruitmen rate anomalies, we identified recruitment failures caused by these natural hazards, but the time series showed that once the effect was removed, the ping shirimp stock responded towards recovery, so these events alone do not explain the stock depletion. Recruitment anomalies were strongly associated with changes in regional primary production, and both series showed a negative shift in the mid-1980s. We concluded that there are two time frames -short-term impacts caused by hurricanes, the volcanic eruption, and even oil spill; and a long-term effect associated with decreasing primary production. We found that this long-term effect is heavily influenced by realted environmental changes, such as increases in water level and temperature, a decrease in salinity, the low intensity of turbulence in this area, the absence of river dischanges that promote primary production, and the low-energy hydrodynamics of the southern Gulf of Mexico. We suggest that the decrease in recruitment rate was strongly influenced by a decrease in salinity and the decrease in primary productivity, wich in the main source of food for the shrimp. This discovery changed the objetives of management policy. Previously, the main objetive was stock recovery, but now management policy is aimed at stock maintenance, especially of reproductive females, while waiting for a favorable change in productivity levels