Portuguese International Bottom Trawl Survey

The dataset includes age- and length-based catch per unit effort data for commercial fish species collected during the Portuguese Ground Fish trawl surveys.
The Portuguese groundfish surveys have been conducted along the Portuguese continental waters since 1979 with the RV “Noruega”. The area extends from latitude 41°20' N to 36°30' N (ICES Div. IXa) and from 20 to 500 m depth. Surveys took place three times a year, in winter, summer and autumn. The winter survey took place in 1992 and 1993, then was interrupted for some years and started again in 2005. The summer survey series has started in 1979 and stopped in 2002. The autumn survey is conducted since 1979 with the exception of the year 1984 when the vessel was in reparation.


Initially (1979) the main objectives of the surveys were to estimate the abundance and study the distribution of the most important commercial species in the Portuguese trawl fishery: hake, horse mackerel, blue whiting, seabream and Norway lobster. Recruitment abundance and distribution for hake and horse mackerel were monitored in the autumn surveys. Additionally, trawl selectivity experiments for hake and horse mackerel with 40 mm mesh size, were also conducted during 1981 surveys using the covered codend method.


At present, the main objective of the Autumn survey is to monitor the abundance and distribution of hake and horse mackerel recruitment, while the Winter survey aims to monitor the abundance and distribution of hake in spawning season. Additionally these surveys estimate (i) abundance indices and biomass of the most important commercial species (ii) biological parameters, e.g. maturity, ages, sex-ratio, weight, food habits. (iii) biodiversity indicators. The primary species are hake, horse mackerel, blue whiting, mackerel, Spanish mackerel, anglerfish, megrim and Norway lobster.


Sampling design was progressively modified in order to improve the precision of the abundance and biomass estimates. During 1979-1980 a stratified random sampling design was adopted with a total of 15 strata. Each stratum was divided into units of approximately 25 nm2. The stratification was based on 3 depth intervals and 5 geographical areas, being the depth ranges 20–100 m, 101–200 m and 201–500 m. During 1979–1980, the number of hauls per stratum performed was based on the previous information of the relative abundance of the target species in each geographical area and on the ship time available. During 1981–1989, the number of strata was increased to 36, (12 sectors and 3 depth ranges) and two random units were sampled by stratum, whenever possible, to allow an estimate of the standard error of the stratified mean by stratum.


In 1989 a fixed stations sampling design was adopted, comprising 97 positions spread over 12 sectors, which one divided into 4 depth ranges: 20–100 m, 101–200 m, 201–500 m and the new 501–750 m, making a total of 48 strata. The positions of the 97 fixed stations were based on common stations made during 1981–1989 surveys and taking into account that at least two stations per stratum could be sampled. A maximum of 30 supplementary stations were planned to be carried out if ship time was available or to replace positions that due to particular factors were not possible to trawl.


A new sampling scheme was implemented in the new Winter series in 2005. The design is a mixture of a fixed sampling scheme with trawl positions distributed over a fixed grid with 5’ per 5’ miles and random trawl positions. The new sampling scheme allows performing future calculations with the former strata and also allows to perform spatial analysis by using geostatistics methods. The 500-750 m depth stratum was removed due to small trawlable and low relevance for the survey purposes. The Autumn groundfish survey plan comprises 96 fishing stations, 66 at fixed (grid) positions and 30 at random. The tow duration is 30 min, with a trawl speed of 3.5 knots, during day light. The Winter groundfish survey plan comprises 75 fishing stations, 66 at fixed (grid) positions and 9 at random; the tow duration is 60 min, with a trawl speed of 3.5 knots, during day light.


The surveys are carried with the Portuguese RV “Noruega”, which is a stern trawler of 47.5 m length, 1500 horse power and 495 GRT. In the Autumn surveys the fishing gear used is a bottom trawl (type Norwegian Campell Trawl 1800/96 NCT) with a 20 mm codend mesh size. The main characteristic of this gear is the groundrope with bobbins. The mean vertical opening is 4.6 m and the mean horizontal opening between wings and doors is 15.1 m and 45.7 m, respectively. The polyvalent trawl doors used are rectangular (2.7 m x 1.58 m) with an area of 3.75 m2 and weighting 650 Kg. In the Winter surveys the fishing gear used is a bottom trawl net (CAR) type FGAV019, without rollers in the groundrope. The mean horizontal opening between the wings is 25 m, the mean vertical opening is 2.5 m and the codend mesh size is 20 mm. The trawl doors used are the same as those used in the NCT gear. In autumns 1996, 1999, 2003 and 2004, the Portuguese RV “Noruega” was unavailable and RV “Capricórnio” was used. R/V “Capricórnio”, is a stern trawler of 46.5 m length, 1200 horse power (880 KW) and 494 GRT. The fishing gear is a bottom trawl (CAR, FGAV019), the same used in Winter surveys.