A historical review of giant kelp harvesting in TasmaniaLong-term Temporal Trends

Hunter Forbes¹, Wouter Visch¹, Scott Bennett¹, J. Craig Sanderson², Jeffrey Wright¹, Cayne Layton¹

¹University of Tasmania, Australia - ²Tassal Group Limited, Australia

Kelps have a long history of human use and exploitation. Knowledge of past harvesting practices can offer insights into environmental baselines and the contemporary management and conservation of these critically important ecosystems. In Tasmania, Australia, giant kelp (Macrocystis pyrifera) was commercially harvested from 1964–1973, but those forests have since undergone precipitous declines due to climate change. We reviewed a collection of archival data and sources to (1) describe the history, methods, and scale of this understudied and largely forgotten industry; (2) gain insight into the historical ecology and extent of Tasmanian Macrocystis forests; and (3) examine the potential ecological impact of the fishery. We calculated that >65,000 tonnes (wet weight) of Macrocystis was harvested from eastern Tasmania over a decade (mean annual harvest = 6,531 t), making it one of the largest wild harvest industries to have ever existed in the region. However, the industry had challenges finding sufficient biomass to sustain operations, ultimately driving its closure. Feasibility surveys had earlier suggested much greater kelp availability than was ultimately realised, perhaps motivating overexpansion, all against a backdrop of climate change in this ocean warming hotspot. Harvest efforts grew wider and more intensive, and during summer months when stocks were lowest almost all exploitable biomass was harvested. It remains uncertain whether harvesting contributed to the decline of Tasmanian Macrocystis forests, but it may have reduced their resilience and exacerbated other stressors and declines, particularly in heavily harvested areas. Ultimately, this historical review provides a rare opportunity to examine the past scale and use of these now-endangered Macrocystis forests and to also help inform the contemporary management and conservation of seaweed resources.