Fisheries Research, 13 ( 1992 ) 293-310 Elsevier Science Publishers B.V., Amsterdam

293

Size selection during trawl sampling of cod and haddock and its effect on abundance indices at

age

Olav Rune Gode and Knut SunnanA Institute of Marine Research, P.O. Box 1870, N.5024 Bergen. Norway

ABSTRACT

Gode, O.R. and Sunnan& K., 1992. Size selection during trawl sampling ofcod and haddock and its effect on abundance indices at age. Fish. Res., 13: 293-310.

Investigations o r length selectivity of the s1~. ",4ard Norwegian bottom trawl has revealed that small cod and haddock were largely under-represented in the catch. To minimize the effect of length selec- tion on abundance indices at age obtained from surveys, a new standard sampling trawl has been used since January 1989. Based on lntraHel trawling experiments, conversion factors for comparing catches from the two sampling gears are available from previous work. Using these factors, data from 1983 to 1988 are adjusted to produce indice~ consistent with those from 1939 and 1990 which were obtained with the new sampling trawl. The new 'rime series appears to be more accurate than the old series, which was biased by large yearly variability in the growth rates of small fish. Cod were more affected than haddock. The validity of combining results from two different sampling trawls into one index time series is discussed.

INTRODUCTION

Trawl selection is size-dependent, while indices of abundance are calcu- lated by age. Consequently, changes in mean length at age from year to year may lead to a different age selection, e.g. in an extreme case a 100% increase in survey trawl efficiency for a particular age group may occur due to an in- crease in mean length at age (GOdz and Walsh, 1992). Bottom trawl surveys have been carried out annually in the Barents Sea and in the Svalbard area since 1981. These surveys yield indices of abundance at age for cod and had- dock which are used in the assessment and management of the stocks (Anon, 1989). In particular, these survey estimates are used as indices of abundance for VPA tuning procedures and as indices of recruitment for catch prediction.

Correspondence to: O.R. Gode, Institute of Marine Research, P.O. Box 1870, N-5024, Bergen, Norway.

© 1992 Elsevier Science Publishers B.V. All rights reserved 0165-7836/92/$05.00

294 O.R. GODe AND t~ SUNNANA

Consequently, inaccurate survey estimates, caused by je~'~j changes in growth rates, may result in mismanagement of the stocks.

The selectivity of the Norwegian standard samp]i~g bottom trawl used in the Barents Sea-Svalbard surveys has been extensively studied (Eng~ and Godz, 1986, 1989a, b; GOdz and EngAs, 1989). These studies were initiated because it was known that the standard trawl was inefficient for catching small cod and haddock (Hylen et al., 1980 ). In addition, the effect of different rig- ging of the standard trawl used in the two areas was examined (EngAs and GOdz, 1989b). Motivated by these studies, the old bobbin ground gear was replaced by a heavier rockhopper groundgear, and in the Svalbard survey, the sweep wires were changed from 80 m to the Barents Sea standard of 40 m. The differences in efficiency due to these changes are well documented and conversion factors by length group are available (Godo et al., 1989).

It is the aim of this paper to derive an index of abundance series for cod and haddock for the entire period, 1983-1990. The effect of changing age selectivity on abundance indices, which is due to changes in mean length at age, is examined, as is the validity of adjusting indices from the old trawl to ones comparable with the new.

MATERIAl AND METHODS

Survey methods

The bottom trawl surveys were conducted in winter in the Barents Sea and during autumn in the Svalbard area. Both surveys had a random stratified design (see Fig. 1 ), and the data analyzed were from the 1983-1990 surveys. The surveys used a Campelen 1800 mesh shrimp trawl with rubber bobbins (old standard) from 1983 to 1988, and the same trawl but equipped with a rockhopper ground gear (new standard) in 1989 and 1990 (see EngAs and Godo, 1989a, for a detailed description). The mesh size of the cod-end was 40 ram. Prior to 1989, 40 m sweep wires were used in the Barents Sea and 80 m ia the Svalbard area. Since 1989, 40 m sweep wires were used in both areas. Tow duration was either 30 or 60 min at a speed of 3 knots.

For each )ear the stratified mean catch per haul k

was calculated, where ~,, is the stratified mean catch per haul, a, is the area of strata i, A is the total area, and ~, is the average catch per tow. These values were then expressed as swept area indices or

i _ ~¢,, "A S

EFFECT OF SIZE SELECTIO~ ON ABUNDANCE INDICES 295

A i s o 2 o ° 2 s o

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IZ

6 g ~ "

iO ° 3 5 ° 4 0 ° 4 5 •

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Fig. 1. The Norwegian stratification for the Barents Sea (A) and the Svalbard area (B).

296 o.g. GODO AND IC SUNNAN~

where S is the area swept by a standard trawl tow. The effective sweep width was assumed to be 25 m.

The species and length compositions of trawl catches were determined either by sorting and measuring the total catch or by taking a representative subsam- pie. For both species, indices at age were calculated based on age-length keys. Bottom trawl survey methods are fully described in Dalen et al. ( 1982 ?, Randa and Smedstad ( 1983 ), and in Hylen etal. (1986).

Recalculation of indices

Recalculations were performed on each haul separately. Numbers caught per length group (5 cm) were adjusted by escapement factors obtained from trawling experiments as follows: ( 1 ) since 40 m sweeps catch smaller fish more efficiently than 80 m sweeps (Engas and Godo, 1989b), tile new stan- dard trawl uses 40 m sweeps, and all data were adjusted to this standard. Consequently, :he catches from the Svalbard surveys before 1989 were ad- justed by an average catch at length relationship derived from comparative trawling experiments using 40 m and 80 m sweeps (Table 1 ); (2) several comparative trawling experiments have demonstrated a large loss of small fish under the old trawl and the superiority of the new trawl in catching these size groups (Eng~s et al., 1988: Eng,~s and Godo, 1989a; Gode etal., 1989). Consequently, data from both the Svalbard and the Barents Sea surveys were adjusted to the new standard, i.e. catches from pre-1989 surveys were raised

TABLE 1

Adjustment factors used in the conversion of trawl catches and recalculation of abundance indices ( from Godo etal., 1989 and Eng,~s and Godo, 1989b)

Length Bobbins to Rockhopper 80 m-40 m sweeps group

Cod Haddock Cod Haddock

!~- i4 7.14 6.66 2.15 2.15 15-19 5.00 4.34 2.15 2.15 20-24 3.57 1.69 !.42 1.00 25-29 2.70 1.22 1.11 0.90 30-34 2.17 I. i I 0.87 0.80 35-39 1.88 1. I ! 0.77 0.70 40-44 1.69 1.15 0.71 0.60 45-49 1.59 1.25 0.65 0.60 50-54 1.49 i.33 0.63 0.60 55-59 !.41 1.33 0.61 0.60 60-64 1.37 !.33 0.60 0.60 65=69 1.33 i.33 0.60 0.60 70 + 1.33 1.33 0.60 0.60

EFFECT OF SIZE SELECTION ON ABUNDANCE INDICES 297

by escapement factors per 5 cm length group estabhshed from trawling exper- iments (Table I ).

The recalculations produced new estimates of catch at length, and the stan- dard procedures for calculating abundance indices at length and age were used on this data set. The term index factor (IF) refers to the total adjustment of an age group, i.e. the relationship between the old and new abunu~,~ce index for that age group. The catchability of the new trawl is assamed constant over time and age groups. For the old trawl, variability of IF for an age group re- flects changes in catchability caused by variation in mean length.

For comparative purposes, the catches from 1989 and 1990 from the new standard trawl were also converted to simulate the old standard indices from the Barents Sea and the Svalbard area using the inverse conversion factors.

RESULTS

The adjusted indices for 1983-1988, together with the indices from 1989- 1990, comprise the new standard time series. Similarly, the back-calculated 1989-1990 indices together with the original 1983-1988 indices constitute the old standard time series. The two sets of indices are calculated for cod in the Svaibard area (Tables 2 and 3), the Barents Sea (Tables 4 and 5 ), and for Barents Sea haddock (Tables 6 and 7 ).

The new standard indices for young cod (age < 2 years) differ more in the Svalbard area from the old standard indices than those in the Barents Sea (Fig. 2). The opposite is true for older fish. This is mainly due to the differ- ences in selection for 40 and 80 m sweeps. The post-1988 employment of 40 m sweep wires in the Svalbard area increased the catch of small fish and re- duced the catch of large fish when compared with 80 m sweep wires (see Ta- ble l ). The adjustments for haddock in the Barents Sea show the same ten- dency as for cod in that area, but with less of a difference between the highest and lowest IF values (Fig. 3). No results for haddock in the Svalbard area were derived since haddock are scarce there. The old cod indices normally increased to a maximum between ages 2 and 4, whereas the new indices to a large extent decrease with age (Tables 2-5, Fi& 4). However, while this seems to be generally true for the Svalbard indices, the new indices for age l- and 2- year-old cod in the Barents Sea are more variable. The old time series for Barents Sea haddock apparently does not have a maximum index at any spe- cific age, whereas for the new time series, the maximum is consistently at age l year.

In both areas there are apparent year-to-year differences in IF values. Changes in IF are caused by variation in mean length at age. These changes can be large. For example, the mean length at age of Barents Sea cod can vary by as much as l 0 cm over time (Fig. 5 ). Changes in mean length at age gen- erally follow the same trend for all immature age groups, with the possible

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EFFECT OF SIZE SELECrlON ON ABUNDANCE INDICES 301

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Fig. 2. index factors ( IF) for I-M-year-old cod from the S¢albard area (SVL) and the Barents Sea ( BAR ). The number after the area code is the age.

exception of 1 -year-old cod. The strongest effect is seen on indices ofcod aged 1-4 and haddock aged 2 ( Figs. 2, 3 ), while for older fish, variable growth rates are of less importance (Tables 2-7). The highest values are more than 100% above the lowest values for age 1 and 2 cod in the Svalbard area, while the differences are less pronounced for Barents Sea cod and haddock (Figs. 2, 3 ).

DISCUSSION

Generally, bottom trawl surveys are assumed to assess age groups with equal accuracy from year to year, i.e. constant catchability over time. But, trawls are highly length-selective sampling tools. The use of abundance estimates from trawl surveys therefore requires the assumption that variation in growth, i.e. size at age, over time does not significantly affect selection by age. The

EFFECW OF SIZE SELECTION ON ABUNDANCE INDICES 30~

6 F

o BAMI

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Fig. 3. Index factors (IF) for I-2-,~ear-old haddock from the Barents Sea (BAR). The number .,fter the area code is the age.

catch adjustments implemented in this paper adjust for known length selec- tion of the old sampl!ng trawl, and the year-to-year variation in IF for an age group is thus caused by changes in the mean length at age. As catchability of the old trawl is inversely related to IF, the estimated abundance indices based on catches from the old equipment are affected accordingly. Large periodic variation in growth rates of north-east Arctic cod is well documented (Nak- ken and Raknes, 1987; Jergensen, 1989; Ajiad et al., 1989). The current re- sults demonstrate that such growth variations of cod and haddock have greatly affected catchability at age for the old Norwegian sampling trawl, and the abundance of an age group was hence estimated with differing accuracy from year to year.

Consequently, for a particular year, mean lengths at age simultaneously above or below average led to over- or under-estimation respectively of both total stock and recruitment before 1989. This was partly the reason for the optimistic prognosis in the mid 1980s, while the opposite occurred in 1988 (Fig. 5, see also Anon., 1989). Also, for a year class, dramatic changes in mean length occur over time. The abundance of this year class will accord- ingly be upwards or downwards biased. This affects the forecast of year class abundance (Anon., 1989). For example, the index of the 1987 year class of cod in the Svalbard area was downwardly biased in 1988 (high IF of age 1 fish) and upwardly biased in 1989 (low IF of age 2 fish, Fig. 2). As prediction of growth is difficult, it is important for stock assessment and prediction that scientific surveys produce indices with constant accuracy from year to year. It should be kept in mind though, that the variation in year class strength during this time period was much larger than the IF variation and thus the overail picture of strong and weak year classes was not altered by variable selection.

~06 OR. CK)I~ AND K. S~JNNANA

INDEX 500['

I

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o.~

A

2 3 4 5 6

AGE (YEARS)

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7

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B 2000

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0 1 2 6 7 3 4 5

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Fig. 4. Old (hatched) and new (open) standard indices for the 1983 year class of cod in the Svalbard area (A) and in the Barents Sea ( B ) for the period 1984-1990.

EFFECT OF SIZE SELECTION ON ABUNDANCE INDICES ~07

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0 ( ) (

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10 - - - - A _ _ l _ , - - - ~ , . . . . . . . . 1983 1984 1985 1986 1987 1988 1989 1990

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Fig. 5. Changes in mean length at age for immature cod in the Barents Sea as observed by the surveys.

Scientific survey strategy and procedures are normally very resistant to changes, which may confound a long series. The current results demonstrate that a time series is not validated by a consistent history, but may have a variable bias due to the inefficiency of the equipment. When changing a sam- pling trawl, c,~nsistency requires that indices should be converted as we have done in this paper. The reliability of the new series is dependent on the pre- cision of the conversion factors. As shown by the trawl experiments, both sweep selection (EngAs and Oodo, 1989b) and escapement under the trawl (Godo et al., 1989) are susceptible to considerable variation, but the effect of varying fish length on selection is highly significant in both cases. Most gadoid surveys apply sampling trawls with relatively Tow efficiency for small fish (Zaferman and Serebrov, 1988; Walsh, 1989; Pjlsson et al., 1989) and will suffer from year-to-year variability in accuracy if changes in growth occur.

Another question which arises is the reliability of the conversion process when catchability is low, as exemplified by catchability < 20% for l-year-old fish. The variability in recorded catches of small fish will be large due to vari- ation in selectivity and because unbiased sampling is difficult if small fish appear in low numbers and are hidden in a large catch. The recalculated Sval- bard indices may be susceptible to large errors as they were adjusted twice

308 O.R. GODI~ AND K. SUNNANA

(sweep and under trawl selection). However, most of these factors are also serious problems for the old indices, due to the inefficiency of the old sam- piing trawl. We think, therefore, that the introduction of an improved stan- dard sampling gear, as done in the Norwegian surveys, is a correct strategy for improving survey reliability. We believe that the future quality of the Nor- wegian indices will be improved, v:hereas the uncertainty of the historic time series will remain independent of the gear changes and become less important as time passes.

The apparent poorer and more variable estimates of the abundance of age 1 and 2 cod in the Barents Sea as compared to the Svalbard survey, may be due to differences in vertical distribution in the two areas. Young cod in the Svalbard area are known to concentrate very close to the bottom during the survey period, and day-night differences in catches are small (Godo, 1990). In the Barents Sea there is extensive diurnal variability in catches of small cod (Eng~s and Soldal, 1991 ). As catches are lowest at night, and as nights are longer than days during the Barents Sea winter survey, small cod may be underestimated as compared to the Svalbard indices. The inconsistent pat- terns of year-class indices by age may be due to variation in vertical distribu- tion over time (see 3akobsen et al., 1989). Thus, availability to the trawl, more than the efficiency of the trawl itself, may be responsible for the Barents Sea IF variability (see also Godo, 1990).

Furthermore, age 1 fish in the Barents Sea, which are partly below 15 cm in the winter, may be susceptible to mesh selection. This selection may have stabilized the mean length (and hence IF) of age 1 fish, if only the largest individuals of that age group are retained in the codend. This effect is ex- pected to be far less during the Svalbard survey in September due to the in- creased size of the fish, and thus more realistic changes in mean length of age 1 fish should be observed.

The results for haddock are parallel to those for cod. The differences be- tween the old and new time series are, however, smaller and the IF is less variable than for cod. Only age 2 haddock scum to be greatly affected. This is mainly because the selection curve for the escapement under the trawl (Table 1 ) rises steeply between age 1 and 2 in haddock. The number of fish affected by this length selection is therefore limited.

For tuning the VPA, for estimating recrjitment and for catch predictions, the Svalbard and Barents Sea abundance indices have been used as indepen- dent measurements of abundance of north-east Arctic cod. Historic informa- tion on the geographic distribution of north-east Arctic cod show year-to-year variations in the proportions of a year class found in the two areas (Garrod, 1967, Randa, 1984), and thus an index of stock abundance should be a com- bination of the indices from the two areas. The sampling gear differences be- tween the two areas have made it difficult to combine the indices. The present conversion of the old indices to a common star,dard together with the new

EFFECT OF SIZE SELECTION ON ABUNDANCE INDICES 309

trawl standardization is a first step towards this goal. We believe that we have minimized the variation in catches between the areas caused by the fishing gear itself. However, as the two surveys are carried out at different times of the year and in areas of different topography, temporal and geographical dif- ferences in the availability and catchability of fish, factors independent of the trawl, may still be an obstacle to combining the two index series. This is a problem that needs further study.

ACKNOWLEDGEMENTS

We thank Michael Pennington, Woods Hole, USA for useful comments and correction of the English text.

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310 O.R. GOD~ AND K. SUNNANA

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