Smooth Bottom Net Trawl Fishing Gear Effect on - New England ...

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NOAA/NMFS Unallied Science Project, Cooperative Agreement NA16FL2264 December 2005 <strong>Smooth</strong> <strong>Bottom</strong> <strong>Net</strong> <strong>Trawl</strong> <strong>Fishing</strong> <strong>Gear</strong> <strong>Effect</strong> on the Seabed: Investigation of Temporal and Cumulative <strong>Effect</strong>s BKAM/CR common. The hemichordate, Phoronis, was among the dominant prey for yellowtail. Three groups of amphipods (caprellids, phoxocephalids and ampeliscids) were important food items at Mud Hole for blackbacks. At Little Tow aroids were again among the dominant prey. November stomach contents were dominated by spionids except for blackback flounder from Little Tow. The average number of organisms found was 47.4 to 443.6. Numbers of species present ranged from 9.0 to 25.7. Cerianthid anemones were the most common prey for blackbacks at this site and in many cases stomachs were filled with a few large individuals of Cerianthus, leaving little room for anything else. Additional common prey items were ampharetids, cirratulids and phoronids. Cerianthids, which can grow to a large size were almost absent in the diet for yellowtail, which have smaller mouths. There was fairly good comparison between the species listed as dominants in the benthic grab samples and those found in the stomachs of flounders. Spionids were abundant in both cases. Phoronids were more abundant in October and November benthic sampling events and they became more common among the prey items. There are other taxa for which there seem to have been some selectivity because they were found among the dominant components of the stomachs but not in the grab samples. Some of these species might have been present in good numbers in grab samples but their relative numerical dominance was greater in fish stomachs. Such taxa include flabelligerids and lumbrinerieds (polychaetes). Others, with low densities in infaunal samples, were among the dominants in stomach analyses; e.g. caprellids, aorids, ampeliscids and phoxocephalids among the amphipods, and the anemone Cerianthus. The total number of individuals (abundance) and numbers of species (richness) found in yellowtail and blackback stomachs were compared (Table 3.6-9) and tested for significant differences. With the exception of the August survey when very little material was found in any of the stomachs, the number of organisms found in yellowtail stomachs was significantly higher than found in blackbacks. Species richness was also significantly higher in yellowtail stomachs than in blackbacks except in August. Yellowtail with their small mouths, apparently select smaller, more abundant organisms as their food supply, and a wider variety of species. Blackback flounder are able to select larger, less abundant species as a significant component of their diet. They do however also consume many of the small polychaetes that are the staple of the yellowtail diet. 61
4.0 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE STUDIES 4.1 Disturbance and Ecological Structure The Massachusetts Bay trawling impact study has addressed the impacts of trawling on physical attributes of the seabed and on diversity, abundance, and successional status of the benthos. Results of our studies in 2001 and 2002 indicate that impacts of net sweep and the ground cables are not great relative to untrawled reference areas. Local impact of trawl door furrows remains moot as the REMOTS® survey apparently did not sample these features. Faunal data also indicate that there were no great differences between trawled and “control” (reference) areas in terms of physical or ecological structure of the seabed. The ambient benthic infauna is adapted to natural disturbance in the form of bed-load transport of sand and the resuspension of fines by tidal turbulence. It is likely that the impacts of trawling on the infaunal benthic communities at Mud Hole and Little Tow are comparable in magnitude to these natural disturbances. This assertion may not hold true for trawl door furrows as these features, although a small proportion of the impacted bottom, were not adequately sampled. 4.2 Disturbance and Ecological Dynamics The 2001 and 2002 trawling studies have focused on seafloor bathymetry, sedimentary structures, benthic invertebrate and fish inventories, and fish stomach contents. Rate dependent processes were not addressed. Any deeper understanding of the effects of trawling will require information about these rate sensitive processes. For example: 1.) What are the rates of infaunal recovery (rate of arrival of colonizing individuals and species per unit time) in disturbed bottom areas affected by both natural and trawling disturbances? 2.) How do these disturbances impact secondary productivity of the bottom (change in prey biomass per unit area per unit time)? If these two questions can be answered, one may be able to determine (in advance) the upper rate of trawling that a site can sustain without compromising bottom secondary productivity. The ecological impact of trawling on the benthic infauna, as described in our Massachusetts Bay study and those cited from Swedish waters (Nilsson and Rosenberg, 2003) and the Mediterranean (Rosenberg, et al., 2003 and Smith, et al., 2003), indicate that disturbance by trawling does not cause total mortality of the impacted areas. Rather, near surface-dwelling organisms tend to be more severely impacted than deeper-living species. By definition, faunal recovery therefore takes place as a secondary succession (primary succession involves repopulation of a substratum representing competition-free space). The rate and mode of recolonization and succession is scale-dependent and also is affected by the kinetic energy of the ambient bottom (McCall, 1977 and Whitlach, Lohrer, and Thrush, 2003). Small-scale disturbances such as anchor scars, trawl door furrows, predator foraging pits, etc., can be very rapidly recolonized on a scale of hours to days by immigration of juvenile/adult organisms from the adjacent ambient bottom (i.e. non-larval recruitment). This is especially 62
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SMOOTH BOTTOM NET TRAWL FISHING GEA
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NOAA/NMFS Unallied Science Project,
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Page 90 and 91: Station LT02-3A Trawl</stro
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Page 100 and 101: Table 3.6-2 2002 Trawl</str
Page 102 and 103: Table 3.6-3 (continued) CRUSTACEA A
Page 106 and 107: Table 3.6-5 Summary of Blackback (W
Page 108: Table 3.6-5 (continued) OTHERS Sarc
Page 113 and 114: Table 3.6-7 Dominant Species in Sed
Page 115 and 116: FIGURES
Page 117 and 118: Figure 1.0-2. Smooth</stron
Page 119 and 120: Figure 1.0-4 Side-scan sonar base m
Page 121 and 122: Figure 3.2.1-2 Bathymetric surface
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Number/minute Number/minute 4 3 4 2
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Number/minute Number/minute 30 20 1
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Number/minute Number/minute 2.5 2 1
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Number/minute Number/minute July -
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Figure 3.4-2 For key species, the a
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Figure 3.4-4 For key species, the a
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Figure 3.4-6: Cluster analysis of M
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Figure 3.4-9 Little Tow 2002 Cluste
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Figure 3.4-12a. Little Tow 2001 and
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Note: [LT02 = Little Tow 2002 sampl
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Results of 2002 REMOTS Surveys to E
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Kg/Tow 200 180 160 140 120 100 80 6
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Kg/1000m2 Kg/1000m2 7 6 5 4 3 2 1 0
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Kg per 1000m2 350 300 250 200 150 1
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Numbe of individuals Number of Indi
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Number of individuals Number of ind
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PHOTOGRAPHS
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Photograph 2.2-2 Christopher Dunbar
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a. Chris Dunbar sieving benthic sam
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Photograph 2.7-1 CR Environmental e
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a. Fisherman Andrew Mirarchi viewin
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2001 2002 Cobble Bottom</st
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MH-3A July 2002 Pre-Trawl</
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MH-4A July 2002- Pre-Trawl<
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LT-1B July 2002- Pre-Trawl<
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LT-3A July 2002- Pre-Trawl<
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LT-4A July 2002- Pre-Trawl<
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Cobble bottom at Little Tow Small r
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2001 2002 Flounder Rock Crabs Sea S
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