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Mortality associated with fisheries other than trawling

All major fishing gear types involve some degree of injury to fish through internal and external wounding, crushing, scale loss and hydrostatic effects, with the severity of the injury depending on the gear type and its operation. Susceptibility to injury varies with species and type of stressor. The aim of this chapter is to explore briefly the general factors that are important for an understanding of stress, injury and mortality associated with capture processes with fishing gears other than trawl. The discussion includes not only fish, but also some other animals affected by contact with fishing gears.

Mortality associated with being hooked and released

There have been several investigations on the survival of fish released from the hook in various longline fisheries. Generally, it appears that hook penetration depth, hooking location and the technique used to remove fish from the hook have major impacts on subsequent survival. A swallowed hook may induce a substantially greater injury than a hooked mouth (e.g. through the jaw, lips or operculum).

Fish removed from hooks automatically (e.g. by a crucifier or gaff) experience a significantly higher mortality than fish removed manually (e.g. Kaimmer, 1994; Milliken et al., 1999). Huse and Soldal (2002) showed that the mortality of undersized haddock released from a hook by means of a gaff was markedly higher (53 percent) than that of those that were torn off by means of a crucifier (39 percent).

Both release methods inflicted severe injuries to the mouth parts of the fish. Fish that were released by a gaff suffered also from punctures to the body wall and damage to the abdomen and intestines. It is worth noting that the gaff can be used to remove the hook without handling the fish, and this is likely to reduce the injury.

Hooking mortality is variable and is affected by many factors, for example, the size and shape of the hook. Trumble, Kaimmer and Williams (2000) conducted a large-scale tagging experiment on Pacific halibut released from longline gear; halibut experienced lower mortality following release from small circle or autoline hooks than from large circle hooks. Neilson, Waiwood and Smith (1989) studied the survival of Atlantic halibut (< 81 cm in size) caught by longline (16/0 circle hooks, at 210 to 300 m). They found that 77 percent of the longline catch survived more than two days in on-board tanks.

The mortality of handline-caught and discarded undersized Atlantic cod was significantly higher in deep water (54 percent) than shallow water (32 percent) (Pálsson, Einarsson and Björnsson, 2003). Parker and Black (1959) and Parker, Black and Larkin (1959) estimated delayed mortalities of 40 to 86 percent for troll-caught chinook salmon, and of 34 to 52 percent for coho salmon.

Wertheimer (1988) measured the mortality of chinook salmon released from commercial trollers. Fish length, injury location and lure type were the three factors influencing post-release mortality (9 to 32 percent). Chopin, Arimoto and Inoue (1996) observed no mortality in released sea bream captured by hook and line, although the fish sustained various types and levels of stress from the capture process.

Fish behaviour during capture included initial flight response, successive struggles of decreasing magnitude, reverse swimming, and finning to maintain position. Struggle activity reduced as the period of capture increased. Chopin, Arimoto and Inoue (1996) argued that hook and line-caught fish were able to exhibit an adaptive response to capture; the cessation of struggling resulted in the captured fish regaining their normal swimming positions and allowed them to recover. This reduced the probability of mortality.

In conclusion, hook-caught fish may suffer a range of injuries, stresses and mortalities depending on the species, size of fish, water temperature, depth of capture, hook type and size, bait type and size, site and depth of hook penetration, and how the fish are released.

Mortality associated with gillnetting

It is commonly assumed that many bycatch fish might be injured and die during the capture process in gillnets or immediately after release from a net. Thompson et al. (1971) recorded mortality of 80 to 100 percent for gillnet escapees (chinook salmon, coho salmon). Thompson and Hunter (1973) separated scale damage mortalities from those associated with combined physical injuries and physiological stress. They suggested that scale damage alone resulted in mortalities of 40 percent, while scale damage and stress accounted for 80 percent of mortalities among salmon escaping from gillnets.

The material of the net is likely to have a substantial effect on the injury and subsequent survival (Van der Haegen et al., 2004). It is worth noting that Suuronen, Ikonen and Siira (2004) observed on average of only seven percent mortality among adult Atlantic salmon released from large floating trap-nets moored along the northern Baltic coast.

Hay, Cooke and Gissing (1986) collected Pacific herring passing through a 57-mm monofilament gillnet with a fyke net attached to the net. The fish were then transported to a cage and monitored for nine months. Mortality was low (< 2 percent) during the first two weeks, but rose afterwards (partly owing to high water temperatures at the cage site). Large herring had severe scale loss of up to 40 percent. Chopin, Arimoto and Inoue (1996) observed relatively high stress levels and mortality of sea breams captured by trammel net. The gill covers of fish were often held closed by the net. The degree of entanglement and constriction caused by netting around the fish’s bodies did not reduce as a result of struggling.

Generally, gillnet fishery appears to cause substantial damage to fish, and fish released from a gillnet may suffer high mortality. Post-release mortality caused by gillnet injuries is variable, and is clearly species- and fishery-dependent.

Other factors that are likely to affect the post-release mortality of gillnet-caught fish include water temperature and fish size and condition. It is also notable that gillnet mortality may continue long after the end of the eventual gillnet fishing season. Gillnets are frequently caught on the bottom and subsequently lost. Trawl gears also drag and cut into pieces the nets. Depending on the conditions, lost gillnets may continue to catch fish several months or even years before they gradually disintegrate (e.g. Tschernij and Larsson, 2003). “Ghost fishing” mortality caused by lost gillnets is an important issue in many fishing grounds. This problem can be partially addressed by the use of biodegradable materials that deteriorate easily or other means to disable unattended gillnets, and by facilitating the quick recovery of lost nets.

Mortality associated with lost pots

Fishing with pots generally results in catches that are alive and uninjured, so in most cases unwanted bycatch organisms can be released with a good chance of survival, although factors such as on deck injury and air exposure, and decompression or thermal shock, may jeopardize the survival of released organisms. However, pots are frequently lost in many types of crab and lobster fisheries (e.g. Smolowitz, 1978; Breen, 1987; Godoy, Furevik and Stiansen, 2003).

The head-line or hauling line may break during hauling, drifting ice may carry the pots away from the set location, the propeller of a vessel may cut the line from the buoy, or strong currents may force the buoy below the surface. The traditional materials used in the construction of pots do not deteriorate easily. Concerns have therefore been expressed as to whether lost pots continue to catch various commercial and non-commercial species, thereby contributing to unaccounted mortality.

Vienneau and Moriyasu (1994) and Hebert et al. (2001) found high mortality among snow crabs trapped in conical pots. Godoy, Furevik and Stiansen (2003), on the other hand, observed relatively low mortality in king crabs from lost pots; crabs escaped from the pots relatively easily.

These authors speculated that this difference may simply be connected to the fact that a snow crab is much smaller than a king crab, and may therefore find it more difficult to escape through the top entrance. Clearly, the overall pot design and the entrance size and shape influence the viability of escaped crabs, and thereby the mortality rates caused by lost pots. To prevent extensive ghost fishing, many pot fisheries are regulated to include galvanic timed releases or biodegradable materials that deteriorate easily.

Mortality associated with purse seine operations

Purse seining is generally known as a non-selective fishing method. Sometimes the catch, or part of it, is released from a purse seine during the haul-back process. Lockwood, Pawson and Eaton (1983) investigated the effect of crowding on mackerel during purse seine operations. They found that the mortality rate was related to fish density and duration in the net before slipping.

Attempts have been made to improve the selection in purse seining by using sorting grids. However, relatively high mortalities (> 40%) were measured in mackerel that escaped through a sorting grid with a bar spacing of 40 mm attached in a purse seine (Beltestad and Misund, 1996; Misund and Beltestad, 2000). Mackerel suffered severe stress and skin injuries during the selection process.

Misund and Beltestad (2000) concluded that the size-selection process in mackerel purse seining causes too high a mortality rate to allow it to be recommended for commercial fishing. Misund and Beltestad (1995) investigated the survival of Atlantic herring after simulated purse seine bursts. Their results indicated that there may be high mortality among herring connected to the net burst, which caused severe scale loss that may have resulted in mortality owing to severe osmoregulation difficulties.

Mortality associated with (scallop) dredging

Caddy (1973) estimated that between 10 and 17 percent of scallops died in the Gulf of St. Lawrence as a result of damage and predation associated with scallop dredging. McLoughlin et al. (1991) in Bass Strait, Australia, assessed dredge damage on dense beds of scallops, and estimated indirect mortality of scallops at 88 percent (owing to bacterial infection spreading through the residual population), which suggests a very high unaccounted post-harvest mortality. The authors estimated that 14 days after dredging, there were still more than 30 percent of the original number of live scallops on the sea bed, but this had decreased to less than 1 percent after 300 days.

Currie and Parry (1999), on the other hand, observed that the proportion of scallops damaged by dredging was generally low in Port Phillip Bay, southeastern Australia. The highest percentage (20 percent) of dead scallops was observed on the firmest (hardest) sediments, where scallops were occasionally trapped beneath the passing dredge skids. Messieh et al. (1991) reported that on the eastern Canadian continental shelf, scallop dredging destroyed (e.g. by crushing) about the same quantities of scallops as were caught by the dredges. Mortality of uncaught clams ranged from 30 to 92 percent.

The density of animals in fishing grounds was reduced by roughly 40 percent immediately after dredging, but returned to its original levels within ten months (Messieh et al., 1991). Within one hour of fishing, fish and crabs were attracted to the dredge tracks in densities that were three to 30 times greater than those observed outside the tracks. It is clear that the long-term mortality caused by dredging operations can be very high.

It is worth noting that bycatch may be another problem for dredge fisheries. For instance, dredging in southeastern Australia periodically catches spider crabs as bycatch (Currie and Parry, 1999); the mortality rates of discarded spider crabs average more than 50 percent.


All major types of fishing gear involve some degree of injury to fish. Stress response and injury vary according to gear type and mode of action. The likelihood of survival for fish that have been released from a hook varies. The injuries caused by a demersal gillnet are different from those caused by a trapnet, where fish can swim without any contact with the gear.

Gears deployed in surface waters do not include the effects associated with depth. Susceptibility to injury varies with species and stressor type. Survival ultimately depends on how well the fish are able to adapt to capture (and release) conditions. Clearly, differences in injury among gear types is an important aspect when considering the most appropriate and sustainable fish capture methods.

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