Offshore spawning of Giant Mud Crabs (Scylla serrata) – insights from satellite tagging and collation of reported sightings

Abstract

Offshore spawning has been the accepted paradigm in life history of the Giant Mud Crab. However, few studies have empirically documented the migration of this important fishery species, despite this behaviour being identified over 70 years ago.

Two lines of evidence were used to quantitatively and qualitatively improve the understanding of the spawning migration: (i) sightings of migrating and/or egg-bearing females, and (ii) the novel application of micro pop-up archival satellite tags (n = 12). The sightings provided information on the pathways and seasonality of migration, while the satellite tags provided insight into the behaviour and movement of migrating females. The study occurred in Queensland waters, both Gulf of Carpentaria and east coast.

Over 100 migrating or egg-bearing female Giant Mud Crabs were opportunistically sighted and reported by fishers. Summary satellite tag data was available from nine individuals, with archived data (i.e., 30-second interval depth, temperature, and light) from two individuals, including one-egg-bearing female.

Combined, the evidence indicates that the offshore spawning migration varies with local bathymetric and oceanographic conditions. This has consequences for larval distribution and the genetic and demographic connectivity of this species. Judicious application of pop-up archival satellite tags could be more broadly applied to brachyuran crabs to provide novel insights into their spawning migrations.