BULLETIN OF MARINE SCIENCE, 79(2): 259–271, 2006 SPatial Patterns of DensitY anD SIZE structure of PenaeiD SHrimPS FARFANTEPENAEUS BRASILIENSIS anD FARFANTEPENAEUS NOTIALIS in A HYPersaline laGoon in THE YucatÁN Peninsula, MEXico Marco Antonio May-Kú and Uriel Ordóñez-López ABSTRACT Spatial variation in the density and size structure of penaeid shrimps Farfante- penaeus brasiliensis (Latreille, 1817) and Farfantepenaeus notialis (Pérez-Farfante, 1967) was investigated in Río Lagartos, a coastal lagoon with hypersaline conditions (mean salinity 58.1). We evaluated the influence of salinity, temperature, and re- cruit density on the density of shrimps. A total of 2060 shrimps belonging to three Farfantepenaeus species was collected on a monthly basis from November 1996 to April 1997. Of the 1349 identified shrimps, F. brasiliensis was the dominant species, accounting for 79.5% of the total catch followed by F. notialis (17.5%) and Farfante- penaeus duorarum Burkenroad, 1939 (3.0%). The remaining 34.5% were small un- identified shrimps classified as recruits (i.e., < 6.0 mm CL). Shrimps were collected only in hydrological zones with a mean salinity < 50 and were completely absent in the innermost zones (mean salinity > 60). We found a common pattern of density and size structure for penaeid shrimp species: decreasing densities and increasing sizes from outer (near sea inlet) to inner zones. Multiple-regression analysis indi- cated that salinity was the most important hydrological variable in the F. brasilien- sis, F. notialis, and recruit catch models, showing a strong negative relationship with density. Recruit density also significantly influenced the density of F. brasiliensis and along with salinity explained 42% of the overall variation of this species. The preference of penaeid shrimps for zones proximal to the sea inlets has implications for management strategies designed to protect this resource during the artisanal shrimp-fishing season. Four sympatric penaeid shrimp species of the genus Farfantepenaeus Burukovsky, 1997 have been reported in coastal lagoons in the Yucatán Peninsula: the brown shrimp Farfantepenaeus aztecus (Ives, 1891), the pink spotted shrimp Farfante- penaeus brasiliensis (Latreille, 1817), the pink shrimp Farfantepenaeus duorarum (Burkenroad, 1939), and the southern pink shrimp Farfantepenaeus notialis (Pérez- Farfante, 1967) (Gracia et al., 1997; Pérez-Castañeda and Defeo, 2001). The adults of these species spawn in offshore waters and the eggs, after hatching, develop through a series of larval stages before becoming postlarvae. The larvae are advected towards the shore by hydrodynamic processes and settle as benthic postlarvae in the juvenile habitats (Dall et al., 1990, Vance et al., 1996). This transition to the benthos (or settle- ment) occurs primarily in structurally complex habitats such as seagrass meadows or algal beds. In these habitats the patterns of density and size structure of penaeid shrimps provide an indicator of the nursery habitat value (Minello and Zimmerman, 1991; Beck et al., 2001). The settlement process is a critical period in the life cycle of shrimps, and densities of benthic postlarvae and recruits can be the most impor- tant factor in determining the density of subsequent life history stages (Vance et al., 1996, 1998; Meager et al., 2003; Pérez-Castañeda and Defeo, 2005). Furthermore, salinity, aquatic vegetation, and distance from the sea inlet are key factors affecting shrimp distribution in juvenile habitats (Zimmerman et al., 1990; Howe and Wallace, Bulletin of Marine Science 259 © 2006 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 260 BULLETIN OF MARINE SCIENCE, VOL. 79, NO. 2, 2006 2000; Hass et al., 2001; Pérez-Castañeda and Defeo, 2001; 2004; Ahmad-Adnan et al., 2002). Hypersaline lagoons occur over a very limited spatial extent in arid and semi-arid regions characterized by low rainfall and high evaporation rates. Salinities in these lagoons are typically > 40 as a result of their minimal exchange with the sea, high wa- ter residence time, and low water flux (Britton and Morton, 1989;H errera-Silveira et al., 1998). In these lagoons the density and diversity of flora and fauna decrease as sa- linity increases, a result of a reduced ability to respond to osmotic stress experienced under hypersaline conditions (Britton and Morton, 1989; Herrera-Silveira et al., 1998; Ortegón-Aznar and González-González, 2000; Vega-Cendejas and Hernández de Santillana, 2004). There are few studies documenting the shrimp fauna in these hy- persaline lagoons and the environmental factors affecting their spatial distributions. The Laguna Madre in Tamaulipas, Mexico, one of the largest hypersaline lagoons in the world, is the main juvenile habitat for F. aztecus, which has been associated with shoalgrass Halodule wrightii Ascherson, 1868. This species, along withF. duorarum, has been collected in salinities > 60 (Britton and Morton, 1989). The Río Lagartos lagoon, located in the northeastern Yucatán Peninsula, is des- ignated a Special Biosphere Reserve. It is the largest coastal lagoon in Yucatán State with hypersaline conditions (mean salinity 57; Herrera-Silveira et al., 1998). This la- goon supports a small but valuable artisanal shrimp fishery, principally exploited between November and February, when hundreds of fishermen participate (SEMAR- NAP, 1998–2001). This fishing period coincides with the major recruitment peak for penaeid shrimps in the Yucatán Peninsula (Pérez-Castañeda and Defeo, 2001). How- ever, there is a lack of data for shrimps in the Río Lagartos lagoon. In this study we describe the spatial pattern of density and size structure for two sympatric penaeid shrimp species (F. brasiliensis, F. notialis) as it relates to salinity, temperature, recruit density from November to April. Methods Study Area.—Río Lagartos lagoon, located in the southern Gulf of Mexico on the north- eastern coast of the Yucatán Peninsula, Mexico (21°26′ N, 87°30′ W; 21°38′ N, 88°15′ W), is part of the Special Biosphere Reserve of Ría Lagartos. The lagoon is connected to the sea through three sea inlets, situated in front of the towns of San Felipe and Río Lagartos (Fig. 1), and measures 80 km long, with a mean depth of 0.5 m and overall area of 96 km–2. Freshwater input from groundwater discharge is negligible and from rivers is nil. The lagoon is divided into three natural basins: Río Lagartos (outer zone), Las Coloradas (middle zone), and El Cuyo (inner zone), each with different hydrological, physical, and biological characteristics H( er- rera-Silveira and Ramírez-Ramírez, 1998; Herrera-Silveira et al., 1998. Ortegón-Aznar and González-González, 2000; Vega-Cendejas and Hernández de Santillana, 2004). Mean water temperature is 26°C, mean salinity is 57 (range: 25–150), with marine conditions near the sea inlets (33–38) and permanently hypersaline conditions (> 70) in the inner zone (Her- rera-Silveira and Ramírez-Ramírez, 1998; Herrera-Silveira et al., 1998). Only the outer zone has submerged aquatic vegetation (SAV) with coverage estimated as 15% of the total surface of the lagoon, represented by the seagrasses H. wrightii, and Thalassia testudinum Banks ex König, 1805, and numerous species of macroalgae, principally Laurencia poiteaui (Lamou- rox) Howe, 1918, and Caulerpa spp., with a mean biomass of 20–500 g dry wt m–2. The bottom of the middle zone is covered by a stromatolithic association of red-bacteria and cyanophyte algae, whereas the inner zone is unvegetated soft substrate (Espinoza-Avalos, 1996; Herre- ra-Silveira et al., 1998; Ortegón-Aznar and González-González, 2000). The climatic regime MAY-KÚ AND ORDÓÑEZ-LÓPEZ: PENAEID SHRIMPS IN HYPERSALINE CONDITIONS 261 Figure 1. Map of the sampling sites (numbered 1–21) in the Río Lagartos lagoon, Mexico. Five hydrologically similar zones (Roman numerals) were identified in the study area using an ag- glomeration analysis. HZ I and II comprise the artisanal shrimp fishing area. has three seasons: dry (March–May: air temperature 36–38ºC, precipitation 0–30 mm mo–1, weak southeast winds < 15 km h–1), rainy (June–October: air temperature 38ºC, precipita- tion 220 mm mo–1, weak southeast winds < 15 km h–1), and “nortes” (November–February: air temperature < 22°C, precipitation 40 mm mo–1, strong winds from the north > 80 km h–1; Herrera-Silveira et al., 1998; Medina-Gómez and Herrera-Silveira, 2003). Sampling Design.—We selected 21 sampling sites distributed throughout the lagoon (Fig. 1). Shrimps were collected monthly from November 1996 to April 1997 (“nortes” and early dry seasons). This period coincides with the major recruitment peak for penaeid shrimps in the Yucatán Peninsula (Pérez-Castañeda and Defeo, 2001). From an anchored boat we hand- hauled one trawl per sampling site along the bottom (0.5–1.5 m depth), covering a distance of 30 m (48 m2 area) using a Renfro beam trawl with a 1.6 × 0.5 m mouth, 1.5 m total length, and 1.0 mm mesh. Samples were taken at night during the new moon to coincide with the period of maximum activity for Farfantepenaeus spp. (Garcia, 1985). The catch efficiency of the Renfro beam trawl, regardless of penaeid shrimp density, has been estimated at 15% over T. testudinum meadows, and over nonvegetated habitats efficiency increases to 28%G ( racia et al., 1994). Although catch efficiency of this gear is low, it was easy to use, produced clean samples, and allowed for a large sampling area (Rozas and Minello, 1997). Before each trawl, salinity and temperature were measured using a field multianalyzer (YSI model 85–25 ft). Shrimp samples were preserved in 10% formaldehyde, kept in plastic bags, and then trans- ported to the laboratory. The carapace length (CL) of each shrimp was measured to the near- est 0.1 mm with an ocular micrometer for organisms < 10.0 mm CL and with a standard vernier caliper for larger organisms. Total length (TL) was also recorded for a representative portion of the samples to determine the TL–CL conversion relationship.