Bottom Trawl Surveys - Proceedings of a Workshop Held at Ottawa ...

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Sample density varies considerably from area to area and usually is dependent on priority, logistics and the overall success of the cruise. In 1980, the year of greatest coverage, the densities were one set per 10.7, 30.4 and 71.7 sq n miles for the Cartwright, Hopedale and Hawke channels, respectively. Coverage was as low as one set per 142.5 sq n miles in the Davis Strait in 1979. These sampling intensities, however, exceed minimum requirements (ICNAF) for groundfish surveys (one set per 350 sq n miles). EFFECTS OF NUMBERS OF STRATA AND DENSITY OF SAMPLING WITHIN EACH STRATUM IN THE GULF OF ST. LAWRENCE When the first sampling plan was designed in 1972 for the Esquiman Channel area it seemed clear that depth would probably be the most useful variable on which to base a stratification scheme. The size of the strata within the depth zones was picked so that the borders conformed to natural boundaries or lines of position and yet contained areas in which, from our relatively scanty knowledge, the distribution and abundance of shrimps in the area would be relatively homogenous. In this it was also recognized that there appeared to be a general decline in abundance north to south and from east to west within the Esquiman Channel and that the size of vessels being used, as well as the higher shrimp abundance, generally limited most of the fishing to the northern part of the area. These relatively small stratum areas were used for trips in 1972, 1973 and 1975 when research vessel trips were mounted which specifically aimed at obtaining a better understanding of shrimp distribution, abundance and biology. In 1976, and during subsequent surveys in the summer, the major target species was juvenile redfish though, because small redfish appear to have similar preferences to shrimps for depth and temperature, these species often occur together and can be conveniently sampled with the same gear. Furthermore, because the intention of the new surveys was to cover the complete deep water area of the Gulf of St. Lawrence, it became necessary to reduce the density of stations in the Esquiman Channel area. Thus, the stratification scheme was modified, reducing strata from 24 to 10 and using the 50"N parallel as the division between north and south. To examine the implications of this change on the estimates of biomass obtained, the data obtained in the 1975 survey have been · re-analyzed in some detail. Some of the results of this analysis are summarized in Tables 6 to 8. In the area north of 50"N, five separate biomass estimates are available (Table 7): (a} Estimates using all available small strata are shown in Figure 1. {b) Catch rates from strata within the same depth zone were then subjected 135 to an analysis of variance which showed that {with the exception of one stratum {4)) differences did not exist (P=0.05) between strata and that it was reasonable to combine strata within each depth zone. This was done by maintaining Stratum 4 as a separate stratum, and an estimate of biomass was obtained using the strata combined as follows: (1, 2, 3}, (17, 18, 22, 23), (9, 10, 14, 15, 16), (5, 6, 7, 8), and ( 4 )·. (c) The new strata (based on the metric depths) were used and estimates of biomass obtained based on a fixed distance of tow of 1.5 miles (assuming a good concordance between towing speed used and distance covered by the trawl). (d) The new strata were used but the distance towed as calculated from the positions obtained for the beginning and end of each set was used to adjust the catches to that equivalent to a tow of 1.5 miles. (e) The area was treated as a single stratum and the estimate of biomass derived as the mean of 44 sets made within the area. 8 10 12 14 16 18 20 22 24 CARAPACE lENGTH (MM) SEPTEMBER 1978 (78,595) OCTOBER 1978 (177,169) Fig. 9. Landings in number/hours fishing in fall of 1978 - Esquiman Channel, Gulf of St. Lawrence. (Total number caught.)
136 Table 6. Esquiman Channel, Gulf of St. Lawrence, various biomass estimates, E.E. PRINCE Trip 155, 1975. Small strata Biomass (kg x 10- 6 ) Mean catch/unit area (kg) No. of Mean Upper Lower Mean Variance 95% ± Units strata all used 12.17 14.23 10.10 35.69 8.44 6.06 340985 16 Small strata combined by depth 12.36 14.65 10.25 36.24 9.07 6.19 340985 7 Small strata two only 12.83 15.36 10.31 37.63 13.71 7.40 340985 2 Large strata (new) variable distance 13.74 16.64 10.84 41.14 16.67 8.70 334122 6 Large strata (new) fixed distance 13.31 15.65 10.97 39.83 11. 72 7.01 334122 6 Random sample (one stratum) 14.43 17.97 10.89 42.32 26.99 10.39 340985 1 Table 7. Esquiman Channel, Gulf of St. Lawrence, various biomass estimates, E.E. PRINCE Trip 155, 1975, north of 50°N. Small strata Biomass (kg x 10- 6 ) Mean catch/unit area (kg) No. of Mean Upper Lower Mean Variance 95% ± Units strata all used 6.596 8.147 5.046 40.075 18.33 9.42 164601 14 Small strata combined by depth 7.107 8.681 5.532 43.176 20.90 9.565 164601 5 Large strata (new) variable distance 7.579 9.084 6.075 52.65 25.52 10.45 143956 4 Large strata (new) fixed distance 7.545 9.359 5.730 52.411 36.26 12.604 143956 4 Random sample 7.512 9.683 5.341 45.638 41.70 13.191 164601 1
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Canadian Special Publication of Fis
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iv TABLE OF CONTENTS/TABLE DES MATI
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Abstract Doubleday; W.G., and D. Ri
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4 Research vessel operating expendi
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6) That sensitivity analyses relati
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List of Participants/Liste des Part
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Division 30 Division 3N Table l. co
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45"00 Yi Fig. 12. Contour diagram o
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66 When departing on a survey, each
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68 ABUNDANCE ESTIMATES OF THE TRAWL
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-:... "' "' 3 _..-bob in 0 35cm Fig
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74 wires length (m) 1250 1000 7 50
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78 200 150 100 50 • • • •
Page 94: CATCH PROJECTIONS AND THEIR RELATIO
Page 102: AGE 3 4 5 6 7 8 9 10 11 12 13 14 15
Page 106: TABLE B. Relative error (expressed
Page 110 and 111: contribute significantly to the est
Page 113: 114 It is seldom the case that surv
Page 123 and 124: 124 GROUNDFISH SURVEY TECHNIQUES AS
Page 125 and 126: 126 The area bounded by the 100-fat
Page 131: 132 Table 2. Biomass estimates, Esq
Page 139: 140 Table 10. Shrimp biomass estima
Page 145 and 146: 146 Dupouy, H., J. Frechette and C.
Page 147: ABUNDANCE ESTIMATORS BASED ON STRAT
Page 167 and 168: to be exact. Monte Carlo simulation
Page 173: EXAHPLE 2 Haddock in NAFO Division
Page 176 and 177: 178 ANALYSIS OF VARIANCE SOURCE CON
Page 179: SUMMER DISTRIBUTION OF GROUNDFISHES
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194 DISTRIBUTION AND SAMPLING VARIA
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Fig. 4 ' cont'd 205
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66' 62' 46' COD 1976 62' 1978 60' 6
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46" u• 46" 66" 64" 62" 64" 62" oo
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216 arhythmic in spring-early summe
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226 Tableau 5 - Pourcentage cumulat
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230 Much further work on fish swimm
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Table 3b. Variation of s (feet/sec
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250 FISH BEHAVIOUR IS A MAJOR FACTO
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professional fisherman. No matter h
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270 10 8 6 4 2 8 6 4 2 10 8 6 4 2 1
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