20 years of Chatham Rise fish surveys

 

NIWA has run the Ministry of Fisheries-funded Chatham Rise survey using R.V. Tangaroa every year since 1992.

The main aim of the surveys is to estimate the abundance of hoki and other commercially important species (such as hake and ling), but during the 20 consecutive surveys NIWA scientists have also been able to study other aspects of deepwater biodiversity on the Chatham Rise, including fish distribution, abundance, and ecology.

Overview

The surveys are conducted by trawling at depths of 200–800 metres. Trawling locations are randomly selected using a specialised computer programme. The trawl is towed for 3 nautical miles (5.5 km) at a speed of 3.5 knots (6.5 km/h) at each location.

The hoki fishery is New Zealand's largest fishery, with a current total allowable catch (TAC) of 120,000 tonnes. The largest fishery area is on the Chatham Rise, with a hoki catch of 39,000 t from this area in 2009–10. The Chatham Rise is particularly important because it is also the major nursery area for New Zealand hoki. Juvenile hoki from both eastern and western hoki stocks mix on the Chatham Rise, so this survey provides an opportunity to get an idea of how many small hoki are out there, before they recruit to their respective areas and are caught by the commercial fishery. This allows the Ministry of Fisheries to set an appropriate catch limit which is responsive to the abundance of small fish coming into the fishery.

Over the last 20 years the proportion of hoki in the catch has declined from nearly 60% in 1993 to 21% in 2004, but has increased again to make up 30–40% of the total biomass in the past six years.The most recent survey in January 2012 recorded good numbers of young hoki (2- and 3-year olds) which should feed into the fishery over the next couple of years. However, the numbers of older hoki have decreased. There were also very few 1-year olds caught in the 2012 survey which indicates that the 2010 year-class was weak. 

As well as providing an essential input into the stock assessment for hoki, the surveys also fulfil an important ecosystem monitoring role by providing additional information on species distribution and biodiversity. Since the surveys began, scientists have recorded a total of 558 species or species groups and analysed more than one million individual fish, squid, crustaceans, and benthic fauna to help establish biomass trends and spatial and depth distributions. They enter a catch record of all species into a trawl database and bring any species unidentified at sea back to NIWA for identification. Rare and new-to-science fish are sent to Te Papa where they are preserved and stored in the National Fish Collection.

The approach

The main aim is to estimate the relative abundance of hoki and other middle depth species on the Chatham Rise using trawl surveys and to determine the relative year class strengths of juvenile hoki (1, 2 and 3 year olds). To achieve this requires about 100 trawls over a period of about 27 days. January was chosen as the optimal time for this survey because we believe that this is a time when most of eastern hoki are present on the Chatham Rise. At other times of the year, adult hoki leave the Chatham Rise to migrate to their spawning areas (mainly in Cook Strait).

All surveys follow standardised and documented protocols. This is vital to ensure that we obtain a consistent series of relative abundance indices. This means that we may not know how many fish are there in absolute terms, but we can tell whether numbers are increasing, decreasing, or remaining the same. Key aspects of this are:

1. Surveying the same area

All surveys cover depths of 200–800 m on the Chatham Rise. Additional deeper regions were also surveyed in 2000, 2002, 2007, 2008, 2010, and 2011. The core survey area is divided into subareas (called strata) based on 200-m depth intervals (i.e., 200–400 m, 400–600 m, and 600–800 m), latitude, and longitude. The stratification has undergone several changes over the time series, as we have learnt more about the distribution of the key species. This has allowed for improved optimisation and a decrease in station numbers (and therefore survey costs).

2. Using the same vessel and gear

Tangaroa is a purpose-built, research stern trawler of 70 m overall length, a beam of 14 m, 3000 kW (4000 hp) of power, and a gross tonnage of 2282 t. The bottom trawl used in the Chatham Rise time series is an eight-seam hoki bottom trawl with 100 m sweeps, 50 m bridles, 12 m backstrops, 58.8 m groundrope, 45 m headline, and 60 mm codend mesh. The trawl doors are Super Vee type with an area of 6.1 m2. 

3. Following the same protocols

Trawl procedures are documented. Trawl positions are selected randomly before the voyage using a Random Stations Generation Program developed at NIWA. A minimum distance between stations of 3 n. miles is used. All core tows are carried out during daylight hours because at night fish move up away from the bottom. At each station the trawl is towed for 3 n. miles at a speed over the ground of 3.5 knots. Tow positions are recorded by GPS and depths from the vessel's echosounder. Measurements of doorspread (from a Scanmar 400 system) and headline height (from a Furuno net monitor) are recorded every 5 minutes during each tow and average values calculated.

4. Consistent catch and biological sampling

At each station all items in the catch are sorted into species and weighed on Seaway motion-compensating electronic scales accurate to about 0.2 kg. Where possible, fish, squid, and crustaceans are identified to species and other benthic fauna to species or family. The level of taxonomic identification at sea has improved over time with development of improved identification guides for fish and benthic invertebrates. The level of biological sampling has also increased over the time series.

In general, an approximately random sample of up to 200 individuals of each commercial, and some common non-commercial, species from every successful tow are measured and the sex determined. More detailed biological data are also collected on a subset of species and included fish weight, sex, gonad stage, and gonad weight. Otoliths (ear bones) are taken from hake, hoki, and ling for age determination. Additional data (e.g., stomach samples, data on hoki liver condition, genetic samples) are collected in some surveys.

5. Standardised analysis

Analyses are carried out using the NIWA custom software SurvCalc. SurvCalc is a C++ computer program developed in 2008 specifically to analyse data from stratified random surveys. Its primary purpose is to calculate estimates of biomass and/or length frequencies, and associated coefficients of variation (c.v.s), from survey data. SurvCalc supersedes, and uses some code from, an earlier program called Trawlsurvey.

Since 2001, Chatham Rise surveys have had an additional aim to collect acoustic (echosounder) data which is used to monitor the abundance of the small midwater fish that hoki eat. This is achieved by collecting and analysing data from the ship's echosounders.

Some surveys have also sampled deeper waters (greater than 800 m) to increase our knowledge about the abundance and distribution of deepwater species.

Results

Since 1992, 558 species or species groups have been recorded from the 20 surveys. The number of species recorded has increased over time, mainly due to improvements in identification of benthic invertebrates such as corals, sponges, crustaceans, and echinoderms. 

Biomass trends and spatial and depth distributions have been estimated for 142 species or groups. For the 49 groups where biomass was relatively well-estimated, biomass has decreased significantly since the start of the time series for only two species: hake and rudderfish. Hoki and arrow squid decreased in the middle part of the time series, but then increased. Eighteen groups increased significantly, 9 increased and then decreased, and 18 showed no clear trend. 

The proportion of hoki in the catch declined from nearly 60% in 1993 to 21% in 2004, but increased again to make up 30–40% of the total biomass in the past 6 years. Two other target species, ling and hake, typically made up 3–4% and less than 2% respectively of the total survey biomass. 

Catch rates and distribution plots provide information on species' distributions. Of 109 species or species groups with sufficient information to draw conclusions about depth distribution:

  • 35 appeared to occur mainly within the core survey depth limits of 200–800 m
  • the distribution of 27 species or groups extended shallower than 200 m
  • 37 extended deeper than 800 m
  • 10 were pelagic.

The relatively high number of groups with depth distributions that extend deeper than 800 m suggests that worthwhile gains in the value of the Chatham Rise survey as an ecosystem monitoring tool will be achieved by extending the survey boundaries deeper, as was trialled in 2010 when strata were added down to 1300 m. 

Over one million individuals of 159 species have been measured on Chatham Rise trawl surveys. If laid end-to-end these would stretch over 500 km! Of these, 45 species had sufficient information to estimate scaled length frequency distributions by year. Most showed no clear trend in mean length over the period for which length measurements were available. In addition 225 852 fish have been individually weighed from a total catch (for the 20 surveys combined) of 3,143 t.

Combined with information from reading otoliths we can estimate how many fish of each age are caught. Recent surveys show that there are good numbers of young (1 and 2-year old) hoki out there which will feed into the fishery over the next couple of years. However, the numbers of older hoki have decreased.

A selection of maps and results from the trawl surveys are available and can be viewed in the image gallery '20 years of Chatham Rise fish surveys - maps and results'.

20 years of Chatham Rise fish surveys - maps and results

See the finished report 'A review of hoki and middle-depth trawl surveys of the Chatham Rise, January 1992–2010' (PDF 601 KB)

Publications

Bagley, N.W.; Hurst, R.J. (1998). Trawl survey of hoki and middle depth species on the Chatham Rise, January 1998 (TAN9801). NIWA Technical Report 44. 54 p.

Bagley, N.W.; Livingston, M.E. (2000). Trawl survey of hoki and middle depth species on the Chatham Rise, January 1999 (TAN9901). NIWA Technical Report 81. 52 p.

Bull, B.; Livingston, M.E.; Hurst, R.; Bagley, N. (2001). Upper-slope fish communities on the Chatham Rise, New Zealand, 1992-1999. New Zealand Journal of Marine and Freshwater Research 35: 795–815.

Horn, P.L. (1994a). Trawl survey of hoki and middle depth species on the Chatham Rise, December 1991-January 1992 (TAN9106). New Zealand Fisheries Data Report No. 43. 38 p.

Horn, P.L. (1994b). Trawl survey of hoki and middle depth species on the Chatham Rise, December 1992-January 1993 (TAN9212). New Zealand Fisheries Data Report No. 44. 43 p.

Livingston, M.E.; Bull, B.; Stevens, D.W.; Bagley, N.W. (2002). A review of hoki and middle depth trawl surveys of the Chatham Rise, January 1992–2001. NIWA Technical Report 113. 146 p.

Livingston, M.E.; Stevens, D.W. (2005). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2004 (TAN0401). New Zealand Fisheries Assessment Report 2005/21. 62 p.

Livingston, M.E.; Stevens, D.; O'Driscoll, R.L.; Francis, R.I.C.C. (2004). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2003 (TAN0301). New Zealand Fisheries Assessment Report 2004/16. 71 p.

O'Driscoll, R. L., Gauthier, S., and Devine, J. (2009). Acoustic surveys of mesopelagic fish: as clear as day and night? ICES Journal of Marine Science 66: 1310–1317.

O'Driscoll, R.L.; MacGibbon, D.; Fu, D.; Lyon, W.; Stevens, D.W. (2011). A review of hoki and middle depth trawl surveys of the Chatham Rise, January 1992–2010. New Zealand Fisheries Assessment Report 2011/47 . 814 p.

Schofield, K.A.; Horn, P.L. (1994). Trawl survey of hoki and middle depth species on the Chatham Rise, January 1994 (TAN9401). New Zealand Fisheries Data Report No. 53. 54 p.

Schofield, K.A.; Livingston, M.E. (1995). Trawl survey of hoki and middle depth species on the Chatham Rise, January 1995 (TAN9501). New Zealand Fisheries Data Report No. 59. 53 p.

Schofield, K.A.; Livingston, M.E. (1996). Trawl survey of hoki and middle depth species on the Chatham Rise, January 1996 (TAN9601). New Zealand Fisheries Data Report No. 71. 50 p.

Schofield, K.A.; Livingston, M.E. (1997). Trawl survey of hoki and middle depth species on the Chatham Rise, January 1997 (TAN9701). NIWA Technical Report 6. 51 p.

Stevens, D.W.; Livingston, M.E.; Bagley, N.W. (2001). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2000 (TAN0001). NIWA Technical Report 104. 55 p.

Stevens, D.W.; Livingston, M.E.; Bagley, N.W. (2002). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2001 (TAN0101). NIWA Technical Report 116. 61 p.

Stevens, D.W.; Livingston, M.E. (2003). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2002 (TAN0201). New Zealand Fisheries Assessment Report 2003/19. 57 p.

Stevens, D.W.; O'Driscoll, R.L. (2006). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2005 (TAN0501) New Zealand Fisheries Assessment Report 2006/13. 73 p.

Stevens, D.W.; O'Driscoll, R.L. (2007). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2006 (TAN0601) New Zealand Fisheries Assessment Report 2007/5. 73 p.

Stevens, D.W.; O'Driscoll, R.L.; Dunn, M.R.; MacGibbon, D.; Horn, P.L.; Gauthier, S. (2011). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2010 (TAN1001). New Zealand Fisheries Assessment Report 2011/10. 112 p.

Stevens, D.W.; O'Driscoll, R.L.; Gauthier, S (2008). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2007 (TAN0701) New Zealand Fisheries Assessment Report 2008/52. 81 p.

Stevens, D.W.; O'Driscoll, R.L.; Horn, P.L. (2009a). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2008 (TAN0801). New Zealand Fisheries Assessment Report 2009/18. 86 p.

Stevens, D.W.; O'Driscoll, R.L.; Horn, P.L. (2009b). Trawl survey of hoki and middle depth species on the Chatham Rise, January 2009 (TAN0901). New Zealand Fisheries Assessment Report 2009/55. 91 p. 

Page last updated: 
5 November 2014
NIWA fisheries scientist Darren Stevens measuring hoki in Tangaroa’s wet lab. Credit: Neil Bagley, NIWA
Hoki in the Pound. [Peter Marriott]