Static and drift net fisheries
Set nets of various types are a particular hazard
to diving seabirds and have been implicated in the
decline of seabird populations in some parts of
the world 7,8,9,17,29. In northern Norway,
for example, the breeding populations of guillemots
at two sites are estimated to have declined by 95%
from the early 1960's to 1989, a figure which could be explained
entirely on gill net mortalities based on observed
catch rates. The numbers of birds killed in nets
depends on their abundance, diving habits and distribution
within the fishery area7. Species which
have been caught in these nets include shearwaters,
red-throated divers, Leach's
petrel, gannet, shag, guillemot, razorbill, and
great northern diver. Ducks such as the common scoter
and long-tailed duck are also known to have become
entangled and die in set nets7.
Inshore gill nets can have a relatively high incidental
by-catch around diving seabird colonies or where
there are high densities gathered on the water surface,
making it inadvisable to set nets in such areas.
Large numbers of razorbills are known to have drowned
in gill nets at the mouth of the Tagus estuary in
Portugal, for example, where this species congregates
on occasions43. Nets set for bass have
caught large numbers of diving birds (mostly razorbills
and divers) and in one incident in the UK an estimated
900 auks were caught over 8 days in nets set below
seabird colonies17. Herring nets and
bottom-set cod nets have also killed large numbers
of diving seabirds (an estimated 25,000 in the southeast
Kattegat between 1982 and 1988), most of which were
found in the bottom-set cod nets45, and
catches of shags in trammel nets may be a threat
to populations of this species in Spain43.
The threat will depend on which species are present
at the time nets are put out, weather, tidal fluctuations
and fishing effort. Gill and tangle net fisheries
in Cardigan Bay, for example, often occur at or
near the cormorant colony but to date there has
been no major entanglement problem10.
High incidental catches of guillemots, razorbills
and divers have been reported in drift nets from
Danish fisheries, and significant catches of auks
in the salmon driftnet fisheries in Ireland and
Denmark43. Anti-predator nets around
aquaculture facilities are also known to entangle
seabirds59,82. Ghost fishing by lost
nets and fragments of nets is also known to entangle
birds but the scale of mortality associated with
this is unknown45. Similarly, the effect
of non-net fisheries, such as long lining and pots,
and in mobile nets is not well known in UK waters
although catches are reported from elsewhere.
The direct and indirect effects of molluscan shellfisheries
and aquaculture on birds are mentioned elsewhere.
An indirect effect of some finfish fisheries has
been an increased food source for some seabirds
resulting from the discarding of by-catch and offal.
The discards are taken by species such as fulmar,
gannet, great skua, common gull, great black-backed
gull and herring gull and appear to have contributed
to the rapid growth of some seabird populations.
It is now considered to be such an important component
of the diet of scavenging seabirds in the North
Sea that changes in the amount of discards may affect
the relative and absolute abundance of various species.
Using fisheries data from the late 1980's
and early 1990's, the number of seabirds potentially supported
by the fishery waste from North Sea fisheries has
been estimated to be around 5.9 million and an area
based analysis suggests that discards may easily
support all scavenging seabirds in southern and
southeastern sub-regions of the North Sea55.
Summary of potential effects of fishing on sea
birds listed in the Birds
Accidental capture of diving birds foraging
for food in and around nets.
Increase in scavenging seabird populations
due to the increased availability of food
caused by discarding of unwanted catch
Entanglement in anti-predator nets
Short term increase in scavenging seabirds
due to increased food
General disturbance of feeding and roosting