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Local Ecological Knowledge Demonstrates Shifting Baselines and the Large-Scale Decline of Sawfishes (Pristidae) in Tanzania

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Local Ecological Knowledge Demonstrates Shifting Baselines and the Large-Scale Decline of Sawfishes (Pristidae) in Tanzania African Journal of Marine Science ISSN: 1814-232X (Print) 1814-2338 (Online) Journal homepage: https://www.tandfonline.com/loi/tams20 Local ecological knowledge demonstrates shifting baselines and the large-scale decline of sawfishes (Pristidae) in Tanzania G Braulik, M Kasuga & G Majubwa To cite this article: G Braulik, M Kasuga & G Majubwa (2020) Local ecological knowledge demonstrates shifting baselines and the large-scale decline of sawfishes (Pristidae) in Tanzania, African Journal of Marine Science, 42:1, 67-79, DOI: 10.2989/1814232X.2020.1728379 To link to this article: https://doi.org/10.2989/1814232X.2020.1728379 Published online: 25 Apr 2020. Submit your article to this journal Article views: 2 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=tams20 African Journal of Marine Science 2020, 42(1): 67–79 Copyright © NISC (Pty) Ltd Printed in South Africa — All rights reserved AFRICAN JOURNAL OF MARINE SCIENCE ISSN 1814-232X EISSN 1814-2338 https://doi.org/10.2989/1814232X.2020.1728379 Local ecological knowledge demonstrates shifting baselines and the large-scale decline of sawfishes (Pristidae) in Tanzania G Braulik1,2* , M Kasuga2,3 and G Majubwa4 1 University of St Andrews, St Andrews, Fife, United Kingdom 2 Wildlife Conservation Society Tanzania Program, Zanzibar, Tanzania 3 Open University of Tanzania, Dar es Salaam, Tanzania 4 Ambakofi, Bagamoyo, Tanzania * Corresponding author, e-mail: [email protected] 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 fishers 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 Introduction The cartilaginous fishes (class Chondrichthyes) face a global entanglements in gillnets and coastal trawl fisheries conservation crisis, with an estimated one-quarter of species (Harrison and Dulvy 2014; Dulvy et al. 2016). Bycaught threatened with extinction (Dulvy et al. 2016). Among the sawfishes are usually retained because the large quantity of most threatened of these are sawfishes (family Pristidae), flesh can be lucrative, the rostra can be sold as curios, and giant coastal, shark-like rays (up to 7 m total length [TL]) the fins are considered valuable within the shark-fin trade that are characterised by a distinctive long rostrum on (Clarke 2008). Their coastal distribution and dependence the front of the head, with numerous large teeth on each on habitats prone to high levels of human impact, such as side. Globally there are five recognised species that occur river mouths, estuaries and mangroves, means they are also throughout coastal and estuarine waters of the tropical susceptible to the impacts of coastal and estuarine habitat Pacific, Atlantic and Indian oceans (Faria et al. 2013). All degradation (Simpfendorfer 2005; Moore 2015). Declines five species are listed as either Critically Endangered or are compounded by the inherent biological vulnerabilities Endangered on the IUCN Red List of Threatened Species of many shark and ray species, in that they are long-lived, (e.g. Carlson et al. 2013; Kyne et al. 2013; Simpfendorfer grow and reproduce slowly, and mature late, with the result 2013). Sawfishes were formerly widespread and abundant, that populations can be slow to recover (Harrison and Dulvy but populations of all species have been declining steadily 2014; Dulvy et al. 2016). Northern Australia and Florida are since the 1960s and 1970s (but possibly earlier) when the the only areas where viable sawfish populations are believed use of gillnets in global marine fisheries started to expand to remain (Harrison and Dulvy 2014; Devitt et al. 2015). dramatically (Klust 1973; Potter and Pawson 1991; FAO Despite the undeniable conservation priority of this group 2012). The elongated, toothed rostrum, or saw, which of charismatic and severely threatened elasmobranchs, can be up to 1.5 m in length, easily becomes hooked and few detailed studies have been published on their status entangled in fishing nets, and in many places sawfishes throughout vast areas of their circumtropical distribution have declined as a result of both targeted and accidental (Moore 2015). There is an urgent need for information on African Journal of Marine Science is co-published by NISC (Pty) Ltd and Informa UK Limited (trading as Taylor & Francis Group) Published online 25 Apr 2020 68 Braulik, Kasuga and Majubwa their status from data-deficient areas, most of which are in Mozambique, and the Rufiji, which is one of the 10-largest developing nations with intensive shallow-water fisheries rivers in Africa and has one of the largest deltas and (Harrison and Dulvy 2014; Dulvy et al. 2016). mangrove stands in the East Africa region (WWF-EAME In eastern and southern Africa, three sawfish species are 2004). Marine fisheries in Tanzania are almost exclusively known to occur: the largetooth sawfish Pristis pristis, green artisanal and are conducted predominantly using gillnets, sawfish P. zijsron, and narrow sawfish Anoxypristis cuspidata purse-seines, hook-and-line, traps and longlines (Ministry (Dulvy et al. 2016). During the 1930s, sawfishes were of Livestock Development and Fisheries 2010). The reported as being common in KwaZulu-Natal Province, South only industrial fishery that existed in Tanzania was the Africa, and Maputo Bay (formerly Delagoa Bay), Mozambique prawn-trawl fishery, which began operating in the Rufiji (von Bonde 1934, as cited by Everett et al. 2015). In Delta in 1988. Effort increased steadily to a peak of 25 KwaZulu-Natal, largetooth sawfish and green sawfish vessels in 2003/4, but the fishery was closed in January were still relatively regular catches in the 1950s and 1960s 2008 owing to the collapse of prawn stocks (Muir and but they have now disappeared, with the last confirmed Ngatunga 2008). Trawling has not been conducted regularly record being from 1999 (Everett et al. 2015). Their local anywhere else along the coast of Tanzania. extinction is thought to be due to anthropogenic changes to the most important estuarine ecosystems, gillnets set for Fisher interviews bather protection, and illegal fish harvesting. Two key sites In 2014, interviews with local fishers were conducted to for sawfishes in Mozambique were identified where recent gather semi-quantitative information on the location and captures and numerous rostra suggest that largetooth sawfish timing of bycatch of marine megafauna. Gillnet fishers still persist, but elsewhere in the country they appear scarce were the primary target of the interviews because this (Leeney 2017). There are no recent sawfish records reported gear type has by far the highest bycatch rates for marine from Kenya (Pierce 2014). Sawfishes are known to occur in megafauna generally, and sawfishes specifically (Lewison Tanzania (Richmond 2011) and dried rostra were noted to be et al. 2004; Harrison and Dulvy 2014). Given that there regularly displayed for sale as curios in mainland Tanzania is no industrial fishery in Tanzania, we obtained all our and the Zanzibar Archipelago in the 1990s (Barnett 1997; information from artisanal fishers. Time and budget Dulvy 2014; Pierce 2014), but recent monitoring of fish allowed for representatives of approximately 3–5% of landing sites in Zanzibar failed to record a single sawfish the fishing fleet to be interviewed (Ministry of Livestock (Temple et al. 2019). Reflecting the general lack of current Development and Fisheries 2010), and a target was set systematic or reliable information, green and largetooth of 15 interviews from each of two villages chosen in each sawfish were documented as ‘presence unknown’ in Tanzania district. The rapid bycatch assessment questionnaire in a recent comprehensive global review (Dulvy et al. 2016). developed by Moore et al. (2010) was used, and interviews The present study was initiated to fill this important were conducted in Kiswahili by Tanzanian researchers data gap, specifically by conducting fisher interviews to not known to the fishers, one-on-one at fish landing sites. gather information to: (i) confirm the occurrence of sawfish The questions related to catches and the use of dolphins, species in Tanzania; (ii) determine which species occur; whales and dugongs (all reported in Braulik et al. [2018]) (iii) examine the spatial and temporal distribution of records as well as sawfishes (reported here).
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