Movement patterns and distributional shifts of Dungeness crab (Cancer magister) in response to hypoxia

Background/Question/Methods

Hypoxia (dissolved oxygen < 2 mg/l) has emerged as a widespread threat to estuaries, coasts, and semi-enclosed seas worldwide. Regionally, hypoxia has been identified as a key threat to the Puget Sound ecosystem, particularly in southern Puget Sound and Hood Canal. Hood Canal is subject to seasonal hypoxia in its southern reaches, and prior work has demonstrated short-term reductions in densities of demersal fish and macro-invertebrates in hypoxia-impacted areas. We interpreted these findings as an indication of distributional shifts that reduce mobile organisms’ exposure to hypoxic conditions, potentially resulting in secondary ecological effects (e.g., enhanced vulnerability to predators, increased competition). We evaluated the nature and extent of these distributional shifts using acoustic telemetry. We deployed an acoustic receiver array in two regions of Hood Canal, a southern highly impacted region and a northern reference region, and tagged 30 Dungeness crab with pressure sensing acoustic transmitters. All observations were accompanied by weekly water quality monitoring. Weekly mean geographic centroids were used to assess large regional southern displacement. Shifts in weekly depth distributions were evaluated using linear mixed-effects models (LMMs) and AIC model selection. We also measured the association between daily activity (number of depth changes/day) and mean depth DO.

Results/Conclusions

Weekly monitoring of water quality revealed strong gradients in dissolved oxygen over time and space, with the vertical extent of low (<1 mg/l) waters increased markedly at the end of summer in the southern site. The south tagged crabs showed no discernible large regional movements to avoid the hypoxic southern areas. In fact, the majority of southern crabs remained in the southern region, while most northern crabs quickly emigrated northward. However, we did capture significant shifts in southern crab depth through time. Southern crabs displayed deeper initial average maximum depths than the north. In addition, southern crab maximum depth decreased more rapidly over time. However, although the southern crabs decreased their depth range, they appeared to remain near the hypoxic waters, potentially increasing their vulnerability to rapid exposure of lethal DO conditions. We also observed increases in range of activity associated with lower DO levels for all crabs. These results have important implications for the overall catchability of southern crabs that are pushed into shallower waters, potentially increasing their susceptibility to fishing pressure. This study ultimately provides further insight into the complex behavioral response of an important commercial species to hypoxia.