Live Spanner Crabs: Alternative handling methods

Abstract

The way that Spanner crabs (Ranina ranina) are treated on boats may explain why commercial operators have difficulty keeping these crabs alive in storage tanks on shore. We stored crabs on boats using three different methods, in baskets (the established industry practice), in cool air and in an aerated flow-through seawater tank. The condition of the crabs was assessed by taking haemolymph samples and measuring lactic acid and ion concentrations and then by recording the mortality of crab's from different treatments during subsequent storage in land-based tanks. If dehydration occurred, it was not reflected in the haemolymph ionic composition. On landing, the crabs held in baskets (25-28°C) had high levels of lactate in the haemolymph (about 40 mmol/L). Road transport prior to tanking saw no further lactate accumulate but many crabs died subsequently. Crabs stored in cool air (20°C) accumulated less lactate and lasted a few days longer before they too began to die. The crabs previously held on the boat in seawater (27-28°C) showed unexpectedly high mortality soon afterwards.
The counter-intuitive result from submerged crabs led us to the option of using cooled seawater sprays on the crabs. In the laboratory, the cool spray (19'C) improved the crab's ability to control haemolymph pH and delayed accumulation of lactate. We recommend that if spanner crabs are stored out of water then they should be kept cool (e.g. 20°C). Use of cooled seawater sprays may further improve physiological condition but this method is yet to be studied on catching boats. While these improvements are probably necessary, none of the parameters we measured adequately predicted onshore mortality. We suggest that the problem with long-term storage may be related to the fact that the claws are not immobilised and that submerged crabs may struggle and injure each other.