112 L
.O. Veale et al. J. Exp. Mar. Biol. Ecol. 255 2000 111 –129
1. Introduction
Mobile demersal fishing gears are known to have a detrimental effect on benthic epifaunal and infaunal communities Dayton et al., 1995; Thrush et al., 1995; Jennings
and Kaiser, 1998, often dramatically increasing local mortality in the wake of the gear Kaiser and Spencer, 1994b; this may have long-term implications for the structure of
the community Thrush et al., 1998. Both those animals disturbed or damaged by the passage of the gear and left on the seabed, and those retained in the catch and
subsequently discarded, are known to attract mobile predators Kaiser and Spencer, 1994a, 1996, which act as facultative scavengers Britton and Morton, 1994. This
ability to feed opportunistically on dead and dying animals, coupled with robustness to capture and damage in the fishing gear, may confer an increased survivorship on certain
species that, in turn, may lead to an enhanced population size Polis et al., 1996; Ramsay et al., 1997b, e.g. the flatfish, Limanda limanda L. Kaiser and Ramsay, 1997. In
intensively fished areas, such as the North Sea, this carrion will inevitably subsidize some marine food webs Furness, 1996; Ramsay et al., 1997b. The levels of fishing
effort presently exerted by scallop Pecten maximus and queen scallop Aequipecten opercularis dredging in the northern Irish Sea approx. 177,000 metre hours for the
1994–95 fishing season will potentially introduce a large amount of by-catch material [approximately 231 tonnes for the 1994–95 season L. Veale, unpublished data] into the
food web; a proportion of this will become available to benthic predators and scavengers.
Aggregations of scavenging species after the passage of towed demersal fishing gears have been recorded in several previous studies, mainly by submersible, video or diver
observations made along a fished track e.g., Medcof and Bourne, 1964; Caddy, 1973; Chapman et al., 1977; Murawski and Serchuk, 1989; Kaiser and Spencer, 1994b. These
have all noted increased densities of several fish and invertebrate scavenging species in response to fishing activities, but details such as the relative attractiveness of the
different prey species, the importance of prey damage, and the relationship between direction of attraction and water movement, have not been addressed. It is of particular
importance to ascertain whether apparently undamaged animals are equally likely to be preyed upon after discard, as this will have implications for the potential benefits of gear
modifications designed to reduce animal damage.
Some studies have used baited time-lapse stills cameras deployed from research vessels to investigate the scavengers attracted to damaged animals typical of material
discarded from fishing gears e.g., Kaiser and Spencer, 1996; Ramsay et al., 1997b. Here, the maximum scavenger activity occurred within 24 h. This approach allows a
detailed investigation of the arrival times of different scavenging species and the rate of bait consumption. However, a stills camera will inevitably return fewer frames than a
video camera, and its deployment from a ship will either limit the duration of the investigation, or incur high ship-time costs. Invertebrate scavenger species are generally
slow moving, and slow at ingesting and digesting food, so it is important that such investigations are of sufficient duration to adequately record the arrival of all species
attracted to the prey, and to monitor their subsequent departure.
A static time-lapse video system was deployed in this study to identify the major
L .O. Veale et al. J. Exp. Mar. Biol. Ecol. 255 2000 111 –129
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species involved in scavenging dead and damaged benthos typical of that either discarded from scallop dredges, or damaged and left on the seabed. A 1000 m long cable
allowed the camera to be positioned offshore, on an area of seabed typical of that supporting the local scallop fishery, but within an area closed to commercial fishing
Bradshaw et al., 2000. Recordings totalling over 2000 h were made, which would have been impossible to achieve using SCUBA divers or cameras deployed from a ship.
These extended time periods ensured that the aggregation and dispersion of slow-moving invertebrate scavengers were adequately monitored.
Firstly, the relative merits of using white and red light to record nighttime footage were compared. Next, the aggregation of scavengers on mixed damaged benthos was
examined, and then the attractiveness of different by-catch species was investigated using four mono-specific baits: damaged Aequipecten opercularis, Pecten maximus,
Asterias rubens and Buccinum undatum L. These species were chosen as they represented some of the most abundant members of the catch assemblage of the north
Irish Sea scallop fishery Veale et al., 2000. The composition and size of scavenger aggregations attracted to both damaged and undamaged A
. opercularis were then compared. An exploration of the interaction of water current direction and distribution of
olfactory stimuli was conducted for Asterias rubens.
2. Materials and methods
