Richard L. O'DRiscoll

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.

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.

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.

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.

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 4–6 week period, centred on the middle date of the spawning season. The other major source of uncertainty was the occurrence of 40–70% 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.

New Zealand fish: a complete guide. By Chris Paulin, Clive Roberts, Andrew Stewart, and Peter McMillan. First published in 1989. Reprinted in 2002 by Te Papa Press, Museum of New Zealand Te Papa Tongarewa, Wellington, New Zealand. 279 p. ISBN: 0–909010–85–4. Price: NZ$39.95 (email: mcmedia@xtra.com.nz). Hook up on kingfish. By Mark Kitteridge. Published in 2001 by Reed Publishing (NZ) Ltd, 39 Rawene Rd, Birkenhead, Auckland, New Zealand (www.reed.co.nz). 188 p., limpbound. ISBN 0–7900–0819‐X. Price: NZ$34.95 (email: kpower@reed.co.nz).

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.