Defenders' Experts
Fishing Gear and its Deadly Impact on the California Sea Otter
Introduction
Fishing gear used by commercial fishermen to catch specific types of fish can be lethal to non-target species. The problem of bycatch -- the capture of nontarget species -- is particularly troubling when it involves the take of listed and/or declining species, such as the southern sea otter, which is both federally listed as threatened and has been declining in recent years. Off the California coast, by-catch of sea otters has resulted from the use of gill and trammel nets and is suspected to have occurred from the use of live fish traps.
Gill Nets
Gill nets are single-walled nets made of nylon or monofilament that are hung without slack and used to fish for variety of species depending on where the nets are set. Two major gillnet fisheries in California have been known to have serious impacts on marine mammals, turtles and birds. The drift gillnet fishery targets broadbill swordfish, common thresher shark, and shortfin mako shark. The California set gillnet fishery targets California halibut and Pacific angel shark. The nets hang like curtains in the water, with weights on the bottom and floaters on the top. Gill nets are designed to snare bigger fish and allow smaller ones to escape. However, the nets, which are strung out for lengthy periods at a time (12 to 24 hours), have proved deadly to marine mammals and shore birds too large to slip through.
A recent year-long survey of fishing impacts on coastal animals revealed that the set gillnet fleet killed an estimated 5,200 common murres (a diving seabird) and more than 100 harbor porpoises each year (these are projected totals for the calendar years 1999-2000 and with 23% observer coverage). Common Murres are also an imperiled species with low numbers. Sea otters, California sea lions, harbor seals and elephant seals have also been killed by the nets. This occurs when these animals are diving and foraging for food and become entangled in the nets and drown.
Trammel nets
Trammel nets are nets constructed with more than one wall of webbing. Though fishermen today seldom fish using the old three-walled trammel nets, fishermen fishing for California halibut create de facto trammel nets by attaching suspenders to the nets, creating slack in the net.
These large mesh nets are generally fished for 24-hour periods (overnight), and have and continue to result in the entanglement and drowning of non-target species such as the sea otter.
Live fish traps
Live fish traps do just what the name implies: catch fish live. Live fish traps have large openings that lead down to a much smaller chamber – fish can swim in but they can’t swim out. Though a dead sea otter has not been documented in a live fish trap off the California coast, it is believed that California sea otters do have encounters with live fish traps that may prove deadly. Drowned otters have been found in similar traps used to fish for crabs in Alaska. In addition, dead otters have been found in California with bruising patterns in the armpit area consistent with having swum into a fish trap and having difficulty freeing themselves. In experiments conducted at the Monterey Bay Aquarium, otters living in captivity swam into live fish traps placed into their aquarium. These otters were immediately rescued from the traps by observers, but the experiment supports the likelihood that, in the wild, sea otters also swim into and become ensnared in these traps.
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