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Geographical Variation in the Vertical Distribution of Cod (Gadus Morhua L.) and Availability to Survey Gears

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Geographical Variation in the Vertical Distribution of Cod (Gadus Morhua L.) and Availability to Survey Gears See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/228908659 Geographical variation in the vertical distribution of cod (Gadus morhua L.) and availability to survey gears Article · January 2002 CITATIONS READS 0 17 2 authors, including: Boonchai Stensholt Institute of Marine Research in Norway 9 PUBLICATIONS 193 CITATIONS SEE PROFILE All content following this page was uploaded by Boonchai Stensholt on 22 July 2014. The user has requested enhancement of the downloaded file. GIS/Spatial analyses in Fishery and Aquatic Sciences Stensholt and Stensholt - Geographical variation in the vertical distribution of cod (109-126) © Fishery and Aquatic GIS Research Group, 2004 1 2 3 Geographical variation in the 4 vertical distribution of cod (Gadus 5 morhua L.) and availability to 6 survey gears 7 8 9 Boonchai K. Stensholt 10 Institute of Marine Research 11 P.O. Box 1870 N-5817, Bergen, Norway 12 Phone: 47(Norway)-55-23-8667 13 Fax: 47(Norway)-55-23-8687 14 E-mail: [email protected] 15 16 Eivind Stensholt 17 The Norwegian School of Economics and Business Administration, 18 Helleveien 30, N-5045 Bergen, Norway 19 20 21 Abstract 22 23 Vertical density distributions of cod (Gadus morhua L.) are expressed in 24 terms of their free vertical range to normalize for the variation in vertical 25 extent that is related to change of pressure with depth. Thus the relative 26 cumulative vertical profiles obtained from different sources and from 27 places with different bottom depth become compatible for a discussion of 28 the relationship between the vertical distribution and other environmental 29 conditions. Of particular interest are the environmental conditions that 30 influence the cod’s vertical distribution and thereby cause a large 31 discrepancy between availabilities of fish to different survey gears, and 32 how these discrepancies and conditions vary with season and location. 33 Such information may be used to assess bias and variance in the 34 estimation of fish abundance. Analysis of acoustic and trawl catch data 35 from demersal fish surveys in the Barents Sea during winter and summer 36 show that in deep waters or along the polar front a large fraction of fish 37 are found to distribute high in the water column and acoustic surveys 38 detect more fish than the trawl surveys. When the density distribution 39 stretches beyond mid-water, this typically happens at the warm side 40 along the polar front, in agreement with the data storage tag records. 41 Acoustic samples with large loss in the acoustic bottom dead zone are 109 GIS/Spatial analyses in Fishery and Aquatic Sciences 42 found consistently over years around the Svalbard Bank. Loss in the 43 acoustic bottom dead zone is estimated by combining information in the 44 vertical profiles from acoustic and tag data. 45 46 47 Key words 48 49 acoustic bottom dead zone, capelin, cod, data storage tag, free vertical range, pelagic 50 living, polar cod, vertical distribution. 51 52 53 1. Introduction 54 55 Acoustic dead zone loss and avoidance reactions to vessel and survey 56 gear are major factors causing uncertainty in the abundance estimates of 57 demersal physoclistous fish based on acoustic and trawl survey data. 58 (Harden Jones and Scholes, 1981; Ona and Godø, 1990; Aglen, 1994; 59 Aglen, 1996; Ona and Mitson, 1996; Aglen et al., 1999; Hjellvik et al., 60 2002). The catch in a bottom trawl is partly fish in the acoustic bottom 61 dead zone and partly fish from a zone where they could be detected by 62 acoustics. However, the fraction of fish from each zone is not known. 63 Moreover, these fractions vary according to the vertical distribution 64 patterns and behaviour of the target species. Fish modify their distribution 65 and behaviour, within their own physiological limitation, in response to 66 different environmental conditions, which vary over different locations and 67 seasons (Neilson and Perry, 1990; Stensholt et al., 2002). Therefore 68 availability of fish to each survey gear varies accordingly. Thus spatial 69 and temporal variation in environmental conditions contributes to the bias 70 and variance in abundance estimates. Understanding these processes 71 may lead to a better survey design and analytical methods to assess and 72 reduce this part of the variance and bias. 73 74 In this paper we analyse and discuss the similarities and 75 discrepancies between acoustic, trawl, and data storage tag (DST) data 76 of northeast Arctic cod (Gadus morhua L.) in relation to geographical 77 location and environmental condition. A method for estimating the data 78 loss in the acoustic bottom dead zone based on acoustic and DST data is 79 proposed. 80 81 82 2. Material and methods 83 84 Environmental and physiological factors influencing vertical profiles are 85 investigated by analysis of acoustic, trawl, and temperature data together 86 with depth and temperature time series from data storage tags (DST). 110 Stensholt and Stensholt - Geographical variation in the vertical distribution of cod (109-126) 87 Tags were attached to adult northeast Arctic cod (53-81 cm) released into 88 the Barents Sea in mid March 1996. Acoustic samples, which reflect the 89 vertical distribution of all length groups combined, trawl and temperature 90 samples are from four series of annual scientific surveys in the Barents 91 Sea, namely, (1) summer demersal fish surveys during July-August, 1995 92 to 2001 (Aglen, 1999), (2) winter demersal fish surveys during 93 February-March, 1996 to 2002 (Mehl, 1997), (3) 0-group surveys in 94 August-September, 1996 to 2001 (Anon 1996), and (4) the pelagic 95 surveys in September-October, 1996 to 2001 (Anon, 2001). 96 97 For the summer and winter surveys, acoustic values are 98 allocated, mostly according to species composition from neighboring 99 bottom trawl, to cod (Gadus morhua), haddock (melanogrammus 100 aeglefinus), saithe (Pollachius virens) and redfish (Sebastes), and in 101 addition to the non-target pelagic species blue whiting (Micromesistius 102 poutassou), capelin (Mallotus villosus), and herring (Clupea harengus) 103 where their distributions overlap with the target species. The sA-values 104 were aggregated in blocks of 1 nm length and 10 m depth from 10 m and 105 downward. In demersal surveys, pelagic trawl hauls are few, and are 106 decided according to acoustic indication. For the 0-group and pelagic 107 surveys, the prey species of adult cod, e.g. young fish, capelin, herring, 108 and polar cod (Boreogadus saida), are the target species. Their acoustic 109 samples were aggregated in blocks of 5 nm length and 5 m depth. 110 111 At a location ‘s’ and for a selected species, say cod, the 112 theoretical sA-value of the trawl catch (Ts) (Aglen, 1996) was compared to 113 the observed acoustic sA-value (As) along the trawl track. Let as = -1 114 As⋅(As+Ts) ; then as < 0.5 indicates a loss of acoustic value in the bottom 115 dead zone as the bottom trawl catch exceeds the total observed acoustic 116 sA-value. 117 118 Since cod is a demersal physoclistous fish, i.e. with a closed 119 swimbladder, the vertical distribution of samples from various depth 120 ranges is expressed in terms of relative pressure reduction level (RPRL) 121 with reference to the bottom pressure. A depth of X m is transformed to 122 RPRL = (B-X)(B+10)-1, where B is the bottom depth. The bottom 123 restricted free vertical range (FVRbot) is defined as a free vertical range 124 that has its deepest end at seabed (Stensholt et al., 2002). For all bottom 125 depths, the FVRbot of cod ranges from RPRL=0 to RPRL=0.5 (Harden 126 Jones and Scholes, 1985). Thus samples from various bottom depths are 127 normalized for variations in the extent of the free vertical range. Taking 128 into account the fish’s physiological limitation to rapid vertical movement, 129 the FVRbot is a natural yardstick suitable in a discussion, ‘from the fish’s 130 point of view’, of factors that influence the vertical distribution of demersal 131 physoclists. A fish that can reach above the FVRbot must be adapted to 111 GIS/Spatial analyses in Fishery and Aquatic Sciences 132 ‘pelagic living’ in the sense that its current FVR has departed from the 133 seabed (Stensholt et al., 2002). Since the adaptation status is unknown, 134 in practice a fish is identified as adapted to pelagic living when it is 135 observed above the FVRbot. For pelagic species the RPRL is sometimes 136 calculated with reference to the deepest end of the distribution range 137 (Stensholt et al., 2002). 138 139 The availability of fish to survey gears depends on the fish’s 140 vertical density distribution, which in turn varies with geographic location. 141 The variation is visualized by selecting samples which have total acoustic 142 sA-values greater than 5 and satisfy one of the following criteria: 143 144 z More than 20% in summer survey samples (10% in winter survey 145 samples) of the observed total sA-value allocated to cod distributed 146 above the FVRbot, and more than 70% of the observed total sA-value 147 allocated to cod distributed more than 10 m above seabed. 148 z The entire distribution is within 20% of the FVRbot (below RPRL = 0.1), 149 and more than 90% of the observed total sA-value allocated to cod is 150 within 10 m above seabed. 151 152 At sampling time, some cod adapted to pelagic living happens to be in the 153 lower part of their current FVR, which overlaps with FVRbot. Those cod 154 are not counted under criterion 1, and thus the true fraction of cod 155 adapted to pelagic living is generally higher than the observed 156 percentage.
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