Discrete spawning aggregations of loliginid squid do not represent genetically distinct populations

Establishing whether heavily fished spawning aggregations of squid represent genetically distinct populations is important for fisheries management, especially in light of recent efforts to apply ecosystem-based management methods and the importance of squid as both predator and prey. Most squid species have the potential for high dispersal rates due to lengthy planktonic paralarval stages and highly migratory adult stages. Such life-history traits lead to predictions of genetic homogeneity (i.e. panmixia) of squid populations across large geographical areas. However, testing this hypothesis can be difficult, because spawning populations of squid are highly mobile and spawning sites are either unknown or spread sparsely over large geographical areas. Loligo reynaudii and Doryteuthis (Amerigo) pealeii are 2 squid species that are commercially fished on inshore spawning grounds located off the coasts of South Africa and the eastern USA, respectively, and for which highly localised spawning aggregations have been documented. We sampled discrete spawning aggregations of these 2 neritic species, so that the highest likelihood of sampling true reproductive populations was achieved, in order to determine whether such spawning aggregations represent discrete genetic populations. As has been reported for many squid species, the levels of genetic diversity detected at nuclear microsatellite DNA loci, within both L. reynaudii and D. pealeii, were high and consistent across all samples. Our results for D. pealeii indicated that adjustments of allele frequencies using MICROCHECKER to take the presence of null alleles into account may introduce bias, due to the presence of loci with small numbers of common alleles; this leads to a conclusion that there is significant genetic differentiation among populations where none exists. For both species, our results indicated no significant genetic differentiation of populations and, thus, no association of spawning aggregations with distinct genetic subpopulations, across the main spawning ranges sampled.