Are Sponges Damaged by Trawls
Three trawl paths exposed to one pass with the trawl in 1996 in about 200 m of water on the eastern Gulf of Alaska continental shelf were revisited in July 1997, 1-year post-trawl. Many big, erect sponges, the taxa affected most significantly, had been removed or damaged by the trawl. Sponges from the cold, deep water of the Gulf of Alaska were slow to recuperate from trawling effects.
These findings contrast with healing times for shallow, warm water sponges and might have fishery management consequences for cold-water regions. There’s growing concern that anthropogenic disturbance, especially commercial fishing activity, affects the seafloor and associated biological communities. In response to such concerns, an assortment of investigations regarding effects of mobile and nonmobile fishing equipment on biotic and abiotic elements of seafloor habitat has been completed.
Auster and Langton (1999) reviewed most, of those studies and noticed that certain kinds of commercial fishing usually have caused changes in species composition and diversity and a decrease in habitat complexity. Mobile fishing gear can reduce habitat complexity by eliminating emergent epiflora (Peterson et. Al., 1983; Fonseca et al., 1984; Peterson et al., 1987; Guillen et al., 1994) and epifauna (Bradstock and Gordon, 1983; Van Dolah et al., 1987; Collie et al., 1996, 1997; Engel and Kvitek, 1998) that provide structural habitat elements, and also by smoothing biogenic (Auster et al., 1996; Currie and Parry, 1996) and non-biogenic (Bridger, 1970, 1972) bedforms.
Work carried out at the hard bottom (boulder, cobble, pebble) habitat on the continental shelf in the Gulf of Alaska (Freese et al., 1999) revealed that one trawl pass could displace boulders and eliminate or harm large epifaunal invertebrates. Both these changes reduce habitat complexity on this substrate in the Gulf of Alaska. Within this region, the invertebrate taxa most likely to be damaged or removed by bottom trawling include gorgonian corals (Krieger, 2002) and many species of giant, erect sponges, which, together with boulders, provide the majority of the three-dimensional relief on the seafloor.
Sponges account for the majority of the invertebrate biomass due to their large size and relatively large population density. The capacity of damaged sponge communities to regenerate could have long-term management consequences for Essential Fish Habitat.
Van Dolah et al. (1987) and Tilmant (1979) showed that experimental trawling in hard bottom habitat in warm, shallow waters of the southeastern United States led to similar instant effects to those found by Freese et al. (1999). Van Dolah et al. (1987) returned to their study website 1-year post-trawl and discovered that sponge population densities had restored to pre-trawl amounts and that damaged sponges had regenerated. However, no quantitative data exist about the retrieval of sponges in profound, Coldwater habitats like the Gulf of Alaska following trawling. Objectives of the study were to determine 1) whether reductions in numbers of sponges observed by Freese et al. (1999) in experimental trawl paths instantly post-trawl lasted after one year, and 2) whether individual sponges ruined by the trawl showed signs of recovery or delayed mortality during this period.
The submersible vehicle Delta was chartered in July 1997 from the NMFS Alaska Fisheries Science Centre to revisit websites in the Gulf of Alaska that was subjected to experimental trawling in August 1996 (Freese et al., 1999). The Delta is a two-person research submersible capable of diving to a depth of 365 m; it can travel up to 6 km/h for up to 4 h. It’s equipped with external halogen lighting, an outside Hi-8 video camera, a handheld digital video camera, magnetic and gyro compasses, sub-to-tender communications, and an acoustic transponder for monitoring the vehicle (Freese et al., 1999).
In 1996, Freese et al. (1999) finished eight tows from a 42.5 m commercial trawl vessel towing a 4-seam, high-opening polyethene Nor’easter bottom trawl. The trawl was altered with 0.6 m tires at the bosom and fitted with 0.45 m rockhopper discs and steel bobbins across the wings. This sort of trawl was selected because it is very similar to equipment used over rough bottom from the commercial rockfish fishery in the Gulf of Alaska (Stark and Clausen, 1995).
Due to time constraints, only three of those eight 1996 trawl paths were revisited in 1997. These trawl paths were selected for further study due to a lot of sponges there in 1996. The start and end of trawl paths were indicated in 1996 with numbered orange flags on elastic whips fitted to lead weights; differential global positioning system (DGPS) coordinates of marker locations and compass beatings involving markers were reported to facilitate movement of the trawl paths in 1997.
The area sampled within trawl paths was estimated by using coordinates in the markers to ascertain trawl path length and multiplying this figure by 5 m, the width of the trawl tire equipment. The tire gear and trawl doors were the only areas of the trawl which were always in contact with the seafloor. Trawl tows in 1996 had been kept relatively short to preclude the cod end of the net from filling and thus narrowing the width of the mouth. Only those sponges which were inside the footprint of the trawl motorcycle equipment were counted.
After descending into the sea floor, the submersible pilot found a trawl path marker using coordinates provided by the aid vessel monitoring system. The submersible then followed the trawl path for its entire length, with the starboard side of the car remaining just outside the trawl marks on the sea floor. Continuous images of the trawl paths were obtained with the external camera. When sponges were situated in the trawl paths, the submersible pilot approached and circled them while the audience recorded pictures of the sponges with the digital video camera.
The soundtrack on the camera was used to record descriptions of damage to sponges, in addition to general observations. Sponges were categorised as
2) Vertical but ripped, or with pieces of the sponge body missing
3) Lying on the floor, either torn or unharmed.
Tears into the shape of the sponge were counted only if the tears were more than 10 per cent of the length of the longest axis of the body of the sponge. Likewise, sponges with less than 10 per cent of their bodies missing weren’t counted as ruined. Damage category three comprised sponges which were still attached to cobble or boulders that was rolled or dragged by the trawl, so the sponges were lying or touching on the substrate.
Only sponges >20 cm high were counted. Sponges were also analysed (both in situ and with videotapes from the lab ) for regrowth or repair of damaged body parts, according to rounding of jagged wounds or partial recovery of tears. After finishing operations in the trawl path, the submersible pilot maneuverer the vehicle approximately 100 m away and completed a reference transect roughly parallel to and equal in length to the trawl path.
Observations were recorded in a way identical to those made in the trawl path. Because trawl gear tire marks weren’t present in the reference transects, a distance of 5 m (the diameter of the tire gear on the trawl) in the submersible’s viewing port was estimated visually, and sponges within just that space were counted and analysed.
Water depths over the three trawl paths and three reference transects were 206-209 m. Lengths of trawl paths between markers were 350, 570, and 860 m. Lengths of reference transects were 370, 500, and 770 m. Lateral underwater visibility along all trawl paths and reference transects were approximately 10 m. Trawl course markers were relocated easily and seemed undisturbed. Furrows in the substratum brought on by the trawl tire gear were still notable after one year and showed little signs of backfilling. Boulders that was transferred by the trawl in 1996 were identified readily by the deep gouges they left from the substratum where they were dragged; the gouges revealed no signs of backfilling. Furrows and hauled boulders made the trawl paths readily recognisable.
Sponge morphology is highly variable, and visual identification in the field is problematic. Taxonomists frequently resort to a combination of histological, cytological, embryological, and biochemical procedures for identification of specimens (Brusca and Brusca, 1990). Bearing this caveat in mind, sponges encountered in this study were tentatively identified by gross morphology by comparing video pictures to plates and photos in the literature (especially Pavlovskii, 1955) and compared with specimens in the Alaska Fisheries Science Centre Auke Bay Laboratory reference collection. The species identified included the demosponges Esperiopsis sp., Mycale sp., and Geodia sp., and the hexactinellid glass sponge Rhabdocalyptus sp..
Esperiopsis and Mycale are similar in appearance (brownish-yellow, using a delicate basket or bell shape) and grow as large as 1 m. When nudged from the submersible, the two species appeared to be flexible, fast assuming their original shape after deformation. Esperiopsis and Mycale were discovered growing on large cobbles and boulders. Geodia was greyish-white and of a stronger basket form. Specimens were smaller than Esperiopsis and Mycale, attaining a maximum height of approximately 50 cm, and were more rigid, getting broken or upended when nudged by the submersible. Geodia climbed on cobbles and boulders.
Rhabdocalyptus was dark brown, columnar or barrel-shaped, and climbed to a height of over 1 m, even though the average height was about 40 cm. Specimens usually had long, needle-like spicules protruding from the body. Rhabdocalyptus was elastic and is generally found growing on the more exceptional substrate, such as small pebbles and gravel. In this paper, the four species are called basket sponges (Esperiopsis and Mycale), glass sponges (Rhabdocalyptus), and Geodia.
A total of 246 sponges >20 cm high was current in the three trawl paths (Table 1). These sponges were those who hadn’t been eliminated by the trawl in 1996. No new colonisation of sponges was evident in any of those three trawl paths. Mean density of the four species for all trawl paths combined was 2.76 sponges/100 [m.sup.2] (2.20-3.37 sponges/ 100 [m.sup.2]). Basket sponges, Geodia, and glass plantations comprised 66.7%, 26.0%, and 7.3percent of total sponges in the three trawl paths. We observed 115 (46.8percent ) sponges that showed signs of damage.
The incidence of harm for basket sponges, Geodia, and glass sponges was 43.3%, 59.4%, and 33.3%. Of the damaged sponges, 31.3percent were vertical, but tom and 68.7percent were lying on the floor, either torn or undamaged. Approximately equal amounts (34 vs 37) of basket sponges were in groups 2 and 3. In contrast, nearly all ruined Geodia (36 vs 2) and all six damaged glass plantations were in class 3.
Of the damaged basket sponges, 9.9% showed some degree of narcotization of the sponge body. Necrotic areas ranged from 5 to 90 per cent of the whole organism. Necrosis appeared to start with bruising and following discolouration of the pinacoderm from the immediate vicinity of the area where the sponge was tom, crushed, or otherwise damaged and then radiated away from the site of damage. The mesohyl was finally necrotised, leaving vast regions of the supportive framework of fibrous collagen (spongin) observable. Only the basket sponges seemed to be changed; Geodia and glass sponges showed no signs of necrosis.
Sponges were analysed for signs of repair or regrowth of damaged or missing body parts. Not one of the 115 damaged sponges in the trawl paths revealed signs of repair or regrowth. All wounds and tears seemed to be fresh with irregular surfaces, and no signs of rounding because of regrowth were noted. On the other hand, many sponges that were knocked over, or parts of sponge which had been tom free and were lying on the floor, still seemed workable after one year.
A total of 287 sponges of the very same species observed in the trawl paths exist in the reference transects (Table 2; mean density = 3.50 sponges/100 [m.sup.2] (2.63-4.79 sponges/100 [m.sup.2])). Basket sponges, Geodia, and glass sponges made up 74.6%, 20.9%, and 4.5percent of total sponges, respectively. Just four sponges (1.4percent ) were slightly damaged, having tears or missing body parts that were partially regenerated, according to rounding of jagged edges of wounds. No narcotization of any sponge at the reference transects was detected.
This study demonstrates that damage to some species of large, erect sponges brought on by trawling in deep waters on the continental shelf break off the coast of Alaska may persist for lengthy periods. In August 1996, instantly post-trawl, the density of sponges in eight trawl paths was 16% lower than the density of sponges from the eight benchmark transects (3.15 sponges/100 [m.sup.2] vs 3.73 sponges/100 [m.sup.2]; Freese et al., 1999). In July 1997, 11 months post-trawl, density of sponges from the three trawl paths was 21% lower (2.76 sponges/100 [m.sup.2] vs. 3.50 sponges/100 [m.sup.2]) than of sponges from the three reference transects.
In addition to the persistent decrease in population density of sponges, damage to person sponges also seems to be long-lasting. In 1996, Freese et al. (1999) observed that 67 per cent of large sponges (therein called”vase” sponges) in eight trawl tracks were ruined, compared to 2% in reference transects. The incidence of harm to the sponges from the three trawl paths analysed in this study was 47%, compared to 1 per cent in the benchmark transects.
That the incidence of damage to sponges from the three 1997 trawl paths is 20 percentage points less than the prevalence of injury observed in 1996 shouldn’t be taken as proof that human sponges had repaired wounds such as rips, or regenerated missing body parts. Evaluation of the video information yielded no case of a sponge undergoing regeneration or repair of damaged or missing body parts. All tears and other wounds appeared jagged and recently produced during the 1997 survey, 11 weeks post-trawl. No signs of rounding or smoothing of rough wound borders, which would indicate the incidence of recovery, was noted.
A probable explanation for the seeming decrease in the prevalence of damaged sponges between 1996 and 1997 is that the narcotization of ruined sponges found in 1997. Approximately 10 per cent of ruined basket sponges were partly to almost wholly necrotised in 1997. Along with necrotised sponges, remnants of the inviting skeletons (spongin) of basket sponges were observed lying on the sea floor in the trawl path. Probably a proportion of damaged basket sponges can’t repair wounds or regenerate body parts quickly enough to counteract ongoing necrosis (presumably due to fungal or bacterial agents) and finally succumb. Therefore, the observed decrease in the incidence of damage to sponges over time is most likely the consequence of delayed mortality of damaged specimens.
It’s worth noting that the sort of damage sustained by sponges varies according to species, likely due to differences in skeletal structure and substrate preference. Sponges with elastic skeletons which are usually attached to big boulders, such as basket sponges, are more likely to sustain tears or be severed from their foundations by the trawl. More rigid species, like Geodia, often seen attached to cobbles, would likely be crushed or rolled over; and glass sponges, flexible but usually associated with finer substrates and connected by tufts of spicules extending to the substrate (Brusca and Brusca, 1990), could be hauled in the substrate in their entirety.
Little information is available about the retrieval of sponges after damage by trawling. Van Dolah et al. (1987) conducted an experimental trawling research on hard-bottom habitat away from the southeastern U.S. shore. The results of the study generally agree with the findings of Freese et al. (1999), because sponges in trawl paths suffered aa linear post-trawl decrease in density and increased incidence of harm to individual specimens.
After 1 year, however, numbers of sponges in these trawl paths equalled or exceeded pre-trawl densities, and individual sponges that were damaged by the trawl couldn’t be relocated because wounds had healed and severed parts had regenerated. Additionally, no moribund or necrotized sponge has been noted. These findings contrast sharply with the results of the study and indicate that reductions in habitat complexity because of elimination of sponges by trawling activity in deep, cold-water habitats such as are located in the Gulf of Alaska may be more persistent than in warmer, warmer waters because of faster growth rates in warmer, warmer waters.
The 1996 Magnuson-Stevens Fishery Conservation and Management Act requires that regional fishery management councils identify and take steps to safeguard Essential Fish Habitat for managed species. The Act defines Essential Fish Habitat as “waters and substrate necessary to fish for spawning, breeding, feeding, or growth to maturity.” The substrate, as described in the Act, includes sediment, hard bottom, structures, and associated biological communities.
The North Pacific Fishery Management Council has designated living substrates in deep waters as habitat areas of particular concern (HAPC) because these regions offer high micro-habitat diversity and are considered easily impacted by fishing activities (NPFMC, 2000). Seafloor habitat with higher population densities of large sponges could qualify as HAPC. I discovered that such habitat is very vulnerable to commercial trawling in the shod-term (1 year), suffering immediate declines through immediate removal of sponges and additional reductions in population densities of sponges because of delayed mortality.
Unlike sponge communities in warm, shallow waters, sponge communities in the Gulf of Alaska don’t seem to be able to go back to pre-trawl population levels after 1 year, nor do human sponges appear to be able to recuperate quickly from wounds suffered by trawl gear. However, this study covers only a 1-year period, and recovery rates for some species could be in excess of several years; consequently, real long-term recovery rates are indeterminate now.
Little is known about the biogeography or community institutions of sponges in deep waters in the Gulf of Alaska. However, due to the intricate habitat that they provide and because of the demonstrated vulnerability of sponge communities to trawling, additional research must be carded out to record the geographic distribution and abundance of these organisms in the Gulf of Alaska, to determine the relative value of sponges as habitat for commercially important species, and to ascertain long-term effects of trawling on healing rates of sponges.