Offshore movement by the portunid crab Scylla serrata (crustacea: decapoda): new insights into the behaviour and ecology of females migrating to spawn from micro pop-up satellite archival tags

Movement to spawn in offshore waters provides numerous benefits to inshore marine fish and crustacean species, but there is often limited empirical data on the behaviour and ecology of this key aspect of their life history. This is the case for Portunid mud crabs of the genus Scylla, which have swimming abilities and are widely distributed throughout the Indo-West Pacific. Two lines of evidence were used to quantitatively and qualitatively improve the understanding of the spawning migration of Scylla serrata, the largest of the four species of mud crab: (i) the novel application of the smallest pop-up satellite archival tag available to twelve females considered to have advanced ovarian development, and (ii) the collation of requested sightings of migrating and/or egg-bearing females. The satellite tags provided empirical data on the movement and behaviour of migrating females, whilst the collated sightings (n = 101) provided information on the direction and seasonality of migration. The study occurred between October 2020 and June 2024 in the waters of Queensland, Australia, that included the continental shelves of the Gulf of Carpentaria (approximately 12–18°S, 138–142°E) and the Queensland’s east coast (approximately 11–28°S, 142–153.5°E). Satellite tag data was obtained from nine individuals, with 30 second interval depth, temperature, and light archived data available from two individuals, including one egg-bearing female. This data indicated three types of behaviour associated with the spawning migration: (i) estuarine benthic behaviour in shallow water (<10 m) where these benthic dwelling crabs remain mostly on the sea floor of tidal estuarine habitats; (ii) active swimming behaviour when the crabs alternated between near surface positioning interspersed with sedentary behaviour at increasing depth likely indicative of swimming to deeper offshore waters; and (iii) offshore benthic behaviour in deeper waters (>20 m). Over 100 sightings of spawning females provided a broadscale insight into their movement and possible offshore destinations. Results indicated the offshore spawning migration of giant mud crabs is variable, depending on local bathymetric and oceanographic conditions, which has consequences for larval distribution and the genetic and demographic connectivity of this species. Pop-up satellite archival tags can provide novel insights into the spawning migration of brachyuran crabs, providing additional information to inform fisheries management.