- National Institute of Water and Atmospheric Research
- Centre for Fisheries
- Richard Lyell O'Driscoll
Research Items (45)
- Dec 2018
Antarctic silverfish ( Pleuragramma antarctica Boulenger) are a keystone species in the Ross Sea. Silverfish eggs and larvae are abundant during spring amongst the sub-surface platelet ice in Terra Nova Bay. It is not known whether the eggs are spawned elsewhere and accumulate under the ice or whether there is mass migration of silverfish to coastal spawning sites in winter. To test the latter hypothesis, an upward-looking 67 kHz echo sounder was moored in Terra Nova Bay to observe potential silverfish migration. The echo sounder was deployed at 380 m in a seabed depth of 550 m and ran for 210 days from 15 May until 11 December 2015. Acoustic reflections consistent with silverfish were observed at depths of 230–380 m during 9–22 September. This timing is consistent with the presence of eggs typically observed in October. Adult silverfish were also detected with an echo sounder and camera deployed through the ice in McMurdo Sound on 10 November 2015. Juvenile silverfish, but not adults, were observed through the ice in Terra Nova Bay during 11–16 November 2017. This paper provides a proof of concept, showing that innovative use of acoustics may help fill important observation gaps in the life history of silverfish.
- May 2017
- The Antarctic Silverfish: a Keystone Species in a Changing Ecosystem
Acoustic methods have the ability to detect and quantify distribution and abundance of Antarctic silverfish (Pleuragramma antarctica) across a range of spatial and temporal scales. The main advantage of acoustic surveys over traditional net-based sampling methods for silverfish is that the larger adult fish are unlikely to avoid the echosounder. Acoustic surveys also allow relatively wide coverage over the whole water column in a short time period because data are collected while the vessel is steaming at 8–10 knots. The key uncertainties, as with most acoustic surveys, are target identification and target strength. These uncertainties are compounded because silverfish do not have a gas-filled swim-bladder and so are a relatively weak acoustic target. Use of multi-frequency acoustic data helps discriminate silverfish from krill and other associated species, and broadband acoustics has considerable potential in this regard. Acoustic target strength has been derived from scattering models and in situ and ex situ measurements. Adult silverfish exhibit different scattering properties to post-larvae and juveniles. In the Ross Sea, adult silverfish are distributed widely over the shelf and tend to form layers at 100–400 m depth. Juvenile silverfish of 50–80 mm standard length occur shallower and were observed as a weak layer centred at about 80 m depth.
- Sep 2015
- Hakes: Biology and Exploitation
An overview of the commercial fishery for hoki (Macruronus novaezelandiae) in New Zealand with current knowledge about the biology, and life history characteristics of the fish, is provided. Hoki form New Zealand's largest finfish fishery, and stock abundance has been closely monitored to enable catch adjustments to be made when necessary. The total allowable commercial catch (TACC) for the 2012/2013 fishing year was set at 130,000 t. In New Zealand, hoki are taken almost exclusively as a target species by large commercial trawlers. Fishing occurs throughout the year with over half of the catch taken during the spawning season in the austral winter (July/August). Most of the spawning season catches are taken with mid-water trawls, and outside the spawning season with bottom trawls. Fishers complete a comprehensive catch return system to monitor tow and catch effort, and the government implements an Observer Scheme that independently reports on the finfish by-catch and protected species by-catch in the fishery. Although a single TACC is set for hoki under New Zealand's Quota Management System, a catch split arrangement is implemented each year to reflect the stock structure and the status of the two sub-populations of hoki. Assessment models suggest that the spawning biomass of both sub-populations declined to a minimum in about 2005 but has since re-built. Current spawning biomass of both sub-populations is currently estimated to be within the target zone of 35-50% B0.
- May 2015
Annual acoustic surveys of spawning southern blue whiting (Micromesistius australis) on the Bounty Platform, southeast of New Zealand, have been carried out using industry vessels since 2004. In most years, surveys were carried out from a single vessel, while in 2009, acoustic data were collected from three vessels. The survey approach in all years was the same—vessels with calibrated Simrad ES60/ES70 echosounders and hull-mounted 38 kHz transducers conducted aggregation-based surveys using an adaptive design. Surveys attempted to cover all areas of high southern blue whiting density. In most years there were multiple snapshots of the same aggregation. The resulting biomass was used as a relative estimate of spawning southern blue whiting abundance. There was a very large (seven-fold) increase in estimated biomass of southern blue whiting at the Bounty Platform from 2006 to 2007, which was due to the recruitment of one very strong year class (2002) into the spawning population. The estimated biomass from 2008 was also high, but biomass declined by a factor of four in 2009. The observed decline in acoustic estimates between 2008 and 2009 was too great to be explained solely by fishing and average levels of natural mortality. The very large changes in estimated abundance between years, and also between snapshots within a year, are related mainly to changes in survey temporal and spatial coverage, and illustrate an important limitation on interpretation of aggregation-based acoustic abundance estimates. In each snapshot an unknown proportion of the spawning aggregation is surveyed, and almost certainly not the entire spawning stock. Survey coverage depended on both the amount of survey time available (which is often limited by commercial constraints) and the behaviour of the fish (e.g., the extent and density of the aggregation, and the timing of spawning). It is therefore difficult to incorporate the resulting series of abundance estimates into a formal stock assessment model as a time series. Despite this, industry acoustic surveys of the Bounty Platform have led directly to management decisions and changes in catch limits.
- Feb 2015
Hoki (Macruronus novaezelandiae) is New Zealand's largest fishery. Acoustic surveys of spawning hoki provide abundance indices for informing management of the stocks. For estimating fish abundance using acoustic methods, relationships between the acoustic backscatter from a single fish, commonly known as the acoustic target strength (TS), and fish length are required. Here we present in situ data on TS from individual live hoki, collected using a trawl-mounted acoustic-optical system (AOS). Deriving TS–length relationships from measurements of individual fish is complicated because of the very large stochastic variability in TS from the same fish, and from individual fish of similar length. A new TS–total length relationship of TS = 24.5log10(TL) − 83.9 was derived based on a weighted non-linear least-squares fit to acoustic backscattering cross-section measurements from 62 New Zealand hoki between 35 and 93 cm TL. It was necessary to use a weighted fitting method to appropriately account for variance, as not doing so can introduce a bias to the estimated mean TS. Using the new TS–TL relationship decreases absolute estimates of hoki abundance by 50–60% from those currently used in stock assessment, but has little impact on relative indices. Estimated 90% confidence intervals around the new TS–TL relationship based on bootstrapping suggested uncertainty in mean TS of ±1.5 dB at typical New Zealand hoki lengths, with resulting uncertainty in abundance estimates of up to 50%. Further in situ measurements and modelling are required to reduce this uncertainty and to investigate potential differences in TS between AOS-measured and ‘undisturbed’ fish.
- Apr 2014
The Chatham Rise is a submarine ridge east of New Zealand. The subtropical convergence occurs over this ridge and it is an area of high productivity, associated with important deepwater fisheries. Acoustic recordings suggest that there is a relatively high biomass of mesopelagic fish over the Chatham Rise, but little is known about the species composition, distribution, and spatio-temporal dynamics of the organisms responsible for the acoustic scattering layers. Two research trawl and acoustic surveys were carried out in May and June 2008 and November 2011 to assess the distribution and acoustic signatures of mesopelagic animals on the Chatham Rise. A total of 171 species or species groups were caught in 86 midwater trawls to a maximum depth of 1000 m. Seven species assemblages were described corresponding to different acoustic mark types. These 7 assemblages were dominated by euphausiids, Maurolicus australis (Stomii formes), Lampanyc-todes hectoris, Symbolophorus boops, Electrona carlsbergi, Diaphus danae and Gymnoscopelus piabilis (all Myctophiformes). Euphausiids could be separated from mesopelagic fish species based on their multi-frequency acoustic response, but the 6 fish groups could not be separated based on their frequency response alone. These different fish assemblages were distinguished based on their unique acoustic mark characteristics (amplitude and dimension features of the volume backscatter), vertical distribution and ancillary information. Classification results were synthesised in a decision model. M. australis and L. hectoris were associated with shallower water on the central and western Chatham Rise, D. danae occurred mainly in deeper water on the north side of the Rise, and E. carlsbergi was associated with cooler water on the south Rise.
The Southern Ocean is undergoing significant change. Underst this change is a globally relevant issue that requires circump represents a group of scientific institutes and industrial partner observations of the under‐sampled mid‐trophic level (Fig. 1) in t BACKGROUND & METHOD The taxonomically diverse mid‐trophic level organisms (Fig. 1) play a critical role in the Southern Ocean ecosystems by transferring biomass and energy through the food web. Yet despite their huge biomass and importance, there is little information on their spatial 3D distribution. Fig. 2. Echogram of a krill swarm (120 kHz, Simrad EK60 RRS James Clark Ross (BAS)). Acoustic observations offe high resolution data on pe Most scientific research fishing vessels now have ec A key aim of SONA is to standards and methodolo processing. These will bu Fig. 1. Antarctic food web. Mid‐trophic level organisms are highlighted in expertise (e.g. ICES WGFA yellow and include mesopelagic fish squid and Antarctic krill Image: BAS BASOOP). SONA specifica fish, krill. BAS. calibrated water column ba AIMS 1. Lead, coordinate and support the Antarctic scientific community in creating an open access database of acoustic observations of the mid‐trophic level. 2. Develop and adopt common standards and methodologies for acoustic data collection and processing, aligned with existing protocols. 3. Implement a self‐sustaining long‐term acoustic observing strategy framed within the international observing and Fig 5 Summary of BAS acoustic data file Fig. 5. modelling communities (e.g. SOOS, South Geo CCAMLR, ICED). Acknowledgements: SONA is Environment Research Council funds come from the ANR Mycto‐3 for Marion Dufresne acoustic instr www.sona.aq Jenny Thomas (SONA Data Manager) Email: email@example.com ustics - A circumpolar database of rophic level in the Southern Ocean N. Behagle4, G. Skaret5, R. Korneliussen5, R. O’Driscoll6, A. Dunford6, C. Reiss7 UK; 2Australian Antarctic Division, 203 Channel Highway, Kingston, Tasmania 7050, Australia; 3CSIRO Marine and Atmospheric pus Ifremer, 29280 Plouzané, France; 5Institute of Marine Research, PO Box 1870 Nordnes, N‐5817 Bergen, Norway; 6National search Division, NOAA Fisheries, Southwest Fisheries Science Center, La Jolla, CA 92037, USA. tanding and predicting how the marine ecosystem responds to polar scale analyses and multidisciplinary observations. SONA rs who have united to build a circumpolar database of acoustic the Southern Ocean.
Two surveys were carried out in the Ross Sea region during February and March 2004 and 2008 from the New Zealand RV Tangaroa. Fishes were sampled on the continental shelf and slope of the Ross Sea, and on adjacent seamounts to the north, mainly using a large demersal fish trawl and a large mesopelagic fish trawl. Parts of the shelf and slope were stratified by depth and at least three random demersal trawls were completed in each stratum, enabling biomass estimates of demersal fish to be calculated. Fish distribution data from these two surveys were supplemented by collections made by observers from the toothfish fishery. A diverse collection of over 2500 fish specimens was obtained from the two surveys representing 110 species in 21 families. When combined with previous documented material this gave a total species list of 175, of which 135 were from the Ross Sea shelf and slope (to the 2000 m isobath). Demersal species-richness, diversity and evenness indices all decreased going from the shelf to the slope and the seamounts. In contrast, indices for pelagic species were similar for the slope and seamounts/abyss but were much lower for the shelf.
- Jan 2013
The diet of Antarctic silverfish Pleuragramma antarcticum was evaluated by examining stomach contents of specimens collected in the Ross Sea (71°-77° S; 165°-180° E) in January to March 2008. Pleuragramma antarcticum (50-236 mm standard length, L(S) ) and prey items were analysed for stable-isotopic composition of carbon and nitrogen. According to index of relative importance (I(RI) ), which incorporates frequency of occurrence, mass and number of prey items, the most important prey items were copepods (81%I(RI) over all specimens), predominantly Metridia gerlachei and Paraeuchaeta sp., with krill and fishes having low I(RI) (2·2 and 5·6%I(RI) overall). According to mass of prey (M) in stomachs, however, fishes (P. antarcticum and myctophids) and krill dominated overall diet (48 and 22%M, respectively), with copepods being a relatively minor constituent of overall diet by mass (9·9%M). Piscivory by P. antarcticum occurred mainly in the extreme south-west of the region and near the continental slope. Krill identified to species level in P. antarcticum stomachs were predominantly Euphausia superba (14·1%M) with some Euphausia crystallophorias (4·8%M). Both DistLM modelling (PRIMER-permanova+) on stomach contents (by I(RI) ) and stepwise generalized linear modelling on stable isotopes showed that L(S) and location were significant predictors of P. antarcticum diet. Postlarval P. antarcticum (50-89 mm L(S) ) consumed exclusively copepods. Juvenile P. antarcticum (90-151 mm L(S) ) consumed predominantly krill and copepods by mass (46 and 30%M, respectively). Small adult P. antarcticum (152-178 mm L(S) ) consumed krill, fishes and copepods (37, 36 and 15%M, respectively). Large adult P. antarcticum (179-236 mm L(S) ) consumed predominantly fishes and krill (55 and 17%M, respectively), especially in the north (near the Ross Sea slope) and in the SW Ross Sea. Amphipods were occasionally important prey items for P. antarcticum (western Ross Sea, 39%M). General concordance between stomach contents and trophic level of P. antarcticum and prey based on δ(15) N was demonstrated. Pleuragramma antarcticum trophic level was estimated as 3·7 (postlarval fish) and 4·1 (fish aged 3+ years).
- Dec 2012
Grenadiers Macrourus spp. are the main bycatch species in the exploratory longline fishery for toothfish Dissostichus spp. in the Ross Sea, Antarctica. Ongoing monitoring tools are needed to assess stock status of grenadiers in the Ross Sea and to ensure ecological relationships are maintained. There may be potential to use fisheries acoustic methods to obtain estimates of grenadier abundance. Acoustic data collected during New Zealand’s International Polar Year Census of Marine Life survey in February–March 2008 provide evidence that single targets close to the bottom over the Ross Sea slope are grenadiers. The acoustic target strength distribution of single targets was very similar to that predicted based on the measured size range of grenadiers. There are also positive correlations between acoustic backscatter and trawl and longline catches of grenadiers. Key uncertainties of the acoustic method are mark identification away from the bottom, and technical issues with low signal-to-noise ratio at depths greater than 1000 m and the acoustic dead-zone close to the bottom.
- Nov 2012
Estimates of the acoustic target strength (TS) of southern blue whiting (Micromesistius australis) at 38 kHz were obtained using an autonomous acoustic-optical system (AOS) mounted on a demersal trawl. Data were collected from aggregations of spawning adult [mean fork length (FL) 34.4 cm] and immature (mean FL 24.6 cm) southern blue whiting south of New Zealand. Mean TS was estimated from 162 tracks containing 695 echoes from targets identified from video as southern blue whiting. The mean TS was -37.9 dB with a 95% confidence interval (CI) of -39.7 to -36.6 dB for 21 immature fish and -34.6 dB (95% CI -35.4 to -34.0 dB) for 141 adults. A logarithmic fit through the mean TS values produced a TS-fork length (FL) relationship from optically verified targets of TS = 22.06 log(10)FL - 68.54. This new relationship gives TS values within 1 dB of those estimated using the relationship recently adopted by ICES for blue whiting (Micromesistius poutassou) of TS = 20 log(10)TL - 65.2 (where TL is total length) obtained from in situ measurements, but higher values than those estimated from the previous relationship for southern blue whiting of TS = 38 log(10)FL - 97, which was based on swimbladder modelling.
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. © 2012 International Council for the Exploration of the Sea. Published by Oxford University Press. All rights reserved.
Antarctic silverfish (Pleuragramma antarcticum) is a key link between plankton and the community of top predators in the shelf waters of the Ross Sea. In spite of their abundance and important role in Antarctic food chains, very little is known of many ecological and biological aspects of this species. A combined trawl and acoustic survey of silverfish was carried out on the western Ross Sea shelf during the New Zealand International Polar Year Census of Antarctic Marine Life research voyage on R.V. Tangaroa in February–March 2008. Multi-frequency acoustic data (12, 38, 70, and 120 kHz) allowed discrimination of silverfish marks from those of krill and other associated species. Mark identification was achieved using targeted midwater trawls. Additional midwater and demersal trawls were carried out at randomly selected locations over the shelf as part of the core biodiversity survey.
The Southern Ocean is delimited by major frontal zones which influence pelagic life at the spatial macroscale. There is a sharp ecological segregation of pelagic fish that inhabit this ocean with some families living in the neritic zone and others in the oceanic zone. The neritic zone is dominated by fish of the Notothenioid suborder. In the oceanic zone, mesopelagic species are dominated by myctophids. Their spatial distribution is highly influenced by meso- or sub-mesoscale oceanographic features. Myctophid presence/absence records from historical surveys and from the Census of Antarctic Marine Life were used to model species assemblages in the Indian sector of the Southern Ocean by using generalized dissimilarity modeling. This statistical technique is data-driven and is used in conjunction with Geographic Information Systems for creating models between communities and environmental factors. Application of these models in large unsurveyed areas is possible and helps in delineating regions of potential similar assemblages. This will allow us to move from the bioregionalization of the Southern Ocean based on only abiotic factors and chlorophyll, to its ecoregionalization by adding species assemblages.
The Southern Ocean is delimited by major frontal zones which influence pelagic life at the spatial macroscale. There is a sharp ecological segregation of pelagic fish that inhabit this ocean with some families living in the neritic zone and others in the oceanic zone. The neritic zone is dominated by fish of the Notothenioid suborder. In the oceanic zone, mesopelagic species are dominated by myctophids. Their spatial distribution is highly influenced by meso- or sub-mesoscale oceanographic features. Myctophid presence/absence records from historical surveys and from the Census of Antarctic Marine Life were used to model species assemblages in the Indian sector of the Southern Ocean by using generalized dissimilarity modeling. This statistical technique is data-driven and is used in conjunction with Geographic Information Systems for creating models between communities and environmental factors. Application of these models in large unsurveyed areas is possible and helps in delineating regions of potenti
O'Driscoll, R. L., Gauthier, S., and Devine, J. A. 2009. Acoustic estimates of mesopelagic fish: as clear as day and night? – ICES Journal of Marine Science, 66: 1310–1317. The pelagic ecosystem on the Chatham Rise east of New Zealand has been monitored annually using acoustic surveys since 2001. Most of the acoustic backscatter comes from diurnally migrating mesopelagic fish, which are the major prey of hoki ( Macruronus novaezelandiae ) and other valuable commercial species. Mesopelagic schools and layers typically occur at 100–400 m depth during the day and migrate above 200 m depth at night. Previous acoustic estimates of the biomass of mesopelagic fish on the Chatham Rise were based only on night-time estimates from the upper 200 m and may significantly underestimate actual biomass. Paired day–night comparisons found that an estimated 20% of the total daytime backscatter migrates to depths of 0–14 m at night, where it is too shallow to be detected by hull-mounted acoustic instruments. In contrast, there is a positive bias associated with acoustic estimates of mesopelagic organisms during the day because they can occur close to the bottom mixed with demersal fish. Two alternative approaches for estimating biomass of diurnally migrating mesopelagic species are suggested. The first applies a correction based on the estimated proportion of backscatter migrating into the surface deadzone to night-time estimates of backscatter in the upper 200 m. The second uses night-time estimates of demersal backscatter, which remains deeper than 200 m at night, to correct daytime estimates of total backscatter. Both methods gave similar estimates of abundance and demonstrated that there was no statistically significant trend in mesopelagic fish biomass on the Chatham Rise over the past seven years.
- Dec 2006
- AGU Fall Meeting Abstracts
Seamounts are prominent features of the worlds underwater topography. They are widely regarded as productive, but fragile, habitat. They are the focus of commercial fishing for a number of demersal and pelagic fish and invertebrate species. Most fishing operations have some impact, either on the target species, associated bycatch species, or the benthic communities and habitat. Longlines, gillnets, traps and pots can all have some effect on the seafloor, but bottom trawling is the most well-known for causing considerable impact on the benthic habitat. There are few published studies on seamounts specifically, and recent research in New Zealand will be described. This has focused on deepwater fisheries for species such as orange roughy, which can form large aggregations over seamount features. The research includes analysis of the distribution of commercial catch and effort data from deepwater seamount fisheries, and "compare and contrast" surveys of seamounts that indicate the effects of bottom trawling can be severe on benthic invertebrate fauna. Fishing has clear consequences for structural complexity of the benthic habitat, and can alter species composition, and abundance. The results of such research are discussed with respect to management of seamount habitat in New Zealand, and the search for a balance that can allow sustainable seamount fisheries, and biodiversity conservation.
New Zealand seamounts support major fisheries for several deepwater fish species, including orange roughy (Hoplostethus atlanticus) and smooth oreo (Pseudocyttus maculatus). Although a high proportion of features in the depth range 500–1000 m have been fished, very little is known about the ecological impacts of bottom trawling on seamounts. The potential impact is likely to be influenced by the spatial extent and frequency of fishing. A new index is presented to assess the relative intensity of trawling on New Zealand seamounts. The fishing effects index (FEI) incorporates information on the density of fishing on the seamount as a proportion of the seabed area and also on tow direction. Detailed fisheries data from more than 250 000 tows were examined to calculate FEI for New Zealand seamounts. The most intensively fished seamounts were on the south Chatham Rise, an area characterised by a large number of relatively small features which were fished serially for orange roughy in the 1980s and 1990s. Other seamounts with high FEI were on the north Chatham Rise, Challenger Plateau, and off the east coast of the North Island. A range of sensitivity analyses indicated that the general rankings of seamounts were relatively robust to the choice of arbitrary thresholds used to assign tows to seamounts.
- Apr 2005
In some fisheries large factory freezer trawlers have periods of down time as the catch is processed. By utilizing this time, scientific acoustic surveys can be carried out between commercial-fishing operations without compromising fishing success. Examples are presented from three acoustic surveys for hoki (Macruronus novaezelandiae) in New Zealand waters during 2002 and 2003 conducted from a commercial vessel fitted with a scientifically calibrated SIMRAD ES-60 echosounder. These surveys confirmed the presence of a new spawning area for hoki and provided biomass estimates from known fishing grounds. The approach described works well for small-scale acoustic surveys adjacent to areas of high catch rates and is cost-effective because the vessel “pays for itself” by fishing commercially. The major limitation is that the boundaries of the survey area are determined by the time available during processing, which is related to the size of the catch and the time required to search for a suitable location for the next commercial trawl. In the New Zealand hoki surveys, processing time was typically 3–8 h, which was sufficient to carry out about 10–70 km of acoustic transects. Acoustic research was also limited to periods of relatively good conditions by the use of a hull-mounted transducer.
- Feb 2004
O'Driscoll, R. L. 2004. Estimating uncertainty associated with acoustic surveys of spawning hoki (Macruronus novaezelandiae) in Cook Strait, New Zealand. e ICES Journal of Marine Science, 61: 84e97. Eleven acoustic surveys carried out between 1991 and 2002 provided estimates of the relative abundance of spawning hoki in Cook Strait, New Zealand. The precision and bias of each survey were estimated using a new Monte Carlo simulation method, which combined uncertainties associated with survey timing, sampling error, detectability, species composition, target strength, calibration coefficients, and missing strata. Because hoki have a long spawning season (more than 2 months) with turnover of fish in the survey area, survey timing was the most important source of uncertainty. Uncertainty was reduced by having a number of sub-surveys (snapshots) over a 4e6 week period, centred on the middle date of the spawning season. The other major source of uncertainty was the occurrence of 40e70% of hoki in mixed species: ''hoki fuzz'' marks. The acoustic analysis assumed all acoustic backscatter from hoki marks was hoki, so the presence of other species caused a positive bias in relative-abundance indices. The magnitude of this bias differed between years because the proportion of hoki in the ''fuzz'' marks was variable. There was additional uncertainty from the variability in the species composition of mixed marks that affected survey precision. The abundance indices were corrected for estimated bias, and the estimate of precision, expressed as the coefficient of variation or c.v., was applied to weight the results from each acoustic survey in the assessment model used to set commercial-catch limits.
- Oct 2003
Deepwater fisheries for orange roughy (Hoplostethus atlanticus) and oreos (Pseudocyttus maculatus) in New Zealand waters have been established for 2030 years. Over time the fisheries have become more focused on seamounts, where aggregations of the fish can occur for spawning or feeding. The catch of orange roughy in particular from these features has increased from about 30% of the total catch in 1985 to 80% by 1995, and has since stabilized at 6070%. There has been active searching for seamount habitat, and by 2000 about 80% of known seamounts in the appropriate depth range had been fished. In New Zealand there are widespread concerns that seamount habitat needs to be managed carefully. In May 2001, 19 seamounts were given some protection through a ban on bottom trawling. Heavy bottom trawl gear is known to have a direct physical effect on the seabed. Seamounts are often small in size, and trawling can rapidly become concentrated in a very localized area. Seamount habitat is thought to be productive, but may also be fragile, and there is growing concern about the effects of fishing on biodiversity and ecosystem productivity. In this paper we report on ongoing research to examine the extent of impact on seamount habitat. Detailed fisheries data were analysed to determine the numbers and distribution of individual trawls on seamounts to measure how intensive trawling has been. Photographic surveys of the seafloor and mapping the distribution of benthic fauna have also been carried out to quantify the area of seamounts impacted by trawl gear. Comparisons have been made between heavily-fished and unfished seamounts.
- Jun 2003
O'Driscoll, R. L. 2003. Determining species composition in mixed-species marks: an example from the New Zealand hoki (Macruronus novaezelandiae) fishery. - ICES Journal of Marine Science, 60: 609-616. A model-based method has been developed for partitioning acoustic backscatter from mixed-species marks. This method uses catch-composition data to partition the echo integral, but allows for differences in trawl catchability and acoustic vulnerability between species. It was applied to estimate the biomass of New Zealand hoki (Macruronus novaezelandiae) from trawl and acoustic surveys on the Chatham Rise and Campbell Plateau in 2001. Mixed-species layers containing up to 20 different species were present in both survey areas. A total of 224 bottom-trawl surveys (123 on Chatham Rise and 101 on Campbell Plateau) were carried out to determine the species composition and relative densities. Simultaneous acoustic recordings made during each of these trawls were used to estimate vulnerability ratios for the two methods, i.e. acoustic as opposed to trawl surveys, (acoustic : trawl) by non-negative, least-squares minimization. The best-fit model for each survey attributed 14-22% of the backscatter in mixed layers within 10 m of the bottom to hoki. This produced hoki biomass estimates 1.3-1.8 times higher than the standard approach, which divides the echo integral in proportion to the catch assuming equal trawl catchability. The precision of the estimated acoustic : trawl vulnerability ratios depended on the contrast in trawl catch composition, and the ratios for the same species differed between areas. A major problem on the Chatham Rise was the acoustic contribution of small mesopelagic species, which are not caught by the bottom trawl. Despite these difficulties, the model-based approach has good potential for determining the biomass of the target species in a mixed- species mark when the different species cannot be discriminated acoustically.
The stock of northern cod (Gadus morhua )o ff Newfoundland and Labrador is at its lowest level in recorded history, with no rebuilding of northern spawning aggregations since the fishing moratorium in 1992. Cod diet was historically dominated by capelin (Mallotus villosus), which have been scarce in the northern areas since 1990. Using the study areas Hawke Channel and Trinity Bay within the historical northern cod range, and Placentia Bay (south coast), we examine growth, condition, and reproductive potential with respect to capelin diet in 18 000 cod sampled primarily in January and June of the years 1996-2000. Overall diet weights differed among areas and seasons (Placentia=Hawke>Trinity in January; Placentia>Trinity=Hawke in June). However, just 7 of 3383 cod stomachs (0.2%) from Hawke contained capelin (PFI=0). In contrast, 10% of cod of ages 3-7 in Placentia and Trinity Bays preyed on capelin and overall capelin partial fullness indices (PFI) were 0.2-0.4. Capelin PFIs were correlated with an index of availability (potential contact of capelin within 40 km of cod) calculated from acoustic surveys. Seasonally adjusted cod liver condition was strongly associated with capelin availability (pTrinityHawke). 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.
Acoustic estimates of capelin, Mallotus villosus (Müller), density were compared with catches in 97 Campelen 1800 bottom and 113 IYGPT midwater fishing sets to assess catching efficiency (qe) relative to acoustic estimates, and the ability of trawl and acoustic surveys to index capelin abundance. Catches in experimental IYGPT sets targeted at capelin and towed at constant depth or undulated over 40 m did not differ from acoustic predictions over a range of densities from 0.00001 to 0.2 fish per m2 (qe=1; p<0.05). Catch and qe did not differ between fishing methods (p>0.05, paired t-test). Campelen catches were typically bigger than acoustic estimates in the trawl zone at low or medium densities, likely a consequence of dead-zone non-detectability. Campelen qes were strongly density-dependent (>1 at densities >0.05 fish per m3; <1 at higher densities), and ranged over several orders of magnitude, making catches representative only of presence/absence. For the IYGPT trawl undulated at fixed survey stations, not targeted at capelin, mean qe averaged 11.6, and high variability, in part related to vertical distribution (s.e.=3.76), limited usefulness as an abundance index. A recognition threshold caused acoustic integrations based on a priori allocation of backscatter to underestimate capelin distributed at low densities (<0.05 fish per m3), as did dead-zone bias. A posteriori allocation reduced bias. It is concluded that acoustic integration supported by directed trawling is the most reliable method of counting capelin.
- Sep 2001
The diet of capelin (Mallotus villosus Müller) from six areas off the Newfoundland and Labrador coast was compared over three seasons (January, May-June, August-September) in 1999. A total of 1247 stomachs were examined. Of these, 837 (67 %) contained food. The proportion of empty stomachs was higher in winter (55 %) than in spring (28 %) or autumn (20 %). Copepods were the major prey over all areas and seasons, occurring in 90 % of non-empty stomachs. Hyperiid amphipods, euphausiids, larvaceans and chaetognaths were also important, occurring in 30 %, 11 %, 9 % and 7 % of non-empty stomachs respectively. The importance of these other prey groups increased with increasing capelin size. Larger capelin contained larger prey. There were also spatial and temporal differences in diet. Capelin from Placentia Bay, southeastern Newfoundland, consumed smaller copepods and a higher proportion of amphipods than capelin from other areas. Diet composition, particularly the incidence of lipid-rich Calanus species, may influence capelin growth.
- Feb 2001
A dispersed, monospecific aggregation of juvenile (0+) capelin was detected acoustically in shallow (20–70 m) water in Bonavista Bay, northeastern Newfoundland in January 2000. This provided a rare opportunity to measure acoustic target strength (TS) of very small (mean length=51 mm) capelin in situ. Mean observed TS at 38 kHz was −61.0 dB. Observed TS was similar to TS predicted by the Norwegian-Icelandic capelin TS-length relationship (TS=19.1 log L (cm)−74), but ∼2 dB lower than predicted by the existing TS-length relationship for capelin in Newfoundland waters at 38 and 49 kHz (TS=20 log L (cm)−73.1). Combining present data with previous 38 kHz data indicates the relationship TS=23.3 log L (cm)−77.1 (r2=0.95, n=6) for capelin of lengths 5–14 cm in Newfoundland waters.
Analysis of simulated data showed that potential contact statistics could be used to describe spatial pattern in sample density data. Potential contact is a new method, analogous to Ripley's K function for mapped point pattern analysis. Potential contact can be used to describe spatial pattern and association over a range of scales without grouping data and is robust against the presence of zeros. The statistical output is ecologically interpretable, as a measure of the degree of contact between individuals. This new technique was applied to examine changes in the spatial distribution of Atlantic cod (Gadus morhua) off Newfoundland, Canada, from 1985 to 1994, a period that encompassed a collapse of the cod stock. Sample data from bottom-trawl surveys indicated that cod were aggregated in patches with dimensions of 100-250 km. During the period of cod decline in the 1990s, spatial structure changed in three ways: the number of patches decreased, patch size shrank, and contact with conspecifics at small (10-20 km) scales fell. Cod were broadly associated with capelin (Mallotus villosus), a major prey species. Spatial distribution of capelin changed over the same time period as changes in cod distribution, and there was no evidence that contact between cod and capelin decreased.
- Oct 1998
Acoustic scattering experiments involving simultaneous acquisition of broadband echoes and video footage from several Antarctic krill were carried out to determine the effect of animal orientation on echo spectral structure. A novel video analysis technique, applied to extract krill angle of orientation corresponding to each insonification, revealed that echo spectra from krill near broadside incidence relative to the incident acoustic wave exhibited widely spaced, deep nulls, whereas off-broadside echo spectra had a more erratic structure, with several closely spaced nulls of variable depth. The pattern of changes in echo spectra with orientation for the experimentally measured acoustic returns was very similar to theoretically predicted patterns based on a distorted wave Born approximation (DWBA) model. Information contained in the broadband echo spectra of the krill was exploited to invert the acoustic returns for angle of orientation by applying a newly developed Covariance Mean Variance Classification (CMVC) approach, using generic and animal-specific theoretical and empirical model spaces. The animal-specific empirical model space was best able to invert for angle of orientation. The CMVC inversion technique can be implemented using a generic empirical model space to determine angle of orientation based on broadband echoes from individual zooplankton in the field. (C) 1998 Acoustical Society of America. [S0001-4966(98)05109-1]
Side-scan sonar allows mapping of near-surface schools of fish, but it has seldom been used for this purpose. Data were collected off the coast of Otago, New Zealand for 21 days during 1994 to 1996 using a 130 kHz Klein 595 digital side-scan sonar. A total of 2198 schools were detected. Of these, 348 schools of barracouta (Thyrsites atun), 67 schools of jack mackerel (Trachurus murphyi), and 17 schools of slender tuna (Allothunnus fallai) were identified. Barracouta schools were significantly smaller than schools of jack mackerel or slender tuna, but the size of schools was not related to state of tide, location, water depth, salinity, temperature, or density of krill. Fish schools were detected throughout daylight hours, but dispersed at night. Analysis of stomach contents revealed barracouta, jack mackerel and slender tuna were feeding on krill, Nyctiphanes australis, and fish schools occurred in areas with high densities of N. australis. Variability in the abundance of N. australis was related to salinity; catches were highest in a band of low-salinity water which was present in the study area following periods of high river runoff.
Backscattering measurements of 14 live individual Antarctic krill (Euphausia superba) were made at a frequency of 120 kHz in a chilled insulated tank at the Long Marine Laboratory in Santa Cruz, CA. Individual animals were suspended in front of the transducers, were only loosely constrained, had substantial freedom to move, and showed more or less random orientation. One thousand echoes were collected per animal. Orientation data were recorded on video. The acoustic data were analyzed and target strengths determined from each echo. A method was developed for estimating the three-dimensional orientation of the krill based on the video images and was applied to five of them, giving their target strengths as functions of orientation. Scattering models based on a simplified distorted-wave Born approximation (DWBA) method were developed for five animals and compared with the measurements.Both measured and modeled scattering patterns showed that 120 kHz acoustic scattering levels are highly dependent on animal orientation. Use of these scattering patterns with orientation data from shipboard studies of E. superba gave mean scattering levels approximately 12 dB lower than peak levels. These results underscore the need for better in situ behavioral data to properly interpret acoustic survey results. A generic E. superba DWBA scattering model is proposed that is scalable by animal length. With good orientation information, this model could significantly improve the precision and accuracy of krill acoustic surveys.
- Jun 1998
The occurrence, abundance, and distribution of seabirds was studied in a physically dynamic region off the coast of Otago, New Zealand. Eleven line‐transect surveys were conducted in late summer and autumn of 1994–96, when surface swarms of “krill”, Nyctiphanes australis, were present in the study area. Twenty species of seabird were recorded. The abundance and occurrence of species varied between sitting and flying counts. The most numerous species were sooty shearwaters (Puffinus griseus), red‐billed gulls (Larus novaehollandiae), black‐billed gulls (L. bulleri), and black‐backed gulls (L. dominicanus). Most species were recorded throughout the study area, and different species were commonly observed together. Spatial similarity matrices revealed strongest association between red‐ and black‐billed gulls and black‐backed gulls. The small (kilometre) scale distribution of seabirds varied between surveys, between transects, and between repeated runs of the same transect 1–3 h apart. Correlations between seabird abundance, salinity gradient, and krill density were weak. Counts of seabirds were highly positively skewed. This skewness, together with spatial and temporal variability in the distribution of flocks, would make abundance estimation at sea difficult.
- May 1998
Description of spatial pattern is an important step towards understanding the underlying, pattern-generating processes, but few statistical techniques adequately describe both the grain and intensity of a spatial distribution. Counts of seabirds collected along line transects are particularly problematic because the data may contain a high proportion of zero counts, and the pattern at increased spatial scales is dependent on how the counts are grouped. Statistics based on Ripley's K function seem well-suited to characterising scale-dependent spatial structure in such data. The spatial distributions of seabirds and fish schools off the Otago Peninsula, New Zealand, are described to illustrate the application of these statistics. Aggregation characteristics of sooty shearwaters Puffinus griseus, red-and black-billed gulls Larus novaehollandiae and L. bulleri and black-backed gulls L. dominicanus were similar. Sitting seabirds were highly clustered, with patch lengths between 145 and 2850 m. The distributions of flying seabirds were less crowded, with patch lengths from 135 to 5290 m. Patches of krill-eating red-and black-billed gulls were longer along transects with high salinity structure. This may reflect a change in foraging strategy in response to visible surface fronts. There was no clear association between the distribution of sitting birds and the presence of schools of predatory, pelagic fish.
To test the hypothesis that fish in schools forage more successfully than individual fish, an analysis was made of the stomach contents of barracouta (Thyrsites atun), a facultatively schooling species of fish, in a wild fish population. Schooling (n = 29) and non-schooling (n = 86) barracouta were captured during a side-scan sonar survey of pelagic fish off the coast of Otago, New Zealand. The proportion of fish with empty stomachs was lower and the mean wet mass of gut contents was higher in barracouta from schools. The increased feeding success of fish in schools was due to increased consumption of krill, Nyctiphanes australis. In regions where the density of krill in net tows was high (>1000 individuals km-1 tow length) or moderate (100–1000 individuals km-1), the mean wet mass of krill in the stomachs of schooling barracouta was 2–4 times higher than in the stomachs of non-schooling barracouta. Few schools of barracouta were observed in areas with low densities of krill (<100 individuals km-1). Schooling by barracouta seems to be a feeding strategy to exploit surface swarms of krill.
- Nov 1997
Acoustic surveys are commonly used to estimate the biomass of Antarctic krill (Euphausia superba). Accurate biomass estimates depend upon knowing the acoustic backscatteringproperties of the animals. A generic scattering model for individual E. superba was developed and tested against laboratory measurements. The model was based on morphometric measurements of live krill and employed a simplified form of the distorted wave Born approximation (DWBA) method. The model included the effects of animal length and orientation. Backscattering measurements of live individual Antarctic krill were made at 120 kHz as part of the August, 1995 Bioacoustical Oceanography Workshop in Santa Cruz, CA, jointly sponsored by the National Science Foundation and the Office of Naval Research. Animals were suspended one by one in an acoustic beam, with substantial freedom to move and thus with more or less random orientation. One‐thousand echoes were collected per animal, and target strength was calculated for each echo. Behavior was recorded on video. The video data were used to estimate three‐dimensional orientation for five of the animals, thus giving their target strengths as functions of orientation. The measurements supported the major features of the scattering model.
Anderson, J. T., Dalley, E. L., and O'Driscoll, R. L. 2002. Juvenile capelin (Mallotus villosus )o ff Newfoundland and Labrador in the 1990s. - ICES Journal of Marine Science, 59: 917-928. Juvenile capelin, Mallotus villosus (Muller), were distributed primarily on the northern Grand Banks and secondarily along the northeast coast of Newfoundland during late summer, 1994-1999. In some years, distributions extended to coastal Labrador. Capelin were seldom observed over deep water of the northeast Newfoundland Shelf or in shallow water of the southern Grand Banks. There were large areas encompassing wide ranges of capelin densities measured by the IYGPT trawl where the acoustic system did not detect capelin. Mean growth rate of capelin from 0- to 2-group was represented by length (mm)=90.9 age (years) 0.57 (r 2 =99%). The 1-group capelin formed discrete schools that varied significantly in fish length from 65 to 110 mm (p
Statistical analysis of large-scale (1-1,000 km) predator-prey associations between cod (Gadus morhua) and capelin (Mallotus villosus) is difficult because the spatial distributions of both species are heterogeneous and temporally dynamic. Statistics based on Ripley's K-function were used to describe the spatial association between cod and capelin off Newfound-land. The number of capelin prey around cod predators (potential con-tact) was computed from acoustic survey data for a range of possible cod ambits, from 5 to 100 km. Potential contact between cod and capelin var-ied seasonally in Placentia Bay, southeastern Newfoundland in 1998, be-ing highest in June and lowest in January. This seasonal difference was largely attributable to an increase in the spatial association of cod and capelin at scales of 10-50 km in spring. A similar pattern was observed off the northeast Newfoundland shelf in 1991-1994 where postspawning cod migrated inshore in spring and encountered groups of capelin. As a con-sequence of this migration pattern, spatial association and potential con-tact between cod and capelin were dependent on survey timing relative to the timing of cod spawning.
"Date: 4 July 1997" Thesis (Ph. D.)--University of Otago, 1997. Includes bibliographical references.