Members of the genus
Alopias, thresher sharks, are threatened from a combination of slow life history characteristics, hence low capacity to recover from moderate levels of exploitation, and high levels of largely unmanaged and unreported mortality in target (for fins and their valuable meat) and bycatch fisheries.
Many countries fish thresher sharks commercially throughout their extensive ranges, with A. vulpinus probably being the most important species (Compagno 1990). This species is frequently caught by offshore longline and pelagic gillnet fisheries (Maguire et al. 2006), is also fished with anchored bottom and surface gillnets, and is a bycatch of other gear including bottom trawls and fish traps (Maguire et al. 2006). Although sometimes referred to as a bycatch in fisheries for other pelagic fishes, this bycatch is normally utilised and would better be described as a secondary target catch. They are also an important sport fishery resource, the meat is considered excellent for consumption, and the large fins highly valued. The growing and largely unregulated shark fin trade also represents a serious threat to thresher sharks. Clarke et al. (2006) report that thresher sharks compose at least 2-6% of the trade in a market study using DNA-based species identification techniques.
The impact of fisheries on A. vulpinus on a global scale, while difficult to assess, has most likely been significant. For example, Japanese and Russian vessels fish the northwest Indian Ocean and central Pacific, and Mexican-Japanese joint ventures have operated longline vessels off Baja California, Mexico, for many years. Furthermore, the number of pelagic sharks landed by fishing fleets in all oceans has become increasingly important in recent years (Mejuto et al. 2006). However, catch statistics are not available (Compagno 2001, Holts 1988, Smith 1998) and where they are, they are under-reported. A recent FAO review of the status of highly migratory pelagic species states: "unless demonstrated otherwise, it is prudent to consider Alopias species as being fully exploited or overexploited globally" (Maguire et al. 2006).
Eastern Central Pacific
A target pelagic gillnet fishery for this species developed off the west coast of the USA, Eastern Central Pacific (particularly California, and also Washington and Oregon) in the late 1970s (Goldman 2005, Maguire et al. 2006). This fishery serves as a well documented case of population depletion and provides strong evidence that there are numerous isolated subpopulations or stocks globally. Starting with 15 vessels in 1977, the fishery expanded to over 225 vessels in 1982 (Holts 1988, Hanan et al. 1993). The fishery peaked in 1982 with reported landings of 1,089.5 t (Anonymous 1993), declining due to overfishing to less than 300 t by the late 1980s (Maguire et al. 2006). Fishing had heavily reduced the number of juvenile and subadult A. vulpinus off central and southern California, virtually eliminating them from the catch. In 1996, California catches of Common Thresher Shark were down to one-fifth of former levels (Smith 1998). This fishery was effectively eliminated by restrictions on the use of gill nets by 1990 (Bedford 1992, Smith 1998, Maguire et al. 2006). It was originally believed that a Pacific-wide distribution of the species would act as a buffer against over-harvesting (Bedford 1992, Smith 1998). However, this was shown not to be the case, as that portion of the population remained at low levels for several years, and is only reappeared in the catch records and in market places some time later (Bedford 1992, Smith 1998). The species is still caught as bycatch or as a secondary target, although to a far lesser extent, of the swordfish gillnet fishery and may be sold for higher prices in the market than swordfish (Bedford 1992, Smith 1998, Maguire et al. 2006).
While the majority of fishery data for this species in the Pacific has come from California, it is fished in numerous locations throughout its range there. The Spanish pelagic longline fishery for swordfish and sharks is expanding rapidly in the Pacific, with effort expanding from the traditional grounds in the southeast Pacific into the central south Pacific and areas of the north Pacific in recent years (Mejuto 2005). A. vulpinus is taken in both artisanal and commercial longline fisheries in areas off South America, including Peru and Chile (M. Romero pers. comm., Bonfil et al. 2005). Hong Kong customs data shows that Peru is amongst the 20 countries that export the most dried fins to Hong Kong (Bonfil et al. 2005).
Indo-West Pacific
Little information is currently available on A. vulpinus in the Indo-West Pacific. Whereas records of A. superciliosus and A. pelagicus are recorded in the catches of fisheries operating in this region, albeit very under-reported, very little information is available on catches of A. vulpinus. Although pelagic fishing effort in this region is high, with reported increases in recent years, A. vulpinus is more characteristic of cooler waters and further information needs to be collected on records and catches of the species in this region.
Northwest and Western Central Atlantic
Thresher sharks are also an important pelagic species in the north Atlantic, although A. vulpinus is only sporadically recorded in the northwest and western central Atlantic and A. superciliosus is the more common thresher shark in this area. Thresher sharks are generally recorded by genus by observers as well as in logbooks. Analysis of U.S. Atlantic pelagic longline data from their scientific observer program for 1992-2005, suggest that the region of the US east coast where A. vulpinus is somewhat common is from about 35°N-40°N (approximately North Carolina to Philadelphia), where A. vulpinus: A. superciliosus are caught in a ratio of ~1:3 (Baum unpublished data). The first longline fisheries in the Atlantic were begun by the Japanese in 1956 in the western equatorial waters (Uozumi and Nakano 1996). The fleet expanded rapidly in the 1960s, and covered almost the entire Atlantic by the late 1960s (Bonfil 1994), including the areas currently fished by the American fleet. Fishing pressure is high and ongoing and as A. vulpinus is an incidental catch in these fisheries monitoring of catches of this species is extremely limited. Currently, there are no management measures specific to this species in any EEZ or within international waters, and no stock assessments have been done. In Canada and the U.S., less than 5% of the pelagic longlining fleets are monitored by observers, making it difficult to elucidate reliable trends in abundance from these data. Fisheries monitoring in international waters is even more limited. The pelagic longline fishing grounds for the US fleet extend from the Grand Banks (about 45°N) in the northwest Atlantic to 5-10°S off the South American coast, within which geographical areas of longline fishing are defined for classification (Cortés et al. 2007).
Baum et al. (2003) concluded from their analysis of Northwest Atlantic pelagic longline data that the relative abundance of all thresher sharks (A. vulpinus and A. superciliosus combined) had declined 80% from 1986-2000. This analysis is based on estimates of trends in abundance from standardized catch rate indices of the U.S. pelagic longline fishery logbook data, and the fifteen year time period is over one generation length for this species. Although the analysis is not species specific, the sample size of thresher sharks in this data is over 20,000. Furthermore, the area covered by the dataset analysed, ranging from the equator to about 50°N, encompasses the confirmed range of A. vulpinus in these two regions (Compagno 2001). An alternative analysis of the same logbook dataset for 1986-2005 that also combined A. vulpinus and A. superciliosus, resulted in an overall decline of 63% (Cortés et al. 2007). Fishing pressure on thresher sharks began over two decades prior to the start of this time series, thus the estimated declines are not from virgin biomass.
A more recent analysis of Alopias species trends from scientific observer data between 1992 and 2005 in the same U.S. pelagic longline fishery found an almost identical instantaneous rate of decline (-0.12 up to the year 2000) as in the logbook analysis (Baum et al. unpublished manuscript). For this nine year period (1992-2000), the decline amounts to 68%, therefore the decline back to when the fishery started in the 1960s (less than three generation period of 51 years) would be much greater. However, because of recent increases in the catch rates in 2004 and 2005, the overall trend from 1992-2005 of -0.024 was non-significant, and would amount to only a 26% decline (Baum et al. unpublished manuscript). Cortés et al. (2007) also conducted an alternative analysis of this same observer dataset for the same time period that also combined A. vulpinus and A. superciliosus. This analysis of the observer dataset showed a trend opposite to that of the logbook analysis, with a 28% increase since 1992. In contrast, the nominal observer series showed a 39% decline and the logbook index for the same time period showed a decrease of 50%. Furthermore the sample size in the observer analysis was much smaller (n=14-84) than that in the logbook analysis (n=112-1,292) and thus the trend estimated should be regarded with caution. Cortés et al.'s (2007) observer analysis was restricted to four out of the 11 geographical areas covered by the pelagic longline fishing fleet to keep a balanced statistical design (Cortés et al. 2007). Their full logbook analysis, which showed an overall decline of 63%, had much larger sample sizes and is thus better to estimate trends with more certainty (Cortés et al. 2007).
Southwest Atlantic
The thresher shark A. vulpinus is not as common in regional longline catches compared as the bigeye thresher A. superciliosus. Amorim et al. (1998) document its occurrence in the Santos (São Paulo) tuna longline fishery as "low" with only six specimens observed from 1974 to 1996. Gadig et al. (2001) reported on small numbers of juveniles taken by gillnet off São Paulo State.
Mediterranean Sea
Adults and juveniles of Alopias vulpinus are regularly caught as bycatch in longline, purse seine and mid-water fisheries throughout the Mediterranean Sea, as well as in recreational fisheries (Lipej et al. 2004). This species has some important parturition and nursery areas in the Mediterranean (Adriatic and Alboran Seas). Moreno and Moron (1992) observed aggregations of pregnant females of A. vulpinus in the Strait of Gibraltar.
Even though driftnetting is banned in Mediterranean waters, this practise has continued illegally (WWF 2005). The Moroccan swordfish driftnet fleet in the Alboran Sea operates year round, resulting in high annual effort levels (Tudela et al. 2005). Even though sharks are a secondary target or bycatch of this fishery, some boats deploy driftnets 1-2 miles from the coast where the chance of capturing pelagic sharks is higher. The catch rate for A. vulpinus is higher in boats actively fishing for sharks (from 0.7 to 1.5 N/fishing operation and 0.09 to 0.11 catch per km net). Both annual catches and mean weights of Alopias vulpinus have fallen as a result of fishing mortality in the Moroccan driftnet fishery, illustrating the likely impact of this illegal fishery on stocks in the Alboran Sea and adjacent Atlantic (Tudela et al. 2005). Valeiras et al. (2003) also report that pelagic sharks are forming an increasing proportion of the catch of Spanish swordfish sleets. Pelagic fishing pressure is high and ongoing throughout the Mediterranean Sea (Tudela 2004, Megalofonou et al. 2000).
Northeast Atlantic
A. vulpinus is caught primarily as a bycatch of longline fisheries for tuna and swordfish in the northeast Atlantic, and are also taken in driftnets and gillnets (ICES 2005, 2007). As a highly valuable species, it is very likely that this bycatch is retained (ICES 2005). Limited information is available on thresher shark catch in this region. ICES 2006 reports estimated landings of thresher shark at 13-107 t from 1996 to 2005 in the ICES area, however these data are still considered incomplete. Prior to 2000, estimated landings fluctuated at 17-13 t, in 2000-2001 they exceeded 100 t, after which they dropped to 4 t in 2002 and have not exceeded 7 t since. Increased targeting of pelagic sharks by Moroccan drift-netters in the Alboran Sea and Strait of Gibraltar (Tudela et al. 2005), mentioned above, has also likely impacted A. vulpinus in this area.
