Marine reptiles

16.7.2 Marine reptiles

Captures of sea turtles have been recorded in long- line, set-net and towed-gear fisheries. The survival of some turtle species is also threatened by loss of suitable habitat in which to lay their eggs through coastal tourist developments, egg stealing and ille- gal fishing for wild turtles. Turtles are particularly vulnerable to entanglement and drowning in gill- nets and associated ghost-fishing gear as the rough skin on their head and flippers catches easily on the meshes of these nets (Carr 1987). Baited long- lines set for swordfish in the Mediterranean take approximately 20 000 endangered species of tur- tles every year. Similarly shrimp trawl fisheries are

a significant source of mortality for turtles (Poiner et al. 1990).

Ecosystem Effects of Fishing

359

Chapter 16

To date, relatively little information exists on tive with respect to the size class of animals cap- the biology or population dynamics of sea snakes. tured, gill-nets are associated with high numbers As a result, it is difficult to gauge whether the esti- of incidental captures of cetaceans. In the Sri mated 120 000 sea snakes, comprising 10 different Lankan gill-net fishery, one dolphin is caught for species, caught in the prawn trawl fisheries in every 1.7 to 4.0 tonnes of tuna landed. This com- the Gulf of Carpentaria, Australia, represents a sig- pares very poorly with one dolphin for every 70 nificant threat to the survival of these species. tonnes landed in the eastern Pacific purse-seine Post-capture mortality varies from 10% to 40% fishery. Hence, while the use of gill-nets may have depending on species, and in common with sea tur- conservation benefits for target fish stocks, the tles, increases with tow duration (Ward 1996a,b; negative aspects of bycatches of larger organisms Brewer et al. 1998).

may be counterproductive (Hall 1998).

16.7.3 Marine mammals

16.7.4 Ghost-fishing

It is an inescapable fact that wherever fishers pur- Static fishing gear, such as pots and set-nets, are sue prey fish species they are likely to encounter usually deployed and then left unattended for peri- larger predatory organisms such as marine mam- ods of up to several days (Misund et al. Chapter 2, mals. As industrialization of fisheries has replaced this volume). During this time the gear is vulnera- less intensive forms of fishing, so incidental catch- ble to damage from bad weather or can be snagged es of species that are closely associated with the accidentally by passing vessels or even deliberate- target fish species has increased. Such was the case ly vandalized. Once fishing gear has been lost it in the eastern Pacific tuna purse-seine fishery that could continue fishing indefinitely. This phenom- began in the late 1950s (Misund et al. Chapter 2, enon is termed ‘ghost-fishing’. this volume). This fishery replaced the highly

Just how long a gear is likely to continue ghost- species-specific pole and line fishery that had oper- fishing depends upon the circumstances of its loss. ated previously. It is thought that schools of tuna Gear that has been dragged off by trawlers or lost in follow herds of dolphin as they pursue smaller fish rough weather is likely to become tangled and will species, but the exact reason for the association is fish less effectively when it is eventually cut free not entirely understood (Hall 1998). In the early and allowed to sink back to the seabed. In other cir- days of this tuna fishery dolphin were inevitably cumstances, the gear may be held in an open fish- captured since their presence within the purse ing position when it becomes snagged on rocks or seine net ensured the capture of the tuna. Crude es- similar seabed features. In sheltered or deep-water timates of the number of dolphin killed during the situations, lost gear is likely to continue fishing for 1960s indicated fatalities of hundreds of thousands many years as it will not be subjected to damage by per year, which caused a population decline until wave action and abrasion (Carr et al. 1992). In con- the late 1970s. This sparked the ‘tuna–dolphin’ trast to the numerous records of incidental catches debate that culminated in the Marine Mammal in drift-nets and other set gears few incidents of Protection Act of 1972 passed by the US Congress. ‘ghost-fishing’ are reported or studied (Dayton et This act necessitated the adoption of good fish- al. 1995). The few studies that have been under- ing practices that involved ‘backing-down’, which taken reveal large differences in the behaviour of is a process to release dolphins from the net, and lost gear according to the circumstances and envi- the provision of independent observers aboard ronment in which the gear is lost. Kaiser et al. tuna vessels to record dolphin mortality (Hall (1996) reported that a net that was snagged on 1996).

rocky substrata in the Irish Sea continued to catch Gill-nets are also used to catch tuna in inshore crustacea and fish for over a year. In the shallow areas. United Nations legislation has banned the clear waters off the Algarve, Portugal, Erzini et al. use of high-seas gill-nets. Although highly selec- (1997) observed that similar lost nets were over- rocky substrata in the Irish Sea continued to catch Gill-nets are also used to catch tuna in inshore crustacea and fish for over a year. In the shallow areas. United Nations legislation has banned the clear waters off the Algarve, Portugal, Erzini et al. use of high-seas gill-nets. Although highly selec- (1997) observed that similar lost nets were over-

Pots or traps tend to be constructed of robust man-made materials and incorporate a rigid struc- ture. This means that pots are likely to maintain their shape and hence capture efficiency for much longer than lost nets. Animals that are trapped and die within pots act as bait that attracts yet more scavengers into the lost pot. The ‘ghost-fishing’ po- tential of pots also varies according to their design. For example, Parrish and Kazama (1992) found that the majority of Hawaiian spiny lobster (Palinurus marginatus ) and slipper lobster (Scyllarisdes squammosus ) were adept at escaping traps. In contrast, Smolowitz (1978) studied mortalities of lobster (Homarus americanus) in parlour-type traps that have non-return entrances to minimize animal escape. They found that 12–25% of trapped individuals died in these pots (Smolowitz 1978).

Little is known about the frequency of net or pot loss, which no doubt results from the reluctance of fishers to report such incidents and the difficulty in undertaking quantitative surveys to estimate the prevalence of lost gear. Estimates of gear loss would indicate that these are substantial in some fisheries. The fact that fishers have complained about the problem of ghost-fishing certainly indicates the scale of the problem. These com- plaints prompted a grapnel survey of the seabed on Georges Bank, which yielded 341 actively fishing ghost nets from 286 tows (Brothers 1992). The phe- nomenon of ghost-fishing was clearly perceived to have negative effects on commercial stocks of Greenland halibut (Reinhardtius hippoglossoides) by commercial fishers involved in this fishery. As a result, they instigated their own voluntary clean- up programme (Bech 1995). The adverse effects of lost pots are relatively easily averted by incorpo- rating escape panels into pot design and by using biodegradable materials. These technical mea- sures are cheap and simple to instigate and are extremely effective.