| Citation |
Rigby, C.L., Harry, A.V., Pacoureau, N., Herman, K., Hannan, L. & Derrick, D. 2020. Rhizoprionodon acutus. The IUCN Red List of Threatened Species 2020: e.T41850A68642326. https://dx.doi.org/10.2305/IUCN.UK.2020-3.RLTS.T41850A68642326.en. Accessed on 07 February 2022. |
Description |
JUSTIFICATION
The Milk Shark (Rhizoprionodon acutus) is a small (usually to 110 cm total length) shark that occurs in tropical and sub-tropical waters across the Indo-Pacific and in the Eastern Atlantic Oceans. It inhabits continental shelves from inshore to a depth of 200 m and is a highly productive species that breeds annually and matures early. The species is taken as target and bycatch by industrial and small-scale fisheries with multiple gears including trawl, gillnet, and longline and is retained for the meat and fins; it is one of the most commonly consumed tropical and sub-tropical coastal sharks globally. The population is reported to have increased in northwest Australia over three generation lengths (15 years), and to be both stable over two years and have declined by 99% over the past three generation lengths (15–22 years) in two areas of India, respectively. It is inferred to have declined by 67–80% in Sri Lanka over the past three generation lengths and inferred to have declined in Southeast Asia. In some other parts of its range, the Milk Shark is reported to have undergone a population increase, possibly due to meso-predator release. The species productivity likely provides some resilience to fishing pressure, however, the lower productivity where it is heavily exploited in West Africa is of concern. It is heavily fished throughout its range (except in Australia) in mostly unregulated fisheries, and steep declines over the past three generation lengths have been reported. It is suspected that the Milk Shark has undergone a population reduction of 30–49% over the past three generation lengths (15–22 years) due to levels of exploitation, and it is assessed as Vulnerable A2bd.
RANGE DESCRIPTION
The Milk Shark occurs across the Indo-Pacific from South Africa to Australia, and in the Eastern Atlantic from Madeira and Mauritania to Angola; there are two records of this species in the Mediterranean Sea (Ebert et al. 2013, Amor et al. 2016).
DESCRIPTION
Genetic data suggest that the Milk Shark is a complex of four species that require further taxonomic investigation (Naylor et al. 2012). Species-specific population trend data of standardized catch-per-unit-effort (CPUE) was available from northwest Australia (Braccini et al. 2019). The fishery-independent survey with droplines and longlines from 2002 to 2017 found the CPUE to be fluctuating but stable. The area of the surveys was closed to commercial fishing from 1993 to 2005 onwards and the stable pattern is suggested to reflect either no major detrimental fishing effects on the population even prior to the closures or naturally low abundance that precludes detecting abundance patterns (Braccini et al. 2019). The CPUE was analysed over three generation lengths (15 years) using a Bayesian state-space framework (Winker et al. 2020). This analysis yielded a 3.8% annual rate of increase, consistent with an estimated median increase of 69.3% over three generation lengths (15 years), and the highest probability of an increase over three generation lengths (see the Supplementary Information).
A stock assessment of this species based on landings during 2012–2014 in Gujarat, India showed that the fisheries exploitation level between 2012–2014 reduced stock biomass and spawning stock biomass to 55% and 34% of unexploited levels, respectively (Sen et al. 2017). The catch-per-unit-effort from 2012 to 2014 was stable and the assessment considered the population to be underexploited (Sen et al. 2017). This assessment should be regarded with caution due to limited time-frame of sampling and some simplistic assumptions. Market surveys from the same region indicate that all life stages of Milk Shark are vulnerable to capture by fisheries (Barnes et al. 2018).
Elsewhere in India, data from Mumbai indicate declines in landings of this species from 92.9 t in 2000 to 18.5 t in 2005 (Raje et al. 2016). Although landings data are not a direct measure of abundance, these can be used to infer population reduction where landings have decreased while fishing effort has remained stable or increased. In India, there is evidence of increasing fishing effort and decreasing catch-per-unit-effort of sharks in general (K. V. Akhilesh pers. comm. 09/02/2017). When this decline is scaled to three generation lengths (15–22 years) it represents a decline of 99%. In Sri Lanka, shark catches have decreased by 30% between 1994 and 1999 from 13,000 t to 9,000 t and have been steadily declining since 2001 despite increasing fishing effort (Dissanayake 2005). This represents a decline of 67–80% when scaled to three generation lengths of the Milk Shark (15–22 years). In Kuwait, this species represented 12.2% of elasmobranch landings by number in 2008 but this declined to 2.6% in 2011 (Moore et al. 2012). In the Iranian waters of the Arabian/Persian Gulf and Oman Sea, the Milk Shark has declined in abundance by more than 85% over the past 10–20 years; some of this decline may be due to an increase in illegal fishing despite a ban on target shark fisheries in Iran introduced ~15 years ago (T. Valinassab pers. comm. 03/06/2020). In Pakistan, landings of the Milk Shark have declined by 75–80% over the past 20 years due to overfishing (M. Khan pers. comm. 03/06/2020).
In Southeast Asia, catches of sharks and rays are very high but are declining and fishers are travelling much further from port in order to increase catches (Chen 1996). Net and trawl fisheries in Indonesia (especially the Java Sea) and elsewhere are very extensive and as a result, many shark and ray species are highly exploited and stocks of most species have declined by at least an order of magnitude (Blaber et al. 2009). Trawl and gill net fisheries are also moving further afield. For example, in Jakarta the gillnet fishery at Muara Baru travels to waters around Kalimantan due to the decline in local populations (W.T. White pers comm. 22/05/2007). While species-specific data on long-term declines in elasmobranchs in the Southeast Asian region are lacking, declines of the Milk Shark in Southeast Asia and elsewhere in the Indo-West Pacific are inferred given the widespread historical and continuing declines of demersal fisheries in this region (Stobutzki et al. 2006).
Across its range, the population was reported to be increasing in northwest Australia over three generation lengths (15 years), stable over two years in Gujarat, India and to have declined by 99% across three generation lengths (15–22 years) in Mumbai, India. It is inferred to have declined by 67–80% in Sri Lanka and inferred to have declined in Southeast Asia. In some other parts of its range, increases in abundance of the Milk Shark have been reported, possibly due to meso-predator release. The Milk Shark is a productive species which likely increases its resilience to fishing pressure, although the lower productivity in West Africa is of concern as it is heavily exploited in that region. With the exception of Australia, the Milk Shark is exposed to increasing and intense fishing pressure across its range, with steep declines reported in some areas. Overall, it is suspected that the species has undergone a population reduction of 30–49% over the last three generation lengths (15–22 years).
HABITAT AND ECOLOGY
The Milk Shark occurs on the continental shelf in tropical and sub-tropical waters from close inshore to depths of 200 m (Weigmann 2016). It is found throughout the water column but is mainly near the sea floor and is often occurs off sandy beaches and sometimes in estuaries (White et al. 2006, Ebert et al. 2013). The species is usually <110 cm total length (TL), although one specimen of 178 cm TL was recorded off Africa (Last and Stevens 2009, Ebert et al. 2013, Weigmann 2016). Life history traits vary regionally with males mature at 54–82 cm TL and females mature at 62–92 cm (Harry et al. 2010, Moore et al. 2012, Ba et al. 2013, Ebert et al. 2013). Reproduction is placental viviparous with litter sizes of 1–8, an annual reproductive cycle and size-at-birth of 25–45 cm TL (Harry et al. 2010, Ebert et al. 2013, Shaaban et al. 2018). In northeast Australia, female age-at-maturity is two years and maximum age is eight years, resulting in a generation length of five years (Harry et al. 2010). In Senegal, female age-at-maturity is 5.8 years and maximum age is nine years, resulting in a generation length of 7.4 years (Ba et al. 2015).
THREATS
The Milk Shark is caught globally as target and bycatch in industrial, small-scale, and recreational fisheries by multiple fishing gears including trawl, gillnet, trawl, hook and line, and longline. The species is generally retained for the meat and fins (Last and Stevens 2009, Fields et al. 2018). Commercial at-vessel-mortality (AVM) has only been reported for this species in trawl fisheries where it is highly variable ranging from 29–82% in the Indian Ocean and North Australia, respectively (Ellis et al. 2017).
In the Eastern Atlantic, this species is heavily fished throughout its range and frequently caught in Cape Verde, Mauritania, Senegal, Gambia, Guinea Bissau, and Guinea (Diop and Dossa 2011). It is the most commonly caught shark landed at fishing sites along the Senegalese coast (Capape et al. 2006, Ba et al. 2015). The predominance of juvenile Milk Shark in landings during all seasons could be an indication that the species is overexploited in Senegalese waters (Ba et al. 2013). It is among the most commonly caught shark by artisanal fisheries in the waters of Mauritania (Valadou et al. 2006). In the Banc d’Arguin National Park (Mauritania) 70–85% of individuals caught from April to June are females, more than half of which are pregnant with fully-developed embryos (Diop and Dossa 2011); this suggests that fishers target a pupping aggregation in this region. Declines in coastal shark assemblages have been reported throughout the eastern Atlantic region, although recreational fishers in the Bijagos Archipelago (Guinea-Bissau) have noticed an increased abundance of Milk Shark that has been attributed to declines in the numbers of large, predatory sharks (Diop and Dossa 2011).
In South African waters, the Milk Shark is caught incidentally by prawn trawl fisheries. The estimated annual catch of the Milk Shark in trawl fisheries of South Africa was <1 tonne for 2010–2012 (da Silva et al. 2015). The species is also important to recreational fisheries and was the second most commonly caught shark (18% of total catch) in the Kwa-Zulu Natal competitive shore fishery from 1977 to 2000 (Pradervand 2007). Van der Elst (1979) hypothesized that an increase in abundance of small sharks, including the Milk Shark, in the Kwa-Zulu Natal recreational fishery was due to the removal of large, predatory sharks by the Natal Sharks Board protective gillnet program. Extensive artisanal fisheries operate in coastal waters of the southwestern Indian Ocean (Le Manach et al. 2012, Benkenstein 2013, Cripps et al. 2015). It is an important component of commercial and artisanal shark fisheries in mainland Tanzania and Zanzibar where it has been recorded as the most abundant in market surveys (Fowler et al. 2002, Schaeffer 2004). The species is also an important component of artisanal shark fisheries in Madagascar waters (Robinson and Sauer 2013).
In the Arabian Seas region, surveys of fish markets and landings sites have revealed this species to be a major part of elasmobranch landings, often as bycatch in gillnet fisheries (Henderson et al. 2007, Moore et al. 2012, Moore and Peirce 2013, Jabado et al. 2015, Spaet and Berumen 2015). The high level of exploitation on its habitat in the region is of concern. For example, in Eritrea catch and effort data showed that total fishing effort as well as total annual catch increased more than two-fold from 1996 to 2002 (Tsehaye et al. 2007). While no accurate numbers are available, there has been an uncontrolled expansion of industrial trawling in the Red Sea through licenses issued to foreign industrial trawlers (particularly off Yemen) which has resulted in the depletion of marine resources (PERSGA 2002). In Somalia, illegal and unregulated fishing by foreign trawlers and longliners is widespread and impacting shark populations (Glaser et al. 2015). In Iran, there is increasing fishing effort with the number of fishermen almost doubling from 70,729 in 1993 to 109,601 in 2002 (Valinassab et al. 2006). In some areas (e.g. Pakistan, India) fishing effort on this species has been increasing in recent years. In Pakistan waters, about 2,000 trawlers operate in shelf waters, targeting shrimp in shallow waters and fish in outer shelf waters (M. Khan pers. comm. 06/02/2017). CMFRI (2010) reports that 5,767 trawlers operate in Tamil Nadu waters, and fishing pressure is intense with severe declines in fish stocks, including elasmobranchs (Mohanraj et al. 2009, Karnad et al. 2014). Trawling is also intense in Kerala where CMFRI (2010) reported 3,678 trawlers. There were about 6,600 trawlers operating in the Indian state of Gujarat in the early 2000s (Zynudheen et al. 2004). This number increased to 11,582 trawlers in 2010 (CMFRI 2010). In India, there are over 13,400 gill netters operating along the west coast, with many other types of net gear also deployed in coastal areas (CMFRI 2010).
In Southeast Asia, the Milk Shark is one of the most commonly caught sharks in trawl fisheries of peninsular Malaysia and is commonly caught in gillnet fisheries of Indonesia (Department of Fisheries Malaysia 2006, White et al. 2006). The extensive loss and degradation of habitats such as coastal mangroves are also a threat to coastal and inshore species; Southeast Asia has seen an estimated 30% reduction in mangrove area since 1980 (FAO 2007, Polidoro et al. 2010).
In Australia, the Milk Shark is a bycatch in gillnet and prawn trawl fisheries with low to moderate fishing pressure in managed fisheries. It is usually not retained but post-release mortality is likely high (Stobutzki et al. 2002, Tobin et al. 2010). Northwest Australia was closed to commercial fishing of sharks and rays from 1993 to 2005 onwards to protect breeding stocks of Dusky Shark (Carcharhinus obscurus) and Sandbar Shark (C. plumbeus) (Braccini et al. 2019). Ecological risk assessments of the Milk Shark have considered it to be at low to moderate risk from fishing in Australia (Stobutzki et al. 2002, Zhou and Griffiths 2008, Tobin et al. 2010).
USE AND TRADE
The Milk Shark is one of the most consumed sharks in tropical and subtropical coastal waters globally. The flesh is consumed fresh or dried, salted, and smoked and used as fishmeal (Last and Stevens 2009, Roy 2010). Fins are of limited value due to their small size although trade in small, low-value fins has increased in Southeast Asia in recent years (Cardeñosa et al. 2019). Milk Shark fins comprised 1.4% of the fins sampled in Hong Kong in 2015 yet were the most common of the small-sized shark fins sampled in Hong Kong in 2019 (Fields et al. 2018, Cardeñosa et al. 2019).
CONSERVATION ACTIONS
There are few species-specific regulations for the Milk Shark. In South Africa, for small-scale fishing there is a bag limit of one individual Milk Shark for consumption only (DAFF 2016). In Iran, a ban on target shark fisheries introduced ~15 years ago (T. Valinassab pers. comm. 03/06/2020) may benefit the Milk Shark. The introduction of bycatch reduction devices in Australian trawl fisheries is likely to have benefited this species (Griffiths et al. 2006), as would have Australias extensive network of Marine Protected Areas. Research is needed on the species taxonomy and population size and trends, and catch rates should be monitored. |
