Huge Krill Fishery Population Offers Aquaculture Feed Potential

The Fish Site
by The Fish Site
16 March 2006, at 12:00am

Understanding the population structure of commercially valuable species is extremely important for identifying stocks, defining fishing boundaries, and managing exploitation of fishery resources [1,2]. The definition of limits for fisheries requires reliable information on gene flow and the number of migrants exchanged between different areas, since populations affected by natural or human pressures may or may not be reestablished by individuals from neighboring populations [2,3].

Genetic markers are commonly used to evaluate the degree of connection among populations of marine invertebrates, since the minute size of these organisms during their larval phase hinders direct observations of dispersal between areas [4,5]. Microsatellites are a type of genetic marker frequently applied in studies involving population and conservation genetics, and consist of tandem repetitions of short nucleotide motifs (2–6 bp) found abundantly in the genome [6,7]. Due to their high polymorphism, these markers are very useful in differentiating populations and inferring dispersal patterns [8–10]. For instance, microsatellites have been employed to better understand populations of animals with highly dispersive larvae such as the prawn Penaeus monodon [10] and the crab Carcinus maenas [11], as well the blue crab Callinectes sapidus [12]. This information can be applied to management strategies, since genetically structured populations should be considered separate management units for maintenance of genetic diversity [7].

The blue crab is a marine-estuarine crustacean [13,14] that lives for up to three and a half years [15]. Mating occurs inside estuaries and is closely coordinate with the molt cycle, which is controlled by temperature [16,17]. Gonadal maturation can occur at temperatures above 10°C, when females become active to forage and can therefore mature their ovaries [18]. Timing and duration of their spawning season is influenced by salinity, and therefore varies temporally and spatially [19]. In temperate areas with marked seasonality, spawning occurs during spring and peaks in summer, when salinity is usually higher [19,20]. At these zones, the reproductive cycle is characterized by copulation in estuarine waters, after which males remain in the upper estuary while inseminated females migrate to the high salinity waters of lower estuarine and shelf areas for egg deposition from the end of spring to the end of summer [16,19,20]. Larvae then hatch in the ocean, where they are influenced mainly by nearshore wind-generated surface currents. Eventually they return to the estuary through selective tidal stream transport identified using sensorial cues [21]. In this manner, during the larval phase blue crabs are subject to the oceanographic processes that occur in the coastal zone.

The eastern coast of Brazil in influenced by two main ocean current systems: the Brazil Current (BC) and the Malvinas Current (MC) [22]. The BC is formed at around 10°S, and is the western boundary current of the South Atlantic subtropical gyre, transporting warm, high salinity waters poleward and influencing most of the eastern coast during the entire year [23–25]. The MC is formed at around 55°S and flows northward carrying cold, low salinity waters, influencing mainly the South Brazilian coast in the winter when it is intensified [26,27]. The BC and MC interface at between 28–36°S in what is known as the Subtropical Confluence Zone; the latitude where this zone occurs can vary seasonally according to mass transport of both currents, as well as wind forcing [28]. These currents, as well as their seasonal variations, can influence regional transport of C. sapidus larvae.

Blue crabs represent an important commercial and recreational asset valued at approximately US$185 million worldwide in 2013 [29], and are the most exploited portunid species in Brazil [30]. In some regions, this species is commonly used as an alternative during closed seasons of other fishery resources, such as the anchovy Anchoviella lepidentostole in the southeast and the pink shrimp Farfantepenaeus paulensis in South Brazil, when it is commonly caught using banned fishing gear [30]. Despite its economic importance in the region, few studies discuss the ecology and populations of C. sapidus in the Western South Atlantic [31]. Such studies are extremely important for producing baseline data and establishing management strategies of blue crab fishery stocks.

In this context, the present work aimed to evaluate the genetic diversity and connectivity of blue crab populations in Western South Atlantic. Since coastal ocean currents are likely to influence larval transport and gene flow between areas, we tested the hypothesis that there are seasonal differences in gene flow between populations due to variations in currents.