Local ecological knowledge demonstrates shifting baselines and the large-scale decline of sawfishes (Pristidae) in Tanzania

Abstract

Sawfishes are coastally distributed rays that grow to very large sizes. All five species are assessed as Endangered or Critically Endangered as a result of population declines caused mainly by entanglements in fishing gear and by habitat degradation. Three species occur in the western Indian Ocean, but their populations are believed to have declined drastically in many areas and little is known about their presence in Tanzania. Fisher interviews were conducted to provide an overview of the status of sawfishes in Tanzania and to highlight where conservation actions should be focused to conserve sawfishes nationally. In total, 386 interviews were conducted: 296 at 31 villages in four regions on Tanzania’s mainland, and 90 interviews at 12 villages in four regions of Pemba Island. Less than half (46%) of the fishers were familiar with sawfishes and could identify one from a photograph. A precipitous decline in sawfish abundance in Tanzania could be inferred from the age-related decline in local Knowledge of their  existence; all fisheres aged between 70 and 80 years were familiar with sawfishes, whereas only 10% of fishers younger than age 20 had ever heard of them. A total of 29 sawfish records were compiled, comprising 16 rostra and 14 catches that occurred between 2002 and 2014 (one record  included both a catch and a rostrum). All rostra examined were from the largetooth sawfish Pristis pristis. Since 2002, the sawfish catches occurred in three locations, all on the mainland coast: in the southern Mtwara region, in the vicinity of the Rufiji Delta, and at Bagamoyo on the Zanzibar Channel. Interview data and rostra evidence suggest that the Rufiji Delta may be a largetooth sawfish nursery area, and confirmation of this should be a priority.

Keywords: biodiversity conservation, Endangered marine megafauna, elasmobranch, extinction, gillnets, participatory research, Pristis pristis  rostrum, rapid bycatch assessment