Benthic Habitats and the Effects of Fishing

<strong><em>Abstract. </em></strong>Subtidal benthic habitats from the Piscataqua River, New Hampshire and Maine, have been delineated by an automated segmentation technique using bathymetry derived from multibeam echo sounder data. The map, produced by segmentation of seafloor textures, represents a hypothetical benthic habitat map that requires ground-truthing. Video mosaics are being used to ground-truth substrate composition and transitions apparent in the bathymetry data map and to describe biological features and organism occurrences and densities. Here, we describe the utility of video mosaics for ground-truthing benthic habitat characteristics and present two examples of their use. Video mosaics acquired along two transects in the Piscataqua River were used to detect substrate transitions apparent in the bathymetry that were identified as distinct hypothetical habitat types and to quantitatively assess coverages of distinct sediment conditions, density of megafaunal organisms (lobsters), and bioturbational features (crab feeding pits).

Benthic Habitats and the Effects of Fishing

Laser-based underwater imaging sensors have been developed and matured in the last decade that provide high resolution optical imagery of the sea floor. Laser Line Scan (LLS) and Streak Tube Imaging Lidar (STIL) have been particularly successful. A prototype Fluorescence Imaging Laser Line Scan (FILLS) sensor has been deployed in several underwater environments, yielding highresolution (~1 cm pixel size) imagery of the associated benthic habitats. The prototype FILLS sensor illuminates the sea floor with 488 nm laser light, and constructs four independent images from light collected at 488 nm, 520 nm, 580 nm, and 685 nm, respectively. The 488 nm image is formed from elastically scattered light (i.e., light scattered with no change in photon energy), while the other images are formed by inelastically scattered light. (The FILLS sensor is routinely operated during nighttime hours so that ambient illumination is negligible). Fluorescence is the primary physical mechanism giving rise to the inelastically scattered light sensed by FILLS. Coral reef environments produce particularly strong (and spectacular!) fluorescence imagery. FILLS was developed primarily for the detection, classification, and identification of man-made objects in underwater environments. In addition it can serve admirably for the characterization of underwater habitats. Examples of FILLS imagery relevant to fish habitat evaluation will be presented.

Benthic Habitats and the Effects of Fishing

<strong><em>Abstract. </em><strong>Fishers have often complained that standard United Kingdom groundfish survey data do not adequately reflect the grounds targeted by commercial fishers, and hence, scientists tend to make overcautious estimates of fish abundance. Such criticisms are of particular importance if we are to make a creditable attempt to classify potential essential fish habitat (EFH) using existing data from groundfish surveys. Nevertheless, these data sets provide a powerful tool to examine temporal abundance of fish on a large spatial scale. Here, we report a questionnaire-type survey of fishers (2001–2002) that invited them to plot the location of grounds of key importance in the Irish Sea and to comment on key habitat features that might constitute EFH for Atlantic cod <em>Gadus morhua</em>, haddock <em>Melanogrammus aeglefinus</em>, and European whiting <em>Merlangius merlangus</em>. Plotted grounds were cross-checked using records of vessel sightings by fishery protection aircraft (1985–1999). A comparison of the areas of seabed highlighted by fishers and the observations made on groundfish surveys were broadly compatible for all three species of gadoids examined. Both methods indicated important grounds for cod and European whiting off northern Wales, the Ribble estuary, Solway Firth, north of Dublin, and Belfast Lough. The majority of vessel sightings by aircraft did not match the areas plotted by fishers. However, fishing restrictions, adverse weather conditions, and seasonal variation of fish stocks may have forced fishers to operate outside their favored areas on the (few) occasions that they had been recorded by aircraft. Fishers provided biological observations that were consistent among several independent sources (e.g., the occurrence of haddock over brittle star [ophiuroid] beds). We conclude that fishers’ knowledge is a useful supplement to existing data sets that can better focus more detailed EFH studies.

Benthic Habitats and the Effects of Fishing

<strong><em>Abstract. </em></strong>In late 1994, substantial portions of Georges Bank were closed to commercial fishing to assist with stock rebuilding. These areas were Closed Area I (CAI), located on the western portion of the bank, and Closed Area II (CAII), on the eastern portion. After about 5 years of closure, the southern portion of CAII and the central portion of CAI, having exhibited substantial increases in biomass and density of sea scallops <em>Placopecten magellanicus</em>, were reopened to scallop fishing. Before the industry was allowed entry, we conducted surveys to monitor the recovery of benthic habitat and fauna inside both areas. Sampling sites were selected in a paired station design for an inside–outside comparison representative of major habitat types in each closed area; other stations were chosen to survey the remainder of the closed areas. At each station, we examined a suite of biotic and abiotic variables ranging from substrate type to benthos to nekton. Our results suggest few differences between the inside–outside paired stations in both closed areas for nekton and benthic species composition and species richness. Fish abundance and biomass were similar inside and outside the closed areas. However, individuals of species such as skates (<em>Raja </em>spp.), haddock <em>Melanogrammus aeglefinus</em>, and flounders (Pleuronectiformes) were generally larger inside than outside the closed areas. Additionally, habitat type was important in determining the distribution, abundance, biomass, size, and feeding ecology for some of the more benthic-oriented species studied. In CAI, the differences we observed in the suite of biotic metrics are likely a result of the high diversity of habitat types, with many of the habitat types composed of higher-relief material (e.g., cobble, gravel, etc.) in the region. The seabed in the southern portion of CAII is a relatively high-energy sand habitat of low to moderate complexity and has a relatively low vulnerability to trawling and dredging, which may explain why there were less pronounced differences in abundance or biomass across habitat types in that closed area as compared to CAI. Other parts of closed areas on the northeastern shelf may exhibit more obvious changes in the same biological metrics due to the presence of more complex habitats and increased vulnerability to bottom tending fishing gear. Those differences we observed for CAI and CAII may have implications for the population dynamics of commercially valuable benthic species, yet that question remains a major challenge.