(Pecten Maximus (L.)) Dredge Discards Using a Static Time-Lapse Camera System
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Journal of Experimental Marine Biology and Ecology 255 (2000) 111±129 www.elsevier.nl/locate/jembe An in situ study of predator aggregations on scallop (Pecten maximus (L.)) dredge discards using a static time-lapse camera system L.O. Vealea,* , A.S. Hill b , A.R. Brand a aUniversity of Liverpool Port Erin Marine Laboratory, Port Erin, Isle of Man IM96JA, UK bSEPA, Clearwater House, Heriot-Watt Research Park, Edinburgh EH14 4AP, UK Received 25 May 2000; received in revised form 19 September 2000; accepted 21 September 2000 Abstract The impact of demersal ®shing gears on benthic habitats and species has been the subject of much attention recently, and suggestions have been made that scavenging epifaunal species may bene®t at the population level from the additional food source provided by discards. This paper investigates some aspects of this process, including the relative attractiveness to predators of different discard species, and the role of damage in scavenger attraction. A time-lapse video system with a 1000 m long cable was positioned in an area closed to ®shing, adjacent to the most heavily ®shed scallop (Pecten maximus) ground in the Irish Sea. A variety of undamaged and damaged by-catch animals were positioned in front of the camera, and the subsequent predator aggregations investigated. Densities of scavenger species up to 200 times that of the background population were observed, and aggregations of some species persisted for up to 3 days. The most frequently recorded scavengers, and therefore presumably those species most likely to bene®t from discards as a food source, were: Asterias rubens L., Astropecten irregularis (Pennant), Liocar- cinus spp Stimpson, Pagurus spp Fabricius and Callionymus lyra L. Predator attraction to apparently undamaged queen scallops, Aequipecten opercularis (L.), was almost as high as to damaged A. opercularis. Of all the prey species studied, queen scallops were the most attractive to scavengers. A directional relationship was found between the ambient water current and the arrival of the star®sh, Asterias rubens. 2000 Elsevier Science B.V. All rights reserved. Keywords: Fisheries by-catch and discards; Fishing impact; Pecten maximus; Predator aggregation; Time-lapse video *Corresponding author. Tel.: 144-1624-831-017; fax: 144-1624-831-001. E-mail address: [email protected] (L.O. Veale). 0022-0981/00/$ ± see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(00)00295-1 112 L.O. Veale et al. / J. Exp. Mar. Biol. Ecol. 255 (2000) 111 ±129 1. Introduction Mobile demersal ®shing 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 ®shing 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 ¯at®sh, Limanda limanda (L.) (Kaiser and Ramsay, 1997). In intensively ®shed areas, such as the North Sea, this carrion will inevitably subsidize some marine food webs (Furness, 1996; Ramsay et al., 1997b). The levels of ®shing 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 ®shing 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 ®shing gears have been recorded in several previous studies, mainly by submersible, video or diver observations made along a ®shed 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 ®sh and invertebrate scavenging species in response to ®shing 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 bene®ts of gear modi®cations 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 ®shing 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 suf®cient 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 ±129 113 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 ®shery, but within an area closed to commercial ®shing (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-speci®c 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 ®shery (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 From the 14th June to 5th October 1996, a static video camera system was deployed in the area closed to ®shing off Port Erin, Isle of Man. This was during the closed season for the great scallop, which runs from 1st June to 31st October inclusive: no commercial dredging occurred in the vicinity during the study. A Rovtech Systems low-light colour camera with two 250 W lights was used. The signal was transmitted to a terrestrial control box via a booster unit and 1000 m of cable. The image was recorded on a Panasonic SVHS time-lapse video recorder, and real time video was viewed on a JVC colour monitor. The camera was situated approximately 600 m offshore in 25 m depth, due west of the Marine Laboratory (Fig. 1), mounted on a galvanised Dexian frame at an angle of 458, 1 m above the seabed. The camera was orientated facing east, perpen- dicular to the prevailing currents in the area, which travel north or south, depending on tidal state. Throughout the study period divers positioned a variety of baits loose on the seabed in front of the camera. Mixed bait comprised a variety of dredge-caught epifaunal species, including Aequipecten opercularis, Pecten maximus, Neptunea antiqua (L.), Buccinum undatum, Echinus esculentus Lamarck, and Asterias rubens, which were damaged just prior to baiting: damage was applied with stones and metal bars to simulate that observed in damaged by-catch animals. Baited periods were interspersed with unbaited (control) periods, and the camera was repositioned three times to reduce the number of animals permanently associating themselves with the frame, although no differences in background (unbaited) abundance were noted after repositioning. Initially, the relative merits of using white and red light to record nighttime footage were compared in single 3-day unbaited trials with each type of light. Then the 114 L.O. Veale et al. / J. Exp. Mar. Biol. Ecol. 255 (2000) 111 ±129 relocated over three positions throughout the study. Fig. 1. Approximate positions of the ®xed camera within the area closed to ®shing. Diamonds indicate the positions of navigation buoys. The camera was L.O. Veale et al. / J. Exp. Mar. Biol. Ecol. 255 (2000) 111 ±129 115 aggregations on mixed damaged benthos were investigated, with two unbaited/baited comparisons. Mono-speci®c baits of Aequipecten opercularis, Pecten maximus, Asterias rubens and Buccinum undatum were also studied in single unbaited/baited trials, and then ®nally the effect of damage incurred by Aequipecten opercularis on the ensuing aggregation was investigated by a single undamaged/damaged comparison.