Underwater Video Camera Research Articles

Influence of tides on assemblages and behaviour of fishes associated with shallow seagrass edges and bare sand

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 456:187-199 (2012) - DOI: https://doi.org/10.3354/meps09695 Influence of tides on assemblages and behaviour of fishes associated with shallow seagrass edges and bare sand Alistair Becker1,*, Christine Coppinger1,2, Alan K. Whitfield1 1South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa 2Department of Ichthyology and Fisheries Science, Rhodes University, PO Box 94, Grahamstown 6140, South Africa *Email: a.becker@saiab.ac.za ABSTRACT: Seagrass beds and associated fish assemblages have been the focus of a large number of past studies. However, the influence of small water depth changes due to tides on fish species along shallow seagrass bed edges is unknown. In this study an underwater video camera was deployed to examine the fish community at 3 seagrass edge (Zostera capensis) and 3 sand sites over incoming spring and neap tides in the Bushmans Estuary, South Africa. Small changes in tidally driven water depth had significant effects on the abundance of common fish taxa, with gobies decreasing and others, including mullets, increasing in abundance. These general patterns were consistent in both seagrass and sand sites, as well as during spring and neap tidal cycles. There were inconsistent differences in the abundance of common fish taxa between seagrass edges and bare sand. Importantly, when differences did occur, these were contingent on tidal stage or linked to different tidal cycles (neap or spring), highlighting that small changes in depth may influence edge effects. The behaviour of fish did not appear to be influenced by water depth, but for some taxa there were distinct differences in swimming behaviour between those individuals associated with seagrass beds and those situated over bare sand. These small temporal scale changes in water depth are a previously overlooked aspect of littoral seagrass landscapes that may be important in structuring associated fish communities. KEY WORDS: Underwater video · Water depth · Estuary · Fish movements · Zostera capensis · South Africa Full text in pdf format PreviousNextCite this article as: Becker A, Coppinger C, Whitfield AK (2012) Influence of tides on assemblages and behaviour of fishes associated with shallow seagrass edges and bare sand. Mar Ecol Prog Ser 456:187-199. https://doi.org/10.3354/meps09695 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 456. Online publication date: June 07, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Fish predation on sea urchins on the Great Barrier Reef

Predators are important for regulating adult sea urchin densities. Here, we employ remote underwater video cameras to record diurnal predation on tethered sea urchins at Lizard Island on the Great Barrier Reef (GBR). We identified four fish predators of adult sea urchins (Balistoides viridescens, Balistapus undulatus, Lethrinus atkinsoni and Choerodon schoenleinii). Predator activity appeared to be site-specific. Balistoides viridescens and B. undulatus (f: Balistidae) were the two most important predators of Echinometra mathaei with the former handling E. mathaei significantly faster (mean 0.7 min) than B. undulatus (5.2 min). Balistoides viridescens also successfully preyed on 70 % of detections, while C. schoenleinii, B. undulatus and L. atkinsoni preyed on just 33, 17 and <1 %, respectively. Additionally, B. viridescens were behaviourally dominant among predator species and were observed as aggressors in 30 encounters with B. undulatus and 8 encounters with L. atkinsoni. In only one encounter was B. viridescens the recipient of any aggression (from B. undulatus). In terms of relative vulnerability, of the three sea urchin species examined, E. mathaei were more vulnerable to predation than Diadema setosum or Echinothrix calamaris, with mean handling times of 1.2, 4.8 and 10.3 min, respectively. Balistoides viridescens and B. undulatus both appear to be able to play an important role as predators of sea urchins on the relatively intact coral reefs of Lizard Island. However, B. viridescens emerge as the most efficient predator in terms of handling speed and the proportion of detections preyed upon. They were also the behaviourally dominant predator. This preliminary study of the predators of sea urchins on the GBR highlights the potential significance of relatively scarce but functionally important species.

Species identification in seamount fish aggregations using moored underwater video

Abstract O'Driscoll, R. L., de Joux, P., Nelson, R., Macaulay, G. J., Dunford, A. J., Marriott, P. M., Stewart, C., and Miller, B. S. 2012. Species identification in seamount fish aggregations using moored underwater video. – ICES Journal of Marine Science, 69: 648–659. Acoustic surveys of New Zealand deep-water seamounts often show fish aggregations up to 150 m high on the summit. Although bottom trawls on the seamount slopes catch predominantly orange roughy (Hoplostethus atlanticus), species composition in the midwater plumes is extremely uncertain. In June 2010, moored underwater video cameras were deployed on the summit of the Morgue seamount (summit depth 890 m), a feature that has been closed to fishing since 2001. Cameras and lights were timed to come on for 2 min every 2 h. Fish response to the mooring was monitored using vessel-mounted echosounders. Moored cameras confirmed that orange roughy were present up to 70 m above the seamount summit. Orange roughy made up 97% of the fish identified from the video. Other species observed included smooth oreo (Pseudocyttus maculatus), spiky oreo (Neocyttus rhomboidalis), deep-water dogfish, cardinalfish (Epigonus spp.), and squid. Total along-track backscatter from the plume varied by a factor of 25 over a period of hours. Peak acoustic densities in the plume (equivalent to 20 orange roughy m−3) were an order of magnitude higher than peak visual estimates (0.64 orange roughy m−3), but relative densities between paired video and acoustic observations were generally consistent.

Feeding and social behavior of the piabanha, Brycon devillei (Castelnau, 1855) (Characidae: Bryconinae) in the wild, with a note on following behavior

Knowledge concerning the behavior of wild freshwater fishes in Brazil is restricted to a few studies, despite such studies being able to answer fundamental questions about conservation. Species of Brycon are amongst the most threatened in the Neotropics, particularly in southeast Brazil, due to anthropogenic activities in this region. This study investigated the feeding and social behaviors of the endangered fish, Brycon devillei in the Preto River, Jequitinhonha basin, Minas Gerais State, Brazil. Behavioral data were collected by snorkeling with four spatially separated groups (habituated), and direct observations of shoals were made using an underwater video camera (a total of 448 hours of observations). This species showed diverse tactics to obtain food. However, the species proved to be predominately a specialist surface-picker, which adopted alternative tactics to find food at certain times of the year, most notably when food items on the water surface became low. Feeding frequency was shown to be negatively correlated to agonistic behavior between conspecifics. Feeding associations were also recorded between the anostomid Leporinus garmani, acting as a nuclear species, and B. devillei, as follower species. The data presented here showed the importance of conserving the riparian environment to protect B. devillei populations. Furthermore, the present study included rare observations of nuclear-follower feeding association among freshwater fishes, especially between medium-sized characiforms, being the first observations of such kind in a Cerrado stream.

Underwater LNG release test findings: Experimental data and model results

An underwater LNG release test was conducted to understand the phenomena that occur when LNG is released underwater and to determine the characteristic of the vapor emanating from the water surface. Another objective of the test was to determine if an LNG liquid pool formed on the water surface, spread and evaporated in a manner similar to that from an on-the-surface release of LNG. A pit of dimensions 10.06 m × 6.4 m and 1.22 m depth filled with water to 1.14 m depth was used. A vertically upward shooting LNG jet was released from a pipe of 2.54 cm diameter at a depth of 0.71 m below the water surface. LNG was released over 5.5-min duration, with a flow rate of 0.675 ± 0.223 L/s. The wind speed varied between 2 m/s and 4 m/s during the test. Data were collected as a function of time at a number of locations. These data included LNG flow rate, meteorological conditions, temperatures at a number of locations within the water column, and vapor temperatures and concentrations in air at different downwind locations and heights. Concentration measurements were made with instruments on poles located at 3.05 m, 6.1 m and 9.14 m from the downwind edge of the pit and at heights 0.46 m, 1.22 m, and 2.13 m. The phenomena occurring underwater were recorded with an underwater video camera. Water surface and in-air phenomena including the dispersion of the vapor emanating from the water surface were captured on three land-based video cameras. The lowest temperature recorded for the vapor emanating from the water surface was −1 °C indicating that the vapor emitted into air was buoyant. In general the maximum concentration observed at each instrument pole was progressively at higher and higher elevations as one traveled downwind, indicating that the vapor cloud was rising. These findings from the instrument recorded data were supported by the visual record showing the “white” cloud rising, more or less vertically, in air. No LNG pool was observed on the surface of water. Discussions are provided on the test findings and comparison with predictions from a previously published theoretical model.

Development and observations of a spiny dogfish Squalus acanthias reduction device in a raised footrope silver hake Merluccius bilinearis trawl

A spiny dogfish Squalus acanthias excluder grate (grid) within the extension of a silver hake (whiting) Merluccius bilinearis trawl net was designed and tested in Massachusetts Bay, USA between October 2008 and August 2009 using a live-fed underwater video camera. Grates with 50 mm spacing were investigated for effects from color (white or black), angle, and direction (leading to a top or bottom escape vent). Spiny dogfish numbers were greatly reduced for all gear configurations based on video observations and data collected from the codend, while target species were caught in commercial quantities. Four tows (of various gear configurations) resulted in spiny dogfish blockages in front of the grate. The reduction of spiny dogfish led to increases in the quality of marketable catches, likely reductions in non-target species mortality, and decreases in the codend catch handling times.

New series of corers for taking undisturbed vertical samples of soft bottom sediments

A new series of sediment corers - a Single Tube Gravity Corer and a Multiple Tube Gravity Corer has been developed for taking undisturbed vertical soft sediment samples, and a Hydraulic Pressure Multiple Tube Corer has been preliminarily designed. The Single Tube Gravity Corer can be operated by hand easily without using a winch, and it might be the simplest and the cheapest core sampler available. The Multiple Tube Gravity Corer was developed based on the Single Corer, which normally consists of three coring tubes, however, it is possible to install up to six tubes. The Hydraulic Pressure Multiple Tube Corer has been designed based on the Multiple Corer, which would take undisturbed sediment samples, and would be controlled by a computer and monitored by an underwater video camera. The pore water collection and the sediment Eh measurement-subsampling process through the transparent plastic tube wall is straightforward. Compared to other types of coring devices, such as the famous KC Kajak sediment core sampler made in Denmark; these corers have greater reliability, being simple in structure, relatively light weight, easy and efficient to operate, highly adaptable, and relatively inexpensive to produce.

Temporal variation in coral reef ecosystem processes: herbivory of macroalgae by fishes

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 422:239-251 (2011) - DOI: https://doi.org/10.3354/meps08916 Temporal variation in coral reef ecosystem processes: herbivory of macroalgae by fishes Carine D. Lefèvre*, David R. Bellwood Australian Research Council Centre of Excellence for Coral Reef Studies and School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia *Email: carine.lefevre@jcu.edu.au ABSTRACT: Despite the widely accepted importance of fish herbivory on coral reefs, few studies have considered the temporal variability in the nature of algal–herbivore interactions. We therefore quantified monthly feeding intensity on Sargassum sp. bioassays for 12 mo with remote underwater video cameras deployed to identify the herbivores responsible for macroalgal removal on an inshore island of the Great Barrier Reef, Australia. Significantly higher removal rates were observed during the summer months whereas winter months were characterized by 4 times lower removal rates. However, rather than being simply changes in the feeding activity of a single species, this temporal pattern in herbivory also incorporated changes in the species responsible for the removal of Sargassum. Video analyses revealed that, of the 43 herbivore species recorded from the bay, only 3 played a significant role in Sargassum removal: Kyphosus vaigiensis, Naso unicornis and Scarus rivulatus. K. vaigiensis, a rudderfish, was primarily responsible for the removal of Sargassum during the summer months (83% of the total recorded bites; 85553 bites). There was almost no feeding activity on Sargassum by K. vaigiensis during the winter months (82 bites). However, there was a reciprocal increase in feeding intensity on Sargassum by parrotfishes in the winter months, particularly S. rivulatus (71 bites during summer versus 2884 bites in winter). This temporal variability in herbivore functional roles suggests that functional redundancy on reefs may be less than previously assumed in that the feeding activities of fishes may be both spatially and temporally constrained. KEY WORDS: Regime shift · Kyphosus vaigiensis · Trophic status · Phase shift · Resilience · Functional redundancy · Macroalgae · Sargassum · Great Barrier Reef Full text in pdf format Supplementary material PreviousNextCite this article as: Lefèvre CD, Bellwood DR (2011) Temporal variation in coral reef ecosystem processes: herbivory of macroalgae by fishes. Mar Ecol Prog Ser 422:239-251. https://doi.org/10.3354/meps08916 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 422. Online publication date: January 31, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Effects of turbulence on the transport of individual particles as bedload in a gravel‐bed river

AbstractThis study investigates the association between mean and turbulent flow variables and the movement of individual particles in a gravel‐bed river. The experimental design implemented in the Eaton‐North River (Québec, Canada) is based on the simultaneous observations at a high temporal resolution of both particle movements as bedload using an underwater video camera and of the streamwise and vertical flow velocity components using a vertical array of three electromagnetic current meters (ECMs). The frequency and distance of displacement of particles larger than 20 mm that were sliding or rolling on the bed were measured from a 10 minutes long film. Mean and turbulent flow properties obtained for periods without sediment transport are compared to those when particles were sliding and rolling. When particles are sliding, weak differences are present for the mean streamwise velocity and normal vertical stresses. Instantaneous Reynolds shear stresses are significantly lower for sliding events which was not expected but could be explained by the important dominance of Quadrant 3 events (inward interactions). When particles are rolling, only the vertical normal stresses show a weak difference from those observed in the absence of transport but they tend to occur when Quadrant 2 (ejections) dominate the flow field. For both sliding and rolling particles, vertical and/or streamwise fluid accelerations show high magnitude values when compared to periods without transport. For sliding particles, streamwise acceleration is mostly negative and combines most of the time with a positive vertical acceleration. For rolling particles, streamwise and vertical acceleration are predominantly of opposite sign. These results suggest that fluid acceleration or deceleration in the streamwise and vertical directions is affecting the pressure field around particles. In this study, fluid acceleration seems to play a more important role than Reynolds shear stress or normal stresses for bedload movements. Copyright © 2010 John Wiley &amp; Sons, Ltd.

A Digital Underwater Video Camera System for Aquatic Research in Regulated Rivers

AbstractWe designed a digital underwater video camera system to monitor nesting centrarchid behavior in the Tallapoosa River, Alabama, 20 km below a peaking hydropower dam with a highly variable flow regime. Major components of the system included a digital video recorder, multiple underwater cameras, and specially fabricated substrate stakes. The innovative design of the substrate stakes allowed us to effectively observe nesting redbreast sunfish Lepomis auritus in a highly regulated river. Substrate stakes, which were constructed for the specific substratum complex (i.e., sand, gravel, and cobble) identified at our study site, were able to withstand a discharge level of approximately 300 m3/s and allowed us to simultaneously record 10 active nests before and during water releases from the dam. We believe our technique will be valuable for other researchers that work in regulated rivers to quantify behavior of aquatic fauna in response to a discharge disturbance.

Size matters: macroalgal height influences the feeding response of coral reef herbivores

Algal size has often been related to shifts in the susceptibility of algae to herbivores. Within coral reef systems, macroalgal assays have been widely used to quantify herbivory; however, the effect of macroalgal size has been largely overlooked. I used Sargassum polycystum assays that spanned the range of available heights (10 to 80 cm) and underwater video cameras to examine the effect of macroalgal height on the rates and agents of herbivory on an inner-shelf reef of the Great Barrier Reef. Rates of herbivory were high and displayed limited variation among heights, ranging from 86.4 to 95.2% 5h–1. There was, however, significant variation in the species recorded feeding from S. polycystum of different heights. Naso unicornis dominated feeding on the shortest (10 cm) assays, while Kyphosus vaigiensis dominated feeding on the larger (20 to 80 cm) assays. This variation appears to be related to interspecific differences in foraging behaviour, and suggests that the roles of these fishes may change at different stages of benthic community succession.

A quantitative assessment of the response of mobile estuarine fauna to crab-tiles during tidal immersion using remote underwater video cameras

Laying artificial materials on the shore to create refugia that attract crustaceans is a ‘fishing’ technique known mainly from South America and the UK. In the UK, this method of fishing is known as ‘crab-tiling’, which involves laying of roof tiles, pieces of guttering or car tyres (‘crab-tiles’) intertidally in estuaries to provide habitat for Carcinus maenas. Crabs are then harvested for use as angling bait. C. maenas are known to reside under crab-tiles during low tide, however the extent to which crab-tiles influence the distribution and behaviour of crabs and other estuarine fauna during high tide is not clear. To investigate this, fixed underwater video cameras were deployed on a mudflat over two separate occasions in control (non-tiled) and tiled sites. Crabs were significantly more abundant in crab-tiled sites than control sites, and they remained relatively stationary, travelling less in tiled sites than those in control sites. The abundance of mobile fauna (e.g. benthic gobies, mysids, crabs and pelagic fishes) was greater in control sites than in tiled sites during July. The same trend was observed in March but was not significant. Diversity (number of taxa) tended to be greater in control sites than tiled sites on both occasions, but the trend was not significant. The faunal assemblage was not different between control and tiled treatments in either March or July. Crabs occupied crab-tiles throughout high tide immersion, and qualitative observations indicated a tendency to aggressively defend particular tiles. It also seems likely that the tiles provided some protection for the crabs from fish and diving birds.

Hydro- and morpho-dynamic modeling of breaking solitary waves over a fine sand beach. Part I: Experimental study

The objectives of this work are (1) to examine the mechanisms that influence tsunami erosion and deposition mechanisms in the littoral zone via physical simulations of breaking solitary waves over a fine sand beach, and (2) to provide experimental data for validation of numerical models to predict tsunami erosion and deposition processes. The experiments were carried out in a 48.8 m × 2.16 m × 2.1 m wave flume. The flume was instrumented to observe free surface elevations, cross-shore velocities, suspended sediment concentrations, vertical and cross-shore pore-pressure gradients near the shoreline, and morphological changes. In addition, wave–sediment interactions were observed via underwater video cameras. The results are systematically analyzed to investigate the roles of wave breaking, bore runup, wave drawdown, and wave-induced pore-pressure variations on tsunami erosion and deposition processes. The studies showed that the wave plunging on a thin layer of water prior to reaching the shoreline did not cause much sediment suspension, while the waterjet impinging directly on the beach entrained substantial amounts of sand. The suspended sediments were subsequently pushed up the slope by fluid momentum as the broken wave transformed to a turbulent bore. A small net deposition region was observed near the maximum runup point where both the flow velocity and the water depth were near zero. A significant amount of the sediment transport occurred during the wave drawdown in the form of thick sheet flow, which resulted in net erosion of the shore face and the beach. A hydraulic jump formed near the wave breaking region toward the end of the drawdown, which caused most of the suspended sand to deposit in the wave breaking region. Consequently, breaking solitary waves over a sloping fine sand beach led to net erosion of the shore face and the beach, net deposition in a small region immediately seaward of the max excursion point, and net deposition in the wave breaking zone.

Analysis of the vertical distribution of the abundance of small pelagic fish larvae in the Gulf of California using submarine videocameras.

Two types of underwater video cameras were used to study the distribution and vertical abundance of larvae of the small pelagic fish Engraulis mordax, Etrumeus teres and Sardinops sagax at 1 m resolution, in a locality in the north of the Gulf of California with calm conditions and high density of adult sardines. The highest average abundance (900 larvae m -1 min -1) was found immediately above the thermocline (33 m) and the pycnocline (36 m), apparently not associated with the maximum chlorophyll detected on the surface, nor to the highest density of fish adults (10 -20m). Video observations made it possible to determine the vertical distribution at a resolution impossible to obtain. through sampling with nets; but nevertheless, This is not a very useful technique in areas with high current speed.

Long-term culture of Atlantic salmon ( Salmo salar L.) in submerged cages during winter affects behaviour, growth and condition

In the search for alternative farming methods, we investigated whether large salmon submerged below 10 m in winter conditions behaved normally and performed as well as control fish held in standard surface cages. On average, 2345 salmon of ~ 3.5 kg were kept in each of six 2000 m 3 sea-cages for 6 weeks; three of which were submerged to 10–24 m depth and three acted as surface controls (0–14 m). Behaviour during both day and night was studied with echo-sounders, and underwater video cameras fitted with infra-red lamps. A sub-sample of fish from each cage was weighed, measured and assessed for fin and snout condition prior to and after the experimental period. In addition, the vertebral column of 50 fish from the control and submerged treatments were dissected and X-rayed to assess vertebral deformities. The submerged salmon seemed unable to re-fill any gas into the swim bladder, as a linear decrease in echo reflection to < 5% of pre-submergence levels after 22 days of submergence indicated loss of almost all gas from the physostomous swim bladders and negatively buoyant fish. Around day 22, submerged salmon swam at night time with a distinct ‘tail-down, head-up’ tilt (26°) compared to the horizontal swimming position of control fish (− 3°). Average swimming speed (body length per second) of submerged salmon were 1.3–1.4 times faster (day: 0.77 ± 0.02; night: 0.46 ± 0.02, (mean ± SE)) than control fish (day: 0.54 ± 0.01; night: 0.37 ± 0.02) both during day and night. Almost no mortality was seen, and the submerged salmon maintained similar diurnal vertical migrations as the surface fish, indicating that deep submergence did not exhaust the fish. However, submerged fish fed less efficiently, resulting in lower growth and reduced feed utilization. Fins and snouts of the submerged fish had small, but significantly more erosion than the control fish. Vertebrae in the tail region were significantly compressed in the submerged fish compared to control fish. This could be an early symptom of development of vertebral deformities. The results suggest that continuous submergence below 10 m for longer than 2 weeks reduces the welfare and performance of Atlantic salmon.

Using dual-frequency sonar to detect juvenile goliath grouper Epinephelus itajara in mangrove habitat

We tested alternative methods to diver visual censuses of juvenile goliath grouper Epinephelus itajara in turbid mangrove habitat. We used a dual-frequency identification sonar (DID- SON) acoustic camera. DIDSON generates high resolution images of fish with a high-frequency multibeam sonar. The system can record the shape, size and movement of fish under low visibility conditions. A paired DIDSON-digital underwater video camera system was used to survey the man- grove shorelines of Niles Channel, in the Lower Florida Keys, USA. In high visibility conditions, the specific E. itajara DIDSON acoustic signal was obtained and used to evaluate DIDSON's performance for E. itajara surveys in turbid conditions. Under both clear and turbid conditions DIDSON enabled the quantification and measurement of juvenile E. itajara in mangrove undercuts and submerged dead trees that were not visible or detected using video or by a diver. DIDSON could not detect immobile juveniles, as it was unable to discern them from the substrate or the underwater structures where they rested. We demonstrate that mangrove microhabitat underwater structures do not inter- fere with acoustic fish signals. This study represents the first known successful application of a DID- SON acoustic camera in mangrove shorelines to achieve juvenile E. itajara detection and sizing.

Measurements of acoustic backscatter and density of captive Atlantic cod with synchronized 300-kHz multibeam and 120-kHz split-beam echosounders

Abstract Gurshin, C. W. D., Jech, J. M., Howell, W. H., Weber, T. C., and Mayer, L. A. 2009. Measurements of acoustic backscatter and density of captive Atlantic cod with synchronized 300-kHz multibeam and 120-kHz split-beam echosounders. – ICES Journal of Marine Science, 66: 1303–1309. Effective management strategies for Atlantic cod (Gadus morhua) in the Gulf of Maine require stock assessments based on accurate estimates of its abundance and distribution. If multibeam echosounders are to provide data for such estimates, the relationship between acoustic backscatter and fish biology must be better understood. Working towards this goal, a series of acoustic measurements was made using a 120 kHz, split-beam echosounder (Simrad EK60) and a 300 kHz, multibeam echosounder (Kongsberg EM3002). The transducers from both systems were fixed to a platform over a submerged 98 m3 cage made of 5 cm stretched-nylon mesh. After standard-sphere calibrations, the cage was stocked with live, mature Atlantic cod, with a mean total length of 80.7 cm (range: 51.5–105.0 cm). The echosounders synchronously collected acoustic data, while the cod were monitored with two underwater video cameras. Cod were incrementally removed from the cage to provide a time-series of acoustic backscatter at four densities (n = 128, 116, 66, and 23). Backscatter measurements of cod are compared between echosounders and over time, and the factors affecting the acoustically derived density estimates are discussed. The benefits and limitations of the EM3002 are highlighted.

Using underwater cameras to assess the effects of snorkeler and SCUBA diver presence on coral reef fish abundance, family richness, and species composition

The results of underwater visual fish censuses (UVC) could be affected by fish changing their behavior in response to the snorkeler or diver conducting the survey. We used an underwater video camera to assess how fish abundance, family richness, and community composition were affected by the presence of snorkelers (n = 12) and self-contained underwater breathing apparatus (SCUBA) divers (n = 6) on a coral reef in Thailand. The total number of families, abundance of some fish families, and overall species composition showed significant differences before and during snorkeling disturbances. We did not detect significant and consistent changes to these parameters in the presence of a SCUBA diver; however, this could be a result of lower statistical power. We suggest that the use of a stationary video camera may help cross-check data that is collected through UVC to assess the true family composition and document the presence of rare and easily disturbed species.

Ex situ target-strength measurements of Japanese anchovy (Engraulis japonicus) in the coastal Northwest Pacific

Abstract Kang, D., Cho, S., Lee, C., Myoung, J-G. and Na, J. 2009. Ex situ target-strength measurements of Japanese anchovy (Engraulis japonicus) in the coastal Northwest Pacific. – ICES Journal of Marine Science, 66: 1219–1224. The Japanese anchovy (Engraulis japonicus) is an important species in regard to the fisheries and ecology of the coastal Northwest Pacific. Measurements of ex situ target strength (TS; dB re 1 m2) were made on live anchovy using 38, 120, and 200 kHz split-beam echosounders. The fish were tethered using small hooks attached to their mouths. During the acoustic measurements, an underwater video camera was used to continuously monitor fish behaviour and tilt-angle (θ). Data for 35 individual anchovy ranging from immature to adult sizes (total lengths LT = 4.8–12.2 cm) were analysed. Least-squares regression fits of TS vs. log(LT) were: TS38 kHz = 20 log(LT) − 65.8 (r2 = 0.82), TS120 kHz = 20 log(LT) − 68.4 (r2 = 0.84), and TS200 kHz = 20 log(LT) − 69.1 (r2 = 0.71). The LT vs. wet weight (W; g) relationship for these fish was W = 0.0036 LT3.204. The mean θ for anchovy swimming freely in a large seawater tank was 9.1° (s.d. = 13.1°). These ex situ measurements of TS, LT, W, and θ can be applied to improve acoustic estimates of Japanese anchovy biomass.

Sediment‐mediated suppression of herbivory on coral reefs: Decreasing resilience to rising sea‐levels and climate change?

We describe a mechanistic basis for maintaining an alternative degraded stable state on coral reefs: sediment‐laden algal turfs. Using remote underwater video cameras we quantified rates of herbivory by coral reef fishes on epilithic algal turfs with natural and experimentally reduced sediment loads. Removal of sediment increased overall fish feeding rates 3.8‐fold, and resulted in a decrease in mean algal turf length of 64% within 4 h. After 4 h, sediment accumulated in the treatment plots, but only returned to 41% of the original depth. A total of 20 species actively fed on the sediment removal plots, compared with 12 species in control plots. Of the five numerically abundant herbivorous fish species, all increased feeding by at least 225% in the absence of sediment. Only juvenile Scarus spp. fed to any extent (28% of bites) on control plots. We suggest that naturally occurring sediment loads in epilithic algal turfs can suppress herbivory and that sediment‐laden algal turfs may be an alternative stable state on coral reefs. This may provide a mechanistic basis for the geological evidence of a sediment‐induced turn‐off of coral reef growth. With projected global sea‐level rises due to climate change, reef‐based sediment loads may be a critical factor in differentiating the relative resilience of coral reefs and identifying reef ecosystems that are at highest risk to rising sea levels.